Steel Fibre Concrete Composites for Special Applications

Normal and High Volume Steel Fibre Concrete Composites for Special Applications

Dr. V.S. Parameswaran, President & Chief Executive, Design Technology Consultants, Chennai Chief Executive, International Centre for FRC Composites (ICFRC), Chennai, Former Director, SERC & Past President, ICI.

In recent times, the sustained efforts of researchers all over the world to innovate and incorporate unmatched excellence in construction have led to development of several advanced concrete construction materials. Of these, composites containing steel fibres have come to stay and deserve special mention. This paper, besides outlining the properties and applications of normal fibre reinforced concrete (SFRC), also describes the emergence and potentials of high-volume fibre composites such as slurry infiltrated fibrous concrete (SIFCON), slurry infiltrated mat concrete (SIMCON), compact reinforced concrete (CRC) and reactive powder concrete (RPC).

Steel Fibre Reinforced Concrete (SFRC)

Concrete is the most widely used structural material in the world with an annual production of over seven billion tons. For a variety of reasons, much of this concrete is cracked. The reason for

concrete to suffer cracking may be attributed to structural, environmental or economic factors, but most of the cracks are formed due to the inherent weakness of the material to resist tensile forces. Again, concrete shrinks and will again crack, when it is restrained. It is now well established that steel fibre reinforcement offers a solution to the problem of cracking by making concrete tougher and more ductile. It has also been proved by extensive research and field trials carried out over the past three decades, that addition of steel fibres to conventional plain or reinforced and prestressed concrete members at the time of mixing/production imparts improvements to several properties of concrete, particularly those related to strength, performance and durability.

The weak matrix in concrete, when reinforced with steel fibres, uniformly distributed across its entire mass, gets strengthened enormously, thereby rendering the matrix to behave as a composite material with properties significantly different from conventional concrete.

The randomly-oriented steel fibres assist in controlling the propagation of micro-cracks present in the matrix, first by improving the overall cracking resistance of matrix itself, and later by bridging across even smaller cracks formed after the application of load on the member, thereby preventing their widening into major cracks (Fig. 1).

The idea that concrete can be strengthened by fibre inclusion was first put forward by Porter in 1910, but little progress was made in its development till 1963, when Roumaldi and Batson

carried out extensive laboratory investigations and published their classical paper on the subject. Since then, there has been a great wave of interest in and applications of SFRC in many parts of the world. While steel fibres improve the compressive strength of concrete only marginally by about 10 to 30%, significant improvement is achieved in several other properties of concrete as listed in Table 1. Some popular shapes of fibres are given in Fig.2.

In general, SFRC is very ductile and particularly well suited for structures which are required to exhibit:

Resistance to impact, blast and shock loads and high fatigue Shrinkage control of concrete (fissuration) Very high flexural, shear and tensile strength Resistance to splitting/spalling, erosion and abrasion High thermal/ temperature resistance Resistance to seismic hazards.

The behavior of SFRC under fatigue loading regime as compared to conventional concrete is shown in Fig. 3, while Fig. 4 illustrates the improvement in impact resistance of SFRC with the increase in the fibre content. The high ductility exhibited by normal SFRC and polymer-impregnated SFRC over conventional concrete is shown in Fig. 5.

The degree of improvement gained in any specific property exhibited by SFRC is dependent on a number of factors that include:

Concrete mix and its age Steel fibre content Fibre shape, its aspect ratio (length to diameter ratio) and bond characteristics.

The efficiency of steel fibres as concrete macro-reinforcement is in proportion to increasing fibre content, fibre strength, aspect ratio and bonding efficiency of the fibres in the concrete matrix. The efficiency is further improved by deforming the fibres and by resorting to advanced production techniques. Any improvement in the mechanical bond ensures that the failure of a SFRC specimen is due mainly to fibres reaching their ultimate strength, and not due to their pull-out.

Mix Design for SFRC

Just as different types of fibres have different characteristics, concrete made with steel fibres will also have different properties.

When developing an SFRC mix design, the fibre type and the application of the concrete must be considered. There must be sufficient quantity of mortar fraction in the concrete to adhere to the fibres and allow them to flow without tangling together, a phenomenon called ‘balling of fibres’ (Fig. 6). Cement content is, therefore, usually higher for SFRC than conventional mixes Aggregate shape and content is critical. Coarse aggregates of sizes ranging from 10 mm to 20 mm are commonly used with SFRC. Larger aggregate sizes usually require less volume of fibres per cubic meter.

SFRC with 10 mm maximum size aggregates typically uses 50 to 75 kg of fibres per cubic meter, while the one with 20 mm size uses 40 to 60 kg.

Smaller sections less than about 100 mm in thickness should be considered as requiring 10 mm aggregate size only.

It has been demonstrated that the coarse aggregate shape has a significant effect on workability and material properties. Crushed coarse aggregates result in higher strength and tensile strain capacity.

Fine aggregates in SFRC mixes typically constitute about 45 to 55 percent of the total aggregate content.

Typical mix proportions for SFRC will be: cement 325 to 560 kg; water-cement ratio 0.4-0.6; ratio of fine aggregate to total aggregate 0.5-1.0; maximum aggregate size 10mm; air content 6-9%; fibre content 0.5-2.5% by volume of concrete. An appropriate pozzolan may be used as a replacement for a portion of the Portland cement to improve workability further, and reduce heat of hydration and production cost. The suggested mix proportions for making SFRC mortars and concretes is given in Table 2.

The use of steel fibres in concrete generally reduces the slump by about 50 mm. To overcome this and to improve workability, it is highly recommended that a super plasticizer be included in the mix. This is especially true for SFRC used for high-performance applications.

Generally, the ACI Committee Report No. ACI 554 ‘Guide for Specifying, Mixing, Placing and

Finishing Steel Fibre Reinforced Concrete’ is followed for the design of SFRC mixes appropriate to specific applications.

Fibre Shotcreting

“Shotcreting” using steel fibres is being successfully employed in the construction of domes, ground level storage tanks, tunnel linings, rock slope stabilization and repair and retrofitting of deteriorated surfaces and concrete. Steel fibre reinforced shotcrete is substantially superior in toughness index and impact strength compared to plain concrete or mesh reinforced shotcrete.

In Scandinavian countries, shotcreting is done by the wet process and as much as 60% of ground support structures (tanks and domes) in Norway are constructed using steel fibres. In many countries including India, steel fibre shotcrete has been successfully used in the construction of several railway and penstock tunnels (Fig. 7).

Typical mix proportions for making fibre shotcrete with sand only, and with a combination of sand and coarse aggregate, is given in Table 3.

Applications of SFRC

The applications of SFRC depend on the ingenuity of the designer and builder in taking advantage of its much enhanced and superior static and dynamic tensile strength, ductility, energy-absorbing characteristics, abrasion resistance and fatigue strength.

Growing experience and confidence by engineers, designers and contractors has led to many new areas of use particularly in precast, cast in-situ, and shotcrete applications. Traditional application where SFRC was initially used as pavements, has now gained wide acceptance in the construction of a number of airport runways, heavy-duty and container yard floors in several parts of the world due to savings in cost and superior performance during service.

The advantages of SFRC have now been recognised and utilised in precast application where designers are looking for thinner sections and more complex shapes. Applications include building panels, sea-defence walls and blocks, piles, blast-resistant storage cabins, coffins, pipes, highway kerbs, prefabricated storage tanks, composite panels and ducts. Precast fibre reinforced concrete manhole covers and frames are being widely used in India, Europe and USA.

Cast in-situ application includes bank vaults, bridges, nosing joints and water slides. “Sprayed-in” ground swimming pools is a new and growing area of shotcrete application in Australia. SFRC has become a standard building material in Scandinavia.

Applications of SFRC to bio-logical shielding in atomic reactors and also to waterfront marine structures which have to resist deterioration at the air-water interface and impact loadings have also been successfully made. The latter category includes jetty armor, floating pontoons, and caissons. Easiness with which fibre concrete can be moulded to compound curves makes it attractive for ship hull construction either alone or in conjunction with ferrocement.

Use of SFRC for repair work is also a growing market. Several tunnels and bridges have been repaired with spraying of layers of shotcrete after proper surface preparation. A few most common applications of SFRC are illustrated in Fig. 8.

SFRC shotcrete has recently been used for sealing the recesses at the anchorages of post stressing cables in oil platform concrete structures. Recent developments in fibre types and their geometry and also in concrete technology and equipment for mixing, placing and compaction of SFRC and mechanized methods for shotcreting have placed Scandinavian and German consultants and contractors in a front position in fibre-shotcreting operations world wide.

Laboratory investigations have indicated that steel fibres can be used in lieu of stirrups in RCC frames, beams, and flat slabs and also as supplementary shear reinforcement in precast, thin-webbed beams. Steel fibre reinforcement can also be added to critical end zones of precast prestressed concrete beams and columns and in cast-in-place concrete to eliminate much of the secondary reinforcement. SFRC may also be an improved means of providing ductility to blast-resistant and seismic-resistant structures especially at their joints, owing to the ability of the fibres to resist deformation and undergo large rotations by permitting the development of plastic hinges under over-load conditions.

Slurry Infiltrated Fibrous Concrete (SIFCON)

SIFCON is a high-strength, high-performance material containing a relatively high volume percentage of steel fibres as compared to SFRC. It is also sometimes termed as ‘high-volume fibrous concrete’. The origin of SIFCON dates to 1979, when Prof. Lankard carried out extensive experiments in his laboratory in Columbus, Ohio, USA and proved that, if the percentage of steel fibres in a cement matrix could be increased substantially, then a material of very high strength could be obtained, which he christened as SIFCON.

While in conventional SFRC, the steel fibre content usually varies from 1 to 3 percent by volume, it varies from 4 to 20 percent in SIFCON depending on the geometry of the fibres and the type of application. The process of making SIFCON is also different, because of its high steel fibre content. While in SFRC, the steel fibres are mixed intimately with the wet or dry mix of concrete, prior to the mix being poured into the forms, SIFCON is made by infiltrating a low-viscosity cement slurry into a bed of steel fibres ‘pre-packed’ in forms/moulds (Fig. 9).

The matrix in SIFCON has no coarse aggregates, but a high cementitious content. However, it may contain fine or coarse sand and additives such as fly ash, micro silica and latex emulsions. The matrix fineness must be designed so as to properly penetrate (infiltrate) the fibre network placed in the moulds, since otherwise, large pores may form leading to a substantial reduction in properties.

A controlled quantity of high-range water-reducing admixture (super plasticizer)may be used for improving the flowing characteristics of SIFCON. All types of steel fibres, namely, straight, hooked, or crimped can be used.

Proportions of cement and sand generally used for making SIFCON are 1: 1, 1:1.5, or 1:2. Cement slurry alone can also be used for some applications. Generally, fly ash or silica fume equal to 10 to 15% by weight of cement is used in the mix. The water-cement ratio varies between 0.3 and 0.4, while the percentage of the super plasticizer varies from 2 to 5% by weight of cement. The percentage of fibres by volume can be any where from 4 to 20%, even though the current practical range ranges only from 4 to 12%.

Uniaxial Tensile Strength

Unlike the cracks which form in continuous reinforced cementitious composites such as ferrocement, the cracks in SIFCON generally do not extend through the whole width of the specimen. Instead, they can be short and randomly distributed within the loaded volume, i.e. on the surface and through the depth of the specimen. The ultimate tensile strength of SIFCON typically varies from 20 to 50 MPa, depending on the percentage of steel fibres and the mix proportions used.

Compressive Strength

The cement slurry (without fibres) used in the making of SIFCON generally develops a one-day strength of 25 to 35 MPa, and a 28-day strength of 50 to 70 MPa. The corresponding values for SIFCON composites are 40 to 80 MPa and 90 to 160 MPa, respectively, depending on the percentage of steel fibres incorporated in the matrix. Generally, SIFCON exhibits an extremely ductile behavior under compression.

Flexural Strength

The ultimate flexural strength of SIFCON is found to be very high and is in the order of magnitude higher than that of normal SFRC. The values observed by several researchers range from 25 to 75 MPa with an average of about 40 MPa. SIFCON is found to possess excellent ductility both under monotonic and high-amplitude cyclic loading.

Shear Strength

Investigations carried out in USA, Denmark and India have shown that the ultimate shear strength of SIFCON specimens were 30.5, 28.1, 33.3 and 31.8 MPa, respectively, for fibre lengths of 30, 40, 50 and 60 mm, indicating thereby that the fibre length does not seem to affect the shear strength. The average shear strength of SIFCON can be taken as about 30 MPa as compared to just about 5 MPa for plain concrete.

Resistance to Abrasion, Impact, Fatigue, and Repeated Loading

SIFCON possesses extremely high abrasion and impact resistance, when compared with plain concrete and SFRC specimens. The resistance improves further drastically with the increase in the percentage of fibres. It is several times that of ordinary plain or reinforced concrete.

Design Principles

The design methods for SIFCON members must take into account their application or end-use, the property that needs to be enhanced, mix proportion, strength, as well as its constructability and service life. In general, a high-strength SIFCON mix can easily be designed and obtained

with virtually any type of steel fibres available today, if the slurry is also of high strength.

Like conventional concrete, the strength of the slurry is a function of the water-cement ratio; because the slurry mixes used in SIFCON usually contain significant percentages of fly ash or silica fume or both, the term “water-to-cement plus admixtures” is used when designing the slurry mix. In addition, the ratio of the “admixtures to cement” is also an important parameter in the design of SIFCON. It is also to be noted that higher volume percentages of fibres need lower viscosity slurry to infiltrate the fibres thoroughly. In general, the higher the strength of the slurry, the greater is the SIFCON strength.

Applications of SIFCON

SIFCON possesses several desirable properties such as high strength and ductility. It also exhibits a very high degree of ductility as a result of which it has excellent stability under dynamic, fatigue and repeated loading regimes (Figs. 10 a & 10 b). It is also quite expensive. Because of this, SIFCON should be considered as an efficient alternative construction material only for those applications where concrete or conventional SFRC can not perform as may be expected/required by the user or in situations where such unique properties as high strength and ductility are required.

Since properties like ductility, crack resistance and penetration and impact resistance are found to be very high for SIFCON when compared to other materials, it is best suited for application in the following areas:

Pavement rehabilitation and precast concrete products Overlays, bridge decks and protective revetments Seismic and explosive-resistant structures Security concrete applications (safety vaults, strong rooms etc) Refractory applications (soak-pit covers, furnace lintels, saddle piers) Military applications such as anti-missile hangers, under-ground shelters Sea-protective works Primary nuclear containment shielding Aerospace launching platforms Repair, rehabilitation and strengthening of structures Rapid air-field repair work Concrete mega-structures like offshore and long-span structures, solar towers etc.

Compact Reinforced Concrete (CRC)

CRC is a new type of composite material. In its cement-based version, CRC is built up of a very strong and brittle cementitious matrix, toughened with a high concentration of fine steel fibres and an equally large concentration of conventional steel reinforcing bars continuously and uniformly placed across the entire cross section (Fig. 11).

CRC was initially developed and tested by Prof.Bache at the laboratories of Aalborg Portland cement factory in Denmark. The pioneering experiments carried out at this laboratory established the vast potential of CRC for applications that warrant high strength, ductility and durability.

CRC has structural similarities with reinforced concrete in the sense that it also incorporates main steel bars, but the main bars in CRC are large in number and are uniformly reinforced. Owing to this and also because of the large percentage of fibres used in its making, it exhibits mechanical behavior more like that of structural steel, having almost the same strength and extremely high ductility.

CRC specimens are produced using 10-20% volume of main reinforcement (in the form of steel bars of diameter from about 5 mm to perhaps 40 or 50 mm) evenly distributed across the cross section) and 5-10% by volume of fine steel fibres. The water-cement ratio is generally very low, about 0.18% and the particle size of sand in the cement slurry is between 2 and 4mm.The flow characteristics while mixing and pouring is aided by the use of micro silica and a dispersant. High-frequency vibration is often resorted to for getting a the mix compacted and to obtain homogeneity. Prolonged processing time for mixing, about 15-20 minutes, ensures effective particle wetting and high degree of micro-homogeneity.

Such highly fibre-reinforced concrete typically has compressive strengths ranging from 150 to 270 MPa, and fracture energy from 5,000 to as much as 30,000 N/m.

CRC beams exhibit load capacities almost equivalent to those of structural steel and remain substantially uncracked right up to the yield limit of the main reinforcement (about 3 mm/m), where as conventional reinforced concrete typically cracks at about 0.1-0.2 mm/m.

Some of the properties of CRC as obtained from extensive experiments carried out on CRC specimens are given in Table 4.

Design of CRC

The development and design of CRC is based on fracture mechanics principles/theories, that takes into account the coherent and ductile phase of the composite, cracked pattern and ultimate failure mode. The theories assume that, as in the case of metals, any single, micro crack developed owing to the presence of a local flaw can not propagate and cause sudden tensile failure because of the interlinked pattern of main steel and fibres, thereby rendering the composite highly elastic, ductile and strong.

Applications of CRC

CRC can probably be used especially in the form of large plates or shells designed, for example, to resist very large local loads with unknown attack position (from explosives, say, or mechanical impact) or to resist uniformly distributed pressure, either as pure compression or pure tension (e.g. large pressure tanks).

Because CRC has very high “strength-density ratios” (often greater than those of commonly used structural steel), it offers particularly interesting possibilities for members, where weight and inertia loads are decisive. It could, for instance, be used for different forms of transport (ships, vehicles, etc.), where low weight is essential, or for rapidly rotating large machine parts, where the performance is limited by the capacity of the materials to resist their own inertia loads.

The high degree of ductility of CRC, even at very low temperatures, will make CRC very interesting for large objects that have to resist large loads at low temperatures, where steel will fail due to brittleness or suffer functional deficiency due to progressive corrosion damage.

Because of the far better possibilities of forming CRC and combining it with several other components than those afforded by steel, CRC finds its principal use in hybrid constructions – for example, load-carrying parts in large machines, or special high-performance joints in conventional steel and concrete structures, where large forces have to be concentrated in small volumes.

Slurry Infiltrated Mat Concrete (SIMCON)

SIMCON can also be considered a pre-placed fibre concrete, similar to SIFCON. However, in the making of SIMCON, the fibres are placed in a “mat form” rather than as discrete fibres. The advantage of using steel fibre mats over a large volume of discrete fibres is that the mat configuration provides inherent strength and utilizes the fibres contained in it with very much higher aspect ratios. The fibre volume can, hence, be substantially less than that required for making of SIFCON, still achieving identical flexural strength and energy absorbing toughness.

SIMCON is made using a non-woven “steel fibre mats” that are infiltrated with a concrete slurry. Steel fibres produced directly from molten metal using a chilled wheel concept are interwoven into a 0.5 to 2 inches thick mat. This mat is then rolled and coiled into weights and sizes convenient to a customer’s application (normally up to 120 cm wide and weighing around 200 kg).

As in conventional SFRC, factors such as aspect ratio and fibre volume have a direct influence on the performance of SIMCON. Higher aspect ratios are desirable to obtain increased flexural strength. Generally, because of the use of mats, SIMCON the aspect ratios of fibres contained in it could well exceed 500. Since the mat is already in a preformed shape, handling problems are significantly minimised resulting in savings in labour cost. Besides this, “balling” of fibres does not become a factor at all in the production of SIMCON.

Investigations using manganese carbon steel mats (having fibres approximately 9.5 in long with an equivalent diameter of about 0.01 to 0.02 in) and stainless steel mats (produced using 9.5 in long fibres with an equivalent diameter of about 0.01 to 0.02 in) have revealed that SIMCON has performed very well compared with SIFCON specimens that had a steel fibre content of 14% by volume as illustrated in Table 5.

It is clear from the table that the energy-absorption capacity of SIMCON is far superior to SIFCON. A reinforcement level in SIMCON of only 25% of that of conventional SIFCON is found to provide as much as 75% of the latter’s ultimate flexural strength.

Applications of SIMCON

SIMCON offers the designer a premium building material to meet the specialised niche applications, such as military structures or industrial applications requiring high strength and ductility.

While the use of SIFCON is presently limited only to specialised applications owing to high material and labour costs involved in the incorporation of a very high volume of discrete fibres that are required for achieving vastly improved performance, SIMCON broadens these market applications by cutting the fibre quantity to less than half and there by substantially reducing the product cost.

Reactive Powder Concrete (RPC)

Another recent development in concrete technology is the production of reactive powder concrete (RPC) containing steel fibres as macro-reinforcement. First developed by Bouygues-SA, Paris, its processing has been patented. A high degree of strength, compactness, refined microstructure and homogeneity is achieved by using dense and powder-like particles smaller than 600 microns, and in some cases 300 microns, and by the addition of 2 to 5% of steel fibres. RPC, therefore, do not contain any aggregates, and traditional sand is replaced totally by finely ground quartz of particle size less than 300 microns.

The compactness of an RPC mix is enhanced further by pressing the mix before and during setting, while still in the moulds/forms and by using a very low water-cement ratio (about 0.2%). By subjecting the material to low or high pressure steam curing and by applying pressures up to 50 MPa, the pozzalolanic reaction of the silica fume is accelerated resulting in further modifying of the structure of the hydrates and in concrete strengths as high as 500MPa.

Even though RPC is very strong, it exhibits a brittle failure when fibres are not present. By confining RPC (with steel fibres) in mild steel /stainless steel tubes and applying pressure-cum-heating techniques during its casting, the compressive strength and ductility can be improved tremendously. It is reported that very high strengths of 200 to 800 MPa can be obtained for RPC with cement contents of 955 to 1000 kg/m3. Typical composition of an RPC mix used in the construction of the very first RPC pedestrian bridge built in 1997 in Sherbrooke, Quebec, Canada is given in Table 6. A view of another bridge built in Japan using RPC filled stainless tube supporting columns is shown in Fig. 12.

In due course of time, RPC is expected to outperform normal high performance concretes (HPC) as illustrated in Table 7.

Indian Scenerio

In India, SIFCON, CRC, SIMCON and RPC are yet to be used in any major construction projects. For that matter, even the well-proven SFRC has not found many applications yet, in spite of the fact that its vast potentials for civil engineering uses are quite well known. The reason for these materials not finding favour with designers as well as user agencies in the country could be attributed to the non-availability of steel fibres on a commercial scale till a few years ago. The situation has now changed. Plain round or flat and corrugated steel fibres are presently available in the country in different lengths and diameters. It is, therefore, possible now to use new-age construction materials like SIFCON and CRC in our country in the construction of several structures that demand high standards of strength coupled with superior performance and durability.

Acknowledgment

Some of the pictures and tables included in the paper have been freely extracted from the Keynote paper presented by the author at the National Conference on Advances in Construction Materials, Methodologies & Management organized by the Chaitanya Bharathi Institute of Technology at Hyderabad during 21-22 January, 2009. The author thanks the organizers of the Conference for giving consent to make use of them in the preparation of this paper.

Cementitious Composites with Steel Reinforcement

Response of Engineered Cementitious Composites with Steel Reinforcement and Concrete in Moment Resisting Frames

Dr. S. C. Patodi, Professor, J. D. Rathod, Applied Mechanics Department, Faculty of Technology & Engineering, M.S.University Baroda.

With the advent of new materials, there is a constant need for designers to find innovative ways to incorporate these materials into new applications. The field of civil engineering is currently at a cross roads of equal significance with development of new materials termed as High Performance Fiber Reinforced Cementitious Composites (HPFRCC). These materials with tensile performance magnitudes higher than Reinforced Concrete (R/ C), allow designers to create structures previously impossible due to limitations of minimum reinforcement, minimum clear cover or excessive cracking in R/C. The replacement of brittle concrete with an Engineered Cementitious Composite (ECC), which represents a class of HPFRCC, micro structurally tailored with strain hardening and multiple cracking properties, has shown to provide improved load-deformation characteristics in terms of reinforced composite tensile strength, deformation mode and energy absorption. This paper reports, investigation of response mechanism of composite moment resisting frame system with large energy dissipation capabilities. Plain cementitious matrix is used in frame specimens to estimate deformation behavior and formation of plastic hinges. Expected plastic hinge regions are properly detailed by steel reinforcement. Deformation mechanism of plain cementitious matrix suggested economic use of ECC by replacement with concrete in some areas. Load-displacement curves are plotted and compared for damage tolerance evaluation. Crack width is measured as a function of load for damage reduction evaluation and toughness index is found out for post peak performance evaluation. Compatibility of ECC with reinforcement and concrete in terms of deformation and strength is discussed.

Introduction

In earthquake resistant design, the structural system performance requirements can be specified in terms of minimum ductility ratio, number of load cycles, sequence of application of load cycles and permissible reduction in strength at the end of loading. At the beam column connection level, the following performances are desirable:

i. Ductile plastic hinge behavior under high shear stress, ii. No congestion of transverse reinforcement for confinement and for shear,

iii. Concrete integrity under load reversals andiv. Concrete damage contained within a relatively short hinging zone. These performances

are difficult to achieve with ordinary concrete, although some encouraging results have been obtained with Fiber Reinforced Concrete (FRC)[1].

Desirable performance of the plastic hinge is not easy to translate directly into numerical quantities of materials property requirement. In general, however, it may be expected that the following properties of the concrete material in the plastic hinge should be advantageous:

i. High compression strain capacity to avoid loss of integrity by crushing,ii. Low tensile first cracking strength to initiate damage within the plastic hinge,

iii. High shear and spall resistance to avoid integrity loss by diagonal fractures and iv. Enhanced mechanism that increases inelastic energy dissipation. ECC is a class of ultra

ductile fibre reinforced cementitious composite used to achieve above objectives without introducing ductile detailing in a structure. ECC can undergo upto 5% strain in tension, yet at the lower fibre volume of 2% with flexible processing. ECC can be used in some fused zones so that with the above performance, overall performance of the structure can be enhanced[2].

ECC when used with ordinary reinforcement detailing replacing the concrete at some key places, interact with reinforcement and concrete. Both, reinforcing steel and ECC can be considered as elastic-plastic material capable of sustaining deformation up to several percent strains. As a result, the two materials remain compatible in deformation even as steel yields. Compatible deformation implies that there is no shear lag between the steel and the ECC, resulting in a very low level of shear stress at their interface. As a result of low interfacial stress between steel and the ECC, the bond between ECC and reinforcement is not as critical as in normal R/C, since stress can be transmitted directly through the ECC via bridging fibers even after microcraking. In contrast, in R/C members the stress must be transferred via interface to the concrete away from the crack site. After concrete cracks in an R/C element, the concrete unloads elastically near the crack site, while the steel takes over the additional load shed by the concrete. This leads to incompatible deformation and high interface shear stress responsible for the commonly observed failure modes such as bond splitting and/or spalling of the concrete cover. ECC has excellent shear capacity. Under shear ECC develops multiple cracking with cracks aligned normal to the principal tensile direction. Because the tensile behavior of ECC is ductile, the shear response is correspondingly ductile. As a result, R/ ECC elements may need less or no conventional steel shear reinforcement. With tensile strain hardening and ultra high tensile strain capacity, ECC can sustain very large deformation without damage localization. When ECC structural element is loaded in flexure or shear beyond the elastic range, the inelastic deformation is associated with micro cracking with continued load carrying capacity across these cracks[3]. The tight crack width in ECC has advantageous implications on structural durability and on the minimization of repair needs subsequent to severe loading of an ECC member. ECC can eliminate premature delamination or surface spalling in an ECC/concrete combination.

In the present work, the effects of cementitious composite ductility on the steel reinforced behavior are experimentally investigated and contrasted to the unreinforced composite. Interaction between ECC and concrete is observed and possibility of replacing ECC with concrete is explored. Tight crack width control in ECC is examined. L-type plane frame and portal frame specimens are used for the experimental investigation.

Material Composition

Recron 3S brand synthetic fibers of triangular cross section produced by Reliance Industries were used with cementitious matrix. Fiber volume fraction of 4% was used which was found as optimum fiber volume fraction by pilot tests. Kamal brand 53 grade OPC, 300 m passing silica sand, 2% dose of concrete super– plasticizer of conplast SP430 brand with w/c ratio as 0.35 and

sand/cement ratio as 0.5 were used for the preparation of samples of ECC in the present experimental investigation. In addition, Kamal brand 53 grade OPC, silica sand confirming to zone III, 12.5 mm size coarse aggregates with w/c ratio of 0.35 and 0.5% dose of superplasticizer were used to produce concrete for use in combination with ECC in C-ECC specimens. Mix proportion for concrete used was 1:1.295:2.407. Mild steel reinforcement having yield strength of 250 N/mm2 was used in R-ECC specimen.

Specimen Configuration

L-type plane frame specimens, 3 specimens each, were cast with plain cementitious matrix and ECC with 4% fiber. Portal frame specimens, 3 specimens each, were cast with plain cementitious matrix (PCC), ECC with 4% fiber, steel reinforced ECC (R-ECC), and combination of ECC and concrete (C-ECC). Specimen configuration of LFigure type frame and portal frame specimens is shown in Figure 1.

Experimental Programme

For the preparation of specimens, the ingredients in required proportion were mixed in Hobart type mixer machine. Flow table test was performed to satisfy workability criteria in fresh state. After filling the mould with the matrix, it was compacted and demoulded after 24 hours. All the specimens were kept in curing tank for 28 days at room temperature. After putting proper identification mark, specimens were fixed into prefabricated experimental set up on MTS machine. Basic Testware available on computer supervised controller was used to conduct the test. All the specimens were tested in flexure at a displacement control rate of 0.005 mm/sec. Load and displacement at the first crack and at ultimate load were recorded during the test. Loaddisplacement curves were plotted and data were automatically recorded using basic Testware data acquisition facility. Crack width was measured for the first initiated crack during the test with the help of travelling microscope having least count of 0.01 mm. Test setups for plane frame and portal frame specimens are shown in Figure 2.

Discussion of Test Results

L-type frame specimens were tested under flexure. First crack load and ultimate load results are reported in Table 1 for plain matrix (L-0%) and ECC with 4% (L-4%) fiber matrix. Reserved strength refers to increase in strength of the member upto ultimate load over the first crack strength. This criterion is used to represent residue strength of the material. Deflection hardening refers to increase in deflection of the member upto ultimate load over the first crack deflection. This value shows inelastic deformation capability of the member which represents ductility of the material. Plain matrix failed suddenly with no reserved strength and deflection hardening. ECC-4% matrix has load and displacement values higher than plain matrix. First crack displacement is almost 2 times and ultimate displacement is almost 3.5 times than plain cementitious matrix. There is marginal increase in reserved strength but considerable improved performance in deflection hardening over plain cementitious matrix is observed. Maximum deflection

hardening achieved is more than 100%. Ultimate flexural strength of ECC-4% is found as 3.63 N/mm2 against 2.69 N/mm2 that of plain matrix.

Load-displacement curves are plotted for all the three specimens of ECC-4% as shown in Figure 3. Strain hardening is observed in all the specimens with very little linear portion in the beginning.Toughness index I5 is calculated as area under the load displacement curve for 3 times first crack displacement divided by the area under load displacement curve for first crack displacement. Post peak performance of the material can be represented by this value, which is also indicative of energy absorption capacity of the material. Toughness index I5 for ECC- 4% specimen is found as 4.21 which for a plain matrix could not be represented as it failed suddenly after the formation of first crack.

One can utilize design strength up to ultimate strength of ECC matrix in strain hardening zone. Development of cracks and crack width are therefore important in strain hardening zone. ECC matrix is well known for its tight crack width control which is utmost important for the durability of a member. One should make sure that migration of aggressive substances into matrix should be eliminated so that corrosion of reinforcement and subsequently spalling of matrix and delamination can be prevented. According to ACI committee 224, ultimate crack width should be limited to 150 μm when member is exposed to an environment of seawater and seawater spray in wetting and drying [4]. Rate of increase of crack width as a function of load gives information about consideration of design load for particular crack width criteria. Crack width was measured of the first visual crack and then crack width development with increase in load in the column of L- frame was recorded and was found within 150 μm at ultimate load.

Crack generation history in the column of L frame is tabulated in Table 2 in which crack number along with its location from bottom of the beam is presented. Failure of L-type specimens took place due to rotation of column in the middle at crack number 4. First crack generated right below the bottom of the beam and subsequent cracks appeared below first crack with spacing of about 2 cm up to middle of the column as the load increased. Spacing of the cracks was more below the crack number 4. Number of crack formation with increase in load in the column of L–type specimen is shown in Figure 4.

Portal frame specimens were tested under flexure to evaluate ECC performance along with combination of reinforcement and concrete. Strain hardening was not observed in plain cementitious matrix. First crack load and ultimate load of PCC, ECC, RECC and C-ECC frames are given in Table 3.

Lower first crack strength and then large amount of plastic hinge formation is desirable for seismic response so as to have large energy dissipation. This behavior is reflected in ECC sample number 1 and 3. Sample number 3 of ECC-4% performed well in both and showed reserved strength as 365.44% and deflection hardening as 331.38% which is the maximum among ECC, R-ECC and C-ECC. In R-ECC samples nominal mild steel reinforcement of diameter 4 mm and 6 mm were used as shown in Figure 1(D). Shear reinforcement was not used looking to the enhanced shear capacity of ECC material. R-ECC specimens showed consistent enhanced performance with percentage reserved strength and percentage deflection hardening. Also, the deformation compatibility between ECC and reinforcement was observed.

Concrete of compressive strength 58.89 N/mm2 [5] was used alongwith ECC matrix as per the plastic hinge formation and compression zone requirement in plain cementitious matrix. Replac–ement of ECC by concrete is indicated by dark portion in Figure 1(C). C-ECC specimens render economy in strength perfor– mance as is clear from the higher first crack and ultimate strength compare to ECC. Deformation compatibility between ECC and concrete and enhancement of strength perfor– mance after first crack is, however, questionable which can be observed from the poor results of percentage reserved strength and percentage deflection harde– ning. Bending moment and shear force at the base of column (AB), top ofcolumn (BA), and end of the beam (BC), along with bending moment at the center of the beam are calculated and tabulated in Table 4.

Ultimate flexural strength and shear strength in PCC, ECC, RECC and C-ECC are calculated and tabulated in Table 5. Contribution of mild steel in flexure and associated consistent compatible deformation is highlighted in the result of R-ECC. Shear reinforcement is not provided in R-ECC specimen. Shear resistance is contributed by ECC material only. Ultimate shear strength of ECC material for ECC-4% is 7.51 N/mm2 [5] which is approximately double than M20 concrete. Calculated shear strength in

sample number 1 is 12.02 N/mm2 which is higher than the ultimate shear strength of ECC. Therefore, shear failure of beam in R-ECC specimen is observed as shown in Figure 5.

Crack width as a function of load was measured on the column of RECC sample number 2 and results are given in Table 6. Crack width remained 100 mm at a load of 19,494 N. Approximately, 20,000 N load is found to act for the threshold crack width of 150 μm. Structural element should be loaded corresponding to maximum permissible crack width of 150 μm from durability point of view. Development of the crack width upto 20,000 N load is slow but then it becomes fast.

Development of crack width was also measured in beam of RECC sample number 3. There was a slow crack width development upto 20,833 N load but then suddenly it became fast. Approximately, 21,000 N causes crack width within limit of 150 μm.

Crack development along with its location in the column from bottom of a beam for R-ECC was studied and is represented here in Figure 6 and Table 7. First crack initiated right at the bottom of the beam and new cracks generated below the first crack at approximately constant spacing with increase in load unlike ECC specimen.

Load displacement curves are plotted in Figure 7 for PCC, ECC-4, R-ECC and C-ECC specimens. PCC and CECC could not show strain hardening. ECC-4 specimen showed well defined strain hardening and post peak performance with less first crack load. R-ECC specimen performed the best with respect to strength, strain hardening and post peak behavior. Toughness indices are found out for ECC, R-ECC and C-ECC and tabulated in Table 8. As load displacement curve of ECC indicates the best post peak performance, the toughness index of 12.67 could be obtained for ECC.

Crack patterns for PCC, ECC-4, R-ECC and C-ECC are shown in Fig. 8. Single crack formation at the center of the beam and top of the columns were responsible for failure of the PCC specimen. This crack pattern gave information about reinforcement detailing and concrete substitution. Rotation of the beam in the center and at the top of the column was seen in ECC, R-ECC and CECC specimens. The crack pattern of ECC, R-ECC and C-ECC were distinctly different from that of PCC. The first crack started at the midspan of the beam on the tensile face, and multiple cracks developed from the first cracking point and spreaded to the outside of the midspan. The multiple cracks at the outside of the midspan were inclined similar to the shear cracks in the R-ECC beams. As the ultimate load approached, one of the cracks from the midspan started to open up after the development of large damage zone. Horizontal parallel cracks starting from the top of the column at the constant spacing of 2 to 5 cm developed upto the center of the column as shown in Figure 6. The first crack at the top of the column widened and rotation took place from this crack.

R-ECC specimens having larger resistance to rotation due to reinforcement did not fail due to rotation. Cracks were not seen along the reinforcement even after such large inelastic deformation which indicates good compatibility between reinforcement and ECC. Shear strength of the beam at support became more than ultimate shear strength of ECC material. Shear reinforcement was not provided in the beam. Eventually, beam of RECC failed due to shear from one of the ends as shown in Figure 5. Fractured surface of combination of ECC material with concrete revealed that there is good bond between two materials, without any delamination and spalling.

Conclusion

Plastic hinges were formed at beam column junction in L and Portal frames. ECC plays significant role in rotation of such plastic hinges in ductile manner. Therefore, energy absorption capacity of plastic hinges in such cases is greatly enhanced. Total collapse of structure can be much delayed or damage can be minimized with the help of such fused zones made with ECC and thus the overall performance of the structure can be improved.

ECC has compatible deformation and good bond strength with steel reinforcement. Debonding of ECC with steel reinforcement due to shear, spalling, punching was not observed. R-ECC renders maximum improvement in structural performance. Shear resistance of ECC is also quite large. Shear reinforcement can thus be minimized or eliminated, but it requires careful design.

C-ECC has no problem with flexural strength compatibility. However, it has poor deformation compatibility. It requires further investigation for proper interface behavior.

In R-ECC, ECC and C-ECC, vertical and inclined multiple cracks with close spacing are observed in beam portion while horizontal cracks with 2 to 3 cm spacing are observed in columns of portal frame specimens. Damage zone is large in column compared to beam. This strong column-weak beam concept can be used for specimen configuration and thus hinge formation in the column can be avoided.

Tight crack width control is the key property of ECC for durability performance. Ultimate crack width of ECC matrix remains within 150 mm upto quite large load considered to be sound for concrete durability. Thus, ECC can be effectively used in cover with less thickness.

The additional cost of ECC over normal concrete is mostly because of the use of fibres, higher cement content and use of high performance super– plasticizer. This is the reason why optimization of the composite to minimize the fibre content is so important. Finally, economy of ECC should be based on cost/benefit analysis. The life cycle cost of structure includes not only the initial material cost but also the construction and maintenance cost.

Acknowledgment

The authors would like to thank Reliance Industries Ltd., Grasim Industries Ltd., and Fosroc Chemicals Ltd. for supporting this research work by providing Recron 3s fibers, Kamal brand 53 Grade OPC cement and Conplast Super Plasticizer respectively. Thanks are also due to the funding agency DST, New Delhi for providing a grant of Rs. 25.6 Lakhs, under FIST Project, to Prof. S. C. Patodi for upgrading the testing facilities used in this investigation.

References

Fischer, G. and Li, V. C. “Intrinsic Response Control of Moment-Resisting Frames Utilizing Advanced Composite Materials and Structural Elements,” ACI Structural Journal, Title No. 100-S18, March-April 2003.

Li, V. C. “Large Volume, High-Performance Applications of Fibers in Civil Engineering,” ACE-MRL, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan, DOI 10.1002/ app. 2263, 2000.

Fischer, G. and Li, V. C. “Effect of Matrix Ductility on Deformation Behavior of Steel Reinforced ECC Flexural Members under Reversed Cyclic Loading Conditions,” ACI Structural Journal, No. 99-s, pp. 79, 2002.

Li, V. C. “On Engineered Cementitious Composites (ECC)- A Review of the Material and its Applications,” Journal of Advanced Concrete Technology, Vol. 1, No. 3, pp. 215- 230, Nov. 2003. Rathod, J. D., Patodi, S. C., Parikh, B. K. and Patel, K. H. “Study of Recron 3S Fibers Reinforced Cementitious Composites,” National Conference on Emerging Technology and Developments in Civil Engineering, Amravati, pp. I-88 to I-95, March 2007.

High Performance Concrete - Novel Approach with Pre blended Material

Dr. S. K. Manjrekar, Chairman and Managing Director, Sunanda Speciality Coatings Pvt. Ltd, Mumbai.

The development of high performance concrete is a giant step in making concrete a high-tech material with enhanced characteristics and durability. High performance concrete is an engineered concrete obtained through a careful selection and proportioning of its constituents. The concrete is made with the same basic ingredients but has a totally different microstructure than ordinary concrete. The low water/binder ratio of high performance concrete, that is its universal characteristic, results in a very dense microstructure having a very fine and more or less well connected capillary system. high performance concrete’s dense microstructure make the migration of aggressive ions more difficult, consequently high performance concrete are more durable when exposed to aggressive environmental conditions. This fact has been endorsed by a case study of the use of specially formulated HPC in an aggressive chemical environment at a fertilizer plant in Gujarat.

What is High Performance Concrete?

The concrete that was known as high-strength concrete in late seventies is now referred to as high performance concrete because it has been found to be much more than simply strong.

The Strategic Highway Research Programme (SHRP) is a $150,000.00 product-driven research program under the Federal-aid highway program in U.S.A. SHRP was developed in partnership with the State Departments of Transportation, American Association of State Highway and Transportation (AASHTO), Transportation Research Board (TRB), industry, and the Federal Highway Administration (FHWA).

SHRP defined HPC as :

1. Concrete with a maximum water-cementitious ratio (W/C) of 0.352. Concrete with a minimum durability factor of 80%, asdetermined by ASTM C 6663. Concrete with a minimum strength criteria of either4. - 21 Mpa within 4 hours after placement (Very Early Strength, VES),5. - 34 MPa within 24 hours (High Early Strength, HES), or6. - 69 MPa within 28 days (Very High Strength, VHS)

High performance concrete can hence be defined as an engineered concrete with low water/binder concrete with an optimized aggregate/binder ratio to control its dimensional stability and which receive an adequate water curing.

Water/Cement or Water/Binder Ratio

Both expressions were deliberately used above, either singly or together, to reflect the fact that the cementitious component of high performance concrete can be cement alone or any combination of cement with supplementary cementitious materials, such as, slag, flyash, silica fume, metakaolin, rice husk ash, and fillers such as limestone. Ternary systems are increasingly used to take advantage of the synergy of supplementary cementitious materials to improve concrete properties in the fresh and hardened states and to make high performance concrete more economical.

Despite the fact that most high performance concrete mixtures contain at least one supplementary cementitious material, which should favor the use of more general expression water/binder ratio, the water/ binder and water/cement ratios should be alongside each other. This is because most of the supplementary cementitious materials that go into high performance concrete are not as reactive as portland cement, which means that most of the early properties of high performance concrete can be linked to its water/ cement ratio while its long-term properties are rather linked to its water/binder ratio.

Concrete compressive strength is closely related to the density of the hardened matrix. High performance concrete has also taught us that the coarse aggregate can be the weakest link in concrete when the strength of hydrated cement paste is drastically increased by lowering the water/binder ratio. In such cases, concrete failure can start to develop within the coarse aggregate itself. As a consequence, there can be exceptions to the water/binder ratio law when dealing with high performance concrete. In some areas, decreasing the water/binder ratio below a certain level is not practical because the strength of the high performance concrete will not significantly exceed the aggregate’s compressive strength. When the concrete’s compressive strength is limited by the coarse aggregate, the only way to get higher strength is to use a stronger aggregate.

High performance Concrete: A Composite Material

Standard concrete can be characterized solely by its compressive strength because that can directly be linked to the cement paste’s water/cement ratio, which still is the best indicator of paste porosity. Most of concrete’s useful mechanical characteristics can be linked to concrete compressive strength with simple empirical formulas. This is the case with elastic modulus and the modulus of rupture (flexural strength), because the hydrated cement paste and the transition zone around coarse-aggregate particles constitute the weakest links in concrete. The aggregate component (especially the coarse aggregate) contributes little to the mechanical properties of ordinary concrete. As the strength of the hydrated cement paste increases in high performance concrete, the transition zone between the coarse aggregate and the hydrated cement paste practically disappears. Since there is proper stress transfer under these conditions, high performance concrete behaves like a true composite material.1

Making HPC

High performance concrete can not be made by a casual approach. Each ingredient viz : cement, supplementary cementitious materials, sand, course aggregates, superplasticizer, and the other admixtures must be carefully selected and checked, because their individual characteristics significantly affect the properties of the final product.

Particular attention must be paid to water content. Even seemingly insignificant volumes of water present in the aggregates or admixtures must be accounted for. Compressive strengths from 50 to 75 MPa can usually be achieved easily with most cements.2

Concrete Shrinkage

If water curing is essential to develop the potential strength of cement in plain concrete, early water curing is crucial for high performance concrete in order to avoid the rapid development of autogenous shrinkage and tocontrol concrete dimensional stability, as explained below.

Cement paste hydration is accompanied by an absolute volume contraction that creates a very fine pore network within the hydrated cement paste. This network drains water from coarse capillaries, which start to dry out if no external water is supplied. Therefore, if no drying is occurring and if no external water is added during curing, the coarse capillaries will be empty of water as hydration progresses, just as though the concrete was drying. This phenomenon is called selfdesiccation. The difference between drying and selfdesiccation is that, when concrete dries, water evaporates to the atmosphere, while during selfdesiccation, water stays within concrete means it only migrates towards the very fine pores created by the volumetric contraction of the cement paste.

In ordinary concrete with a high water/cement ratio greater than 0.50, for example, there is little cement and more water than is required to fully hydrate the cement particles present. A large amount of this water is contained in well connected large capillaries, in ordinary concrete. This means that the hydrated cement paste does not shrink at all when selfdesiccation develops.

In the case of high performance concrete with a water/binder ratio of 0.30 or less, significantly more cement and less mixing water have been used, so that the capillary network that developed within the fresh paste is essentially composed of fine capillaries. When self-desiccation starts to develop as soon as hydration begins, the menisci rapidly develop in small capillaries if no external water is added. Since many cement grains start to hydrate simultaneously in high performance concrete, the drying of very fine capillaries, can generate high tensile stresses that shrink the hydrated cement paste. This early shrinkage is referred to as autogenous shrinkage. Of course, autogenous shrinkage is as large as the drying shrinkage observed in ordinary concrete when these two types of drying develop in capillaries of the same diameter.

But, if there is an external supply of water, the capillaries do not dry out as long as they are connected to this external source of water. The result is that no menisci, no tensile stress, and no autogenous shrinkage develops within the high performance concrete.

Therefore, an essential difference between ordinary concrete and high performance concrete is that ordinary concrete exhibits no autogenous shrinkage whether or not it is water-cured, whereas

high performance concrete can experience significant autogenous shrinkage if it is not water-cured during the hydration process. Autogenous shrinkage will not develop in high performance concrete if the capillaries are interconnected and have access to external water. When the continuity of the capillary system is broken, then and only then, will autogenous shrinkage start to develop within the hydrated cement paste of a high performance concrete.

High performance concrete must be cured quite differently from ordinary concrete because of the difference in shrinkage behavior described above. If HPC is not water-cured immediately following placement or finishing, it is prone to develop severe plastic shrinkage because it is not protected by bleed water, and later on develops severe autogenous shrinkage due to rapid hydration reaction. While curing membranes provide adequate protection for ordinary concrete (which is not subject to autogenous shrinkage), they can only help to prevent the development of plastic shrinkage in high performance concrete. They have no value in inhibiting autogenous shrinkage. Therefore, the most critical curing period for any HPC runs from placement or finishing up to 2 or 3 days later. During this time, the most critical period is usually from 12 to 36 hours. In fact, the short time during which efficient water curing must be applied to HPC can be considered a significant advantage over ordinary concrete. Those who specify and use HPC must be aware of the dramatic consequences of skipping early water curing. Initiating water curing after 24 hours is too late because, most of the time, a great deal of autogenous shrinkage will already have occurred and, by this time, the microstructure will already be so compact that any external water will have little chance of penetrating very deep into the concrete.

Water ponding, whenever possible, or fogging are the best ways to cure HPC; one of these two methods must be applied as soon as possible immediately following placement or finishing.

The water curing can be stopped after 7 days because most of the cement at the surface of concrete will have hydrated and any further water curing will have little effect on the development of autogenous shrinkage due to compactness of the HPC microstructure. Moreover, after 7 days of water curing, HPC experiences little drying shrinkagedue to the compactness of its microstructure and because autogenous shrinkage will have already dried out the coarse capillaries pores. Even then, the best thing to do is to paint HPC with an sealing agent so that the last remaining drops of water in the concrete can hydrate more cement particles. There is no real advantage to paint a very porous concrete since it is impossible to obtain an absolutely impermeable coating; painting HPC, however, is easier and more effective.

Durability of HPC

The durability of a material in a particular environment can only be established by time. Based on years of experience with ordinary concrete, we can safely assume that high performance concrete is more durable than ordinary concrete. Indeed, the experience gained with ordinary concrete has taught us that concrete durability is mainly governed by concrete impermeability and the harshness of the environment.

A specially designed high performance, selfleveling, nonshrink pre-blended high performance concrete was formulated and was put into use against the aggressive chemical environments at a fertilizer plant in Gujarat – Gujarat Narmada Fertilizers Ltd. (GNFC).

This pre-blended high performance concrete was specially formulated to meet the MES & VES proportion as defined in the SHRP programme.

GNFC is a world largest single stream manufacturer of ammonia and urea. Subsequently, for diversification various products viz. Ammonium Nitro-phosphate (ANP), Calcium Ammonium Nitrate (CAN) etc. were added. CAN is a physical mixture of ammonium nitrate and lime mixed at a particular temperature to form granules. As the mixture is not a chemical reaction, it results into availability of free lime in CAN granules. Lime is inert and remains in dormant condition as far as effect on concrete structure is concerned, but CAN which is available in free form in the CAN granules, reacts with hydration products of concrete and deteriorates concrete. CAN also reacts with reinforcement present in RC member and causes corrosion.

Problem

The signs of damages/ deterioration on concrete particularly in CAN plant were first observed in the form of cracks on edges of RC member which started widening within a span of 6 to 8 months. Concrete in cover portion started sounding hollow which would ultimately result into debonding. As such, this type of failure in RC members can be due to many reasons but one observation which narrowed down the probabilities was observation of watery droplets around these members. The droplets were chemically analysed and they were found to be containing CAN. It was found on further investigation that CAN is highly hygroscopic and hence it would attract moisture from atmosphere and form watery layer all around the surface on which CAN is present. In addition to this hollow sound and cracks, diminishing of cement slurry and erosion like failure was also observed. Coarse aggregates could be seen on the surface of RC member. These are the signs of medium corrosion of RC members wherein cracks, loss of external finish, leaching of liquid and progressive reduction in strength etc. would occur

These observations were immediately followed by the signs of corrosion wherein debonding and spalling of concrete, corrosion of reinforcement and disintegration of concrete by dissolution of cement slurry were observed.

Conventional Solution

Two alternatives were initially decided to be implemented. One was to build up the thickness of damaged/removed concrete by concrete of higher grade after water washing of exposed surface of beam, application of good bond coat and repairing of the reinforcement bars by welding was carried out. Second method was to build up the thickness with epoxy screed after similar preparations. In first method, the thickness was to be built up by pouring concrete after providing suitable shuttering and in second method, thickness was to build up in layers. Both the alternatives were tried but they failed. First method failed earlier as compared to second method. Additionally, huge wastage was observed in second method which made the second method uneconomical.

Limitations of Conventional Solution

a. Repairing was carried out in running plant where airborne CAN dust and humidity were present.

b. CAN, present in core of concrete which was not apparently visible and hence it was not removed.

c. Immediate coating of repaired surface, stopping the breathing of repair mortar and implemented.

d. Stresses resulting in debond of the repair mortar Unapproachability to surface ofcut outs due to their covering by ducts/equipment which enclosed a part of surface and could not be approached and repaired.

e. Minor vibrations transmitted from the equipment during repairing activity.f. Repair system limitation was that the repairing was in layers each of 25 mm thickness

which sandwiched CAN dust in-between every layer and did not allow to establish a proper inter layer bond.

Innovative Solution

All these advertise were examined jointly with the representatives of GNFC. Considering the overall view of the problems as well the limitations involved the job demanded a robust, fast setting, non-shrink, impermeable high performance concrete. POLYCRETE is a high strength, fast setting and non shrink specially formulated HPC. Besides combining all the above properties it also is free flowing and self levelling. Its application procedures are much simple than conventional methods.

Based on all the above properties and parameters coupled with ease in application and fast

strengths which will delay CAN particles from depositions again POLYCRETE was suitably selected for the project.

Repair procedures were suggested which included a Low viscosity Bonding agent and shear keys as per design requirements.

Present Scenario

This system has been applied in end of September, 1997. Repaired area was continuously observed and there are no signs of deterioration observed since then. Strength of the repaired mass was measured in December 1997 and it was found to be around 650 Kg/cm2. This system has ensured that the repaired portion has an excellent mechanical strength and all chances of its getting debonded from original surface are eliminated.

References

1. Baalbaki, W., Benmokrane, B., Chaallal, 0., Aitcin, P.-C., (Sept.- Oct. 199) “Influence of Coarse Aggregate on Elastic Properties of high performance concrete,” ACI Materials Journal, Vol. 88, No. 5, pp. 499-503.

2. Aitcin, P.-C., (1993) “Durable Concrete–Current Practice and Future Trends,” ACI SP-144, pp. 83-104.

3. Nilsen, A.U., Aitcin, P.-C.(Vol. 14, No. 1, Summer, 1992), “Properties of High-Strength Concrete Containing Light-, Normal–and Heavyweight Aggregate,” Cement, Concrete and Aggregates, pp. 8-12.

4. Lessard, M., Dallaire, E., Blouin, D., Aitcin, P.-C (Sept. 1994)., “High Performance Concrete Speeds Reconstruction of McDonald’s,” Concrete International, Vol. 16, No. 9, pp. 47-50.

5. Aitcin, P.-C., Neville, A.M., Acker, P., (Sept., 1997) “The Various Types of Shrinkage Deformation in Concrete: An Integrated View,” to be published in Concrete International, Whiting, D., “In-Situ Measurements of the Permeability of Concrete to Chloride Ions,” ACI SP-82 1984, pp. 501-524.

6. Kreijger, P.C. (1987), “Ecological properties of Building Materials,” Materials and Structures, Vol. 20, pp. 248-254.

Concrete Containing More Than Two Admixtures

Effect of Sustained Elevated Temperature on the Properties of Concrete Containing More Than Two Admixtures

D. K. Kulkarni, Selection Grade Lecturer, Civil Engineering Department, Rajarambapu Institute of Technology, Rajaramnagar, Islampur. Dr. K.B. Prakash, Professor Civil Engineering Department K. L. E Society’s College of Engineering & Technology, Belgaum.

Concrete is a material often used in the construction of highrise buildings. In case of unexpected fire, the concrete elements such as columns, beams, etc. will be subjected to extreme temperatures and needs assessment of their performance after fire. Hence, it is important to understand the changes in the concrete properties due to extreme temperature exposures.

In this paper, an attempt is made to find out the effect of sustained elevated temperature on the properties of concrete containing more than two admixtures. The following combinations of admixtures are used in this experimentation work.

Superplasticiser + Air Entraining Agent + Accelerator Superplasticiser + Air Entraining Agent + Retarder Superplasticiser + Air Entraining Agent + Waterproofing Compound Superplasticiser + Air Entraining Agent + Shrinkage Reducing Admixture Superplasticiser + Air Entraining Agent + Viscosity Modifying Admixture

The tests are conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength, and impact strength of concrete when it is subjected to a temperature of 600°C for 6 hours.

Introduction

One of the greatest advantages of concrete as a building material is its remarkable resistance to fire. The distress in concrete due to fire manifests in the form of cracking and spalling of the concrete surface1. Concrete though not a refractory material is incombustible and has good fire resistant properties2. The property of concrete to resist the fire reduces damage in a concrete structure whenever there is an accidental fire. In most of the cases the concrete remains intact with minor damages only. The reason being low thermal conductivity of concrete at high temperature and hence limiting the depth of penetration of fire damage. But when the concrete is subjected to high temperature for long duration, the deterioration of concrete takes place3.

Concrete has been widely used as construction materials in buildings and other industrial structures for a long time. The recent technological advances have extended its use to special applications like aircraft engine test cells, tube jet runways, nuclear reactor vessels and missile launching pads, which have to endure higher tempratures4.

Chemical admixtures play a key role in the production of concrete with enhanced performance also known as High Performance Concrete or HPC. In conjunction with mineral additives, such as silica fume, chemical admixtures have enabled major improvements in many of the properties of concrete, particularly, compressive strength and durability.

Now-a-days the concrete is called upon for the use in various tricky situations and the concrete has to show a resistive nature for all the special situations for which it is used. In such circumstances, it becomes necessary to use two or more than two admixtures simultaneously in concrete.

Experimental Programme

The main aim of this experimentation work is to find the effect of sustained elevated temperature on the properties of concrete containing more than two admixtures. The following combinations of admixtures have been selected for the studies on concrete:

Superplasticiser + Air Entraining Agent + Accelerator (S+AEA+A) Superplasticiser + Air Entraining Agent + Retarder (S+AEA+R) Superplasticiser +Air Entraining Agent + Waterproofing Compound (S+AEA+W)

Superplasticiser +Air Entraining Agent + Shrinkage Reducing Admixture (S+AEA+SRA)

Superplasticiser +Air Entraining Agent + Viscosity Modifying Admixture (S+AEA+VMA)

Portland pozzolana cement and locally available sand and aggregates were used in the experimentation. The specific gravity of fine and coarse aggregate was 2.66 and 2.85 respectively. The experiments were conducted on a mix proportion of 1: 1.26:2.51 with w/c = 0.41 which corresponds to M20 grade of concrete. The admixtures and their chemical content and dosages used in the experimentation are shown in Table 1.

The fine aggregate, cement and coarse aggregates were dry mixed in a mixer for 60 seconds. The required quantity of fibers and hybrid fibers were added into the dry mix and again the entire mass is mixed homogeneously for another 60 seconds. At this stage approximately 80% of calculated quantity of water (w/c = 0.41) was added into the dry mix and agitated for 3 minutes. Now the superplasticiser was added in the remaining 20% water and this liquid was added to the concrete. The concrete was mixed again in the mixer, after which the remaining two more admixtures were added and homogeneously mixed. This homogeneous concrete mass was poured into the moulds which were kept on the vibrating table. The concrete was consolidated in three layers by using just the required vibration time needed for a good compaction. After consolidation the top surface was finished smooth and covered with wet gunny bags. After 12 hours, the specimens were demoulded and transferred to the curing tank wherein they were allowed to cure for 28 days.

For compressive strength test, the cubes of dimensions 150 X 150 X 150 mm were cast and were tested under compression testing machine as per I S 516-19595. For tensile strength test, the cylinders of diameter 100 mm and length 200 mm were cast and were tested under compressive testing machine as per I S 5816- 19996. For flexural strength test the beams of dimensions 100 X 100 X 500 mm were cast and were tested on an effective span of 400 mm with two point loading as per I S 516-19595. For impact test four different test methods are referred in the literature7. Drop weight method being the simple method, was adopted to find the impact energy. Impact strength specimens were of dimensions 250 X 250 X 30 mm. A steel ball weighing 13.03 N was dropped from a height of 1 m on the centre point, which was kept on the floor. Number of blows required to cause first crack and final failure were noted down. From these number of blows, the impact energy was calculated as under. Impact energy = w h N (N-m)

Where w = Weight of steel ball = 13.03 N

h = Height of drop = 1 m

N = Number of blows required for first crack or final failure as the case may be.

After 28 days of curing, the specimens were transferred to the electric furnace wherein they were maintained at 6000 C for 6 hours. After 6 hours they were cooled to room temperature and then tested for their respective strengths.

Test Results

Table 2 gives the compressive strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of compressive strength w.r.t. reference mix. The variation of compressive strength is depicted in the form of graph as shown in Figure 1.

Table 3 gives the tensile strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of tensile strength w.r.t. reference mix. The variation of tensile strength is depicted in the form of graph as shown in Figure 2.

Table 4 gives the flexural strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of flexural strength w.r.t. reference mix. The variation of flexural strength is depicted in the form of graph as shown in Figure 3.

Table 5 gives the impact strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of impact strength w.r.t. reference mix. The variation of impact strength is depicted in the form of graph as shown in Figure 4.

Discussion on Test Results

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum compressive strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), ( S + A E A + W ) , (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least compressive strength. The percentage increase in the compressive strength of the above said combinations w.r.t. reference mix are respectively 45.07%, 32.65%, 25.07%, 15.76%, and 7.48%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum tensile strength when subjected to 600°C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least tensile strength. The percentage increase in the tensile strength of the above said combinations w.r.t. reference mix are respectively 55.35%, 53.02%, 51.63%, 47.91%, and 13.48%.

It has been observed that the concrete produced from the combination of admixtures

(S+AEA+R) show maximum flexural strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least flexural strength. The percentage increase in the flexural strength of the above said combinations w.r.t. reference mix are respectively 111.03%, 77.93%, 35.17%, 30.34%, and 9.65%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum impact strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least impact strength. The percentage increase in the impact strength of the above said combinations w.r.t. reference mix are respectively 77.77%, 55.56%, 44.43%, 33.33%, and 11.10%.

This may be due to the fact that the addition of combination of admixtures induce more workability thus making the compaction a perfect one. This makes the concrete more dense which is ultimately responsible for increase in the strengths. The addition of AEA creates small air bubbles in the concrete. These induced air bubbles can resist the expansion of concrete due to temperature.

Conclusions

It can be concluded that the combinations of admixtures used in the experimentation such as (S+AEA+R), (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA), do not have any compatibility problems either with respect to the properties of fresh concrete or hardened concrete. It can also be concluded that the maximum strength of concrete can be obtained with the combination of admixtures (S+AEA+R) when subjected to 6000C for 6 hours. This is followed by the combinations of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). Hence it can be recommended to use any combinations of admixtures on the site to suite the situations.

Acknowledgment

The authors would like to thank Dr.(Mrs) S. S. Kulkarni, Principal, RIT, Sakharale and Dr.S.C.Pilli, Principal, KLE Society’s College of Engg. & Technology, Belgaum for giving all the encouragement needed which kept our enthusiasm alive. Thanks are also due to the management authorities and others who constantly boosted our morale by giving us all the help required. Thanks are also due to authorities of MBT Pvt.Ltd(Degussa) Mumbai India for supplying the required admixtures.

References

Lakshmipathy M and Balachandar M, “Studies on the effects of elevated temperature on the properties of high strength concrete containing supplementary cementatious materials,” Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp. 539-554

Balamurugan P and Perumal P, “Effect of thermoshock on bond strength of HPC, “Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp. 555-556

Sashidhar C, Sudarsana Rao H, Ramana N.V and Vaishali Gorpade, “Studies on SIFCON subjected to elevated temperature,” Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp.567-576

Anbuvelan K, Dinesh M, Kumaravel K, Thiyagarajan A and Sureshkumar N, “Sustained elevated temperature effects on post peak flexural strength of high strength concrete containing polypropylene fibers,” Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9,2005, SRMIST, Chennai, India. pp. 577-590

I S : 516-1959 “Methods of tests for strength of concrete,” Bureau of Indian Standards, New-Delhi.

I S : 5816-1999 “Splitting tensile strength of concrete method of test,” Bureau of Indian Standards, New-Delhi

Balsubramanain, K. et al, “Impact resistance of steel fiber reinforced concrete,” The Indian concrete Journal, May 1996, (pp 257-262).

Engineering of Self Compacting ConcreteSubrato Chowdhury, & Sandeep Kadam, UltraTech Cement Limited, Andheri (East), Mumbai

Cementitious material is the lifeline of modern infrastructure. Increasing demand for concrete in newer applications leads to engineer the properties of concrete at fresh and hardened state.

One of the most important performance criteria for concrete is the fluidity at fresh state. Appropriate fresh state properties are achieved by engineering suitably the theology of concrete. Such engineering is achieved by incorporating chemical & mineral admixtures into cementitious system. The development of self-compacting concrete is primarily achieved by designing the appropriate theology using different cementitious system, admixtures, etc.

Self-compacting (or consolidating) concrete (SCC) is a particular concrete mix which has a special performance requirement of self–consolidation or compaction at the time placement. However, at the hardened state, there is not much difference in terms of mechanical properties

and durability between SCC and other type of concrete mixes viz. high performance concrete (HPC), normal strength concrete (NSC), etc.

The important aspects of achieving the functional requirements (filling ability, passing ability and resistance to segregation) of SCC are related with:

Appropriate characterization of ingredients Mix proportion Mixing method Placement

This paper would discuss the effect of characteristics of individual ingredients, different approaches for mix proportioning and the mixing method on the overall performance of the SCC mix in fresh state, especially on its theology. The effect of method of placement, especially in terms of the pressure exerted on the formwork will also be discussed.

Introduction

Concrete is a suspension of aggregates in cement paste (1). A suspension is self-flowing if it flows under its own weight. Additionally, it is to ensure–uniform suspension of solid particles during casting and thereafter until setting (2). The above perspective induces the definition of self-compacting (or, consolidating) concrete (SCC), as a concrete mix, which in fresh state, has the ability to fill the formwork and encapsulate reinforcing bars only through the action of gravity i.e. self-weight at the time of placement without any external energy inputs from vibrators, tampering or similar actions and with maintained homogeneity at the time of placement (3). SCC can be used in most application where traditional vibrated concrete, such as conventional normal strength concrete (NSC), high performance concrete (HPC) is used. Two principal advantages of SCC are improved homogeneity of fresh concrete that leads to more durable concrete at hardened state as well as higher productivity in terms of pouring of concrete, and improvement in working condition and less noise pollution (4, 5).

The difference between the SCC and vibrated concrete exists in the performance requirements during fresh state; not much in terms of properties at harden state such as strength, durability. SCC is engineered to fill all the space within the formwork passing through the reinforcements or other obstruction without segregation. This attributes to three important functional requirements related to workability of the concrete mix: filling ability, resistance to segregation, and passing ability (3). Filling ability is the high fluidity and deformability to ensure adequate flow under selfweight. Resistance to segregation is the ability of the particle suspension (in fresh state) to maintain homogeneity throughout the mixing, transportation, and placement process. Passing ability is the ability to pass obstacles, narrow opening and closely spaced reinforcement bar without getting blocked by interlocking of aggregate particles (3). Filling ability and passing ability of a fresh concrete mix depend on its fluidity and resistance of segregation on the homogeneity. Additionally, the paste or mortar has to deform well too. The yield stress and plastic viscosity generally characterizes such theological behavior of fresh concrete mix. Fluidity is inversely proportional to the yield stress, while plastic viscosity has direct proportionality on homogeneity. Contact and collision between aggregates as well as the interparticle friction

increase with the decreases in relative distance between aggregates particles in the concrete mix, resulting in the blockage of aggregate particles (6). Limiting coarse aggregate volume increases inter-particle separation and reduces the inter-particle friction and collisions resulting in minimization of the blockage leading to improvement in passing ability.

The increase of paste volume with emphasis to low water powder ratio (w/p) in presence of compatible chemical admixtures further strengthens the fluidity and helps in attaining homogeneity. Adequate homogeneity improves viscosity of the mix, which in turn enhances the segregation resistance. An optimum balance between fluidity and viscosity is the key to achieve efficient selfcompacting characteristics of the concrete mix at fresh state. In SCC, the powder contains binder component consisting of ordinary Portland cement (OPC), mineral admixtures like flyash along with/ without filler material like limestone powder, dolomite etc. To achieve moderate plastic viscosity and low yield value, multiple chemical admixtures are required. Special chemical admixture like viscosity modifier admixture (VMA) is used for controlling the viscosity of the mix and superplasticizer for lowering the yield stress. In addition, the characteristics of fine and coarse aggregates play very important role on the yield stress of the mix.

Overview of SCC

The work on SCC had started in 1988 in Tokyo University, Japan. The Japanese concept spread through Asia and to Europe around 1993 (7). This concept is well accepted in USA now. A few points are important with regard to engineering of structures using SCC mix to satisfy the intended specification.

These are:

i. characterization of the ingredientsii. mix proportion technique to achieve desired characteristics

iii. mixing methodiv. effect of method of placement, especially the pressure exerted on formwork.

The above points are deliberated in the following sections of the paper.

Characterization

Ingredient characterization exhibits different aspects depending upon the background of the users. These concepts range from that of the scientist, who thinks of it in atomic terms, to that of the concrete technologists, who thinks of it in terms of properties of concrete in fresh and harden state, procedure of construction and quality assurance, etc. Characterization of an ingredient deal with those features of the material like composition, structure, etc that are significant for a particular preparation, study of properties or use etc. The three basic functional requirements of SCC mix at fresh state, i.e. filling ability, passing ability and resistance to segregation could be assessed in terms of the theological characteristic like yield stress and plastic viscosity.

An appropriate ingredient characterization helps to achieve the performance behavior of SCC at both fresh and hardened states. General-purpose Ordinary Portland Cement (OPC) is suitable to be the main cementitious constituent for SCC. It is also well–established that compatibility between superplasticizer and OPC plays important role on the rheological characteristic of mortar. Certain chemical compounds of OPC clinker such as alkali (Na2O, K2O); sulphate (SO3) has significant influence on such compatibility (8). The presence of mineral admixtures has a definite role on the performance of paste, especially format ion of micro mortar. The micro-mortar formations is involved with all particles below the size of 125¼, chemical admixture and water (6). Flyash is commonly used mineral admixture in SCC. Particle size distribution of flyash, chemistry of flyash and presence of un-burnt coal particles has enough impact on fluidity and deformability of mortar for SCC (10). The bulk solid volume of the fly ash also has significant impact on the rheology. Low lime content flyash improves the fluidity of the paste (9). The flow value increased as the bulk solid volume of flyash is increased (9, 10). High belite content OPCwas used at the initial years of SCC without any application of VMA (3). However, high alite content high strength OPC may be desirable for achieving high strength SCC along with appropriate replacement level of OPC by mineral admixture.

The compatibility of multiplechemical admixtures present along with mineral admixtures needs a serious attention towards satisfactory performance of rheological properties as well as hydration kinetics that has bearing on hardened properties.

The fine aggregate is one of the major components of paste formations. Well-graded fine aggregate is desirable. The size of coarse aggregate in SCC is 5 to 20 mm. However, the size of the aggregate is decided based on the size of the opening such a spacing of reinforcement bar (1). Larger the aggregate size more the driving force for flow would be required. Blocking will occur if the maximum size of the aggregate is large as well as the content of the larger size aggregates is high. Crushed stone aggregates require more paste volume for nonblockage criteria compared with the natural gravels. Higher packing density of aggregates reduces demand of superplasticizer (1, 2). Extensive works on characterization of ingredients like OPC, fly ash and fine aggregates for SCC were carried out by authors and are published elsewhere. (8,10,11,12).

Ingredients for self-compacting concrete shall satisfy the respective codal specifications. Findings of the works, on characterization of ingredients, carried out by authors are summerised here.

Ordinary Portland Cement

Clinkers may have different levels of alkali and sulphate concentrations, but the corresponding OPC shows fairly the same levels of sulphate owing to addition of gypsum during grinding process. Alkali and sulphate content of the clinker not that of cement binder, has influence on the rheology of mortar for SCC.

Initial flowability and viscosity of mortar mixes are not influenced by the alkali and sulphate content of the clinkers irrespective of the dosages of flyash. The initial flowability decreases and viscosity increase with elapsed time for all the cement replacement levels and types of OPC.

Low sulphate content of clinker increases the flow ability and reduces viscosity irrespective of alkali content. Alkali content of clinkers has similar trend of effect on flowability and viscosity but this influence is not as significant as that of sulphate. OPC from low sulphate bearing clinkers and cement replacement level of 50% and above by flyash is vulnerable to the risk of segregation. Low sulphate content increases the filling ability of concrete mixes.

Flyash

The flow of the mortar is affected adversely with flyashes having higher percentage of particle size above 90ì, and the mortar becomes unfit for the purpose. The flow is enhanced with fly ashes having higher percentage of particle size below 45ì.

Flyash with high lime and sulphate content is not suitable for producing SCC as it decreases the flow and increases the viscosity; non-cohesiveness of the mortar is also increased significantly. Flyash with higher LOI, i.e. the higher carbon content, is not a suitable mineral admixture for SCC mortar. It affects the rheology adversely making the mix highly viscose aswell as non-cohesive.

Higher quantity of fly ash could result in adjustment of chemical admixture to lower dosages for achieving appropriate flow and viscosity of mix. Flyash of appropriate characteristics acts as flow enhancing and viscosity reducing agent in SCC mortar. Increase in flyash quantity neutralizes the negative impact of high sulphate and high alkali content of OPC clinker as well as the size fraction of fine aggregates on the rheology of mixes. Though quantity of flyash does not significantly influence the initial spread and viscosity of mixes, its increase in value helps in retention of higher spread diameter and lower viscosity.

Fine Aggregate

Initial viscosity of mortar mixes is influenced by the size fraction of fine aggregate. The finer fraction of sand reduces flowability and increases viscosity of mortar mix. Lower quantity of fines in fine aggregate accentuates the possibility of segregation.Ingredients characterized and found suitable by mortar rheology experiments are suitable for selfcompacting concrete.

Mix Proportioning Method

A number of methods for proportioning SCC mix have been developed over the years with primary attention to produce satisfactory self–compacting properties but with less attention to the properties at hardened state. Most of the methods those are presently available may have some inherent limitations, either in terms of ingredients for which they have been shown to be suitable or in terms of the range of concretes that can be produced. These methods are of varying complexity and may require wide range of information on the effect of each ingredients on the mechanics of SCC mixes. In general, the SCC mix proportioning methods consider volume as the key parameter because of the importance of the need to fill over the voids in between the aggregate particles by the paste.

Different mix-proportioning methods can be grouped in having two categories of approaches. The basic steps of first category are determination of quantity of coarse aggregate, and then deriving appropriate quality of mortar compatible for SCC mix. While in the second approach, the suitable mortar mix is first proportioned and then quantity of coarse aggregates is determined. The mixes proportioned by both these categories can further be subdivided in to three types; powder type, VMA type and mixed type. In first type cement content is very high, mineral admixture content is very low to none and no VMA is used. The second type method results in almost equal quantity of cement and mineral admixture, and high quantity of VMA is required for maintaining homogeneity of the mix though superplasticizer requirement comes down significantly compared to the first type. Mineral admixture content in the third type mix is about one–third of the powder content and a lower quantity of VMA is used (13).

Okumara and Ozawa of University of Tokyo developed most probably the first method of SCC mix proportion in 1995 [3, 14]. Their method is also known as general method. This is a step-by-step method in which VMA is not used. First the quantity of coarse aggregate, per unit volume of concrete mix, is set at 50% of the dry rodded weight. The required mortar volume is determined taking into consideration the air content in the mix. The fine aggregate content is worked out about 50% of the resulting mortar volume. The water/powder ratio and superplasticizer dosages of the mortar are adjusted until the minimum relative flow area of 5 and relative flow rate between 0.9 – 1.1 are achieved using mortar spread and V–funnel test respectively [3]. The mix proportion thus arrived at is tested for selfcompactability by concrete funnel test and slump flow test. The mix is considered satisfactory from selfcompatibility consideration if it exhibits slump flow of 650mm and relative flow rate between 0.5 and 1.0. This method is applicable to a limited range of Japanese materials; 5-20 mm sized coarse aggregates, fine aggregates of size less than 5mm, and high belite Portland cement. The air-entraining agent was used. Criterion related to concrete strength is not included in this mix proportioning method. This method falls under first category and produces only powder type mix.

Bui, et al [15] introduced a new approach for the proportioning of SCC that essentially falls under the second category and can produce combined and VMA type mixes. The approach is based on the paste rheology model, which is built on the combination of the criteria of minimum apparent viscosity, minimum flow and optimum flow viscosity ratio. The effect of aggregate properties and content has been considered to develop a new paste model for SCC. The model developed by testing wide range of concrete composition also provides a basis for quality control and further development of mineral and chemical admixtures. Polycarboxylate based superpla–sticizer was used and a viscosity modifying agent was used in some mixes. Relationship between viscosity and flowability of paste, with aggregate spacing were developed using average aggregate diameter 5.675 mm.

For different paste volume, water binder ratio, cement content, flyash content, admixtures, the flow of each paste were plotted against viscosity. The limits for segregation and low deformability zone were also plotted. Bui et al defined three zones for mix proportion with the help of these plots. One extreme zone is segregation zone in which flow is very high and viscosity is low. Other extreme zone is low deformation zone where viscosity is high and flow is low. The satisfactory zone falls in between these two extreme zones. The paste rheology, which is falling within the satisfactory zone, was considered appropriate for the purpose of

selfcompatibility. Subsequently, the unit volume was achieved by addition of aggregates into the paste without any additional adjustment.

Mixing Method

The ingredients of vibrated HPC mix and SCC mix are similar except for VMA. The HPC mix is manufactured adopting multistage mixing method. It has been observed that mixing method has significant influence on the properties of the concrete mix both in hardened and fresh state (16, 17). Hardly any information is available in this respect for SCC mix.

Form Pressure

SCC results in higher form pressure because of its extreme fluidity showing nearly Newtonian behavior (18). The method as well as rate of casting dominates the form pressure (19). The traditional vibrated concrete results in lower form pressure than SCC having same casting rate. The correlation between form pressure and casting rate is relatively linear. When concrete is placed using pump and if the pumping is done from bottom it creates more anchor pressure than that when pumping from top (18). The anchor force due to pump filling from bottom doubles than that when filling from top, the reason is that the pressure from pump adds to the pressure of concrete. The relation between concrete pressure and optimal rate of pouring calls for further study to establish their inter-relation (20).

Leemann and C. Hoffmann investigated the pressure exerted by SCC on formwork both at laboratory scale and at field (20). They studied the formwork pressure caused by SCC with varying workability and conventional concrete filling the formwork from top in the laboratory and the pressure of SCC pumped into the formwork at its base was determined in a field study. The studies conclude that the maximum pressure of filled into a formwork from top is dependent on the casting speed and rate of the continuous pressure decrease of the SCC already cast. SCC pumped into the formwork a tits base can locally surpass hydrostatic pressure.

Concluding Remarks

SCC mix engineering starts with balancing between high fluidity and high segregation resistance to achieve appropriate self-compacting properties, hardened state properties as well as optimized behavior of the suspension within the formwork. Meticulous selection and characterization of locally available ingredients are the key to engineer the rheology of SCC.

The constituents of the material have significant impact on the concrete rheology and hydration kinetics of SCC mix. The approach for characterization of SCC mix leading to defined acceptance criteria needs further work.

The characterization in terms of physical and chemical properties of ingredients of powder, aggregates and their influence on the behavior of SCC is essential.

Selection of appropriate chemical admixtures, its dosages, its chemical compatibility with powder are issues to be addressed further.

A detailed investigation on the effect of curing regime on the properties of SCC at hardened state needs further investigation.

The form pressure in SCC is few folds more and different compared with vibrated concrete. More work is needed to under stand the relation between pump and concrete pressures.

Few of the areas like adjustment for mix proportioning procedure, use of local aggregates, mixing methodology, online controlling of rheology, prediction of strength and durability, need to be looked into.

References

M.A. Rahman, M. Nehdi, “Rheology of Cement Pastes using Various Accessories,” First North American Conference on Design and Use of Self Consolidating Concrete. (November 2002), pp. 49-53.

K. H. Khayat, Chong Hu, Jean- Michel Laye, “Importance of Aggregate packing Density on Workability of Self-consolidating Concrete,” First North American Conference on Design and Use of Self–Consolidating Concrete. (November 2002), pp. 55-62.

A. Skarendahl, O. Petersson, “Self-compacting Concrete-State – of–the–art report 174-SCC,” RILEM Technical Committee, France, Report 23. (2000)

Kamal H. Khayat, “Holistic Approach,” First North American Conference on Design and Use of Self–consolidating Concrete. (November 2002) pp. 9.

K. H. Khayat, “Stability of Self compacting Concrete, Advantage and Potential Application,” First International RILEM Symposium on Self–compacting Concrete, Stockholm, Sweden. (September 1999) , p p. 143–152.

Peter Billberg, “Mix Design Model for Self-compacting Concrete,” First North American Conference on Design and Use of Self Consolidating Concrete. (November 2002), p p.63-68

Preface, Third International Symposium on Self–compacting Concrete, Reykjavik, Iceland. (August 2003).

P.C.Basu, P. P. Biswas, S. Chowdhury, A. K. Ghoshdast idar, P.D. Narkar, “Influence of Components of Portland Cement on Rheology of Mortar for Self- Compacting Concrete,” Second North American Conference on the Design and Use of Self– compacting Concrete , Illinois, Chicago, USA. (October-November 2005).

Pipat Termkhajornkit, Toyoharu Nawa, Hiroshi Ohnuma, “Role of Flyash and Naphthalene Sulfonated Superplasticizer on Fluidity of Paste,” First North American Conference on Design and Use of Self–Consolidating Concrete. (November 2002), p p. 43-44.

P.C. Basu, S. Saraswati, S. Chowdhury, “Effect of Different Fly Ashes on Rheology of Mortar for Self-compacting Concrete,” Second North American Conference on the Design and Use of Self–compacting Concrete, Illinois, Chicago, USA. (October-November 2005).

P.C. Basu, S. Chowdhury, “Influence of Minor Constituents of Portland Cement on Rheology of Mortar for Self–Compacting Concrete,” Proceeding of The Structural Engineering Convention), Indian Institute of Science, Bangalore. (2005), pp. 209-219.

P. C. Basu, S. Chowdhury, “Impact of Fine Aggregate Particle Size on Mortar Rheology for SCC,” The Indian Concrete Journal, Volume 81. (January 2007), pp. 1-8.

Ouchi Masahiro, Nakamura Sadaaki, Osterberg Thomas, Hauberg Svenerik, “Application of Selfcompacting Concrete in Japan, Europe and United States,” Sweden. (2005), pp. 1-1 8.

Okamura H, Ozawa K, “Mix Design for Self-compacting Concrete,” Concrete Library of JSCE 25. (1995), pp . 107-120.

V. K. Bui, S.P. Shah, K. Akkaya, “A New Approach in Mix Design of Self-consolidating Concrete,” First North American Conference on Design and Use of Self– consolidating Concrete. (November 2002), pp. 69-74.

P. C. Basu, S. Saraswati, “Durability of High Performance Concrete: An Overview and Related Issues,” Proceedings of International Symposium on Advances in Concrete through Science an Engineering, Evanston, Illinois, USA. (March 2004).

M. Kakizaki, H-Edahiro, T.Tochigi and T. Nikki, “Effects of mixing method on mechanical properties and pore structures of ultra high strength concrete.” SP 132-54.

Wolfgang Brameshuber, Stephan Uebachs, “Investigations on the Form Pressure Using Self compacting Concrete,” Third International Symposium on Self–compacting Concrete, Reykjavik, Iceland. (August 2003), p p. 281-287.

Peter Billberg, “Form Pressure Generated by Self-compacting Concrete,” Third International Symposium on Self-compacting Concrete, Iceland. (August 20 03), pp. 271-280.

Andreas Leemann, Cathleen Hoffmann, “Pressure of Self Compacting Concrete on Formwork,” Third International Symposium on Self–compacting Concrete, Iceland, (August 2003), pp. 288-298.

Acknowledgment

The article has been reproduced from the SEWC’07 proceeding with the kind permission from the SEWC organisers.

Self Compacting Cocrete

Dr. S.C. Maiti, Ex–Joint Director, National Council for Cement & Building Materials, New Delhi. Raj K Agarwal, Managing Director, Marketing & Transit (India) Pvt. Ltd, New Delhi.

Introduction

Concrete mixtures having high workability and high cohesiveness will be self–compacting concrete. The self–compacting concrete (SCC) is defined as a flowing concrete that can be transported without any segregation and placed without the use of vibrators to construct concrete structures free of honeycombs. Initially such concrete was developed by Japanese researchers. For such concrete which is specially required for heavily reinforced sections, a viscosity modifying agent (VMA) is required along with a polycarboxylic ether (PCE) based superplastisizer. Because of high fluidity, SCC requires higher fines content, in order to resist bleeding and segregation. Natural fine aggregate together with manufactured sand and mineral admixture {flyash or ground granulated blast furnace slag (ggbs) or silica fume} provide higher fines contents in the concrete mix. A cohesive SCC is thus produced in order to flow steadily in the heavily reinforced concrete sections, without any segregation & bleeding.

Materials and Mix Proportions

Besides cement, water and aggregates, the necessary ingredients for producing SCC are superplasticizers (PCE based), viscosity–modifying agents and mineral admixtures e.g. flyash, ground granulated blast furnace slag & silica fume. The proportion of fine aggregates required is higher, may be around 55% and the corresponding proportion of coarse aggregate (generally of smaller size, say 10 or 12 mm maximum size) will be around 45%. The mineral admixtures and fine sand (manufactured sand) are required to make the highworkability concrete mix cohesive.

Typical concrete mix proportions for high strength (74.5 MPa at 28 days) SCC used by Gettu & others (from Spain) (1) are as follows:

Cement (OPC-53 grade) = 428 Kg/ m3 Water = 188 l/ m3 Flyash (2935 cm2 / gm) = 257 Kg / m3 Superplasticizer (vinyl copolymer) = 7.9 Kg / m3 Sand (crushed limestone ) (0-5mm) = 788 Kg / m3 Coarse aggregate (gravel) (5-12mm) = 736 Kg / m3

The water / binder ratio of the concrete mix is 0.27. A look at the materials & mix proportions indicate use of smaller size coarse aggregate (12mm maximum size) & the shape is rounded, being gravel aggregate. In fact, crushed gravel will be a better option in order to obtain high- workability and highstrength SCC.

The workability measured for the above mix is “ slump flow” of 48cm. In our country, still we are carrying out the usual slump test even for high–workability concrete mix. The “flow test” as specified in IS 9103 (2) can be conducted for testing such high–workability concrete mix, but the

“slump flow test” will be better than the “flow test,” as no lifting (15 times in 15 seconds) of concrete is necessary, as the SCC is a flowing concrete mixture.

Vachhani and others (3) used SCC in the prestigious Delhi Metro construction. Concrete mix proportions for M-35 grade of SCC are as follows :

Cement = 330 Kg / m3 Water = 163 l / m3 Flyash = 150 Kg / m3 Superplasticizer = 3.12 l / m3 VMA ( glenium stream 2 ) = 1.3 l / m3 Retarder ( Pozzolith 300 R) = 0.99 l / m3 Sand = 917 Kg / m3 Coarse aggregate: 20mm maximum size = 455 Kg / m3 10 mm maximum size = 309 Kg / m3

Vachhani and others (3) highlighted the mechanism of self compaction, which is based on :

i. Large quantity of “fines” (500 to 650 Kg / m3),ii. Use of high –range water – reducing superplasticizers (with water- reduction of 25%),

andiii. The use of Viscosity Modifying Admixtures.

“Fines” includes cement, flyash and the part of sand of size less than 0.125 mm. This together with water & chemical admixtures constitute the paste in the concrete mix. The paste makes the concrete mix cohesive and controls the segregation–resistance of the mix. The polycarboxylic ether–based superplasticizer ( presently being imported) generally provides water–reduction of the order of 30-40 % in the concrete mix. The VMA improves the segregation–resistance of the mix without changing the fluidity or workability. The retarder in the concrete mix controls the workability–retention, which is specially important in hot climate.

Fresh Self–Compacting Concrete

The characteristics of fresh SCC are fully described by the following properties :

i. Filling ability–ability to completely fill all the spaces in the formwork,ii. Passing ability–ability to flow around reinforcement, and

iii. Segregation resistance–ability to resist segregation of materials during transportation and placing.

Consequently new test methods have been developed to test SCC in the fresh state. The “filling ability” is tested by “ slump flow” and “ V funnel,“ the “passing ability” is tested by “L- Box“ and“ U–Box“ and the segregation–resistance is tested by “ V- funnel”

Hardened Self–Compacting Concrete

The properties and characteristics of hardened SCC do not greatly differ from those of normal concrete, except that SCC can not be used in mass concrete construction using bigger size aggregates, say 75mm or 150mm sizes. Because such concrete always needs to be compacted with needle vibrators, in order to compact thoroughly in the forms.

Any required compressive strength of SCC can be achieved. Vachhani & others (3) obtained 28- day compressive strength of 44-49 MPa in the above–mentioned concrete mix proportions for M-35 grade concrete, for the Delhi Metro construction.

The high – strength SCC can be called “ High–performance concrete,” as such concrete has denser microstructure with lower inherent “porosity” and “permeability,” because of lower water- cementitious materials ratios and use of mineral admixtures in concrete.

Concrete Mix Proportioning Approach

The Self–Compacting Concrete, because of its high–workability and cohesiveness, generally needs higher fines content and lower size (10 or 12 mm maximum size) of coarse aggregate. Smoother and rounded or semi- rounded (may be crushed gravel) coarse aggregate will develop cohesiveness in the concrete mix. Bapat’s (4) suggestion is good. Flakiness & elongation indices of coarse aggregate should be less than 15% each. Large quantity of fines is also required–500 to 650 Kg/m3 of concrete, & therefore crushed stone fine aggregate is also required along with natural fine aggregate. Flyash has also been used as an essential ingredient of SCC. In India, 30 to 50% flyash has been used in SCC. Originally Japanese people (5) suggested water–powder ratio between 0.90 & 1.1 (by volume). But it is the paste that controls the segregation of the concrete mix. The powder & the paste includes finer ( less than 0.125mm) part of the fine aggregate. Vachhani (3) & Bapat (4) used about 35 to 36% paste to produce self compacting concrete. The viscosity modifying agent also controls the segregation– resistance of the concrete mix.They are generally starch, cellulose & gum–based. Preferable & satisfactory VMA is “Welan Gum.” The quantity of such VMA required in SCC is very less, about 0.1% by weight of cementitious materials.

Prof P.K. Mehta (6) included “Welangum,” silica fumes & ultrafine colloidal silica under the list of VMA. Gum or cellulose– based material is capable of modifying the viscosity of SCC, but the silica fume may not be able to modify the viscosity of concrete. Subramanian and Chattopadhyay (7) observed that micro silica at an appropriate dosage may be beneficial in reducing the dosage of “Welan gum.”

The following mix proportioning steps for SCC can be followed.

The target 28-day compressive strength of concrete can be calculated first based on standard deviation value used for the specified grade of concrete.

The water–cementitious materials ratio can be decided based on the target 28–day compressive strength of concrete. This can be in the range of 0.30 – 0.50, 0.30 for a 28

day compressive strength of about 90 MPa, while 0.50 for a 28 day compressive strength of about 30 MPa .

For the high – workability concrete mix, the water content of concrete will be in the range of 180 – 190 l/m3 of concrete.

The maximum size of aggregate for SCC is more or less fixed at 10 or 12 or 16 mm. The sand (natural + manufactured) content can be kept at about 55% & the coarse

aggregate content can be about 45%, by weight of total aggregate. The superplasticizer required is PCE–based and about 1% by weight of total cementitious

material. The cementitious material includes ordinary Portland cement, flyash /ggbs & silica fume (in case of high strength concrete). For normal strength concrete (say from M-25 to M-50), no silica fume will be required, but about 20 to 30 % good quality flyash will be required. If ggbs is used in place of flyash, its percentage can be 40 to 50 %, by weight of total cementitious material. For high strength concrete of M-60 to M-80, about 10% silica fume will be required instead of flyash or ggbs. The dosage of super plsticizer & the viscosity modifying agent can be fixed based on one or two trial mixes in a laboratory.

With the above details in hand, concrete mix proportions for any grade of SCC can be arrived at.

Conclusions

The self compacting concrete, a high workability cohesive concrete mix needs polycarboxylic ether–based superplasticizer and a viscosity modifying agent.

The proportion of fine materials in the concrete mix is also higher than that of normal concrete mixes. Therefore, in addition to natural fine aggregate, manufactured sand and mineral admixture eg flyash, ggbs or silica fume is also to be used.The percentage of fine aggregate is around 55%, while that of coarse aggregate is around 45%, by weight of total aggregate. Smaller size of coarse aggregate (10,12 or 16mm maximum size) having soother surface texture (rounded or crushed gravel) is required for concrete to flow smoothly in the formwork. For normal “standard” concrete grades of M-25 to M-50, about 20 to 30 % flyash or 40 to 50 % ggbs can be used, whereas for high strength self – compacting concrete of grades M-60 to M-80, 10 % silica fume will be required.

References

Gettu,R, Izquierdo, J, Gomes, P.C.C & Josa, A. Development of high – strength self- compacting concrete with flyash: a four – step experimental methodology. 27th conference on OUR WORLD IN CONCRETE & STRUCTURES : 29 – 30 August 2002, Singapore, pp.217 – 224.

IS 9103. Specification for concrete admixtures. Bureau of Indian Standards, New Delhi. Vachhani, S.R, Chaudary, R & Jha, S.M. Innovative use of self compacting concrete in

Metro construction. I.C I Journal, Vol. 5, No 3, Oct – Dec 2004, pp.27 -32. Bapat, S.G, Kulkarni, S.B & Bandekar, K.S. Self- compacting concrete in nuclear power

plant construction. I.C.I Journal, Vol-6, No 3, Oct- Dec 2005, pp- 37- 40.

Okamura,H, Ozawa,K & Ouchi,M. Selfcompacting concrete. Structural Concrete, Vol-1, No1, March 2000.

Mehta,P.K & Monteiro,P.J.M. Concrete-Microstructure, Properties & Materials. Third edition, 2006, Tata McGraw –Hill Publishing Co Ltd, New Delhi, p.478.

Subramanian, S & Chattopadhyay, D. Experiments for mix proportioning of Self – compacting concrete. The Indian Concrete Journal, Jan 2002, pp. 13 – 20.

High Performance Concrete Admixtures High Performance Concrete Admixtures for Improving

the Properties of Concrete Pramod Pathak, Director, Multichem Group, Mumbai.

Admixtures are the ingredients in concrete which are other than the hydraulic cementitious material, water, aggregates or fiber reinforcement that are used as ingredients of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that are added to the batch before or during mixing. Admixtures are usually further defined as a non–pozzolanic (does not require calcium hydroxide to react) admixture in the form of a liquid, suspension or water-soluble solid. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance. Not all admixtures are economical to employ on a particular project.

Also, some characteristics of concrete, such as low absorption, can be achieved simply by

consistently adhering to high quality concreting practices.

Water-reducing admixtures improve concrete’s plastic (wet) and hardened properties, while set-controlling admixtures are used in concrete being placed and finished in other than optimum temperatures. Both, when used appropriately, contribute to good concreting practices. Also, both admixtures should meet the requirements of ASTM C 494, (Table 1).

Water-Reducing Admixtures

Water reducers decrease the amount of mixing water required to obtain a given slump.

This can result in a reduction of the watercementitious ratio (w/c ratio), which leads to increased strengths and more durable concrete.

Reducing the w/c ratio of concrete has been identified as the most important factor to make durable, high-quality concrete. On the other hand, sometimes the cement content may be lowered while maintaining the original w/c ratio to reduce costs or the heat of hydration for mass concrete pours.

Water-reducing admixtures also reduce segregation and improve the flow ability of the concrete. Therefore, they are commonly used for concrete pumping applications as well.

Water-reducing admixtures typically fall into three groups: low-, medium- and high-range. These groups are based on the range of water reduction for the admixture. The percent of water reduction is relative to the original mix water required to obtain a given slump (Table 2). While all water reducers have similarities, each has an appropriate application for which it is best suited. Table 3 presents a summary of the three types of water-reducing admixtures, their ranges of water reduction and their primary uses. Their effect on air entrainment will vary depending on the chemistry.

How They Work? When cement comes in contact with water, dissimilar electrical charges at the surface of

the cement particles attract one another, which results in flocculation or grouping of the particles. A good portion of the water is absorbed in this process, thereby leading to a cohesive mix and reduced slump.

Water-reducing admixtures essentially neutralize surface charges on solid particles and cause all surfaces to carry like charges. Since particles with like charges repel each other, they reduce locculation of the cement particles and allow for better dispersion. They also reduce the viscosity of the paste, resulting in a greater slump.

Table 4 presents some of the most common basic materials used for each range of water

reducer. Other components are also added depending on the requirement of additional properties of concrete. Some water-reducing admixtures have secondary effects or are combined with retarders or accelerators. This will be discussed later.

Effects on Concrete Water-reducing admixtures are primarily used to reduce the water-cementitious content

of concrete, thus increasing strength. In some cases, they can be used to increase the workability or slump of the concrete providing for easier placement. Mid-range water-reducing admixtures were developed to increase the slump beyond the range available with regular water reducers without the excessive retardation that had been known to occur. High-range water reducers, commonly called superplasticizers, were developed for high-strength and high performance concrete applications.

Superplasticizers, e.g., Multiplast Super can take a 3- inch slump concrete to a 9-inch slump without risk of segregation and without compromising its strength. Many precasters can benefit from the use of a superplasticizer, especially because of its improved high early strength development.

All water-reducing admixtures increase strength development as a result of better dispersion of the cement. This increases the exposed surface area of the cement particles, allowing for more complete hydration of the cement.

Set-Controlling Admixtures Set-controlling admixtures alter the rate of the cement’s hydration and, therefore, the rate

of setting (stiffening) of the paste. Coincidentally, they also may affect the hardening or strength gain after the paste has set. Setcontrolling admixtures include retarding and accelerating admixtures.

Retarding Admixtures These admixtures, Multiplast R slow down the hydration process. They may also reduce

the setting time of cement. Retarding admixtures fall into two categories: regular and extended-set. Regular, most commonly referred to as just “retarders,” are used to place concrete in hot climates when long travel times are expected or, in case of emergency, when placement is delayed. They are also commonly used for mass concrete pours to prevent cold joints.

Extended-set control admixtures are those used to delay hydration for many hours or even days. These are usually a twocomponent admixture system. The first component is a retarder (stabilizer) which delays the setting of concrete. The second component is an

accelerator (activator) which overcomes the retarder. The concrete typically reaches initial set in a few hours after the activator is applied.

How they work Retarders essentially slow early hydration by reducing the rate at which tricalcium silicate (C3S) reacts with water. Furthermore, retarders slow the growth of calcium hydroxide crystals. Both reactions develop the early setting and strength gain characteristics of paste. The effect remains until the admixture is incorporated into the hydrated material, thereby removing it from the solution and allowing for initial set to occur. The duration of retardation is based on the dose and chemistry of the retarder, cement composition, temperature and the time it was added to the mix.

Accelerators These admixtures increase the cement’s rate of hydration. Multiplast ACC are designed

to increase the rate of hydration of C3S, thereby increasing early strength. There are two types of accelerators: rapid and normal.

Rapid accelerators can set concrete in minutes and are used in shotcreting applications, to make repairs against hydrostatic pressure or when very rapid setting is required. These are typically not used in precast concrete applications.

Standard or normal accelerators are used to speed up construction in cold-weather concreting conditions; however, it is important to note that they are not antifreezing admixtures.

Effect on concrete: Both retarders and accelerators seem to have negligible effects on air entrainment. However, when water-reducing agents are included, such as lignosulfonates, some air may be entrained.

Retarders tend to reduce one-day strengths and usually increase later-age strengths . Retarders may also increase slump loss and cause an early stiffening of the mixture, even though the strength gain has been delayed. Retarders tend to lose their effectiveness as concrete temperature increases. They also tend to increase the plastic shrinkage.

Accelerators typically increase early strengths. However, laterage strengths may be reduced relative to the same concrete without the accelerator. They also tend to increase early-age shrinkage and creep rates, but tests have shown that ultimate values seem to be unaffected.

Combinations Some admixture chemistries provide for a combination of effects such as water reduction

with retardation or acceleration. Advantages of this include reducing the number of admixtures that have to be stored and added to the concrete; less admixture incompatibility; and cost savings. Disadvantages include less flexibility and limited use when an accelerating or retarding effect is not desired. ASTM C 494 lists specifications for these combination admixtures.

Conceptual Maintenance and Rehabilitation Strategies

Conceptual Maintenance and Rehabilitation Strategies for Bituminous Concrete Pavements

Dr. S.S.Seehra Former Director Grade Scientist & Head, Rigid Pavements Division, CRRI Chief Consultant–Pavement and Geotech, Span Consultant Pvt. Ltd. New Delhi

Introduction

Highway maintenance is an important activity of every highway department. The safety and convenience of traffic using the road are governed to a large extent by the quality of maintenance. The operation – economics of road transport is influenced by the degree of maintenance imparted to the road. The life of an asset can be preserved and prolonged if adequate maintenance measures are undertaken well in time. In developing countries, stage construction of pavements is often resorted to, with lesser pavement thickness and lower specifications than needed for a full design. The proper maintenance of roads, therefore, assumes greater significance in such situations.

This paper emphasizes the need for conceptual mechanism that will ensure the maintenance management procedures to be planned timely with adequate preventive maintenance interventions for effective sustainability of these roads. The financial resources at the command of a maintenance engineers are always short of demands, and it becomes necessary to utilize the same in the most judicious manner, by applying the best engineering practices and managerial skills. Poor road drainage and particularly failure to prevent ingress of water into the subgrade and into the lower pavement layers is considered to be the single main culprit of road failures in India. Maintenance of the drainage system is usually a relatively low cost operation, and one which can significantly reduce the need for far more costly pavement repairs and rehabilitation.

Road maintenance is an essential activity to rejuvenate roads in the safest condition, and to ensure that Pavement Management System (PMS) should also be an integral part of a larger overall Road Maintenance Management System (RMMS).

Need for Maintenance

A bituminous surface deteriorates with the passage of time owing to

i. The action of traffic, especially of overloading of heavy commercial vehicles.ii. Environmental factors, such as ingress of water, oxidation of the binder and loss of

volatiles.

iii. Inadequacies in the initial design, specifications and construction standards of the bituminous layers; and

iv. Lack of adequate support from the lower pavement layers.

Timely and proper maintenance will prolong the life of a pavement system. Road maintenance is a routine, periodic and special activities to be performed to upkeep the pavement, shoulders and other facilities provided for road users, as nearly as possible in its constructed conditions under normal conditions of traffic and forces of nature.

Pavement Management Systems

Modern methods of highway maintenance make use of good management principles, which are invaluable aids in planning and programming of maintenance operations. Many Pavement Management Systems (PMS) have been developed and are extensively used worldwide. A PMS is a computer package, which facilitates advance planning of maintenance operations and optimal allocation of resources. It consists of the following elements

i. A basic road data bank, builtup and updated periodically by road inventories and condition surveys.

ii. A pavement performance model, which predicts the future programme of a given pavement system.

iii. A transportation cost model, which calculates the road user costs for the given condition of the pavement.

iv. Selection of Intervention levels. v. Prioritizing the maintenance needs (renewal and overlay) for a given budget.

Rehabilitation in a Pavement Management System

One of the major reasons that pavement design was historically considered as a one shot process was the lack of an adequate concept for dealing with performance. This need was filled by the serviceability performance concept. Thorough examination of actual highway pavement life histories indicates that this cycle process shown in Figure 1 is more realistic than the so-called one shot design method. As a matter of fact, almost no pavements can be found that serve out a predetermined design life of 20 years or more without some rehabilitation.

Maintenance Management Systems

For a modern road to operate efficiently and effectively for the benefit of all users, it is required to meet defined customer requirements. Maintenance operations and technologies themselves are evolving rapidly to meet the demands of modern road networks. Road drainage performance plays a vital role in ensuring the efficient structural performance of a pavement. Figure 2 illustrates a range of factors, which contribute to the deterioration of a road and to its consequent condition at any given time.

Development of Pavement Management Systems

Efficient and effective maintenance management is most simply expressed as doing the correct thing at the correct time and in the correct place. Pavement Management Systems are most effective if they fulfill a number of essential requirements in relation to the roads and road network to which they are applied. They can assist the engineer in identifying the most cost-effective appropriate treatment on selected sections of the road network through the use of economic analysis, predictive models and time series information. An effective management system must meet a number of core or critical requirements, too much sophistication should be viewed with caution and additional modules should be justified incrementally. Fundamental requirements of maintenance management planning stages are shown in Figure 3.

Maintenance of Flexible Pavements

To meet the various treatments needs of different pavement types, industry and research institutions have developed a range of materials and treatments to offer the engineer a wide variety of effective solutions. The materials are variously required to meet a number of criteria

including strength, resistance to deformation, impermeability, good skidding resistance, low noise and spray generation, efficient drainage system, and value for money. Many modern materials offer a good range of these qualities, but it is fair to say that the perfect materials have yet to be developed. Figure 4 shows the pavement performance modeling which is also applicable to other forms of infrastructure behaviour.

Surface Defects and their Rectification

Pavement tend, under continued trafficking, to lose their anti-skid properties as the texture wears out and the stone aggregates get polished. The skid resistance of a surface may also be reduced by bleeding, or ‘fatting up’ of the road surface with excess bitumen. Many countries carry out skid resistance measurements as part of their maintenance needs assessment. Those pavement sections which have lost their anti-skid properties may be treated with a fresh surface treatment.

Simple maintenance procedure for correcting common distresses in flexible pavements including patching, crack and surface sealing through resurfacing. Skin patches, alligator cracks, deep patches, edge cracks, joint cracks, shallow depressions, hungry and caked surface, reflection cracking, shrinkage cracks and slippery cracks arriving from different technical conditions over a pavement are to be treated differently. This makes it increasingly necessary to find a method of common distress confinement and rehabilitation with a broad based applicability. Figure 5 shows the components of defectiveness profile recorded for surface treatment.

Assessment of Maintenance Needs

Maintenance needs are assessed every year as part of planning of maintenance. The assessment is done on the basis of condition surveys, which can take various forms such as :

Visual rating Roughness measurements Benmkelman Beam Deflection measurements Skid Resistance Measurements

Visual rating is a simple method of inspecting the pavement surface for detecting and assessing the type and severity of the damage. Manifestation of distress or damage occurs in the form of; rutting, corrugations, ravelling, flushing, potholes, transverse cracking, longitudinal cracking, depressions, settlement, polished surface, streaking, hungry surface, edge cracking, reflection cracking, shrinkage cracking, deformation, slippage, shoving, stripping, disintegration, loss of aggregate, alligator cracking and edge failure etc. These serviceability indicators are to be kept intact through proper maintenance cycles methodology and strategies.

Preventive Bituminous Surface Maintenance through Micro-Surfacing

Future maintenance strategies for renewal surfacing activity is the need of the hour at regular intervals of time so that the constructed roads perform satisfactorily throughout their designed service life. Periodic renewals consist of the provision of micro-surfacing layer so as to preserve the required characteristics of the pavement and offset the wear and tear of the surface caused by traffic and weathering etc. Periodic renewals represent preventive maintenance, which is needed to prevent deterioration of the pavement characteristics and to ensure that initial qualities are kept up for the future requirements of traffic during the design life of the pavement. Early detection and repair of noticeable defects can prevent a major break down of the surface. For instance, if symptoms like hungry surface, raveling etc. are noticed at an early stage and suitable preventive action by way of renewal of surface is taken to arrest further deterioration, the life of the pavement can be prolonged. Micro-surfacing is a thin layer of a mixture of a modified bitumen based emulsion, aggregate, water and additives like cement and lime in desired proportions. Microsurfacing is generally applied over a hungry, baked flexible pavement surface and also in case of fretting of aggregate over already laid surface, shallow depressions and fine cracks to moderate cracks (3mm to 6mm wide).

It is relatively thin sections in which the mix is laid over the surface which is called microsurfacing. Micro-surfacing mix can be applied more frequently by machine application up to a thickness of 5mm in one application, where the situation demands there can be two applications of the same thickness. Micro-surfacing can be easily sweeped into cracks and fishers. Since a cationic type modified bitumen emulsion is used in preparing micro-surfacing the surfacing has got the unique feature of resisting action of water and preventing damage due to rains in the sealed pavement which may prove a good remedial measure. Micro-surfacing is a low cost preventive maintenance treatment that retards the deterioration of pavement surface caused by environmental and the associated oxidation of the existing surface. Micro-surfacing is now recognized as most cost-effectiveway to treat the deteriorated surface. The micro-surfacing treatment should only be used on structurally sound pavement without extensive cracking or other deterioration. This preventive maintenance treatment is applied in one or two courses and does not require compaction. Traffic can usually be located back on to the roadways within one hour under ideal condition. The micro surfacing mix provides excellent smoothness and good

friction with minimal increase in pavement noise levels.

Micro-surfacing may be suitably used on cracked pavement in lieu of more conventional rehabilitation such as crack to sealing, fog seal, liquid seal and double surface treatments. Microsurfacing provides a convenient economical way of addressing pavement distress such as raveling and cracking.

Highway Design and Maintenance Standards

The World Bank had developed the highway design and maintenance standards model and its Version-3 has been in use. HDM-4 has been developed and released for use. For the proposed planning model, HDM-4 has been calibrated for Indian deterioration and user cost models and customized for the chosen computer system platform, which will bring the planning process to the state-of-the-art level. National Highway network maps can be digitized using Survey of India (SOI) base maps and the mapping data can be held in the Geographic Information System (GIS) format in a cartographic database. It may also be possible to overlay, the maps with all the other relevant information collected during the socio-economic, road condition and road inventory surveys. The entire highway management system needs to be established on a computer system platform on client server model at the headquarters of Ministry of Road Transport and Highways (MoRT&H). The regional centers could be linked with the headquarters through a communication system so as to enable data transfer from the field to the headquarters. Computer operating system can be selected to ensure compatibility of the hardware and the facility of upgradation at a later date. Software for GIS, HDM, image processing software, terrain modeling software, and Autocad may be required for the proposed planning model.

Future Suggestions of Road Maintenance

There is a need for the guidelines on the strategic maintenance of flexible pavements, which should be easy to use, understandable, and cost-effective and provide uniformity in evaluation, process and management methodology.

There is no doubt that the external influences which affect road operators and authorities will continue to play a major part in shaping the development of road maintenance. Indeed, it is probable that the most successful road maintenance will be that which is noticed least in terms of its impacts on the road user, the environment, and those living and working close to the carriageway. Some general indicators of future challenges for the road maintenance engineer are given below:-

i. Obtaining Good quality information about road conditions is an essential pre-requisite for sound decision making about the need for road maintenance, and type of treatment that is subsequently applied. Increased interest is likely to be paid to the concept of smart roads, which use sensors within road components to feed real – time information regarding their condition and performance.

ii. A greater emphasis is also likely to be placed on improving communication with road users to provide them with up-to-date and reliable advice about the state of the road,

including weather, traffic, safety and other conditions. Capabilities of this type could eventually form the basis of better highway maintenance control if the demand for limited road space grows excessively.

iii. The very wide use of roads, and their impact upon those which they serve, offer great challenges to road maintenance engineers to ensure that theassets for whose upkeep they are responsible are maintained for the benefit and convenience of all the road users.

iv. Most important aspect of the future is where does the future of Pavement Management Systems go from here? How can we upgrade and improve the technology of the Pavement Management System? How can we improve the Pavement Management System itself.

v. No existing system is directly applicable to another agency, do not be afraid to take advantage of the benefits of others experience. Much could be gained from expert sources with previous experience in the Pavement Management Systems.

Conclusions

Efficient and effective maintenance management is simply expressed as doing the correct thing at the correct time and in the correct place.

Making good decision regarding road maintenance is a complex process that involves the right treatment for the right road at a right time.

A thin hot mix asphalt overlay on the principle of microsurfacing will improve the riding quality and skid

resistance, revitalize the existing surface free of ruts and potholes, and other surface defects.

The preventive maintenance approach through microsurfacing will not only save our scare funds but also provide a safe and comfortable ride to our road users.

Micro-surfacing is one of the latest mixtures of surface treatments, such as chip seal, liquid seal, slurry seal and fog seal composed of polymer modified asphaltemulsion, crushed aggregates, mineral filler, water and field control additive as needed.

The traffic on National Highways is likely to increaseenormously in the futuristic scenario. To meet the demand optimally, and thereby to ensure rapid economic progress of the country, it is essential to develop and establish an efficient highway planning and management systems. For this the existing deficiencies in the system need to be overcome and new capabilities need to be developed.

Effort is also required to integrate various systems related to highway management system carried out in

India and abroad. Committed manpower resources will have to be developed and adequate infrastructure will have to be established to bring the highway planning and management system to the state-of-the-art level and comparable to those existing in developed countries.

Maintenance-by-Contract of National Highways and expressways should be privatized or as a part of construction contract to reduce the burden on the exchequer. The best possible way shall be to run pilot projects in every state and on National Highways and State Highways to start with.

We should follow-up strict construction supervision and stringent quality control measures and must protect our investment with minimal maintenance costs.

Proper pavement design, regular inspection and maintenance of drainage system is of utmost importance in preserving the investment made on the construction of highway pavements. The drainage conditions be improved along the highways so that the damage to the sub-grade due to seepage of water be avoided. The special attention is required for the maintenance of roads and highways in the snow and desert areas.

The above highway maintenance strategies would be useful to reduce the losses caused due to bad condition of roads. The saving due toreduction in losses can be used for the construction of new roads and improvement of existingroads and highways.

References

1. Butler, B.C. and L.G. Byrd “Maintenance Management,” Section 25 of Handbook of Highway Engineer, Van NostrandReinhold, 1975.

2. Stacy, A.F., “The determination of Pavement Maintenance Strategies” proceedings, Australian Road Research Board, 1978.

3. Asphalt Technology and Construction, Asphalt Institute Park, Maryland, USA, 1978.4. Local Authority Associations, Highway Maintenance – A Code of Good Practice,

Association of County Councils, London, 1989.5. New Roads and Street Works Act, HMSO, London, 1991.6. IRC:82-1982 “Code of Practice for Maintenance of Bituminous Surfaces of Highways,”

The Indian Roads Congress, New Delhi. 7. Ralph Mass and Ronald Hudson, “Pavement Management Systems” Robert E.Krieger

Publishing Company, Florida, 1982.8. Federal Highway Administration (FHWA), “Construction, Maintenance, Implementation

and Management of Highways,” Washington, D.C. 1972.9. IRC:81-1997, “Tentative Guidelines for strengthening of Flexible Pavements using

Benkelman Beam Deflectio Technique,” Indian Roads Congress, New Delhi–1997.10. Seehra, S.S., “Causes of Failure of the Existing Pavements and their Evaluation for

Strengthening.” The Annual Journal of the Institution of Engineers (India), Vol. 11, 1972.11. Transportation Research Record No. 1597, “Maintenance of Highway Pavements and

structures“Transportation Research Board, Washington, D.C.1997.12. 12. Mookerjee, A.K., “Road Rehabilitation and Maintenance,” International Seminar on

Highway Rehabilitation and Maintenance, Organized by Indian Roads Congress, New Delhi, 1999.

GREEN CONCRETE & GLOBAL WARMINGDr. S.P. Bhatnagar, Tech Dry (India) Pvt. Ltd. Bangalore

There are several causes for global warming including carbon dioxide emission from burning of fossil fuels for the purpose of electricity generation. Coal accounts for 93 percent of the emissions from the electric utility industry. Coal emits around 1.7 times as much carbon per unit of energy when burnt as does natural gas and 1.25 times as much as oil. Natural gas gives off

50% of the carbon dioxide. Carbon dioxide emitted from cars is about 20%. Carbon dioxide emitted from airplanes causes 3.6% of global warming and that the figure could rise to 15% by 2050. Building structures account for about 12% of carbon dioxide emissions.1

It is wellknown that the ecological balance is getting disturbed but we will keep our discussions restricted to the construction industry and utility of the green concrete.

Green building is all about science-physics, chemistry, and biology. It’s really about ecology because ecology is about physics, chemistry, and because it is all about systems and integration of physics, chemistry, and biology.

Building is the shelter creating boundaries between people and the environment. Green building is about creating optimized boundaries between people and the environment.

The green building programme has identified a set of parameters that should be kept into consideration when the building is constructed and materials are chosen for it. It is vast subject to even define the material, which constitutes as environmental friendly or green material.

Worldwide, the construction industry contributes about 9% to the global GDP, and is one of the most important elements of every economy. Today’s demands on buildings, roads, bridges, tunnels, and dams could not be met without construction chemicals. The strength of concrete has risen dramatically as a result of the development of construction chemicals.

The global construction chemical industry is a $20 billion business. The United States and Western Europe are the two largest markets, together accounting for 56% of the total market. Japan, China and India come next and together have a market share of about 21%.

The raw materials needed for the production of construction chemicals are manufactured by the big chemical producers. Polymers are the most important group of raw materials and are found in virtually every construction chemical formulation ranging from adhesives to waterproofing treatments. The development of new construction chemicals in many cases requires interaction by the chemical producer, construction chemical manufacturer and end user. The construction chemical industry spends about 3% of its sales on R&D of new products and applications.2

We often hear that India is going to become world power. It sounds musical to our ears but when you see the state of our Infrastructures, it is disappointing. In this paper, we will deal with one important aspect and that is the construction industry.

The production of bricks required the burning of fuels, either fossil fuels or agricultural wastes. The firing of bricks is to increase the strength and durability of the brick and to decrease water absorption. Concrete requires the manufacture of cement. To produce cement, limestone and clay are heated at 1450°C consuming fossil fuels, and cement is formed. The limestone is converted to calcium oxide and carbon dioxide.

CaCO3->CaO + CO2

For every 1000kg of calcium carbonate used, 440 kg of carbon dioxide is produced.

This production of carbon dioxide raises the question for the world of the desirability and economics of emulating western building practices in these countries, given the huge population requiring housing. For the production of bricks and concrete energy intensive activities are undertaken. In addition, the energy use results in carbon dioxide production. In the case of cement production, the demand for cement worldwide is 800 million tonnes per year. Assuming 500 million tonnes of limestone is used for this purpose each year then more than 220 million tonnes of carbon dioxide is emitted to the atmosphere from cement works alone each year. This is the equivalent of 44 kg of carbon dioxide for every inhabitant of the Earth each year.

Cement

Let us concentrate on some of the major factors contributing to this state of affairs related to construction industry. We believe that every responsible citizen would continue in adopting environmental objectives. The given table will show the energy demand and emissions generated in production of 1kg of cement.

Corrosion

The corrosion of steel reinforcement is by far the single most common cause of structural damage.

The key environmental factors that reduce the passivation of steel are carbonation and chloride. Other factors which may influence either the initiation or rate of reinforcement corrosion include cracks in concrete, temperature, moisture, oxygen and in adequate concrete quality or cover.

There are two major situations in which corrosion of reinforcing steel can occur:

Carbonation Chloride contamination

Carbonation

Carbonation is the process in which the Carbon dioxide (CO2) enters in the concrete as carbonic cid in the presence of moisture and reacts with calcium hydroxide and following reaction takes place

Ca(OH)2 + CO2 -> CaCO3 + H2O

Chloride Ions

Chloride ions can enter concrete in two ways:

They may be added during mixing either deliberately as an admixture or as a contaminant in the original constituents.

They may enter the set concrete from environment pollutants dissolved in rain water/humidity.

Both Carbonation and Chloride ions damage the protective, highly alkaline passive shield around reinforcement. This leads to the corrosion of reinforcement/ malignancy of reinforcement as we call, which makes the building less durable and vulnerable to natural calamities which leads to human tragedy and loss of property.

Toxicity of Admixture

While using admixture, it is very important to be careful and make judicious decision so that the ingredient of cement does not react with admixture and produce undesirable side products.

Plasticizers tend to liberate cancer causing toxic product like formaldehyde.

Tons and tons of admixtures particularly plasticizers and superplasticizers are used in construction. The study shows that approximately 15-25% of sulphonated naphthalene polymers (SNP), lignosulphonate and polycarboxylates and 30-60% of sulphonated melamine polymers (SMP) were leached. Some additional test showed that this is the only part of leached organic substance that comes from superplasticizers and rest of them come from coating and adhesives.

Togero4 shows in his studies that some small fraction of formaldehyde in both SNF and SMF is liberated, which is not only hazardous but also carcinogenic.

lmpregnants

Water vapour permeability is an essential requirement for building materials. A satisfactory water repellent leaves the treated substrate permeable to water vapour while restricting the passage of liquid through the capillaries.

A natural external finish of masonry buildings may be required for aesthetic purposes. Treatment by impregnation does not change the finish appearance and no yellowing is normally developed during use.

Permanent bonding between concrete capillaries and impregnants results in longterm durability.

Solvent Based Impregnants

Impregnants dissolved in a suitable solvent can be used to create a waterproof hydrophobic surface which does not allow the ingress of water. However, solvent being toxic and hazardous has been banned in most of the Western countries. Moreover, solvents are very expensive.

Water Based

Water based impregnants form a zone within the pore of structure after penetration, resulting in a molecular size three to four times the dissolved size, and some impregnants is bound chemically to the silicates in the cement matrix.

Coatings

Coatings have not been successful because they tend to block pores and capillaries with the trapped water underneath it. This trapped water hits the weaker part of the surface and create ingress points in the form of cracks, blisters, honeycombs etc.

The effect of some of the coatings is injurious like:

Asbestos

We are sure that everybody knows that any type of asbestos causes cancer and it is not confined to a specific blue variety.

Asbestos as we all know is the name given to group of minerals that occur naturally as masses of strong, flexible fibers that can be separated into thin threads and woven. These fibers are not affected by heat or chemicals and do not conduct electricity. For these reasons, asbestos has been widely used in many industries.

Membranes

It has become a fashion to use membranes, the word is a misnomer since the elastomeric coating should have the following properties which these membranes do not have.

Tensile strength, Elongation, Crack bridging, Abrasion resistance, Temperature flexibility, Weatherability, Bonding, Flashing attachment, Materials compatibility, Wind uplift resistance.

The conventional membranes or thick toppings are normally bitumen, asphalt, polyurethane, and epoxy based. Besides several disadvantages like blister formation, debonding and other factors which allow water to enter.

Bitumen

The main constituents in bitumen are polyaromatic compounds which undergo photochemical oxidation particularly at high temperature (since they are black, the temperature is much higher).

These photo-oxidation generates gases and strong carcinogenic compounds like Benzo[a]pyrene.

Elastomeric Coatings

Elastomeric coatings should not be misunderstood as the above mentioned membrane since these are normally non-cementitious and are produced by using special polymerization techniques and unlike other membranes they are flexible, breathable with high elongation and weatherability and crack bridging membranes rather than coating which have several problems.

Elastomeric coatings are the latest type of coating which came into roof protection systems and tanking systems in basements.

To understand this concept we must address ourselves to basic questions to why latest membranes in this field are different than the conventional coating membranes.

Let us understand the term elastomers. Elastomers are a class of materials which differ quite obviously from all other solid materials in that they can be stretched easily and almost completely reversibly, to high extensions and before reaching its ultimate breaking elongation – it can be released and will rapidly recover to almost exactly the original length it had before stretching. The material is said to be elastic.

Most synthetic elastomers are not as elastic as natural rubber, but all can be stretched (or otherwise deformed) in a reversible manner to an extent, which easily distinguishes them from all other solid materials.

Elastomers are a special case of the wider group of materials known as polymers. Polymers are not made up of discrete compact molecules like most materials, but are made of long, flexible, chainlike or string-like, molecules. At this scale the inside of a piece of rubber can be thought of

as resembling a pile of cooked spaghetti. In spaghetti, however, the chains, though intertwined, are all separate. But in most practical elastomers each chain will be joined together occasionally along its length to one or more nearby chains with just a very few chemical bridges, known as crosslinks. So the whole structure forms a coherent network which stops the chains from sliding past one another indefinitely – although leaving the long sections of chain between crosslinks free to move. The process by which crosslinks are added is known as vulcanization.

Polymers on the other hand are giant molecules of different chemicals. A polymer or a macromolecule is made up of many (poly) molecules (‘mers’) or monomers linked together like wagons in a train, for example poly(vinyl chloride), poly(ethylene), etc. The polymerization of vinyl chloride (VC), which represents some 500 to 2000 molecules of VC linked together to make a giant molecule of commercial PVC. Monomers may have the same or different chemical compositions

Water in the form of vapour, liquid presents below-grade construction with many unique problems. Water causes damage by vapour transmission through porous surfaces, by direct leakage in a liquid state. Water presence in below grade makes interior spaces uninhabitable not only byleakage but also by damage to structural components as exhibited by reinforcing steel corrosion, concrete spalling, settlement cracks, and structural cracking.

Therefore, all elastomeric membranes are not alike and different parameters like nature of monomer cross-linking agent, polymerization technique, initiators, accelerators and fillers can have an influence on the physical and chemical stability of the final elastomeric membrane.

Grouting

Grouting is the injection of a fluidized material into the soil to enhance its strength, density, or to reduce its permeability. Grouting can be more feasible than the cut and cover method, for example, excavating a trench to put in a tunnel lining and filling in the gap with soil. In the city, traffic may have to be rerouted around the cut and cover project site.

In planning a grouting programme for particular conditions, we need knowledge of various types of grouts and their properties. The basic types of grouts now in use and their properties are discussed. Types of admixtures and fillers used and their effects on the grout are also discussed. The most common types of grout are Portland cement, clay, chemical, and asphaltic grouts. No one grout is suitable for every situation.

Now-a-days excellent chemical grouting products have been developed, which can strengthen the voids whether in basements or otherwise. For example there are 2-component system where the damage is not only treated on the surface of the structure but that the complete centre of damage and the whole section of the building structure are completely treated.

These kinds of products do not effect the environment nor pollute the ground water.

Thermal Insulation

This has been a very misunderstood subject and there has been an understanding that Brick bat coba, surkhi or thermal insulation are preferred as Insulation products while thermal insulation does provide insulation but is not very durable. Surkhi which is now-a-days used as burnt bricks but definitely does not provide any thermal insulation on the concrete.

Heat naturally flows from warm areas to cooler areas, regardless of direction. This flow of heat can never be stopped completely, but the rate at which it flows can be reduced by using materials which have a high resistance to heat flow.

The general guideline for thermal insulation is to understand that thermal resistance of insulating material is directly proportional to the type of material and its thickness measured in terms of thermal conductivity.

Thermal insulation for buildings has been known since long and is one of the serious requirements more because of the climatic conditions in India. Moreover, we in India need any new system, which can contribute in saving energy.

For the last few years, lightweight micaceous minerals like Vermiculite have been used. The problem with this product is that it is very porous in nature and absorbs water and therefore has to be waterproofed. Moreover, it is soft, and laying of tiles over it is often required.

The choice of the insulating material depends on the cost, area to be covered and the cost of heating or cooling. There are large numbers of insulation materials available in the market.

Recently, ceramic microspheres and some natural clay along with redispersable spray dried polymers have played a key role. For example, lightweight waterproofing concrete not only replaces brick bat coba and reduced the weight on the surface of the roof, gives a very good insulation.

It is important that the key persons in the field of real estate and construction industry should appreciate the advantages of green building and its benefits, but unfortunately they mix the cost benefit of these green buildings.

In one of the reports conducted by the World Business Council for Sustainable Development (WBCSD). Respondents to a 1400 person global survey estimated the additional cost of building green at 17% above conventional construction, more than triple the true cost difference of about 5%. At the same time, survey respondents put greenhouse gas emissions by buildings at 19% of world total, while the actual number of 40% is double this.3

Existing technologies combined with common sense design can increase energy efficiency by 35 percent and reduce heating costs by 80 percent for the average building in industrialized markets.

Life cycle analysis shows that 80 to 85% of the total energy consumption and CO2 emissions of a building comes from occupancy through heating, cooling, ventilation, and hot water use. Buildings already represent approximately 40% of primary energy use globally and energy

consumption in buildings is projected to rise substantially in the world’s most populous and fast growing countries such as China and India.

It would also be interesting to note that we can perhaps use environmental friendly green material, some them are:

By-product: Unused or waste material from one manufacturing or energy producing process that can be used in another manufacturing or energy producing process.

Diversion: Avoidance of landfill disposal of a material or product through reuse or recycling.

Embodied Energy: All of the energy required in the raw material extraction, manufacturing, distribution, and transport of a material product up to its point of use.

Global Warming potential: Possible Climate warming effect caused by the manufacture and/ or use of a material or product compared to that of carbon dioxide which has a GWP of 1.0.

Indoor Air quality: Condition of air inside buildings with respect to harmful concentrations of contaminants, volatile organic compounds and particulates.

Life Cycle: All stages of production, including raw materials extraction, manufacturing, distribution, use, maintenance, reuse or recycling, disposal, and all transportation.

Off-Gassing: Releasing of gases or vapours into the air Rapidly renewable: Materials that are replenished relatively quickly, usually in less than

10 years. Recyclable: Having the potential for being recycled by possessing such traits as highly

recoverable, easily separated from other materials, not contaminated by toxic coating etc. Recycled content: Portion of material or product that is made from recovered material. Reused or salvaged materials: Materials or products from building deconstruction or

demolition that are reused ‘as –is’ with little or no processing or modification Solid waste: Material or product, typically long lasting and not biodegradable, disposed

of in landfills or incinerators. Source separation: Separation of waste materials by material type at the point of use to

facilitate recycling. Third party certified: Materials or products that are monitored by independent

organizations for compliance with recognized environmental standards.

Quite often, it is extremely difficult to accurately assess the environmental performance of a building material or product over its entire life cycle. In many cases, the GBP relies on third party certification organization to accomplish this task.

Green Terrace/Green Roofs

We have been emphasizing on green concrete. We would once again say that ecological engineering is an emerging field, it permits us to develop design of sustainable ecosystem with integrate human society.

We can avoid sound pollution by using lightweight minerals. Many home owners and the designers prefer to add bright lights because it gives a better feeling of architecture, it lights up garden and tress but we forget that the by-product of all these is light pollution.

What is light pollution? When the light is shining into your neighbor’s house it creates a sky glow effect, it can cause glare and so many other problems. Light pollution is also harmful to wild life and equally to human beings.

Infact several European countries and the US have very aggressively pursued the project of green roofs or terrace gardens. This would help mitigate the urban heat insland effect, reduce storm water runoff, improve building insulation and increase green space and biodiversity in urban centers.

Progress in horticultural engineering, including improvements in drought-resistant plants, and advances in waterproofing systems aided the gradual development of a viable green roof industry. Germany has been on the forefront in this field and has subsidized green roof costs.

Green roofs are the result of a complete underlying roof build-up system, providing continuous, uninterrupted layers of protection and drainage. Recent advances in technology have made them lighter, more durable and better able to withstand the extreme conditions of the rooftop.

Waterproofing

If waterproofing is not done and is not effective, it can encourage the growth of algae, fungus, mosses which are the natural sources of bacteria and in-house pollution, radon gases which causes disease like asthma and diabetes mainly in children.

Conclusion

The benefits of green buildings are many: greater energy efficiency, reduced water consumption, longer useful life, better health conditions for occupants, and much more. All of these factors can improve the value of a building over the long term and reduce operational costs. However, the mistaken perception exists that green building “costs too much” without a commensurate return on investment. Therefore, we conclude that waterproofing is a critical step but should be based on environmental friendly, non-toxic and energy saving techniques.

References:

1. US Emission Inventory 2004 Executive summary p.102. SRI Consulting SCUP Report 3. World Business Council for Sustainable Development (WBCSD)

4. Togero, 2004

Self Curing Concrete An Introduction

Ambily P.S, Scientist, and Rajamane N P, Deputy Director and Head, Concrete Composites Lab Structural Engineering Research Centre, CSIR, Chennai Excessive evaporation of water (internal or external) from fresh concrete should be avoided; otherwise, the degree of cement hydration would get lowered and thereby concrete may develop unsatisfactory properties. Curing operations should ensure that adequate amount of water is available for cement hydration to occur. This paper discusses different aspects of achieving optimum cure of concrete without the need for applying external curing methods.

Definition of Internal Curing (IC)

The ACI-308 Code states that “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing Water.” Conventionally, curing concrete means creating conditions such that water is not lost from the surface i.e., curing is taken to happen ‘from the outside to inside’. In contrast, ‘internal curing’ is allowing for curing ‘from the inside to outside’ through the internal reservoirs (in the form of saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers) Created. ‘Internal curing’ is often also referred as ‘Self–curing.’

Need for Self–curing

When the mineral admixtures react completely in a blended cement system, their demand for curing water (external or internal) can be much greater than that in a conventional ordinary Portland cement concrete. When this water is not readily available, due to depercolation of the capillary porosity, for example, significant autogenous deformation and (early-age) cracking may result.

Due to the chemical shrinkage occurring during cement hydration, empty pores are created within the cement paste, leading to a reduction in its internal relative humidity and also to shrinkage which may cause early-age cracking. This situation is intensified in HPC (compared to conventional concrete) due to its generally higher cement content, reduced water/cement (w/ c) ratio and the pozzolanic mineral admixtures (fly ash, silica fume). The empty pores created during self-desiccation induce shrinkage stresses and also influence the kinetics of cement hydration process, limiting the final degree of hydration. The strength achieved by IC could be more than that possible under saturated curing conditions.

Often specially in HPC, it is not easily possible to provide curing water from the top surface at the rate required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities often achieved.

Potential Materials for IC

The following materials can provide internal water reservoirs:

Lightweight Aggregate (natural and synthetic, expanded shale), LWS Sand (Water absorption =17 %) LWA 19mm Coarse (Water absorption = 20%) Super-absorbent Polymers (SAP) (60-300 mm size) SRA (Shrinkage Reducing Admixture) (propylene glycol type i.e. polyethylene-glycol) Wood powder

Chemicals to Achieve Self–curing

Some specific water-soluble chemicals added during the mixing can reduce water evaporation from and within the set concrete, making it ‘self-curing.’ The chemicals should have abilities to reduce evaporation from solution and to improve water retention in ordinary Portland cement matrix.

Super-absorbent Polymer (SAP) for IC

The common SAPs are added at rate of 0–0.6 wt % of cement. The SAPs are covalently cross-linked. They are Acrylamide/acrylic acid copolymers. One type of SAPs are suspension polymerized, spherical particles with an average particle size of approximately 200 mm; another type of SAP is solutionpolymerized and then crushed and sieved to particle sizes in the range of 125–250 mm. The size of the swollen SAP particles in the cement pastes and mortars is about three times larger due to pore fluid absorption. The swelling time depends especially on the particle size distribution of the SAP. It is seen that more than 50% swelling occurs within the first 5 min after water addition. The water content in SAP at reduced RH is indicated by the sorption isotherm.

SAPs are a group of polymeric materials that have the ability to absorb a significant amount of liquid from the surroundings and to retain the liquid within their structure without dissolving.

SAPs are principally used for absorbing water and aqueous solutions; about 95% of the SAP world production is used as a urine absorber in disposable diapers. SAPs can be produced with water absorption of up to 5000 times their own weight. However, in dilute salt solutions, the absorbency of commercially produced SAPs is around 50 g/g. They can be produced by either solution or suspension polymerization, and the particles may be prepared in different sizes and shapes including spherical particles. The commercially important SAPs are covalently cross-linked polyacrylates and copolymerized polyacrylamides/ polyacrylates. Because of their ionic nature and interconnected structure, they can absorb large quantities of water without dissolving. From a chemical point of view, all the water inside a SAP can essentially be considered as bulk water. SAPs exist in two distinct phase states, collapsed and swollen. The phase transition is a result of a competitive balance between repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network. The macromolecular matrix of a SAP is a polyelectrolyte, i.e., a polymer with ionisable groups that can dissociate in solution, leaving ions of one sign bound to the chain and counter-ions in solution. For this reason, a high concentration of ions exists inside the SAP leading to a water flow into the SAP due to osmosis. Another factor contributing to increase the swelling is water solvation of hydrophilic groups present along the polymer chain. Elastic free energy opposes swelling of the SAP by a retractive force.

SAPs exist in two distinct phase states, collapsed and swollen. The phase transition is a result of a competitive balance between repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network.

Means of Providing Water for Self–curing Using LWA

Water/moisture required for internal curing can be supplied by incorporation of saturated-surfacedry (SSD) lightweight fine aggregates (LWA).

Water Available from LWA for Self–curing

It is estimated by measuring desorption of the LWA in SSD condition after exposed to a salt solution of potassium nitrate (equilibrium RH of 93%). The total absorption capacity of the LWA can be measured by drying a Saturated Surface Dry (SSD) sample in a dessicator.

Water in LWA for Internal Curing

About 67% of the water absorbed in the LWA can get transported to self-desiccating paste. Some water remains always in the LWA in the high RH range and it becomes useful when the overall RH humidity in concrete is significantly reduced. The water retained in LWA in air-dry condition may not be enough to prevent autogenous shrinkage whose magnitude, however, may be reduced significantly. The fine lightweight aggregate, in saturated condition, produce a more uniform distribution of the water needed for curing throughout the microstructure.

The grain size of the LWA used as curing agent should be less in order to minimise the paste– aggregate proximity, i.e. the distance to which the internal curing water could diffuse. The grain size of down to 2–4 mm are found to be beneficial.

Utility of LWA Near Surface of Concrete

At the surface of the concrete, as the water evaporates from the concrete surface, a humidity gradient develops. This accelerates the appearance of the localized humidity gradients. The water from the LWA near the surface is then used up faster than in the interior of the concrete thus causing the near-surface layer of the concrete to become denser in a shorter time. This helps reduce the amount of water that would normally evaporate and contributes to improve internal curing of the concrete. It also leads to reduced or no stresses due to drying helping in eliminating the surface cracking.

Potential of LWA for Reducing Autogenous Shrinkage

As the cement hydrates, the water will be drawn from the relatively “large” pores in the LWA into the much smaller ones in the cement paste. This will minimise the development of autogenous shrinkage as the shrinkage stress is controlled by the size of the empty pores, via the Kelvin- Laplace equation.

The radii of capillary pores formed during hydration in the cement paste are smaller than the pores of the LWA. When the RH decreases (due to hydration and drying), a humidity gradient develops; with the LWA acting as a water reservoir, the pores of the cement paste absorb water from the LWA by capillary suction. The unhydrated cement particles from the cement paste now have more free-water available for hydration and new hydration products grow in the pores of the cement paste thus causing them to become smaller. The capillary suction, which is the inverse to the square of the pore radius, increases as the radius becomes smaller and thus enabling the pores to continue to absorb water from the LWA. This continues until most of the water from the LWA has been transported to the cement paste.

Crushed LWA for Internal Curing

Crushed LWA could provide a better surface for binder interaction as the pelletising process often produces LWAs with sealed surface. The vesicular surface resulting from the crushing operation allows paste penetration and provides more surface area for reaction between the aggregate and paste. The transition zone associated with a crushed aggregate has advantages over a more smooth and sealed surface.

Water Required for Self–curing

It depends upon chemical and autogenous shrinkages expected during hydration reactions.

Types of Shrinkage Drying

Shrinkages may occur at earlyages or at later ages over a longer period; different types of shrinkages may be identified as :

Drying shrinkage, autogenous shrinkage, thermal shrinkage, and carbonation shrinkage.

Reason for Chemical Shrinkage

Chemical shrinkage is an internal volume reduction due to the absolute volume of the hydration

Products being less than that of the reactants (cement and water). For example: Hydration of tricalcium silicate:

C3S + 5.3 H -> C1.7SH4 + 1.3 CH

Molar volumes

71.1 + 95.8 -> 107.8 + 43 i.e, 166.9 -> 150.8

Therefore,

Chemical shrinkage = (150.8 –166.9) / 166.9 = -0.096 mL/mL = -0.0704 mL/g cement

For complete reaction of each gram of tricalcium silicate, there is a need to supply 0.07 gram of extra curing water to maintain saturated conditions. (A value of 0.053 for 75% hydration at 28 day was experimentally observed by Powers in 1935).

Quantity of Chemical Shrinkage

Portland cement hydration is typically accompanied by a chemical shrinkage on the order of 0.07 mass of water per mass of cement for complete hydration: for silica fume, slag, and fly ash, these coefficients are about 0.22, 0.18, and 0.10 to 0.16, respectively. It can be measured by ASTM standard test method, C1608

Autogenous Shrinkage

It is as a volume change in concrete occurring without moisture transfer from the environment intoconcrete. It is due to the internal chemical and structural reactions of the concrete. Autogenous shrinkage is prominent in HPCs due to the reduced amount of water and increased amount of various binders used.

At early ages (the first few hours), before the concrete has formed a hardened skeleton, autogenous shrinkage is often due to only chemical shrinkage. At later ages (>1+days), the autogenous shrinkage can also result from self-desiccation since the hardened skeleton resists the chemical shrinkage.

The external (macroscopic) dimensional reduction of the cementitious system under isothermal sealed curing conditions; can be 100 to 1000 micro strains.

Self-desiccation

It is the localized drying resulting from a decreasing relative humidity (RH) which could be the result of the cement requiring extra water for hydration. It is the reduction in the internal relative humidity of a sealed system when empty pores are generated.

Potential of Selfdesiccation Prominent in HPC/ HSC

The finer porosity of HSC/HPC (with a low w/c), causes the water meniscus to have a greater radius of curvature, causing large compressive stress on the pore walls, leading to greater autogenous shrinkage as the paste is pulled inwards. Self–desiccation is only a risk when there is not enough localized water in the paste for the cement to hydrate and it occurs the water is drawn out of the capillary pore spaces between the solid particles. At later ages, a strong correlation exists between internal relative humidity and free autogenous shrinkage.

Mineral admixtures, such as fly ash and silica fume, in concrete tend to refine the pore structure towards a finer microstructure thereby water consumption will be increased and the autogenous shrinkage due to self-desiccation will be increased.

Inter-dependance of Autogenous & Chemical Shrinkages

Chemical shrinkage creates empty pores within hydrating paste and stress generated is stimated by equation:

σcap = 2 *γ / r = - In (RH) * R * T / Vm

where γ,Vm = Surface tension and molar volume of the pore solution,

r = the radius of the largest water-filled pore (or the smallest empty pore),

R = the universal gas constant, and T is the absolute temperature

The sizes of empty pores regulate both internal RH and capillary stresses. These stresses cause a physical autogenous deformation (shrinkage strain) given by:

ε = ( S * σcap/ 3 ) * [ (1/K) – (1/Ks)]

where ε = shrinkage (negative strain), S = degree of saturation (0 to 1) or volume fraction of waterfilled pores, K = bulk modulus of elasticity of the porous material, and Ks = bulk modulus of the solid framework within the porous material.

The above equation is only approximate for a partiallysaturated visco-elastic material such as hydrating cement paste, but still provides insight into the physical mechanism of autogenous shrinkage and the importance of various physical parameters The internal drying is analogous to external drying shrinkage.

Early External Water Curing and Cracks in HPC

Reduction of autogenous shrinkage due to external curing in HPCs is possible for first one or two days when the capillary pores are yet interconnected. Early water curing can lead to higher strain gradients when the skin of the concrete becomes well cured (no shrinkage) whereas, autogenous shrinkage, which is generally difficult to control, begins at the interior of the concrete. These problems can be mitigated by use of a pre-soaked LWA.

Monitoring of Self – curing

This can be done by:

i. Measuring weight-lossii. X-Ray powder diffraction

iii. X-Ray microchromatographyiv. Thermogravimetry (TGA) measurementsv. Initial surface absorption tests (ISAT)

vi. Compressive strengthvii. Scanning electron microscope (SEM)

viii. Change internal RH with time ix. Water permeabilityx. NMR spectroscopy

Advantages of Internal Curing

a. Internal curing (IC) is a method to provide the water to hydrate all the cement, accomplishing what the mixing water alone cannot do. In low w/c ratio mixes (under 0.43 and increasingly those below 0.40) absorptive lightweight aggregate, replacing some of the sand, provides water that is desorbed into the mortar fraction (paste) to be used as additional curing water. The cement, not hydrated by low amount of mixing water, will have more water available to it.

b. IC provides water to keep the relative humidity (RH) high, keeping self-desiccation from occurring.

c. IC eliminates largely autogenous shrinkage. d. IC maintains the strengths of mortar/concrete at the early age (12 to 72 hrs.) above the

level where internally & externally induced strains can cause cracking.e. IC can make up for some of the deficiencies of external curing, both human related

(critical period when curing is required is the first 12 to 72 hours) and hydration related (because hydration products clog the passageways needed for the fluid curing water to travel to the cement particles thirsting for water). Following factors establish the dynamics of water movement to the unhydrated cement particles:

i. Thirst for water by the hydrating cement particles is very intense,ii. Capillary action of the pores in the concrete is very strong, and

iii. Water in the properly distributed particles of LWA (fine) is very fluid.

Concrete Deficiencies that IC can Address

The benefit from IC can be expected when

Cracking of concrete provides passageways resulting in deterioration of reinforcing steel, low early-age strength is a problem, permeability or durability must be improved, rheology of concrete mixture, modulus of elasticity of the finished product or durability

of high fly-ash concretes are considerations. Need for: reduced construction time, quicker turnaround time in precast plants, lower

maintenance cost, greater performance and predictability.

Improvements to Concrete due to Internal Curing

Reduces autogenous cracking, largely eliminates autogenous shrinkage, Reduces permeability, Protects reinforcing steel, Increases mortar strength, Increases early age strength sufficient to withstand strain, Provides greater durability, Higher early age (say 3 day) flexural strength Higher early age (say 3 day) compressive strength, Lower turnaround time, Improved rheology Greater utilization of cement, Lower maintenance, use of higher levels of fly ash, higher modulus of elasticity, or through mixture designs, lower modulus sharper edges, greater curing predictability, higher performance, improves contact zone, does not adversely affect finishability, does not adversely affect pumpability, reduces effect of insufficient external curing.

Effect of Particle Size and Content of LWA

Internal curing by saturated lightweight aggregate can eliminate autogenous shrinkage with the smallest possible amount of lightweight aggregate. The grain size of the LWA used as curing agent needs to be reduced in order to minimize the paste– aggregate proximity, i.e. the distance to which the internal curing water should diffuse. The reduction of the grain size (down to 2–4 mm), is shown to be beneficial. However, the further reduction of grain size could result in a

decrease of curing efficiency.

The effectiveness of internal curing depends not only on whether there is sufficient water in the LWA, but also on whether it is readily available to the surrounding cement paste as well. Hence, if the distance from some location in the cement paste to the nearest LWA surface is too great, water cannot permeate fully within an acceptable time interval. This distance can be called the paste– aggregate proximity. Alternatively, aggregate distribution can be described by means of aggregate– aggregate proximity, which is the distance between two nearest LWA surfaces, often called spacing. For a given amount of aggregate, the paste–aggregate proximity can be adjusted by the size of the aggregate. The finer the aggregate size, the closer will be the paste– aggregate proximity.

The LWA can be used for internal curing without considerable detrimental effects on strength when added in the amounts just required to eliminate self-desiccation.

“Protected Paste Volume” Concept in Self-curing

For self-curing, besides providing necessary quantity of water inside the matrix, it is essential to ensure the proximity of the cement paste to the surfaces of the source of water so that required high RH is generated around the cement grains for hydration reaction. In this regard, the “protected paste volume” concept is useful to recognise the effective volume of cement paste. For this, the aggregates are represented by impenetrable spherical or ellipsoidal particles and each aggregate particle is surrounded by a soft penetrable shell representing the interfacial transition zone. Instead of the interfacial transition zones, the saturated LWA (fine aggregate) particles surrounded by a shell of variable thickness can be assumed for evaluation. Then, by systematic point sampling, one can determine the volume fraction of paste contained within these shells and hence the relative proximity of the cement paste to the additional water.

Distribution of Internal Water Reservoirs for Curing

The transport distance of water within the concrete is limited by depercolation of the capillary pores in low w/c ratio pastes. With water-reservoirs well distributed within the matrix, shorter distances have to be covered by the curing water and the efficiency of the internal-curing process is consequently improved. The concept of internal curing was established, based on dispersion of very small, saturated LWA throughout the concrete, which serve as tiny reservoirs with sufficient water to compensate for self-desiccation. The spacing between the LWA particles is conveniently small so that the water travels smaller distances to counteract self-desiccation. The amount of water in the LWA can therefore be minimized, thus economising on the content of the LWA.

Travel of Water from Surfaces of LWA

Estimates of travel of internal water from the surface of water reservoir in the concrete matrix are:

early hydration — 20 mm middle hydration — 5 mm late hydration — 1 mm or less “worst case” — 0.25 mm (250 ìm)

(Early and middle hydration estimates in agreement with x-ray absorption-based observations on mortars during curing).

Size of pores for Internal Water Storage

Water is held in pores primarily by capillary forces. Only pore sizes above approximately 100 nm are useful for storage of internal curing water. In smaller pores the water is held so tightly that it is not available for the cementitious reactions. Since some of the water absorbed by the LWA in the smaller pores will not be released to the hardening cement paste, an amount of water more than sufficient to counteract selfdesiccation should be absorbed in the LWA. A great quantity of water is in fact entrapped in the internal porosity of the larger particles; one should consider that only about half of it is available for internal curing. In case of smaller fraction, the opposite seems to hold: the absorption is lower, but almost 80 % of the water is lost by 85% RH.

Usefulness of IC in Pavements

The major problem of cracking in pavements may be alleviated by internal curing, besides imparting many potential benefits.

Usefulness of IC for Early-Age Cracking

The IC can influence the ‘Early- Age Cracking Contributors’ which are mainly thermal effects and autogenous shrinkage. During initial ages of concrete, hydration heat can raise concrete temperature significantly (causing expansion), subsequent thermal contraction during cooling can lead to early-age (global or local) cracking if restrained (globally or locally). Another prominent effect would be autogenous shrinkage, especially in concretes with lower water-binder ratios where sufficient curing water cannot be supplied externally, the chemical shrinkage accompanying the hydration reactions will lead to self-desiccation and significant autogenous shrinkage (and possibly cracking).

Pore Sizes in Internal Reservoirs & Capillary Pores

IC distributes the extra curing water throughout the 3-D concrete microstructure so that it is more

readily available to maintain saturation of the cement paste during hydration, avoiding selfdesiccation (in the paste) and reducing autogenous shrinkage. Because the autogenous stresses are inversely proportional to the diameter of the pores being emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste.

Quantifying Effectiveness of IC

IC can be experimentally measured by:

Internal RH Autogenous deformation Compressive strength development Degree of hydration Restrained shrinkage or ring tests 3-D X-ray microtomography (Direct observation of e 3-D microstructure of cement-

based materials).

Conclusion

The internal curing (IC) by the addition of saturated lightweight fine aggregates is an effective means of drastically reducing autogenous shrinkage. Since autogenous shrinkage is a main contributor to early-age cracking, it is expected that IC would also reduce such cracking. An additional benefit of IC beyond autogenous shrinkage reduction is increase in compressive strength. As internal curing maintains saturated conditions within the hydrating cement paste, the magnitude of internal self-desiccation stresses are reduced and long term hydration is increased. IC is particularly effective for the highperformance concretes containing silica fume and GGBS. In cement mortar containing a Type F fly ash, the fly ash functions mainly as a dilutent at early ages, and higher and coarser porosity at early ages result in less autogenous shrinkage.

The self-desiccation is the reduction in internal relative humidity of a sealed hydrating cement system when empty pores are generated. This occurs when chemical shrinkage takes place at the stage where the paste matrix has developed a self-supportive skeleton, and the chemical shrinkage is larger than the autogenous shrinkage. Effects of self-desiccation depend on the sizes of the generated empty pores. These pore sizes in turn are dependent on the initial waterto- binder ratio (w/b), the particle size distributions of the binder components, and their achieved degree of hydration. The continuing trends towards finer cements and much lower w/b have significantly reduced the capillary pore “diameters” (spacing) in the paste component of the fresh concrete, and have often resulted in materials and structures where the effects of self-desiccation are all to visible as early-age cracking. Many strategies for minimizing the detrimental effects of selfdesiccation (mainly the high internal stresses and strains that may lead to early-age cracking), such as internal curing, rely on providing a “sacrificial” set of larger water-filled pores within the concrete microstructure that will empty first while the smaller pores in the hydrating binder paste will remain saturated. It may be noted that the effects of self-desiccation are not always detrimental, as exemplified by the benefits offered by self-desiccation in terms of an earlier RH reduction for flooring applications and an increased resistance to frost damage.

IC is useful when ‘performance specifications’ are important than ‘prescriptive specifications’ for concrete. Prime applications of IC could be: concrete pavements. precast concrete operations, parking structures, bridges, HPC projects, and architectural concretes. Concrete, in the 21st century, needs to be more controlled by the choice of ingredients rather than by the uncertainties

of construction practices and the weather. Instead of curing through external applications of water, concrete quality will be engineered through the incorporation of water absorbed within the internal curing agent.

Acknowledgment

The authors thank Dr. N. Lakshmanan, Director, SERC, Chennai, for permitting to publish this paper. Bibliography on Selfcuring (Internal Curing)

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An Overview of Some Development in CONCRETE TECHNOLOGYDr. S.S. Rehsi, Consultant (Building Materials' Former United Nations Expert on Building Materials, Chandigarh

Introduction

There has been rapid advances in concrete technology during the past three decades or so. The improvement in strength and other structural properties achieved earlier through the use of steel reinforcement are now accepted as routine and the reinforced cement concrete and pre–stressed concrete have become conventional materials. Later work led to the development of a variety of concretes in the form of, among others, fibre reinforced concrete, polymer concrete, Ferrocement, sulphur concrete, lightweight aggregate concrete, autoclaved cellular concrete, high-density concrete, ready-mixed concrete, self-compacting concrete, rollercompacted concrete, high strength concrete, super high-strength concrete, high performance concrete, high-volume fly ash concrete, self-curing concrete, floating concrete and smart concrete (1-27). Some of these concretes are briefly discussed here.

Fibre Reinforced Concrete

Different types of mineral, organic and metallic fibres have been used. Among the mineral fibres, use of asbestos in the production of asbestos cement products is well known. Since, water absorption of the asbestos fibre is high, its use in concrete increases water requirement. Consequently, there is reduction in strength of the concrete. Organic fibres such as, coir, jute, rayon and polyester are attacked by the highly alkaline condition in concrete. As a result, concrete containing these fibres loses strength with time. Other organic fibres namely, nylon, polypropylene and polyethylene are alkali-resistant. But, due to their lower modulus of elasticity, the incorporation of these fibres do not increase strength. Concrete containing nylon or polypropylene fibres, however is reported to develop higher impact resistance. Virgin Poly-Propylene fibers of structural grades, such as Forta Ferro Fibres, having high strength and moduls of elasticity are now available from FORTA Corpn USA. These are being extensively used all over world for Pavement/highways/Runway construction, Fibreshotcreting of tunnels, Repair and Rehab jobs and Bridge deck construction incl India. Among all fibres the use of steel fibre in concrete has received far greater attention, in the past but because of the corrosion problem structural grade poly propylene and other synth fibers are taking over now.

The compressive strength, tensile strength, fatique strength, modulus of elasticity, abrasion resistance, skid resistance and thermal conductivity of steel fibre reinforced concrete has been found to be slightly higher than the corresponding plain concrete. While creep and shrinkage are more or less unaffected, there is over 100 percent increase in the flexural strength and impact

toughness of plain concrete when reinforced with steel fibre, 2 percent by volume. At the same fibre content, use of a blend of fibres having different aspect ratio, in place of single aspect ratio fibre, gives greater structural benefits. It has also been found more beneficial as well as economical to use steel fibres only in the tensile zone of the flexural member. Unlike plain concrete, steel, fibre reinforced concrete is not brittle and offers far greater resistance to cracking. The fibres act as crack arrestors and restrict the growth of flaws in concrete from enlarging under stress into visible cracks. The ultimate failure is reached only when some of the fibres get pulled out of the matrix. As compared to plain concrete, the resistance of steel fibre reinforced concrete to thermal shock and heat spalling is also far superior.

The major applications of steel fibre reinforced concrete are in pavements (both for new construction and overlays), precast concrete units, concrete reactor pressure vessels, blast resistant structures, machine foundations, tunnel linings and structures requiring resistance to thermal shocks, such as refractory linings.

Polymer Concrete

Depending upon the method of monomer incorporation into the concrete, the polymer concrete is termed as

i. polymer impregnated concrete, when dried precast concrete is impregnated with monomer and polymerized in-situ,

ii. polymer cement concrete, when cement, aggregate, water and monomer are mixed together and polymerized after laying and,

iii. polymer concrete, when aggregate and monomer are mixed together and polymerized after laying.

A number of factors such as distance to be penetrated, degree of drying, total porosity and pore size in concrete, monomer viscosity, whether or not vacuum and/or pressure is applied, influence the extent of monomer filling in polymer impregnated concrete. The widely used monomers are methly methacrylate, styrene, acrylonitrile and chlorostyrene. The monomer polymerization is done either by thermal catalytic process or by radiation.

As compared to plain concrete, the strength and other properties of polymer concrete are considerably higher. At 6 per cent polymer loading, the mechanical properties of polymer impregnated concrete vis-à-vis corresponding plain concrete were found to be as follows:

Compressive strength, 2 to 4 times higher Tensile strength, about 4 times higher Modulus or Elasticity, About 4 times higher Creep and Permeability, Almost Nil

With almost nil permeability, the polymer impregnated concrete has much greater resistance to the attack of acidic and/or sulphate containing waters

Economics permitting, applications of polymer concrete having good scope are: concrete pipe

manufacture, concrete piles, concrete tiles, tunnel supports and linings, precast concrete decks, precast concrete building units for use in aggressive conditions, desalting structures, lightweight concrete constructions and providing surface protection to cast in-Situ concrete.

Ferrocement

Ferrocement is a kind of reinforced concrete in which the matrix is cement mortar, microconcrete and the reinforcement is in the form of layers of wire mesh or similar small diameter steel mesh closely bound together to produce a stiff structural form. The mix proportions of the cement mortar usually are: cement 1 part, sand 1.5 to 2.5 parts and water 0.35 to 0.5 part, by weight. Admixtures are added in the mix for improving to properties. The maximum size of sand grains depends upon the mesh opening and reinforcing system to ensure proper penetration. Different types of wire mesh such as, hexagonal wire mesh (commonly known as chicken wire mesh), welded wire mesh, woven mesh, expanded metal mesh, are used. Use of Hexagonal Mesh is not preferred due to its poor resistance to loads. The mechanical behavior of Ferrocement is greatly influenced by the type, quantity, orientation and strength properties of the mesh. The thickness of ferrocement elements range from 2 to 3 cm with 2 to 3 mm external cover. When additional strength is required, one or more layers of steel bars are inserted between the inner layers of the mesh. Use of short random fibres in Ferrocement elements at the same steel content has been found to greatly increase the modulus of elasticity and strength. Polymer impregnation of the Ferrocement elements, with and without short random fibres is reported to considerably improve upon these properties.

Ferrocement has a variety of applications. The important among these are: construction of fishing and cargo boats, grain storage bins, water storage tanks, biogas holders and digesters, fermentation tanks, precast roofing and walling units, cooling towers, sewage troughs, septic tanks, irrigation channels, drying pans for agricultural products, shutters and formwork for use in concrete constructions, lining for tunnels and mines, and providing waterproofing treatment over

RCC or RB roofs, lining of surface of tanks or swimming pools. Ferrocement has been successfully used in india by SERC (G)'s Material Science Group for construction of domes, large tanks, manhole covers, Drainage units and for repair and rehab of structures. the new techniques and applications developed by this group are being used on large scale on commercial basis.

High Strength Concrete

IS: 456-2000 designates concrete having 28-day compressive strength of 60 to 80 N/mm2 corresponding to grades M60 to M80 as high strength concrete (HSC).

The production of HSC requires stringent control on the quality of materials used. The Portland cement should, preferrably, be of 53 grade conforming to IS: 12269- 1987. Crushed stone coarse aggregates produced from trap, quartzite or granite give higher strength and are more suitable than rounded gravel for use in making HSC, particularly when the desired concrete strength is 70 N/ mm2 or more. Studies on the effect of the size of coarse aggregate on the strength of concrete showed that smaller size produced higher strength. A maximum coarse aggregate size of 10 mm is considered suitable for use in HSC. The use of mineral admixtures such as fly ash, silica fume, metakaoline in combination with suparplasticizers in HSC matrix greatly enhances impermeability, durability and strength.

The use of HSC in construction offers the advantages of

i. reduction in the size of concrete members with resultant reduction in self-weight,ii. greater stiffness,

iii. early stripping of formwork andiv. lowering of construction cost due to reduction in the concrete member size and self-

weight.

It has, therefore, been widely used in the construction of highrise buildings and bridges in many countries including India. A super high-strength concrete, called reactive powder concrete (RPC) is produced by eliminating the use of coarse aggregate. The concrete matrix consists of cement, finely ground sand with particle size close to that of cement, silica fume and short steel fibres. The water/cement ratio is kept very low, around 0.15. The desired workability is obtained by using higher amounts of super plasticizers. The RPC does not require reinforcement bars. It is suitable for use in building very thin structures meeting different architectural needs.

High Performance Concrete

High performance concrete is defined as concrete that meets special Performance and durability requirements in terms of mechanical properties, volume stability and longer life in severe environmental conditions to which the concrete is exposed during its service life. High performance of concrete is generally linked to strength of the concrete; higher the strength, better the performance. Therefore, in the first place high performance concrete has to be a high strength concrete. Besides high strength, low permeability of concrete is an essential requirement to

prevent ingress of corrosive waters containing chlorides, sulphates and /or other deleterious salts. Low permeability is achieved by using higher cement content, mineral admixtures such as fly ash, Silica fumes, metakaoline or granulated blast furnace slag, and keeping water/cement ratio low at 0.35 or less. Higher amounts of super plasticizers are used to obtain the desired workability in the concrete matrix. Workmanship has to be excellent to ensure full compaction and proper concrete cover over embedded steel reinforcement. All these and subsequent adequate curing of concrete after laying and regular maintenance of concrete construction ensure high performance.

High-volume Flyash Concrete

High-volume fly ash concrete technology was developed at the Canada Centre for Mineral and Energy Technology (CANMET), Ottawa, Canada in 1980’s. It enables minimizing the amount of cement required to produce high quality concrete for different types of applications by incorporating upto 50 to 60 percent fly ash in the concrete mix. The concrete is prepared using a low water/cement ratio of 0.30, and the desired workability is obtained by using super plasticizers. While highvolume fly ash concrete was initially developed for mass concrete construction where low heat of hydration and just enough early strength were required, later work showed that this concrete developed excellent long-term structural properties, namely compressive strength, flexural strength, splitting tensile strength, and Young’s modulus of elasticity. Its durability measured in terms of its low water permeability, resistance to carbonation, alkaliaggregate reactions and penetration of chlorides and sulphates, was also found to be excellent. In view of this, highvolume fly ash concrete is eminently suitable for structural applications, in addition to its use for building of roads and pavements. It is being used for such constructions in Canada, U.S.A. and other countries. In India, Ambuja Cements Ltd, has made a beginning by building two fly ash concrete roads, one at Ropar (Punjab) and another at Ambujanagar (Gujarat) in 2002, using 50 per cent fly ash in the concrete mix. Both these roads are reported to be performing very well.

Self-compacting Concrete

A concrete that gets compacted by itself totally covering reinforcement in the formwork is called self-compacting concrete (SCC). It is highly flowable, selflevelling, self-defoaming and coahesive and can be handled without segregation. Like any other super plasticized concrete, the ingredients in SCC mix consists of cement, coarse and fine aggregates, mineral and chemical admixtures. A limiting value of coarse aggregate as 50 per cent of the solid volume of the concrete, and of fine aggregate as 40 per cent of the solid volume of the mortar fraction in the

SCC mix proportion is suggested for achieving good self-compact ability. Commonly used mineral admixtures are fly ash, silica fume, ground blast furnace slag Chemical admixtures consists of a super plasticizer and a viscosity modifying admixture. The use of one or more mineral admixtures having different morphology and particle-size distribution improves deformability, self-compact ability and stability of the SCC. While the super plasticizer helps achieving high degree of flow ability at low water/ Cementing material ratio, the viscosity modifying admixture increases viscosity of the fresh concrete matrix and reduces bleeding.

The SCC has the advantages of easy placement in thin - walled elements densely reinforced concrete structure, quality, durability and reliability of concrete structures, faster construction and reduced construction cost.

Self-curing Concrete

Curing of concrete by which the concrete, after laying, is kept moist for some days, is essential for the development of proper strength and durability. IS 456-2000 recommends a curing period of 7 days for ordinary Portland cement concrete, and 10 to 14 days for concrete prepared using mineral admixtures or blended cements. But, being the last act in the concreting operations, it is often neglected or not fully done. Consequently, the quality of hardened concrete suffers, more so, if the freshly laid concrete gets exposed to the environmental conditions of low humidity, high Wind velocity and high ambient temperature.

To avoid the adverse effects of neglected or insufficient curing, which is considered a universal phenomenon, concrete technologist and research scientists in various countries including India, are working on the development of self-curing concrete. Different lines of action are being pursued. These include

i. use of water-soaked, surface dry lightweight aggregates which release water when the concrete starts getting dry and losing water,

ii. develop high early strength concrete, which attains a strength of 20N/mm2 in 30 minutes and so may not require further curing, and

iii. develop a system by which some “enteric” coated particles or capsules containing membrane-forming curing compound (or a substance that reacts with water to do so) is distributed over the surface of the concrete slab in the final stages of finishing. The particles will open if the surface becomes dry and a membrane will form while the concrete is still water-saturated upto its top but has no free water on the surface.

Smart Concrete

Smart concrete is a concrete that can take care of its own shortcomings or that can act as a senser to help detecting internal flaws in it. It is produced by incorporating some changes in the ingredients of the concrete mix. For instance,

i. due to its high density, the high strength concrete does not permit water vapours to go out during fire, leading to spalling off concrete cover and damage to concrete members. Addition of 2kg polypropylene fibres per m3 of high strength concrete mix increases fire resistance. At high temperature during fire, these fibres melt and leave pores for water vapours to escape from the concrete surface, thus preventing spalling and damage,

ii. incorporation of 0.5 per cent specially treated carbon fibre in the concrete mix increases the electrical conductivity of the concrete. Under load, the conductivity decreases but returns to original on removal of the load. The concrete could thus act as a senser to

a. measure the number, speed and weight of the vehicles moving on concrete highways, and

b. detect tiny flaws regarding internal condition of concrete construction after an earthquake, and

iii. Use of porous carbon aggregate, available in the form of coke at the steel plant, in the concrete mix imparts good electrical conductivity which can help in room heating, melting of ice on concrete highway and runways by passing low voltage current.

Conclusion

As other areas of research and development (R&D) in concrete technology has been a continuing process, Different types of concretes, as described above, have been developed from time to time, to meet the needs of the construction industry. Technologies for self-curing and smart concrete are still in the development stage, but are expected to be fully developed soon and available for use in constructions.

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Designing Reinforced Concrete Structures for Long Life Span

Dr. Rakesh Kumar, Scientist and Dr. Ram Kumar, HoD, Bridges & Structures Division, Central Road Research Institute, New Delhi.

Innovation in construction industry is highly linked with development of advanced construction materials. In the recent two–three decades lot of research relating to how to enhance the life of reinforced concrete structures has been carried out. As a result of which—it has been possible to design structures having service life span of more than 100 years. This article discusses some aspects of possibility for designing reinforced concrete structures for a very long life.

Introduction

The presence of heavy reinforcement i.e. a high degree of congestion of reinforcement in structural elements significantly hampers concrete placement and its quality due to lack of proper compaction. Adequate compaction of such sections by proper means is essential for durability assurance and often depends on the crew’s ability to ensure it. Inadequate compaction of concrete in such structural elements can lead to surface and structural defects and inadequate bond development with the reinforcement. Durability of reinforced concrete structures is mainly dependent on the quality of the concrete, quality of reinforcing steel, cover depth of reinforcement, compaction and curing of concrete and finally quality management of the construction practices. Notably, the serviceability and the safety of concrete structures have been the prime concern of the structural engineers. The serviceability limit of concrete structures is primarily governed by the extent of damage resulting from daily service loads and various deterioration processes, which might be active throughout the structure’s life. Durability problems in concrete structures may be due to several causes such as errors in design or carelessness in detailing, use of inferior construction materials, inadequate quality control, poor workmanship, heterogeneity of the materials etc. Durability affecting features of concrete structures are observed in the form of cracking, spalling (Figure. 1), corrosion of reinforcing steel bars (Figure. 2), loss of mass (Figrue. 3) and loss of strength. The cause of concrete deterioration can be physical, chemical and in most cases, a combination of both. The net effect of concrete deterioration processes is to weaken the integrity of the complex microstructure of

concrete. The low porosity and dense microstructure of concrete significantly reduce many sources of its deterioration. In concrete, cement paste is the primary active constituent. Therefore, the mechanical properties and performance of concrete is largely determined by the properties of the cement paste. Microstructure characteristics of concrete such as its porosity, pore size distribution, properties of transition zone, and connectivity of pores, govern almost all the gas and liquid transport phenomena through the concrete [1-5]. Therefore, the rate at which a concrete structure may deteriorate is mainly depend on the permeability of the concrete as well as how the concrete is placed, compacted, cured, and allowed to sustain load, cover depth and quality of cover concrete. Contact with, or the presence of certain aggressive chemical ions, such as chlorides, sulphides, acids, carbon dioxide, and even water, causes the deterioration of the concrete. Such deterioration involves either leaching of material from the surface by a dissolution mechanism or by expansion of material inside the concrete. Exposure conditions vary over a wide range including hot and dry desert ambient air, wind, and rain or snow. Higher ambient air temperature may accelerate the chemical reaction of concrete leading to faster deterioration. Furthermore, the concrete quality degradation mechanism may be either a physical effect such as shrinkage, creep, erosion, and similar factors, or a chemical reaction such as sulphate attack, reinforcement corrosion, alkali-silica reaction, carbonation, freezing and thawing, and other similar factors. Designer should throughly understand the interaction of concrete with both exposure environment and service loads.

The broad categories of factors, which determine the durability of a concrete structure, are design, material properties, and construction practice. Errors in design or carelessness in detailing may lead to cracking, leading to premature demise of useful life of a concrete structure. Long-term durability of concrete in civil infrastructures such as road and bridges can be achieved if the construction materials quality, structural detailing and dimensioning, and concreting works are appropriately performed. It is well recognized that the quality of concrete in structures and defects induced at early age due to various reasons are main factors for the long-term durability of concrete. These deterioration processes can be physical, chemical or mechanical or combination of them.

Among various foresaid factors, cracking due to shrinkage, poor workmanship, environmental factors, and over load/overstress initiate the process to reduce concrete durability. Such concrete cracking which cannot be eliminated but can be minimized provides path for the ingress of water/moisture, air to allow reinforcement corrosion to start. Therefore, there is a need for quality management for concrete placement, compaction, and curing. Also reinforcement should be such that it has “sufficient” cover depth protecting the reinforcing bars from deeper and wider cracks; and/or, reinforcement which does not corrode or would corrode only to predetermined minimum amount. Innovation in construction is highly linked with development of advance construction materials and technology. There are materials and technology available to ensure construction of long-life structures.

Mechanism for Enhancing Durability

The fundamental fact that properties of material originate from its internal structure is also valid for concrete as well as steel. The principle of modifying internal structure suitably has been used in developing a number of metals, composites, and other materials [6]. Improvement of

durability of concrete has remained an active research area for concrete technologist for many years. As a result of continuous effort for enhancing durability of concrete structures, high-performance concrete (HPC) and selfcompacting concrete (SCC) have been developed. Improved properties of high-performance concrete are due to the modification of its microstructure. The modification is significantly dependent on the reaction mechanism among the ingredients of concrete, physical process, and curing. Chemical and mineral admixtures augment the reaction mechanism. In high-performance concrete, commonly used admixtures are silica fume [7, 8] and fly ash [9-11]. These materials improve the microstructure of concrete by pozzolanic action as well as a filler effect. Better performance of high-performance concrete is primarily due to refinement of the pore structure of the concrete particularly at the transition zone [7, 11]. Even the proven technology of high-performance concrete can enable the structures to double its useful lifespan in comparison with engineered structures constructed with conventional concrete technology [12].

A water-to-cement ratio (w/c) of 0.4 by mass is required for complete hydration of all the cement particles and for hydration products to fill all the space originally occupied by the mixing water [12]. If the w/c is higher than 0.4 by mass, even if all the cement particles hydrate, there will always be some residual original mixing water-filled spaces that can hold freezable water. If w/c is lower than 0.4 by mass, some of the cement will always remain unhydrated; but, in theory, all of the mixing water-filled spaces could be filled. However, the amount of water that goes into chemical combination with Portland cement is equal to about w/c of 0.2 by mass. The additional Amount of water, i.e., 0.2 w/c by mass [12] is needed to fill gel pores. This extra water must be available if the hydration product is to be formed. On the other hand, the development of superplasticizers has revolutionized technology and has made it possible to make workable and/or very workable concrete with very low water-to-cementitious ratio even less than 0.2 [13-15]. Such concrete not only achieve highstrength but also possess improved durability.

The use of some mineral admixtures, such as coal fly ashes and other pozzolans, work as a filler in addition to contributing pozzolanic activity and fill the spaces occupied by water in capillary pores and make them discontinuous. As a consequence of this, the morphology of hydrated cement changes which favorably affect most of the mechanical properties of concrete in comparison with conventional concrete [4, 7, 10, 16].

Highly Durable Concrete Structures

A greater understanding of concrete behavior at microstructure level and performance under different aggressive conditions has improved the confidence of concrete technologists to think about highly durable concrete lasting for 1000 years. Recently some efforts have been made for designing highly specialized structures, such as bridges, tunnels, and tall structures, for a lifespan of a century or more [17- 19]. Most recently, Mehta and Langley [20] designed an unreinforced, monolith concrete foundation consisting of two parallel slabs, to last for 1000 years. They used high-volume Class F fly ash concrete in the construction of the foundation. The slabs were built with HVFA concrete mixture containing 240 lb/ yd3 of Class F fly ash and 180 lb/ yd3 of portland cement. The petrographic examination of oneyear- old test slab, that was cast and cured under the similar conditions, has shown crack-free nature of the HVFA concrete [21].

At present, this seems to be achievable for concrete without reinforcement to predict/speculate on a 1000-year life. In-depth understanding of microstructural behavior of concrete, and possibility for improvement of it, to overcome shortcomings that cause reduction in durability of concrete, by the use of chemical and mineral admixtures, has given the basis to concrete technologist to think for design of highly durable concrete structures that should last for several centuries. For such structures the following items should be clearly understood and implemented.

Quality management of material, methods, and testing. Manage all design and construction aspects to ensure the structural integrity. Designer should have adequate knowledge of material properties such as strength, creep,

shrinkage, etc., of concrete and their affect on cracking of the concrete. Design adequate depth of cover for the reinforcing steel. Use of fly ash and/or other pozzolonic materials instead of ordinary portland cement

only. Use of high-quality aggregates free from deleterious compounds for preventing alkali-

aggregate reactivity, and similar actions. Aggregates should also have proven reliability. Concrete, from its proportions, mixing, methods of construction, (compacting and

curing), should be given careful attention so that an adequately dense concrete, with full compaction and a desirable pore system may be ensured.

Adequate cover for the reinforcement ensuring highquality compaction and curing of the concrete. High-performance & self-compacting concrete may help in minimizing the potential of corrosion of reinforcement and deterioration of concrete due to poor quality of cover.

Corrosion resistant steel, steel coated with corrosion resistance layer such as cementitious material slurry, stainless steel, or other types of newer steel, may be used.

Concrete should be carefully tested and quality managed to meet long-term tests such as water and air permeability, shrinkage, creep, freezing and thawing, chloride-ion penetration by ponding and chloride diffusivity.

Prediction of life of structures based on corrosion rate of reinforcement.

Conclusion

The possibility for design of reinforced concrete structures for a very long lifespan of several years exist without a proven method (by calculation or experiments). The improved microstructure of concrete by judicious use of mineral admixtures, such as flyash, silica fume, and other pozzolans, as well as new generation of chemical admixtures, have given hope for the RC structures for life span of more than 100 years. Concrete structures for a very long lifespan need materials of high-quality and also comprehensive knowledge about concrete properties and their effects on design aspects of the structure, and a new generation of steel reinforcement.

Acknowledgment

The approval of Dr Vikram Kumar, Director, Central Road Research Institute, Mathura Road, New Delhi to publish the work is acknowledged.

References

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Naik, T. R., Singh, S. S., and Mohammad M. (1995). “Properties of high performance concrete incorporating large amounts of high-lime flyash,” International Journal of Construction and Building Materials, l 9(6), 195-204, Butterworth-Heineman, England.

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Feylessoufi, A., Villiéras., F., Michot, L. J., De Donato, P., Cases, J. M., and Richard, P. (1996). “Water, environmental, and nano-structural network in a reactive powder concrete.” Cement and Concrete Composite, 18(1), 23-29.

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Khayat, K. H., Hu., C., and Laye, J. M. (2002). “Importance of aggregate packing density on workability of self-compacting concrete,” Proceedings, First North American Conference on the Design and Use of Self- Consolidating Concrete, Center for Advanced Cement-Based Materials, North–western University, Evanston, IL, U.S.A., November 12-13, 2002, pp. 53- 62.

Malhotra, V. M. (1995). “Fly Ash, Blast-Furnace-Slag, Silica Fume, and Highly Reactive Metakaolin,” Proceedings, Seminar On Recent Advances in Concrete Technology, UWM Center for By-Products Utilization, University of Wisconsin-Milwaukee USA, Proceedings Complied by Tarun R. Naik and Henry J. Kolbeck.

Dunaszegi, L. (1998). “Highperformance concrete in the confederation bridges.” ACI Concrete International 20(4), 43- 48.

Holley, J. J., Thomas, M. D. A., Hopkins, D. S., Cail, K. M., and Lanctot, M.–c. (1999). “Custom HPC mixtures for Challenging bridge design.” Concrete International 21(9), 43-48.

Langley, W. S., Gilmour, R. A., Turnham, J. Forbes, G., and Mostert, T. (1997). “Quality management plan for the confederation bridge.” Proceedings, Third CANMET/ACI International Symposium on Advances in Concrete Technology, Auckland, New Zealand, August 24-27, 1997, ACI Special Publication SP-171, pp. 73-96, American Concrete Institute, Formington Hills, Michigan, Ed. V. M. Malhotra.

Mehta, P. K., and Langley, W. S. (2000). “Monolith foundation: Built to last 1000 years.” ACI Concrete International, 22(7), 27- 32.

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RMC—A Revolution in Production of Concrete

Dr. Y.P. Gupta, Material Consultant, Allahabad Bypass Project & Professor of Civil Engineering (Rtd.), MNNIT, Allahabad.

Introduction

Ready mixed concrete, by far the most common form of concrete, accounts for more than half of all concrete consumption. Ready mixed refers to concrete that is batched for delivery from a central mixing plant instead of being mixed on the job site. Each batch of ready-mixed concrete is tailormade according to the specifications of the contractor or concrete mix design and is delivered to the site in green or plastic condition, usually in the cylindrical trucks often known as “Transit mixers.”

Concrete constituents occupy a large space for storage at construction site. Further, the builder has to spend a lot of time and effort to source these materials and test their quality before use. Ready Mixed Concrete (RMC) suppliers take care to collect and store all these materials and supply the required quantity of concrete at the specified time and place so that construction can proceed smoothly. Metropolitan cities are hard-pressed for storage space. Therefore, RMC greatly relieves the space problem.

The real advantage for the construction industry accrues from the quality of the concrete because of the expertise and experience of the batching plant QC Engineer. However, the quality of the structure made using RMC largely depends on close coordination between the supplier of RMC and the builder at the site at all stages starting from ordering concrete to discharging and placing of the concrete. Transit Mixers can drive directly onto the site and can mechanically control the positioning of the discharge chute without the help of contractor's personnel.

History

As early as 1909, concrete was prepared by a horse-drawn mixer that used paddles turned by the cart’s wheels to mix concrete en route to the jobsite. In 1916, Stephen Stepanian of Columbus, Ohio, developed a self-discharging motorized transit mixer that was the predecessor of the modern ready–mixed concrete truck. Development of improved readymixed concrete trucks was developed in the 1920s. During the 1940s, the availability of heavier trucks and better engines allowed mixing drum capacities to increase, which in turn allowed ready-mixed concrete producers to meet the high demand for concrete that developed as a result of World War II.

RMC–A Step Forward and Ideal for Many Jobs

Specification of RMCThe builder should specify the grade (strength) of concrete required for his structure. It is also necessary to specify the minimum cement content and maximum permissible water-cement ratio and the workability in terms of slump value. This will ensure that concrete will have required strength on attaining maturity, workable at the time of placing and will be durable. For special jobs, the type of cement or admixture to be used should also be specified.

Types of RMC

RMC can be classified according to ingredients mixed in concrete. These may be on the basis of Cementitous Material i.e. Flyash is a part of Cement or not and Admixture is used or not. Otherwise, there are two principal categories of ready mixed concrete.

1. Dry Concrete: All the ingredients are mixed in dry form without mixing water in it. All these materials are sent in rotating drum and measured water quantity is sent in separate Water container. The water is mixed at site when it reaches there.

2. Green Concrete: All the ingredients are mixed together including the measured water quantity at Concrete Batching Plant itself. They are sent in rotating drum or in transit mixture to the site of concreting.

Code Stipulation

The most important parameter is the time that gets elapsed from the instance of adding water to the placement of concrete. Normally, the concrete has to be placed in about 90–120 minutes or before the rotating drum of transit mixer has made about 300 revolutions. Indian Standard 4926:2003 permits concrete to be discharged from the truck mixer within 120 minutes after loading. It also permits a longer period if suitable retarding admixtures are used or by deliberate chilling.

Mixing Plant

RMC is a specialized material in which the cement aggregates and other ingredients are weigh batched at a plant Figures 1 and 2 in a central mixer or truck mixer, before delivery to the construction site in a condition ready for placing by the builder. Thus, ‘fresh’ concrete is manufactured in a plant away from the construction site and transported within the requisite journey time. The RMC supplier provides two services, firstly one of processing the materials for making fresh concrete and secondly, of transporting a product within a short time.

Sometimes materials such as water and some varieties of admixtures can be transit–mixed (also known as Transit Mixture), that is they can be added to the concrete at the jobsite after it has been batched to ensure that the specified properties are attained before placement. Here materials are batched at a central plant and are completely mixed in the Batching Plant or partially mixed in transit. Transit–mixing

keeps the water separate from the cement and aggregates and allows the concrete to be mixed immediately before placement at the construction site (Dry Concrete). This method avoids the problems of premature hardening and slump loss that result from potential delays in transportation or placement of central–mixed concrete. Additionally, transit- mixing allows concrete to be hauled to construction sites further away from the plant. There are several types of RMC plants varying in type of mixing and capacity of concrete production. These plants are generally available in capacities varying from 15 /hour to 200 / hour. A typical RMC plant is shown here.

The Truck Mixer

While ready mixed concrete can be delivered to the point of placement in a variety of ways, the overwhelming majority of it is brought to the construction site in truck–mounted, rotating drum mixers Figure 3. Truck mixers have a revolving drum with the axis inclined to the horizontal. Inside the shell of the mixer drum are pair of blades or fins that wrap in a helical (spiral) configuration from the head to the opening of the drum. This configuration enables the concrete to mix when the drum spins in one direction and causes it to discharge when the direction is reversed.

To load, or charge, raw materials from a transit mix plant or centrally mixed concrete into the truck, the drum must be turned very fast in the charging direction. After the concrete is loaded and mixed, it is normally hauled to the job site with the drum turning at a speed of less than 2 rpm. The maximum number of revolutions the drum may rotate before delivery is about 300.

Transportation of Concrete

Central–mixed concrete is completely mixed at the plant then transported in a truck or transit mixer or agitator truck. Freshly mixed concrete may be transported in a open dump truck if the jobsite is near the plant or very low slump is required like for pavement quality concrete used in road construction. Slight agitation of the concrete during transit prevents segregation of the materials and reduces the amount of slump loss.

Site Preparation

A fully loaded transit mixer weighs approximately 25 Tons. Hence prior checking of good access to the site of discharge of concrete from transit mixture is essential. This will avoid problems of delay on the day of concreting.

Quality Assurance

For this a sample of concrete must be taken out of Transit Mixture (as shown in Figure 4) to measure the workability by taking the slump. Samples are also taken for determining actual compressive strength of concrete. Three cubes of size 150x150x150 are made on site of this concrete from every or alternate transit mixture depending upon the total quantity of concrete ordered. Samples should be taken from different parts of the load.

Handling and Placing

Efficient use of RMC depends upon a rapid turnaround of truck mixers and proper facilities for rapid discharge and placing of concrete. With proper access and site facilities, the modern truck mixers can position it and discharge the full load in 15 to 30 minutes. They represent a potential delivery rate of nearly 30 m3 per hour. The concrete arrives with the ordered workability and hence no extra water should be added at the site. Concrete that does not arrive within the tolerance limit of ordered workability may be rejected or if permitted, it can be altered by mixing a small dose of Admixture, after judging the condition of concrete.

Ready-mixed concrete is often remixed once it arrives at the jobsite to ensure that the proper slump is obtained. However, concrete that has been remixed tends to set more rapidly than concrete mixed only once. The builder often handles the concrete with only a few manual laborers. Continuous handling methods such as mobile pump and conveyor system help in increasing the turnover. It is best to discharge the concrete from the truck mixture as close as possible to the place where it is required. Concrete can be discharged directly from the truck through chutes or it can be pumped by static or Mobile Pump as shown in Figure 5 at the construction pouring point.

Advantages of Using Ready Mixed Concrete

Ready Mixed Concrete can ensure quality because of the expertise and experience of RMC plant Technical Staff.

There is no botheration of ordering materials like Aggregate, Sand, Cement etc an find place to store them. Then arrange for site mixing machine.

Ready-mixed concrete is particularly advantageous when small quantities of concrete or intermittent placing of concrete are required.

Ready-mixed concrete is also ideal for large jobs where space Figure 4: Taking Sample for Testing is limited and there is little room for a mixing plant and aggregate stockpiles.

Conclusion

RMC is ‘Fresh’ Concrete manufactured in a plant away from the construction site and transported within the stipulated time to the site.

Concrete arrives with the ordered specifications. Do not add water at the site. Modern Truck Mixers can discharge the full load in 15 To 30 Minutes. Concrete can be discharged directly from the truck through chutes or it can be pumped by static

or mobile pump at the pouring point.

Strange are the Ways of Cement and Concrete

Dr. Anil K Kar, Engineering Services International, Kolkata; Arun Kumar Chakraborty, Asst. Professor,Civil Engineering and A K Sarkar, Civil Engineering Bengal. Engineering, and Science University, Shibpur

Introduction

It is commonly recognized that the compressive strength and other useful properties of concrete increase with increasing duration of curing, more particularly moist curing (lower curve in Figure 1). This knowledge of increasing compressive strength with increasing periods of moist curing has been gained from tests over the years where standard cubes or cylinders of concrete are tested on the last day or a day after a specified period of moist curing.

It is commonly known that the rate of gain in strength in the initial period is faster in the case of ordinary portland cement (OPC) than in the cases of portland slag cement (PSC) and Portland pozzolana or flyash cement (PPC). It is generally... observed that after the first few weeks of moist curing, further gain in strength in the case of OPC concrete is insignificant (lower curve in Figure 1), whereas concrete with blended cement (PSC and PPC) can gain considerable strength beyond the first week or two of concreting (Figure 2).

Much of the knowledge, more particularly impression, about concrete and concrete structures is based on the performance of well cured concrete and concrete structures which were built decades ago with OPC of that time. The impressions, many a people have about concrete, is that concrete is impervious and naturally waterproof. Another impression, people carry from the past, is that concrete structures are durable.

Much has changed over the years with cement and construction practices, hastening the decay and distress in modern concrete structures.

This paper is an attempt to study the changed ways of cement and concrete. This is limited to studying the influence of the duration of moist curing on the compressive strength of concrete with OPC, PSC, and PPC.

Though blended cements find a very considerable share of the construction market in India today, the earlier practice was to use mostly OPC.

It was an old practice in the era of OPC to provide 28 days’ moist curing to concrete.

Over the years, OPC went through many modifications in its chemical compositions and physical characteristics, resulting in higher ultimate strength and the development of most of this ultimate strength within a week or two of concreting (lower curve in Figure 1).

This early attainment of much of the ultimate strength and a greater emphasis on early completion of projects made the codes/standards lower the required period of moist curing. The Indian Standard Code of Practice for Plain and Reinforced Concrete, IS:456:20001, as well as its earlier version2 lowered the requirement of the minimum period of moist curing of OPC concrete from 28 days in earlier decades to 7 days.

Increasingly, cement manufacturers in India started marketing blended cements aggressively. Because of slower rates of hydration, it became necessary to set standards at longer periods of moist curing of concrete with blended cements.

In order that concrete of comparable mix proportions with blended cements may yield comparable or higher (than 28-day OPC concrete strength) strengths, researchers generally recommend 56 to 90 days of moist curing of concrete in the case of blended cements with mineral admixtures. The Indian code1, however, considers it sufficient to cure such concrete with blended cements for only 10 days. The code recommends that this minimum period of 10 days may be extended to 14 days. The earlier code required 7 days’ moist curing for concrete with blended cements too.

Though the code1 considers moist curing for periods ranging between 7 to 10 days to be adequate for concrete construction with different types of cement and though the duration of effective curing of concrete during construction may be even less thanthe periods specified in the code 1 , the design and construction of concrete structures are based on compressive strength of well compacted concrete samples after 28 days of moist curing.

Besides the shortcomings, which may arise as a result of the gap between the required (considered desirable by researchers on the basis of attainment of strength) and mandated (by the code) periods of moist curing, and besides the obvious gap between the design (tested after 28 days’ moist curing) and the actual periods of curing, Kar3-7 has pointed out that today’s cement is in many ways different from cements which were used till a few decades ago and which had given durable concrete structures. Furthermore, Kar3-7 has shown that today’s cement in India may contain harmful alkalis to make concrete less durable or even self-destructive (curve for PPC in Figure 2).

In this scenario, it is considered appropriate: (a) to study the influence of curing on the development of strength in concrete and (b) to examine the reasonableness of the codal provisions on the duration of moist curing of concrete. This is done for concrete, made with the three basic types of cement (viz., OPC, PSC and PPC).

Codal Provisions on Curing

Among the different provisions on curing of concrete, IS:456-20001 suggests that “Curing is the process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature regime. The prevention of moisture loss from the concrete is particularly important if the watercement ratio is low, if the cement has a high rate of strength development, if the concrete contains granulated blast furnace slag or pulverised fuel ash. The curing regime should also prevent the development of high temperature gradients within the concrete.

” The code1 also requires that “Exposed surfaces of concrete shall be kept continuously in a damp or moist condition by ponding or by covering with a layer of sacking, canvas, hessian or similar materials and kept constantly moist for at least seven days from the date of placing concrete in case of ordinary Portland Cement and at least 10 days where mineral admixtures or blended cements are used. The period of curing shall not be less than 10 days for concrete exposed to dry and hot weather conditions. In the case of concrete where mineral admixtures or blended cements are used, it is recommended that above minimum periods may by extended to 14 days.”

It appears from the language of the code that the extension of the curing period from 10 days to 14 days is not mandatory.

It is of interest to note here that the provisions in IS :456-20001 were considered reasonable eight years ago, whereas the provisions in IS :456-19782, which permitted 7 days’ moist curing for concrete with OPC, PPC as well as PSC, were considered reasonable at least thirty years ago.

During the intervening 22 years between the two codes, many structures with PSC concrete must have been cured moist for 7 days or less.

During the intervening period between the two codes1.2 and during the period following the more recent code, cement has undergone very significant changes in chemical compositions and physical characteristics3-7.

The resulting effects of such changes include the generation of considerable heat inside concrete at early ages. There is also the exothermic reaction from the high contents of water soluble alkalis in Indian cement of today3-7, further hastening the rate of hydration of cement, thereby leading to still faster gain in strength. All of these can make it possible to lower the required period of moist curing if the attainment of strength in concrete will become the only criterion in the determination of the adequacy of any particular period of moist curing of concrete. This would suggest that even if it may be found that today’s high early strength cements may yield strengths within acceptable ranges on satisfaction of the requirements of the mandated moist curing for short periods, such short duration curing in the past might not have yielded concrete strengths close to the design strengths at 28 days.

As stated earlier, it is studied here whether the stipulated periods of moist curing, set in IS:456-2000 1 for concrete with OPC and blended cements, are reasonable or not. This is done with cement that is available in the Kolkata region today. The tests for compressive strength were

conducted using 150 mm cubes with cements of several nationally and internationally known brands.

The evaluation of the adequacy of the period of curing is made from the consideration of attainment of strength as a percentage of strength at 28 days of moist curing.

It is well–known that moist curing, particularly at the initial periods, reduces the permeability of concrete. It is further known that greater the impermeability, better is likely to be the durability of concrete structures. In clause 8, the code 1 has, however, given four options for lengthening the life of concrete structures. Kar8 has explained that among the four options, given in the code, only the option of providing surface coatings/protection systems to concrete structures is practical in lengthening the life of concrete structures.

Since the provision of surface protection systems will effectively make concrete surfaces or structures impervious to the external agents of decay, any shortcomings in the form of greater permeability of concrete due to any inadequacy in curing loses some or much of any significance. Accordingly, no serious attempt is made to study the effects of curing on the permeability of concrete at 7 or 10 days vis-a-vis permeability of concrete cured moist for 28 days.

Codal Provisions and Effects of Curing

It is a common practice, in the acceptance of cement and concrete, to ensure that concrete has the required compressive strength.

There may be additional tests for setting times for workability and expansion as an yardstick for durability.

Five sets of curves are presented here as a part of the study on the influence of the period of moist curing on concrete strength.

Figure 1 shows the compressive strength of OPC concrete. The lower curve in Figure 1 represents the strength of OPC on the completion of moist curing for 1, 3, 7, 10, 14, 21 and 28 days as is the conventional practice. In this particular case, the 7-day strength of 26.22 MPa is 77.6% of the 28-day strength of 33.78 MPa.

It would appear that the mandated curing period (minimum) of 7 days may or may not lead to an endangerment of the safety of an OPC concrete structure if it would have been designed and constructed in accordance with the 28-day strength but cured moist for 7 days and loaded immediately thereafter. The real situation is better if the structure will not be loaded immediately after 7 days of moist curing, as explained below.

An interesting observation can be made here with the help of the upper curve in Figure 1 which represents the compressive strength of the concrete as that in the lower curve, except that the upper curve represents the compressive strengths at 28 days for cube samples which were cured moist for different periods from 0 to 28 days. It is observed that the peak concrete strength of

37.90 MPa at 28 days is higher than the concrete strength of 33.78 MPa when it is cured moist for a period less than 28 days. In this particular case, it so happens that the peak strength at 28 days is available when concrete is cured moist for 7 days, and then cured in air a further period of 21 days. In the same token, it is observed that in the event the concrete in Figure 1 would not be cured moist at all, but kept covered or in shade, a minimum strength of 30.46 MPa would develop at 28 days, which is 90% of the strength of the concrete, if it would be cured moist for 28 days. This is not too bad a situation where concrete may not be moist cured at all and the attainment of strength of concrete will be the only consideration.

Figure 2 shows the gain in compressive strength of PPC concrete from one batch and PSC concrete from two batches. The 150 mm cubes were cured moist for different durations. The cubes were tested for compressive strength at the end of each period of curing. The samples were cast in the month of April 2008. It is noticed that the compressive strength of concrete at the end of stipulated 1 periods (10 days) of moist curing is 76 to 80 percent of the 28-day strength in the case of PSC concrete whereas it is 91 percent in the case of PPC concrete.

Since structural elements e.g., floor slabs and floor beams are frequently loaded close to their design loads during construction stages, it is recognized here that it may not be unreasonable if the moist curing will be terminated at the end of 10 days in the case of PPC concrete, but the same cannot be said in the case of PSC concrete. It will be seen later that if loading of the structure will be delayed, moist curing of the structure or structural element for 10 days may not be too unreasonable.

It is recalled here that the code1 permits 7 days’ moist curing in the case of OPC concrete and 10 days’ moist curing in the case of concrete with blended cements, whereas (a) the design is based on compressive strength after moist curing for 28 days, and (b) real structures are seldom cured moist for the stipulated (in the code) periods.

In consideration of the above, it would appear that if adequate care will not be taken to limit construction or service loads, immediately upon the termination of moist curing, to within 70% of the design loads, and that too with appropriate margins for various uncertainties, the stipulated periods of curing will prove to be unreasonable. It is seen in the following that the picture is not necessarily unreasonable, if

a. concrete will be cured moist for at least 3 days, and b. the structure will not be loaded until 28 days after concreting or some such days after

concreting as may be determined from tests for particular batches of cement.

This suggestion is made only in the context of strength. Prolonged curing may increase the durability of concrete by minimizing permeability and shrinkage.

In contention of the above, three cases are studied in the following. These include cases:

a. When the design is based on compressive strength of concrete after 28 days’ moist curing but the concrete is cured for the stipulated period of 7 or 10 days and the structural element is not loaded until 28 days.

b. The concrete is cured for 3 days and the structural element is not loaded until 28 days. c. Where the design is based on compressive strengths of concrete after 28 days’ moist

curing but the concrete is not provided with any moist curing and the structural element is not loaded until 28 days.

Figures 3, 4 and 5 show the variations in compressive strength of concrete when concrete is tested at 28 days but cured moist for 0, 3, 7, 10, 14, 21 and 28 days, followed by air curing for 28, 25, 21, 18, 14, 7 and 0 days, respectively. Test results on concrete samples from three batches of concrete are represented in each of the three figures.

It is seen in each case in Figures 3 to 5 that the peak compressive strength is recorded when concrete is tested at 28 days but the moist curing is between 3 to 14 days (Tables 1 – 3).

Among all the three batches of OPC concrete (Table 1), it is seen that strengths equal to or higher than the strengths, at 28 days’ moist curing, can be obtained if the moist curing will be discontinued after 3 days.

It is further noticed in Figure 3 and Table 1 that in two cases the peak strengths in OPC concrete were obtained when concrete was cured moist for 7 days, followed by air curing for 21 days before the test. In fact, the 28-day (moist curing) compressive strength was lower than the peak strength (at 7- day moist curing followed by 21 days’ air curing) by as much as 17.4 percent in one case.

In the case of PSC concrete (Figure 4 and Table 2), the peak strengths were gained when concrete was cured moist for 7 to 14 days, followed by air curing for the remaining days. In the three cases of PSC too, it is seen that the 28-day strength could be reached by discontinuing moist curing after 3 days. It’s noticed that with continued moist curing, there is a drop of strength

(from the peak) by as much as 11.3 percent.

In the case of PPC concrete (Figure 5, Table 3), the peak strengths were obtained after 7 to 14 days’ moist curing, followed by air curing till 28 days from concreting. In two of the cases, the strengths of concrete were higher than the 28-day strengths when the cubes were cured moist for 3 days, followed by curing in air for 25 days. In the remaining case (Case III), the 28-day strength (moist cured) could be obtained with 9 days’ moist curing, followed by 19 days of air curing. In case I (Table 3), there is a drop of 25.1 percent in the peak strength with continued moist curing beyond 10 days.

It is seen that the stipulated1 period of 7 days’ moist curing in the case of OPC and 10 days in the case of blended cements is justified as far as matching the 28- day (moist cured) strength is concerned provided that the structures will not be loaded till 28 days after concreting.

It is seen that the stipulated2 period of 7 days’ moist curing for concrete with blended cements, as in Ref. 2, was not reasonable, particularly when cement of the earlier periods did not gain strength as early as it does today.

Concluding Remarks

There is an increasing trend to shorten the period of moist curing of concrete. The Indian code IS:456- 20001 has lowered the required period of moist curing of concrete to 7 days in the case of OPC and 10 days in the case of blended cements with mineral admixtures. The period of 10 days is, however, higher than what was specified in Ref. 2. In some countries, the minimum period of moist curing has been lowered to 3 days and in real cases in India many concrete structures are not provided any moist curing.

From the performance of four batches of OPC concrete (1 in Figure 1 and 3 in Figure 3), it appears that curing concrete, with today’s OPC, for 3 days in moist condition will be sufficient if matching the 28-day design strength will be the only criterion and the structures will not be loaded until 28 days after concreting.

From the performance of nine batches of PPC and PSC concrete (3 each in Figures. 2,4 and 5), it appears that except in one case of PPC concrete, curing concrete, with today’s PPC and PSC, for 3 days in moist condition will be sufficient if matching the 28-day design strength will be the only criterion to be fulfilled and the structures will not be loaded until 28 days after concreting. In case III of PPC in Figure 5, moist curing of 9 days, followed by air curing of 19 days, leads to a matching of compressive strength of concrete if such concrete will be cured moist for 28 days before loading.

A closer study of the various curves in Figures 1 to 5 show a decreasing trend for the strength of concrete with continued moist curing after the peak strength will have been reached much earlier than 28 days. This challenges the widely held concept about concrete that concrete increases in strength with continued curing in moist condition.

Studies are underway to determine if the declining strength with increasing moist curing has

more to do with impurities in today’s cement3-7 or with any lingering damp/ moist condition of the test samples after the initial period of moist curing.

References

IS:456-2000, Indian Standard, Plain and Reinforced Concrete Code of Practice (Fourth Revision), Bureau of Indian Standards, New Delhi, July 2000.

IS:456-1978, Indian Standard Code of Practice for Plain and Reinforced Concrete, Third Edition, Bureau of Indian Standards, New Delhi, September 1978.

Kar, A. K., “Concrete Structures We Make Today,” New Building Materials & Construction World, New Delhi Vol. 12, Issue 8,February 2007.

Kar, A. K., “The Ills of Today’s Cement and Concrete Structures,” Journal of the Indian Roads Congress, Vol. 68, Part 2, July- September 2007.

Kar, A. K., “Durability of Concrete Bridges and Roadways,” New Building Materials & Construction World, New Delhi, Vol. 13, Issue 3, September 2007.

Kar, A. K., “Woe Betide Today’s Concrete Structures,” Part I New Building Materials & Construction World, New Delhi, Vol. 13, Issue- 8, February, 2008.

Kar, A. K., “Woe Betide Today’s Concrete Structures,” Part II New Building Materials & Construction World, New Delhi, Vol.13, Issue- 9, March, 2008.

Kar, A. K., “IS 456:2000 On Durable Concrete Structures,” New Building Materials & Construction World, New Delhi, Vol. 9, Issue-6, December, 2003.s

New Building Material- Freshly Ground Lime Instead of Cement

Prof M. D. Apte, Pune

By the end of the Nineteenth century, the British rulers had imported ‘Cement’ to India and

commenced discouraging the method of using freshly ground lime for masonry construction that was in vogue in India since ages. If properly used, the cement construction could be made fairly waterproof. The construction could last as well appreciably long and gave hardly any trouble of maintenance, similar to slaked lime construction to the users. The Portland cement age was dawning in India! Local industrialists as well went ahead and established cement factories here. Being a factory manufactured material, it was touted to be always of uniform (and good) quality. By the end of Second World War, the fresh lime grinding as a process of preparing masonry material had been fully relegated into an historical construction activity!

In late fifties when we civil engineering students of Government College of Engineering Pune were taught this subject of cement concrete, it was emphasized that Concrete structures like bridges will last for over 60 years whereas residential accommodation can give satisfactory service for over 100 years! The cement concrete was quite strong and durable, even better than “finely and freshly ground lime” under use then. Even RCC was started being used by engineers with success.

We students were awestricken with the new found material and the technique of its use. The mix design for 1:2:4 (volume batch concrete) RCC, we used to need 15-16 one CWT (112 Lbs) bags of cement to make 100 cft of finished concrete. Sometime the cement consumption could go up to even 17 bags.

The mix design was introduced with a hollow box (3’ x 3’ x 3’) packed fully with coarse aggregate additionally packed with fine aggregate and in turn this interfiled with finer powder of cement. The body of concrete being aggregate the cement was only the binding agent as we could understand.

Any more small voids in this box were supposed to be filled with the expanding cement gel after it reacts with the mixing water in the concrete. The resulting concrete was supposed to be even waterproof.

We the engineering students were really overwhelmed by the good qualities of the cement and began enthusiastically looking forward to design cement concrete structures. By that time British and other foreigners had progressed into pre-stressed and/ or post-tensioned concretes. Indians accepted that as well as a technical gift from West. Concrete designing had become a science and wordy wars about volume batching versus weigh batching were fought in technical journals with gusto. Whatever concrete construction was executed before Second World War in India was by the British Engineers done through the Indian ‘Mestries’. They had no restriction of time or

money for the projects and accordingly these constructions are standing even today as good examples. (Of course, now the British or other Western engineers have relaxed their vigil and got confused due to the large varieties of cements in the market, their output is dropped to ‘average’). Indian engineers used concrete since independence without teaching their masons and mestries the correct techniques needed to use this new material properly (since they themselves were unaware of that aspect). Cement was being used as readymade ground lime only. The engineers took it to increase the strength or durability of concrete, one needs to add cement in excess to the mixture.

Once a boxed structural member was concreted, the sample of the concrete used therein was to be cast in cubes and after proper curing; the cube was to be crushed to determine the quality (compressive strength) of the concrete used in the member. Bureau of Indian Standards brought a standard for this cube test and use of concrete to give impetus to good concrete construction. After they published IS: 456 of 1978 regarding use of plain & reinforced cement concrete giving the direction to use more cement (up to 540 Kgs per CM3 of concrete) for durability consideration, use of more cement for strength as well as durability purposes became rule rather than exception. Most of the ‘experts’ included in the BIS committee on Cement Concrete Section are either representing large construction companies or cement manufacturing companies who were only interested in increasing the use of cement to earn more money. They were not necessarily interested in propagating use of some other material in construction. Isn’t it? Not only cities but even small towns became concrete jungles and no wonder the mother Earth reacted by increasing environmental temperatures everywhere.

Cement factories grew in number as well as size and they manufactured special cements not only for refractory or sulphate resistance purposes but also to give higher strength in compression (instead of 33 N/mm2) of 43, 53 or even higher (at the end of 28 days curing). When the manufacturers professed that the stronger concrete is more economical, gullible people, even engineers, enthusiastically started using it in their designs. This was the time when dangers of cement concrete even with reinforcement were started appearing on horizon. Defects like, rusting of steel and carbonation of concrete, cracking of “stronger” cement concretes after a few days use, deterioration of the concrete members after a few seasons of intensive (though within designed loads) use, destruction of concrete because of alkali-aggregate reaction in certain circumstances, cracking of concrete with excessive cement quantity leading to destruction of the monolith, members failing because of inadequate concrete strength development etc. made frequent appearances. Deterioration of Vashi Bridge near Mumbai and failure of many bridges like Mandovi River Bridge in Goa as well as overhead tanks, multistoried concrete residential buildings etc compelled Indian civil engineers to wake up and study the technology and its application in India thoroughly. According to their thinking, the defects might be due to various reasons like the pour was incorrect, segregation might have taken place while pouring, placement of reinforcement might not be exactly as per design or might have been shifted to wrong places during concreting or the compaction might not have been done effectively or the curing might not have been done correctly or defect might be in erection and/or removal of form work. Moreover, even when the test cubes were cast along with the member being concreted, further progress in concreting was never held up (as obstacle to maintain progress of work) till the 28 days crushing strength of the cubes certifying the strength of the concrete became available to the site engineer. In the name of progress, the constructors gave more importance to the test

cubes being cast and tested successfully than the concreting of the members themselves (to avoid any future problem arising in case the test results were not found satisfactory). This led to having individuals other than site engineers specializing in casting of test cubes. This was found to result in the cubes not really as representative a sample of the concreting of the member as desired. In addition to the use of excessive cement in concrete, this and other shortcomings in use of correct technology and procedures led to the defects in concrete that have surfaced in India over the years.

From the basic principals of concrete technology one can list following essentials of strong and durable concrete by using Ordinary Portland Cement:-

1. Cement is only binding agent and has hardly any inherent strength as a material. It can adhere to surfaces of strong pieces as gum and give strength to the monolith body.

2. For convenience, pieces of stones as aggregates of various sizes are considered suitable to give a body to concrete. To have cost within limits, quantity of cement should be small and sizes of coarse aggregate pieces be as large as possible in the mixture.

3. The cement only binds the various aggregate pieces together to make the concrete monolith. Since cement as binder has no inherent strength, the binding layer should be as thin as possible. Moreover, cement being in very fine form has a high coefficient of thermal expansion/contraction compared to that of aggregates used and hence thinner the layer, safer it is. So cement must be used as least as practicable.

4. Strength of concrete has very little bearing on the quantity of cement in the concrete in the long run. Larger the (than necessary) quantity of cement, the concrete is likely to deteriorate over time faster due to temperature variations in the environment.

5.

To make concrete stronger, less voids or gaps should be permitted in the concrete monolith. This is possible by using all the intermediate sizes of aggregate (to reduce the size of gaps) and adequate compaction of the concrete in-situ after pouring. Water should be just sufficient to make the rich chemical gel with cement. Extra water remaining if any is likely to create voids after evaporation.

6. In the concrete mixture only cement is a manufactured substance and hence is more susceptible to environmental damage and deterioration and hence least durable.

Therefore, thinnest possible gel around the aggregate pieces is all that is needed to make a durable concrete.

7. In short, to make strong and durable concrete what we need is well graded aggregate to fill the volume of concreting (box?), added with minimum required cement to cover the interstices with strong gel formed with little more than essential, say within 40% water as

compared with cement quantity. To make it durable, prevent any voids within the monolith by adequate compaction. You can provide compaction such that the strength of the concrete is as designed.

The mixture must be uniform and before setting time of the cement is reached, compaction must be completed. Once concrete is cast and set, it should be cured with water at least for 7 days and then damp curing may be satisfactory.

8. If possible and convenient, it is suggested that concreting can be done by first filling coarse aggregate in the centering boxes and colloidal mass of sand and (water added) cement is poured to fill the voids before compaction. This will ensure that the semi-elastic gel that is produced by the colloidal mixture will be able to coat the aggregate pieces effectively with less cement at the same time giving better strength.

9. As far as reinforcement is concerned, the quantity of steel as designed must be placed at correct locations to resist tensile stress development in concrete. It must be ensured that the steel reinforcement bars do not shift during pouring and compaction of concrete. Adequate concrete cover must be around the reinforcement to prevent environmental carbon-di-oxide, chlorine or moisture from reaching the bars and corroding them.

In short, it can be seen that once the ingredients of concrete are properly selected, ensuring W/C ratio around 0.4 and adequate compaction to prevent any voids in hydrating (cement in) concrete are the only ways to ensure strong and durable concrete. If these conditions are adhered to, then adding any extra quantity of cement (per cubic meter of concrete) has no positive effect on the strength or durability of concrete. Rather more cement is likely to make the concrete less durable since thicker cement layer will have more shrinkage/ expansion than other ingredients of concrete as environmental temperature wane and wax giving rise to destruction of the monolith.

The cement is required only to surround the aggregate pieces for binding neighboring pieces. The maximum size of aggregate (MSA) will determine the quantity of cement required per CM of concrete. As the MSA decreases, the required cement quantity will increase since the surface area of the smaller aggregate pieces (to be bound together) will increase. Normally, for RCC we use 20 mm MSA. When this size increases (like for road or foundation purposes) to, say 40 mm, then naturally cement quantity will reduce by around 10%. For soil stabilization, cement is mixed in soil (comparatively coarser, even sandy) at not more than 10% by volume. As soil becomes clayey and finer, the cement content may go even up to 25%. This is natural, since surface area of particles to be covered by cement increases appreciably. The unrestrained compressive strength of this stabilized soil becomes about 6 Kgs per mm2. If properly restrained and compacted the resulting compressive strength, it can be comparable with concrete. Thus compressive strength will depend on compaction and W/C ratio only. For a cubic meter of concrete (MSA 20 mm) about 1300 litres of aggregates are required. Accordingly, for cementing purposes, 160 Kgs of cement should be sufficient. Some small additional quantity of cement may be required to cater for inadequate and/or non-uniform mixing of the concrete and to cater for the rough surfaces of the aggregate pieces being bound by the cement paste. Addition of any extra cement cannot make the concrete more durable. In case the mix is found to be non-workable for want of sufficient fines, an odd bag of pozzolanic powder may be added and/ or some plasticizer used. Since the concrete strength will be limited by that of the aggregate used, any lesser strength of the concrete can be achieved by adjusting the compaction suitably. Addition of extra quantity

of cement will not do the trick of giving more strength and/or durability to the concrete, in case adequate control on W/C ratio and/or compaction of concrete could not be maintained.

Amongst the constituents of concrete only cement is a factory manufactured item and therefore susceptible to environmental attacks. Natural materials will always be superior and economical when compared with ‘manufactured’ replacements. Since cement has no intrinsic body and therefore strength, any quantity in excess of binding needs, is likely to make the layers between aggregate pieces thicker. Any exposed cement at the surface may get damaged due to environmental factors in addition. Amongst the constituents of the concrete cement is having the largest coefficient of shrinkage. This will ensure that the thicker cement layers will crack and loosen the aggregate pieces while facing changes of environmental temperature. This finally will result in deterioration of the monolith. Therefore, cement used in the concrete must not be in excess. Cement is factory produced, but its raw materials like lime-stone, clay etc are Natural minerals and therefore cement cannot be (and also is not) a product of identical chemical composition (even from adjacent batches). In short, every bag that one opens, needs field checks for characteristics of the cement before using the same. This makes it further costly.

Thus many defects in concrete may be developed after the structures are in use. This has led to development of construction chemical industry. They have developed chemicals to treat these defects. While treating the intended defects the reactive chemicals create some other side effects (defects?) in the concrete. As a result of all this the cement as a replacement of finely ground lime has become enormously costly and beyond the affordability of common man. Doubts are also cropping up if Cement is really an effective and acceptable replacement for freshly ground lime.

It will be apparent therefore that the New Material (cement) that was initiated (with much fanfare) to replace freshly ground lime is neither advantageous nor economical to anyone (at least in Indian environment) but to the manufacturers of Cement and construction chemicals. The structures constructed in Cement Concrete are non-durable, and cannot be made waterproof by human intervention Even Cement is not an environment-friendly material and its production as well as use add pollutants as well as heat to the atmosphere. The position in foreign countries is also not very good. The costly cement concrete structures need varieties of construction chemicals to add for getting desired results. Lot of technical consideration is needed to determine the type and quantity of the chemical to be mixed. The results are not of required durability or long lasting. The chemicals to be added are not inert and therefore dangerous to life and nature as well. Since however, the Westerners did not have any better method or construction material before cement, they may continue to insist on cement as ‘Best’ building material in use; let them. It is suggested that at this stage of development India should carry out checks on the utility of freshly ground lime against cement. Selection from the large variety of cements and additives in the market and the appropriate practices of complicated processes of designing, mixing, pouring, compacting as well as curing of concrete are confusing even to engineers and the construction is not economical to the consumer i.e. common man. Structures constructed with lime over 60 years ago appear to be still in serviceable state without undue maintenance expenditure.

Cement is manufactured from mixture of lime stone and clay (both ground/crushed) in water (or dry if could be uniformly mixed). The slurry is blended to correct composition. This corrected

slurry fed to rotary kiln heated by powdered coal is converted into clinkers. These clinkers ground in ball mill with addition of 2 to 3 % gypsum (for preventing flash setting). This cement is stored in cement silos for loading in bags or vehicles. The coal requirement of the rotary kiln is 350 Kgs per ton for wet process and 100 Kgs for dry process. Many factories produce cement by dry process but still some use wet process. Let us assume that on the average a ton of cement needs 150 Kgs of coal. 200 MT of cement (quantity produced and used in whole year of 2006) has used 30 million tons of coal. This would have added about 75 million tons of CO2 (Carbon-Dioxide) to the environment to increase the atmospheric temperature. Other countries in the world would have added lot more and thus earth temperature would have been raised to a very high extent. It is possible that this has been taken into account in Industry’s contribution to environmental pollution. It therefore can only be noted here as cement factory’s pollution portion.

Once cement concrete is poured in formwork and starts hydrating, it evolves heat. Total heat that cement can generate during hydration is around 125 calories per gram of cement during its complete activity of hydration. This activity is a long drawn process and for our purpose we take 28 days hydration as full hydration for design purpose. A heat of hydration of 90 calories is given out by every gram of cement during that period. Let us consider that on the average 3 calories of heat is given out per day by one gram of cement. As per reports, in whole year India has consumed 200 million tons of cement during 2006 (or say, 17 million tons per month) for construction/repair of structures. Thus during the year 2006 concrete structures (only under construction/repairs) have given out 34 X 10*12 calories (or 34 trillion calories of heat) to atmosphere EVERY DAY!. More heat at a lower rate is being given out in balance period of the year in addition. This is during construction. Once the structures are in use, their exposed concrete bodies absorb heat from the sun during day and reject to the atmosphere during the evening is another aspect of heat evolution by concrete structures. India is considered to be (still) developing indicating that the use of cement is going to increase continuously as the ‘development’ progresses. Environment is getting heated to a large extent by the use of cement. Thus quite an appreciable quantity of heat is generated (for the environment) by cement consumed by all the nations; more by the developed Nations. Thus one can imagine how much heat is daily given out by the cement use to the environment so that earth temperature goes on increasing. It surely cannot be dismissed as minor aspect while considering the ‘Green- House’ effect on the earth due to human activities.

It will be clear from above discussion that cement as building material has technical problems right from start and could not yet been completely rectified. Rather they appear to be increasing continuously. People using it have perforce to add some (more expenditure?) chemicals to overcome the defects. Similar to modern system, an attempt to rectify some defect creates another one in concrete structures as well. In addition, this material requires lot of energy during production process and adds heat and pollution to the environment while in use. It is thus creating havoc all over the world. Actually, Kyoto round of WTO talks during Nineties could have done better by adding ‘scrapping of this material from production (as well as use) slowly’ to its suggestions for corrective measures to reduce rising temperature of the earth.

While discussing this aspect with my friends most of them agreed with the view that cement is surely a building material quite inferior to burnt and ground lime. However, they did insist on

telling that the lime is incapable of constructing highrise buildings for which cement concrete is only available. Use of cement therefore can immediately be stopped only for buildings lower than 3 stories high. Quite a large amount of pollution can be reduced by use of this rule since only a fraction of buildings are presently highrise structures. This should be immediately implemented.

If we consider the condition of man on this planet since he came, it will be apparent that getting way from Nature’s contact and desire to abjure physical labor are the two tendencies of man which are detrimental to him. Lack of physical labor has made him fall sick frequently for lack of exercising his body adequately and properly. Distance from Nature has kept him away from Nature’s ways to prevent/recover from various mental as well as physiological illnesses. When man stays in highrise buildings he is necessarily away from earth i. e. Nature and he misses all the advantages of it. Therefore, as a rule man need not operate from high rise structures at all. Therefore, he could have done nicely with burnt & ground lime as construction material and avoided manufacture and use of cement at all. All the pollution of environment as discussed above could have been avoided. Now as well disusing this material he can reduce environmental pollution. He will have to use lime as a new building material instead. In old world at least, he will have few persons familiar with it to help him in this aspect.

Mr Joseph Aspdin, a Leeds constructor took a patent for Portland cement {a fine powder of certain earth crust found in nature which was similar to the rock at Portland (a place in England) in color in 1824. Its use in Europe started in right earnest since they had no other (suitable) construction material prior to that time. As good traders they propagated with zeal and force this material in their colonies. The slave population had no choice to refuse it (though they had better material in ‘finely ground lime’) for masonry. During their rule British always discouraged the use of any indigenous materials or systems like Dhaka Mulmul, Handloom weaving, Ayurved and Gurukul System of learning in India and offered their imported versions instead, specifically to kill the indigenous systems and fleece the riches of the enslaved country and people. Same thing happened concerning use of lime in construction during their rule. This led to disuse of lime slowly till WWII, after which it nearly reached its extinction as a building material. As a young boy, I remember to have seen the use of ‘Ghaani’ being used for grinding lime by bullock when our house at Satara was being extended in around fifties (about 55 years ago) as an only instant.

It is quite likely therefore that some oldies aware about use of this material may still be around and can assist us in redeveloping it into a building material superior to Cement in all aspects. The organization of IITians that is coming up in India to develop technical education (and other aspects) Nationwide can do well to serve the World if they can revive the ‘finely ground lime’ as a building material to replace the ‘dirty’ cement. The world will be saving not only money and energy but it will be saving the environment and Nature as well for our future generations. This will really be going back to the (progressive) future! Reference

1. Minimum Cement Content for Strength and Durability of Concrete– Technical Rationale’ by Prof M D Apte published in NBM & CW Jan 2002.

2. Cement concrete text book ‘Concrete Technology’ by Prof M. S. Shetty.

3. Indian Standards concerning Concrete Construction as brought out by BIS 4. Cement Manufacturers Literature and publications5. Experiences of the author during his professional career6. British rulers’ efforts in imposing Western culture

Addition from Editor

Gangaccanal aqueduct known as Solani Bridge near Roorkee in Uttarakhand is a fine example of lime construction (Masonary line mortar) which is more than 150 years old by now with no problems. Large number of Arch Bridges on Ganga canal (constructed along Solani bridge starting at Hardwar are constructed using Bricks masonary in lime mortar.

Experimental Investigation on the Strength and Durability Characteristics of Concrete ContainingDr. S.M.Gupta, Department of Civil Engineering National Institute of Technology, Kurukshetra. The use of silica fume as a mineral admixture for the production of high strength concrete and high durable concrete is gaining importance in recent years. The objective of the present experimentation is to study the effect of silica fume as additive on the strength and durability characteristics of concrete obtained using locally available material. Concrete mix for M20 gradeis designed which serves as basic control mix. Silica fume concrete mixes are obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement. The compressive strength development and durability against acidic and alkaline attack is studied.

Introduction

The present trend in concrete technology is to increase the strength and durability of concrete to meet the demands of the modern world. These factors can be achieved in concrete by adding various blending materials with cement or separately to concrete. The materials suitable for blending are flyash, blast furnace slag, silica fume, etc. Silica fume concrete (SFC) is emerging as one of the new generation construction material. It can be considered as high strength concrete or high performance concrete

The use of pozzolanic admixtures like condensed silica fume, because of its finely divided state and very high percentage of amorphous silica, proved to be the most useful if not essential for the development of very high strength concretes and/or concretes of very high durability. It is recommended that for applications in concrete silica fume should conform to certain minimum specifications such as silicon dioxide content of not less than 85%, spherical shape with a

number of primary agglomerates with particles of size ranging from 0.01 to 0.3 microns (average of 0.1 to 0.2 microns), amorphous structure and a very low content of unburnt carbon.

Silica fume is known to improve both mechanical characteristics and durability characteristics of concrete since, both the chemical and physical effects are significant. Physical effect of silica fume in concrete is that of a filler, which, because of its fineness, can fit into spaces between cement grains in the same way that sand fills the spaces between particles of coarse aggregate and cement grains fill the spaces between sand grains. As for chemical reactions of silica fume, because of high surface area and high content of amorphous silica, this highly active pozzolan reacts more quickly than ordinary pozzolans.

Experiments have revealed that silica fume in concrete essentially eliminates pores between 500 to 0.5 micron sizes and reduces the size of pores in the 50 to 500 micron range. Physical and chemical mechanisms made the silica fume more effective in reducing pore size.

Experimental Programme

An experimental program was carried out to find out the strength and durability characteristic of concrete containing silica fume as an additive. Concrete mix for M20 grade was designed, which served as basic control mix. Silica fume concrete mixes were obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement.

Materials Used

Ordinary Portland cement was used throughout the Experimentation. Silica fume used in the experimentation was obtained from FOSROC Chemicals (India) Limited. The physical and chemical properties of OPC and silica fume (SF) are given in Table 1. Locally available aggregates were used. Coarse aggregates crushed from igneous basalt rock of 20mm and down size having specific gravity of 2.74 and conforming to IS 383-1970 were used. For Fine aggregate local sand having specific gravity of 2.56 and conforming to grading zone I of IS: 383-1970 was used. Superplasticizer based on sulphonated naphthalene formaldehyde was used to impart additional desired properties to the silica fume concrete. The dosage of super plasticizer was 0.7% by weight of cement. Ordinary potable water was used for mixing of the ingredients.

Concrete Mixes

Mix design for M20 grade of concrete was carried out using the guidelines prescribed by IS: 10262- 1982. The designed concrete mix for M20 served as basic control mix (CM). Silica fume concrete mixes were obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement. (viz SFC2, SFC4, SFC6, SFC8, SFC10, SFC12, SFC14, SFC16). The Basic control Concrete mix proportion obtained was 1 part

cement: 1.62 parts of fine aggregate: 3.28 parts of coarse aggregate with water–cement ratio of 0.5 and 0.7% of Superplasticizer.

Batching, Mixing, and Curing

The concrete ingredients viz. cement, sand and coarse aggregate were weighed according to proportion 1:1.62:3.28 and are dry mixed on a platform. To this the calculated quantity of silica fume was added and dry mixed thoroughly. The required quantity of water was added to the dry mix and homogenously mixed. The calculated amount of superplasticizer was now added to the mix and then mixed thoroughly. The homogeneous concrete mix was placed layer by layer in moulds kept on the vibrating table. The specimens are given the required compaction both manually and through table vibrator. After through compaction the specimens were finished smooth. After 24 hours of casting, the specimen were demoulded and transferred to curing tank where in they were immersed in water for the desired period of curing

Tests Conducted

The tests were conducted both on Fresh and Hardened concrete. The tests on fresh concrete was the workability test conducted through Slump test, Compaction factor test; Table 2 and Vee-bee consistometer test. The strength and durability tests conducted on hardened concrete are briefed here:

Compressive Strength Test

The compressive strength test was carried out on cube specimens of dimensions 150 ´ 150 ´ 150 mm. The compressive strength test specimens were cured and tested for 3-days, 7-days, 28-days, and 60-days in compressive testing machine. Three specimens were used for each test.

Durability Test Resistance Against Acid Attack

For acid attack test concrete cube of size 150 ´ 150 ´ 150 mm are prepared for various percentages of silica fume addition. The specimen are cast and cured in mould for 24 hours, after 24 hours, all the specimen are demoulded and kept in curing tank for 7-days. After 7-days all specimens are kept in atmosphere for 2-days for constant weight, subsequently, the specimens are weighed and immersed in 5% sulphuric acid (H2SO4) solution for 60-days. The pH value of the acidic media was at 0.3. The pH value was periodically checked and maintained at 0.3. After 60-days of immersing in acid solution, the specimens are taken out and were washed in running water and kept in atmosphere for 2-day for constant weight. Subsequently the specimens are weighed and loss in weight and hence the percentage loss of weight was calculated.

Resistance Against Alkaline Attack

For alkaline attack test concrete cube of size 150 ´ 150 ´ 150 mm are prepared for various percentages of silica fume addition. The specimen are cast and cured in mould for 24 hours, after

24 hours, all the specimen are demoulded and kept in curing tank for 7-days. After 7-days all specimens are kept in atmosphere for 2-days for constant weight, subsequently, the specimens are weighed and immersed in 5% sodium sulphate (Na2SO4) solution for 60-days. The pH value of the alkaline media was at 12.0. The pH value was periodically checked and maintained at 12.0. After 60- days of immersing in alkaline solution, the specimens are taken out and are washed in running water and kept in atmosphere for 2-day for constant weight. Subsequently, the specimens are weighed and loss in weight and hence the percentage loss of weight was calculated.

Results and Discussions

Workability Test Results

The result of workability of concrete as measured from slump, compaction factor and, Vee-bee degree are shown in Table 2. According to these results, workability of concrete decreases as the silica fume content in concrete increases from to 16%. No wide variations in the slump and compaction factor values for the mixes containing increased amount of silica fume were observed. The silica fume concrete did not show tendencies for seggregation and bleeding. This is due to the fact that as the percentage of silica fume increases the water available in the system decreases thus affecting the workability. As compared to control mix (CM), the mix containing 16% silica fume (SFC16) has a slump reduction of 28% and compaction factor reduction of 5.26%. The effect of silica fume content on the workability with regard to slump of concrete is shown in Figure 1.

Compressive Strength Test Results

The compressive strength of concrete containing silica fume given in Table 3 shows an increasing trend as the percentage of silica fume increases, from 0 to 16%. This is true for 3-days, 7- days, 28-days, and 60-days compressive strength. The strength activity index for 3-days, 7-days, 28-days, and 60-days for 16% of silica fume is 1.65, 1.33, 1.49 and 1.41 respectively. The effect of silica fume content on the Compressive strength of concretes is shown in Figure 2.

Resistance Against Acid Attack

Table 4 shows the change in weight of control mix and silica fume mix when immersed in 5% sodium Sulphric acid (H2SO4) solution. Under a very low pH (0.3 pH) of 5% - H2SO4 Solution, all hydrated products, hydrated silicate and aluminate phases and calcium hydroxide, can easily be decomposed. The control mix was markedly affected by 5% - H2SO4 solution with a significant weight loss. On the other hand, the progress of deterioration in silica fume concrete immersed in 5% - H2SO4 solution varied widely depending on the percentage of silica fume. SFCl6 mix was found to be most effective in preventing the Sulphuric acid attack. It appears that in the Sulphuric acid attack, the early decomposition of calcium hydroxide and subsequent formation of layer amount of gypsum are attributed to the progressive deterioration accompanied by the scaling and softening of the matrix. The percentage weight loss, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.65

The effects of silica fume content on the acidic media durability shown in Figure 3.

Resistance Against Alkaline Attack

Table 4 shows the change in weight of control mix and silica fume concrete when immersed in % sodium sulphate (Na2SO4) solution. The pH value of 5% sodium sulphate (Na2SO4) solution was found to be 12. The percentage weight loss, which is an indication of durability, decreases as the percentage of silica fume in concrete increases.

The weight loss index for SFC16 is 0.00 while for control mix it is 1.00. This may be due to the fact that the silica fume, which also acts as a filler material, increases the density of concrete by

filling the voids. The voids, which are very compactly filled up by the silica fume, do not allow the alkaline media to penetrate into concrete mass and also reduced content of calcium hydroxide in die silica fume concrete due to pozzolanic reaction. Thus the percentage weight loss will be less as the percentage of silica fume in concrete increases. The effect of silica fume content on alkaline media of concretes is shown in Figure 4.

Compressive strength of Silica Fume Concrete after 60-days Immersion in Acidic Media and Alkaline Media

Table 5 shows test result of 60-days compressive strength of silica fume concrete, when exposed to two different media viz Acidic and Alkaline media the strength activity index shows an increasing trend as the silica fume increases from to 0 16%.

The strength activity index for SFC16 is 1.92 for acidic and 1.42 for alkaline as compared to the control mix in the respective media. The effects of silica fume content on the compressive strength after 60-days immersion in Acidic media and Alkaline media of concretes is shown in Figure 5.

Conclusions

These studies have lead to the following conclusions:

1. The workability of concrete as measured from slump, compaction factor and Vee-bee degree decreases as percentage of silica fume in concrete increases. As compared to the control mix, SFC16 has a slump reduction of 28% and compaction factor reduction of 5.26%. Thus the workability of concrete decreases as the percentages of silica fume in concrete increases.

2. The compressive strength of concrete shows an increasing trend as the silica fume content increases, from 0 to 16%. This increasing trend is evident for 3-days, 7-days, 28-days, and 60- days compressive strength. The strength activity index for SFC16 is 1.65, 1.33, 1.49, and 1.41 at 3- days, 7-days, 28-days, and 60- days respectively. Thus silica fume acts as a pozzolanic material, hence the compressive strength of concrete increases as the percentage of silica fume increases.

3. Resistance against acidic attack of silica fume concrete increases as the silica fume content increases from 0 to 16%. The percentage weight loss, which is an indication of durability in acidic media, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.65. Thus since silica fume acts as a filler material and fills up the voids of concrete, the durability of concrete in acidic media increases as the percentage of silica fume in concrete increases.

4. Resistance against alkaline attack of silica fume concrete increases as the silica fume content increases from 0 to 16%. The percentage weight loss, which is an indication of durability in alkaline media, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.00 while for control mix it is 1.00.

5. The 60-days compressive strength of silica fume concrete, when exposed to two different media viz. Acidic and Alkaline media shows an increasing trend as the silica fume

increases from 0 to 16%. The strength activity index for SFC16 is 1.92 for acidic and 1.42 for alkaline as compared to the control mix in the respective media.

References

1. ACI Committee 226R (March- Apr. 1981), “Silica Fume in Concrete,” ACI Material Journal, 84, 3, pp. 158–166.

2. Conferences/Seminars/ Workshops.3. Cook, J.E., “Research and Application of High Strength Concrete, 10.000 psi Concrete,”

Concrete International, Oct., 1989, pp. 67-75.4. Duval R. and E.H. Kadri (1998), “Influence of Silica Fume on the Workability and the

Compressive Strength of High Performance Concrete,” Cement and Concrete Research, 28, 4, pp.533-547.

5. Gupta, S.M., “Experimental Studies on the Behavior of High Strength Concrete,” Ph.D. Thesis, 2001, K.U.Kurukshetra.

6. I.S. : 10262–1962, “Indian Standard Recommended Guidelines for concrete mix design,” BIS, New Delhi.

7. I.S.: 383-1970 (1990), “Specification for coarse and Fine Aggregate from Natural source for concrete,” Bureau of Indian Standards, New Delhi.

8. Leming M.L., “Properties of High Strength Concrete: An Investigation of Characteristics High Strength Concrete Using Materials in North Caroling Research Report FHWA/ NC/88-006,” Department of Civil Engineering, North Carolina State University, Raleigh, N.C., July, 1988.

9. Mehta, P.K., and Gjorv, O.E. “Properties of Portland Cement Concrete Containing Silica Fume,” Cement and Concrete Research, V. 12, No. 5, Sept.1982, pp. 587-595.

10. Moreno, J., “The State–of–the–Art of High Strength Concrete in Chicago, 225W. Wacka Drive. Concrete International, Jan., 1990, pp. 35-39.

11. Neville A.M. (2000), “Properties of Concrete,” Fourth and Final Edition - Pearson Education Asia Ltd.

12. Ojho, R. N., “Use of Flyash and Condensed Silica in Making Concrete,” IE (I), Journal V. 77, November, 1996, pp. 170-173.

13. Rachel J. Detwiler and P. Kumar Mehta (Nov.-Dec. 1989), “Chemical and Physical Effect of Silica Fume on the Mechanical Behavior of Concrete,” ACI Materials Journal, 86, 6, pp. 609-614.

14. Sellevold, E.J. and Nilsen, T. Supp1ementary Cementing Materials for Concrete, Ed. By V.M. Malhotra. CANMET, SP 86-8E, pp. 167-246/1987.

Integral Watertight Concrete Structures- An Insight

Upen Patel, Marketing Manager, BASF Construction Chemicals (India) Pvt. Ltd. Mumbai

Preface

Since the Stone Age mankind has struggled to keep the structures watertight, Even today’s struggle is on to deploy one or other kind of waterproofing system to achieve total water tightness of structures. In spite of good construction practices at site and using branded products, engineers struggle to keep structures watertight. So far the approach has been based on achieving watertight tanking around the structure to guard entry of water in the structure; less attention and focus has been on getting integral water tightness (within the structure). This article explains the main sources of leakages in the structure, and using two technological advancements to enable integral water tightness to structure with a live case study.

Sources of Leakage

There are three main sources of water leakage in concrete structures:

Construction Joints Cracks Porous media

Construction Joints

Large structures of concrete are cast in number of sections. The dividing lines between two sections are the joints between already harden concrete and freshly poured concrete, with continuity of reinforcements. At these joints fresh concrete shrinks and creates a fine crack.

These cracks are normally 0.1 – 0. 3 mm in width and are passages for water to pass through the structure.

Cracks

Due to various reasons such as excessive segregation of concrete mix, high water cement ratios, movements, settlements, rapid variations in ambient temperatures, early de-stripping, etc… concrete structures develop cracks during the construction stage and these cracks are some time deeper and can easily transport water from one side to another.

Porous Concrete Media

Concrete has heterogeneous matrix, a mixture of binding paste and fillers. While mixing the ingredients and placement of fresh concrete, concrete entraps air and the same is attempted to remove by compaction using mechanical means. Achieving uniform compacting throughout the volume of freshly placed concrete is very difficult to achieve. In adequate compaction results in to the air voids. Theoretically, concrete requires only 23 – 25% water by weight of cement for the chemical hydration of the cement. While actual concrete in practice contains 40 – 50% of water by the weight of cement. The extra water is provided to achieve desired workability and easy of placement. This extra water leaves the concrete mass during the hydration reaction resulting in to the formation of pores. An interconnected series of such pores are popularly known as capillary pores. These capillaries make concrete porous. Besides these two features concrete also contains hydration pores which are formed due to volume changes of hydration products and are normally filled with loose lime, one of end product of the hydration reaction. These hydration pores promotes diffusion of corrosive agents in the concrete.

The Integral Watertight Solution

To check the porosity of concrete and leakages through the joints & cracks, integral watertight concept is gaining popularity. The concept is a combination of two systems:

Watertight joints using reinjectable hoses Watertight concrete mass by deploying self–compacting concrete concept

Re-injectable Hoses

The re-injectable hoses are made up of PVC plastic core which enables toughness to the hose. The core has injection channel in the centre, which connects to openings at regular distance in all four directions. The openings are guarded with neoprene seals.

The hose is placed at the central line of the construction joints using clips and the ends are connected to nonperforated hose with termination in near-surface mounted junction boxes. After the casting and destripping of the concrete cover of the junction box is located and marked for future operations. Each length of the hose is first injected with water to assess the leakages at the construction joint. Then injected using water–based lowviscosity, re-swellable, vinylacrylate injection resin. The injection resin pressurized the soft neoprene seals and squeeze out around seals to the openings and travels around the hose and to the crack of the construction joints and other cracks which connects to the construction joints. In the next stage of operation, injection pressure is release and the hose is applied with the vacuum. The neoprene seals now gains original size and seals the openings and prevents the suction of resin from outside of the hose to inside. Also all the resin from the central channel is sucked out and then the hose is rinse using water under low pressure re-circulation stage. Now the resin in the cracks has set and forms an effective seal for passage of water in the future. The hose can be injected with water to verify the effectiveness of the injection. If leakages are noticed then the hose is re-injected with the resin once again. Overall the re-swellable acrylate resin and injection hose provides following main benefits:

Re-injectable hose – permanent access to the construction joint In-build QA system–Test the effectiveness by injecting with water Re-swelling injection resin– swells up to 2.5 time in volume to maintain tight seal even in

the case of movements in the cracks

Watertight Self– compacting Concrete

To obtain proper, robust self– compacting concrete, it is important to include all of following components in the mix. These components enhance the performance of the fresh concrete as mentioned below:

Hyperplasticiser–which is based on PCE polymer and have 30 – 40% water reduction capabilities

Viscosity Modifying Agent – Improve the shear resistance and thickens the paste to achieve effective segregation resistance

Pozzolans – Facilitate increase in the paste volume without increasing the temperature of concrete and enables segregation resistance.

Hence, to achieve a robust mix of self–compacting concrete, it is must that all of these three ingredients are present and are properly included to maximize the benefits they can offer on the hardened properties of the mix.

From the water tightness and durability aspects—these three ingredients enable the following benefits:

Overall by carefully implementing a proper self– compacting mix, achieving watertight concrete mass is possible. Also in the case of large projects, developed mix can be tested for permeability to standards such as DIN 1048 and that can be one of the acceptance criteria. While in the case of smaller projects such special self– compacting mix can be supplied by Ready mix producers who can design and control the ingredients.

Following case history of Hercules Harbor in Monaco enables us an insight in one of such successful implementation of this integral watertight concrete concept.

Case History– Hercules Harbor

Client–Gouvernement of Monaco Location of site–Algeciras Spain Contractor–DRAGADOS BEC V Engineering–DORIS Engineering France R & D–Institute Francais du Petriole Norwegian Technical Institute and many others Technology Supplier–BASF CC Spain (BETTOR MBT)

New studies were made in the 1980‘s to protect and extend the existing harbor. The depth of the sea–bed of 55 meters did not allow conventional construction. Further the Government decided to minimise the impact for the environment during construction. Based upon studies made in France and Norway the Monaco Government decided to build a prefabricated “ semi floating seawal l” a technique common in the offshore oil industry.

The structure was 352 metres long, 44 m wide and about 35 m tall; its design required 2,900 MT

of steel cables for stressing and 45,000 of concrete!

As the structure cannot be constructed in-situ and also near by area was not available for precasting yard, Engineers from DORIS Engineering decided to construct in a dry-dock, 1200 km away in Spain. As such long structure cannot be transported on the ship; it required to be floated in sea and to be transported by towing. This required total watertightness of the structure and all the joints it has.

Any leakage at joint or within the concrete mass would make it sunk below the ocean.

Based upon the construction of TROLLE, an off shore platform built in Norway, DORIS Engineering recommended the use of Self–compacting Concrete. To secure the construction joints, Masterflex 900, the re-injectable hose was specified allowing testing joints and injecting and re-injecting with resin where necessary.

After the construction all the joints were injected using Masterflex 801, water based reswelling resin and tested for watertightness.

No form of external waterproofing treatment was carried out for this structure. No membrane or no accidental drill and grouting were implemented.

Finally, the structure was towed in the sea as it can be carried on the ship and was positioned at the final location in 2004.

Conclusion

The new age technologies and quest of civil engineers have lead to solutions which are long lasting. Such implementations make these technologies time tested and real for the rest of the world to get inspired and to implement.

Trends in Concrete Construction

Brajendra Singh, Chief Consultant, Cement Manufacturers’ Association, New Delhi.

Concrete is one of the oldest and most widely used building materials in the world. In one form or another various types of concrete have been used for construction purpose for around 9,000 years by now. Concrete platforms dating back to 7,000 B.C. have been unearthed in West Asia and concrete structures have been found in a 7,000 years old sunken city, discovered off the coast of Gujarat. These are just two examples, taken at random, from hundreds of concrete structures built throughout known human history.

One of the enduring mysteries of all times, is the answer to the question as to how did the ancient Egyptians, who had no machines worth the name, haul up huge limestone blocks weighing over fifteen tonnes, to construct their massive pyramids. This question has for centuries been very widely debated by archeologists, historians and engineers, and several possible answers arrived at. Leaving aside improbable conjectures like the one that the pyramids were constructed by alien beings who visited our planet from outer space, most other theories focus on methods used to quarry the gigantic blocks, transport them to the building sites, shape and polish them so finely—even though no mortar has been used to join the blocks together, they fit so nicely that even a knife blade cannot be slipped between adjacent stones–and finally haul them into position. Most aver that the blocks were chiselled out of hillside rock formations, floated down the Nile on boats or rafts, moved across land using wooden rollers placed below them, and positioned using long sloping ramps. Human slaves, along with elephants, formed the motive power. Although, eminently feasible, this method would have been painfully laborious and slow. Some years back a new theory of pyramid construction was put forth. According to it, there was no question of quarrying, transporting, shaping and polishing of blocks; nor of hauling them into position. This is because, according to this revolutionary theory, there were no limestone blocks to start with. They were, in actual fact, poured in-situ lime concrete blocks. This process, it is argued, would have saved the almost impossible effort required to construct the 4,500 years old pyramids, especially as the Egyptians of that time apparently had no iron tools, now were aware of the invention of the wheel.

As concrete evolved over the ages, it has become quite clear from recent discoveries, that several

‘modern’ varieties of concrete, may not be so modern after all. Take for instance, lightweight concrete. As far back as 83 B.C. Roman architects used lightweight aggregates formed by the cooling of lava, from volcanoes like Etna, Stromboli and Vesuvius, to build the Temple of Fortune in Palestrina, Italy, whose ruins were discovered some time back. Excavations in Italy have also revealed the remanants of large number of other residential and official buildings, made with lightweight concrete, dating back to between the 1st Century B.C. to the 2nd Century A.D.

Another example of a supposedly modern form of concrete which in actual fact was fairly widely used by the ancients, is fireproof concrete. Sometime during the 3rd Century B.C., the buildings of almost the entire city of Rome, were re-built with fireproof concrete. Then, to give them an aesthetic look, they were given a facing of bricks. When Rome’s first Emperor, Augustus Caesar (after whom the month of August is named), nephew of Julius Caesar, took independent charge of the Roman Empire in 32 B.C., he decided that his capital did not look grand enough. So he had marble facades put on every building, so that Rome literally glittered in the sunshine. Four hundred years later, when the Goths under Alaric looted Rome, they set the entire city on fire. The marble facades and brick burned off, but the basic concrete structures of Rome almost all survived

Innovations

Another innovation that originated over 2000 years ago in Rome, was the blending of reddish volcanic earth with lime. This resulted in a fairly unique product–concrete that set under water. Undersea structures built at that time, are still existing today, though most of them are damaged or broken.

Coming to more modern times, concrete was used for making boats, yes you read that right, it is boats, since 1850s, in France. These boats were made by plastering concrete over an iron mesh boatshaped framework. This composite was named as Fericement in early days and Ferrocement later on. It is still being used for making boats, water tanks, house components, irrigation & sanitation item etc. Such boats had many advantages since they were waterproof and leak proof, did not rot and were also almost damage proof. Right till the 1920s, concrete boats and ships, some as big as 132 metres long and 17 metres wide, weighing over 7,500 tonnes, were plying on ocean going routes. Even today, many colleges in USA, organize regular concrete boat races, which are extremely popular with students.

Next on our list, is fibre reinforced concrete. This product too, is not all that modern as, according to available records, the first fibre-reinforced concrete products were bricks, reinforced with straw fibres, which were in wide use some 3000 years ago. And concrete roofs, reinforced with horse-hair, were all the rage around the 3rd Century A.D. Steelfibre reinforced concrete is a more recent product, since the know-how for the manufacture of steel fibres was not available earlier on. Hence steel fibre reinforced concrete only made its appearance in 1874, a mere 132 years ago.

And do you know when Ready Mixed Concrete (RMC) i.e. concrete mixed in a central batching plant and transported to different work sites, first made its appearance. It was in 1903.

Unfortunately, suitable motorized transport for its conveyance, from batching plant to work site, was not available in those days, since the automobile industry was still in its infancy. So, when centrally batched concrete was carried by horse-drawn container vehicle, it often set on the way, as there was almost no knowledge regarding retarding chemicals available at that time. The manufacture and use of RMC was therefore somewhat slow, till about 1914, when the first petrol engine driven RMC trucks made an appearance. Incidentally, 1914 was also the year when the first concrete road was constructed in our country.

Despite all that is mentioned above, modern-day concrete technologists need not feel that “there is nothing new under the sun.” Today, we have a number of innovative usage for concrete, most of which–as far as current knowledge goes–were unknown or even undreamt of, till just a few decades ago. These include flexible concrete, spun concrete, whisper concrete, ultra-thin concrete and even cementless concrete. Some details of these products and processes are given in the succeeding paragraphs.

Flexible Concrete

The term ‘flexible concrete’ seems to be an anomaly, since concrete is generally considered to be inflexible, as in a rigid road pavement. The requirement for a flexible form of concrete has been felt for many years, due to failure of concrete roads and bridge decks, when subject to severe stress by overloaded trucks going across them. In the mid-1990’s, scientists in USA’s University of Michigan, decided to try and design a flexible form of concrete, which would be ductile and elastic. They gave their new product the name of Engineered Cement Composite (ECC) and started carrying out experiments with different trial mixes.

Eventually, after dozens of hits and misses, they produced a fairly satisfactory mix which, after setting, resulted in a concrete that, when overloaded, bent/sagged but did not crack. The mix was similar to a normal concrete mix, except that there were no coarse aggregates in it. Also it contained around two percent fibres, compared to the normal half percent contained in ordinary fibre-reinforced concrete. Additionally, the fibres incorporated in ECC were specially coated ones; this coating allowed the fibres to slide within the concrete, thus imparting flexibility to it.

Concrete produced by ECC techniques, has already been used in projects in several countries, including Australia, Japan, Korea, Switzerland and USA. The latest formula has given an end product that is 40 percent lighter in weight and 500 times more resistant to cracking, than normal concrete. This latest composite concrete has been used for a 5cm ultra-thin deck on a bridge in

Japan. The 40 percent saving in weight has led to significant economies in construction cost especially in the understructure on which the dead load came. An additional bonus that the deck’s flexibility gave, was that there was no requirement for expansion joints–the entire deck slab was a continuous one. This not only provided a smoother ride for motorists using the bridge, but also saved on the bother and cost of joint filler maintenance/ replacement.

Spun Concrete

Columns are vital part of most buildings. Load bearing columns, unfortunately, tend to be large in size. Though large columns can be fashioned and designed artistically, thus giving a pleasing appearance, they often take up vital space and obstruct free movement as well as vital viewability. Pre-stressing columns imparts additional load bearing capacity to them, thus allowing them to be made slimmer in size and permitting larger spacing between them but even then, their size can create problems.

To provide columns with even more load bearing strength, so that their diameter could be further reduced, a new technique has been conceived, which is basically German in origin. This technique results in the production of what is known as spun concrete. The procedure for making columns of spun concrete is roughly as follows. A steel mould in the shape of the column is made, in two halves. The reinforcement cage for the column is also made in two parts. One part is placed in each half of the mould, anchored to fixing devices, which are a part of the mould, and pretensioned. High strength concrete, up to M-100, is then poured into the mould halves. After that the halves are bolted together and placed in a centrifuge.

The mould, with the concrete in it, is then spun for approximately 10 minutes, at 600 rpm. After that, the concrete is left to set, for between 12 to 16 hours, depending on various factors, such as strength required, column size, ambient conditions and so on. The mould is then removed and the column cured, then transported to the construction site. This process produces a very dense, high strength concrete structure. Heavy reinforcement ratios up to 15 percent, have enabled production of 28 metre high columns, having a diameter of only 70 cm, capable of taking loads up to 360 tonnes, by the use of spun concrete.

Whisper Concrete

One major disadvantage of concrete roads is that they are noisy; vehicles traveling on them produce a ‘swishing’ sound, due to the friction between their tyres and the hard road surface. In European countries, where long stretches of concrete highways exists, this irritating ‘swish-swish’ was, and is, the cause of much annoyance for road users, and for those whose houses are situated in the vicinity of concrete roads. So much so that many countries have made it mandatory to construct sound deflecting fences along concrete roads, wherever they pass through residential areas. In fact in UK, construction of concrete road pavements was actually banned for a few years due to noise pollution.

And that is how ‘Whisper Concrete’ came into being, although it was partly by accident.

In late 70’s and early 80’s, despite predictions that the then quantum jump in oil prices would drastically reduce individual usage of vehicles, traffic on concreted European roads increased by leaps and bounds. Simultaneously, there was an increase in vehicular speed, particularly on inter-city highways. This caused a greater wearing action on road surfaces and also an almost unbearable increase in the level of sound being produced. Smoothened pavements, worn down due to excessive wear and tear, led to skidding of vehicles, causing accidents; and noise pollution gave rise to headaches and other soundrelated psychological problems. These troubles were particularly noticeable near and on autobahns, motorways and freeways, where speeds generally exceeded 120 kmph, and between 75,000 to 1,00,000 vehicles traversed the facility every day.

Among the first countries to take cognizance of motorists complaints was Belgium. Since accidents due to skidding (which led to a large number of deaths and serious injuries) caused much more damage than mere noise pollution, priority was given for mitigating causative factors for the former.

Investigations into the causes of skidding, showed that when concrete pavements were initially laid, they were invariably given a non-skid surface by brooming; a method in which the surface of the road had grooves etched into it, by dragging steel-wire brooms across the top of the concrete pavement, before it had hardened fully. These grooves, which were generally made two millimeters deep, imparted excellent anti-skid properties to the road. However, continuous heavy traffic over on extended period of time, caused the ridges between the grooves to get worn down, thus flattening the surface of the pavement. In those days, re-grooving of the road surface,

though eminently feasible, was a somewhat costly and laborious process (new techniques have made it easier to re-groove the top of a concrete pavement today). Hence road maintenance authorities tended to delay the operations, or even give it the complete go-by. So when it next rained, interaction between vehicle tyres and the wet, smooth road surface, produced a phenomenon known as ‘hydroplaning.’ Hydroplaning is a particularly nasty form of skidding, and normally leads to total loss of control of vehicles by drivers.

An accident rates in the country went up and criticism of official apathy mounted, the Belgian authorities started to take action. They began to look for ways and means to restore the anti-skid surface of concrete roads economically and speedily.

Initially, trial lengths of smoothened road surface, were overlaid with 40-50 mm of concrete having a maximum aggregate size of 6-8 mm. The surface of the new concrete, while still wet, was sprayed with a retarder consisting of glucose, water and alcohol; it was then immediately covered securely with polythene sheeting, to prevent evaporation. This particular retarder, as tests had shown, affected only the top 2 mm of the concrete.

Once partial curing of the remaining concrete had taken place (anything between 8 to 36 hours later, depending on the ambient conditions), the polythene sheeting was removed, and the surface of the road was swept with a machine, which had stiff, rotating wire bristle brushes. These rotating brushes removed the cement mortar from the top 1.5 mm of the pavement, thus exposing the aggregate and making the surface rough enough for safe high-speed driving in wet weather.

When vehicles were driven at expressway speeds over these newly made antiskid surfaces, it was found to every ones surprise that, besides being safer to travel on, such exposed-aggregate pavements were much quieter than normal concrete surfaces. In fact, they eventually proved to be even quieter than blacktopped roads.

Further trials were then carried out, with the emphasis now on reduction of the amount of noise pollution being created. These only served to confirm the earlier findings, that the new type of surface was much quieter than any of the other pavements in service. The delighted public works authorities–who had got two benefits for the price of one–soon labeled the exposed-aggregate pavement as ‘whisper’ concrete, and decided to go in for it in a big way.

However, the Belgians soon discovered that along with its advantages, whisper concrete had one fairly serious drawback. Where overlaying of old smoothened concrete pavements was involved, the cost of using whisper concrete was more or less the same as regrooving, but involved much less effort; and where it was laid on an existing worn-out bitumen pavement, whisper concrete

costs matched those of white-topping (re-surfacing of an old blacktopped pavement with thin concrete slabs). But where new roads had to be built, it was found that the pavement had to be constructed in two layers. A lower layer of 200 mm of ‘normal’ concrete, having a maximum aggregate size of 30 mm; and an upper layer of 40-50 mm of whisper concrete, having a maximum aggregate size of 6-8 mm. This double operation increased both time and cost of construction. Nevertheless, Belgian authorities decided that the advantages of whisper concrete far out-weighted its disadvantages. They therefore went on constructing fresh roads and topping existing ones, with the new material. Between 1981 and 1994 eight million cubic metres of whisper concrete was laid down on the country’s roads. Today, CRCP (Continuously Reinforced Concrete Pavement) with an exposed aggregate surface, is the standard form of road construction in Belgium, for all inter–city highways.

After Belgium, whisper concrete was taken up in a big way by neighboring Netherlands. Extremely happy with its performance, but not too pleased in having to build it in two layers, the Dutch carried out some trials of their own. They soon discovered that if the maximum aggregate size in the entire concrete mass was reduced to 20 mm, and a good percentage of small stone chippings added to the mix, whisper concrete pavements could be laid in a single pass. Though driving on such pavements was not as ‘comfortable’ as on two-layer whisper concrete, the noise produced was somewhat less, apart from the considerable saving in time and money since only a single laying operation was involved.

The next European nation to take up the new road building method was Austria. Austria is by and large a mountainous country, with many of its roads running along the lower portions of valleys. Increasing traffic at greater speeds along these arteries, caused noise to roll up the hillsides in waves. This phenomenon started causing ‘adverse political fall-out.’ Fearing loss of votes, worried Government officials, scanned literature, organized conferences and toured Europe, looking for solutions to the problem. It was not long before they discovered whisper concrete. After due trials and deliberations, the Austrians decided to adopt the Belgian two-layer technique of construction, rather than the Dutch single-layer method. This is because in Austria, despite the country’s middle–of–the–Alps location, suitably tough and hard aggregates are extremely costly. Such aggregates are essentially required for that country’s roads because the heavy snowfalls it experiences, means that most vehicles use studded tyres; and such tyres wear out soft aggregates very fast. Hence the Austrians used soft aggregates for the thicker lower layer of their concrete roads, and hard tough aggregates for the thinner, upper whisper concrete layer. Austria’s selection of the twolayer method of construction, proved to be a wise one, because even five years after the initial whisper concrete roads were built, their surfaces showed no signs of wear and tear, despite their regular use by studded tyre traffic.

The British, traditionalists as usual, waited to see the experience of others and then took up the construction of whisper concrete pavements only in 1995. The guidelines provisionally enunciated by them, are probably the most suitable ones for use by those building whisper concrete roads for the first time. These include:

Under standard highway conditions, a concrete road should consist of a cement-bound sub-base, between 150-200 mm thick. On top of this, there should be 200 mm of CRCP, followed by 50 mm of whisper concrete surfacing.

Existing concrete paving trains should be modified to lay the lower CRCP and the upper whisper concrete surface in the same pass.

Full pavement width (even for double-lane roads) on each side of dual carriageway roads, should be laid in a single operation.

Normally, 8 mm size coarse aggregate should be used in the surface layer. Not more than 3 percent of these should be oversized and 10 percent undersized.

These aggregates should posses a polished stone value greater than 60; this will ensure sufficient hardness to combat wear and tear. The aggregates should also have a ‘flakiness’ index less than 25% which will ensure that they have a fairly uniform shape.

Coarse aggregate should form around 60% of the whisper concrete, which should be airentrained. Sand used should be very fine. The cement used should be OPC (Ordinary Portland Cement).

The whisper concrete layer should be initially levelled by a conventional mechanical float with oscillating beams. This should be followed by further levelling by a ‘super smooth’ float, set longitudinally down the carriageway, at right angles to the first float, which should remove any remaining imperfections or ridges.

Spray the smooth finished surface immediately with a retarder consisting of glucose, water and alcohol. Then cover the surface with a polyethylene ‘cling’ film.

Between 8 and 36 hours later (depending on ambient conditions), remove the polyethylene film and brush the surface with mechanically rotating, stiff bristles; to remove cement mortar from the top 1.5 mm.

Properly planned operations should enable construction of about 3000 linear metres of whisper concrete per day.

Ultra-Thin White- Topping

Until 1991, most white topping projects did not purposely seek a bond between the interface of the concrete and the underlying flexible surface. Rather, the existing bitumen served as base for the new concrete overlay. Today, we refer to this technique as “Conventional” or “Classical” white topping, defined as: “A concrete overlay, usually of thickness of 100 mm or more, placed directly on top of an existing bitumen pavement.

However, a new technology emerged in the early 1990’s, which has dramatically expanded white topping technology and its use. This rehabilitation technique purposely seeks to bond the concrete overlay to the existing bitumen. As a result, the concrete overlay and the underlying bitumen act as a composite section rather than two independent layers. This composite action significantly reduces the load-induced stresses in the concrete overlay. Therefore, the concrete overlay can be considerably thinner for the same loading as compared to a white topping section

with no bond to the underlying bitumen.

When describing pavement thickness, terms such as “thick” and “thin” are relative and depend on the viewpoint and experience of the user. For Ultra-Thin White (UTW) topping, a more definitive description is needed. Based on the international experience, ultrathin white topping can be defined as: “A concrete overlay 50 mm to 100 mm thick with closely spaced joints bonded to an existing bitumen pavement.”

There are three basic requirements for UTW overlays to perform properly. These are:

Availability of an appropriately thick existing bitumen layer. Achievement of a bond between the existing bitumen pavement and the UTW. Provision of short joint spacing.

Bonding allows the concrete and bitumen layers to perform as a composite section. This causes the two layers to act monolithically and share the load. With bonding, the neutral axis in the concrete shifts from the middle of the concrete down toward the bottom of the concrete. This shifting lowers the stresses at the bottom of the concrete and brings the stresses into a range that the thin concrete layer can withstand.

The composite section has opposing effects on corner stresses. There is a decrease in the concrete stresses because the whole pavement section is thicker. However, if the neutral axis shifts low enough in the concrete, the critical load location may move from the edge to the corner depending on the materials and layer characteristics. Essentially, the corner stresses decrease because the bonding action creates a thicker section, but increase because the neutral axis shifts down and away from the top surface.

To combat this effect, close joint spacing is critical. All pavement types must absorb the energy of the applied load by either bending or deflecting. Traditional concrete pavements are designed to absorb energy by bending and thus are made thick enough to resist stresses induced by bending. With UTW, short joint spacings are used so that energy is absorbed by deflection instead of bending. The short joint spacing also minimizes stresses due to curling and warping by decreasing the amount of slab that can curl or warp.

For the UTW overlays, the short joint spacing in effect forms a minipaver block system, which transfers loads to the flexible pavement through deflection rather than bending. Typical joint spacings that have performed well on UTW projects are somewhere between 0.6 and 1.5 m. It is recommended that the maximum joint spacing for UTW be between 12-15 times the slab thickness in each direction.

When performing a UTW project, there must be enough bitumen to protect the concrete (minimize stresses), and enough concrete must be placed to protect the bitumen (minimize strains). A thicker bitumen pavement section improves the load-carrying capacity of the system because it creates a thicker final UTW pavement structure, and also carries more of the load. This shifts the neutral axis down in the concrete, which decreases the concrete stresses.

The construction of a UTW consists of three basic steps:

Prepare the existing surface by milling and cleaning, or blasting with water or abrasive material.

Place, finish, and cure the concrete overlay using conventional techniques and materials. Cut saw joints early at prescribed spacings.

A clean surface is required for proper bond. Milling the surface followed by cleaning improves bond because it opens the pore surface of the bitumen pavement. The milling creates a rough surface that “grabs” the concrete and creates the mechanical bond between the two layers. Once a surface is cleaned it is extremely important to keep it clean until paving commences.

Paving a UTW is no different from paving any other concrete pavement. Conventional slip-form and fixed-form pavers, as well as hand-held equipment–such as vibrating screeds–have all been used successfully without major modifications. The only real change is that the concrete layer is thinner than normal. Normal finishing and texturing procedures are applied to the surface.

Proper curing is critical to avoid shrinkage cracking and debonding between the bitumen and concrete pavements. Curing compound should be applied at twice the normal rate, because the overlay being a thin concrete slab, has high surface area to volume ratio, and can thus lose water rapidly due to evaporation. Care must also be used during application, to avoid spraying curing compound on adjacent uncovered prepared bitumen surfaces, since that would decrease bonding.

Joint sawing should be carried out with lightweight saws, as early as possible, to control cracking. Saw depth should be approximately one–fourth to one third of the total depth of the overlay. Typically, UTW joints are not sealed. Test studies have shown that UTW pavements perform well without sealants because the compactness of the slabs minimizes joint movement.

The concrete mix selected for particular project is matched to the traffic conditions and opened-fortraffic requirements. Synthetic fibers are often added to increase the post-crack integrity of the panels.

Ultra-thin White-topping projects have been carried out in several countries including USA, Brazil and Canada. However, the technique is still regarded to be in its infancy and requires considerable research to streamline and standardize it.

The American Concrete Institute issued ‘Supplement Specification 852’ on 11th July 2000, which laid down specifications for ‘Ultra-thin White-topping Overlay with Steel Fiber Reinforced Concrete.’ As far as is known this is the only existing specification on the subject.

Cementless Concrete

In the late 1980s, Austria was facing a shortage of cement, due to several factors. Shortage of suitable quality limestone was one of them. Another was the extremely stringent emission standards for cement manufacturing plants set by the country’s Government, due to concern about the steadily deteriorating environment.

Both the cement and construction industries were worried, and decided to do something to sort out the problem. Discussions, experiments, laboratory and field trials became the order of the day. Eventually, an absolutely new, novel and unique product was developed, after 15 years of intense effort.

The scientists, technologists and others involved in the project, started off by thinking ‘outside the box.’ They decided that they did not want to produce a modified cement, or even an improved version of OPC. They resolved to create an alternative to cement. This was a tall order indeed, but the experiment team was determined to succeed; and succeed they did. Their basic premise was, that although they did not want cement, their alternative binding material, had to have cementitious properties, if they wanted it to take over cement’s role.

By trial and error, they narrowed down their choice of the base material to slag. Austria, located right in the heart of Europe’s biggest steel producing zone, was ideally situated to procure massive quantities of slag, easily and economically. And blast furnace steel slag is a highly cementitious material.

Once the base element had been identified further experiments and trials were carried out to find ways and means to convert it into a suitable, easy-to-use and economical binding agent. Finally it was determined that by blending gypsum, certain alkaline products and a few other additives with slag, they could obtain a substance that had all the binding properties of cement, yet was superior to it in many ways.

The advantages that this new slag-based binder had included:

a. No burning process was involved in its production. Hence emission of carbon-dioxide and nitrous oxides was reduced to almost zero, making it extremely friendly to the environment.

b. It has a very low heat of hydration. Hence it is ideal for mass concrete applications such as dams and foundations. Also, low heat of hydration means almost no cracks in the finished product, hence eminently suitable for water-retaining structures.

c. High resistance of concrete products made from it, to sulphate and acid attack, as well as damage by alkali-reactive aggregates. Thus can be used with great advantage in aggressive environment.

d. Energy saving of up to 80 percent in its manufacture, since this involves only grinding.

The above-mentioned binder is still not in general production, as its composition was finalized only around five years ago. Trials on concrete items and structures manufactured using this binder, are still being carried out.

The author is grateful to the International Cement Review, BFT International and the Indian Cement Review for some of the information contained in the above article.

The Influence of Flyash Addition on Fresh Properties of Silica Fume ConcreteShweta Goyal, Lecturer, Thapar University, Maneek Kumar, Head, Civil Engineering Department, Thapar University, Patiala, Professor Bishwajit Bhattacharjee, Head, Civil Engineering Department IIT Delhi. This paper deals with the effect of granular characteristics of mineral admixtures like silica fume and flyash added in binary or ternary combinations on the water requirement of resultant concrete. The role of superplasticizers in modifying the rheology has been investigated. Superplasticizers are the admixtures that are added to concrete in very small dosages and modify the water requirement of resultant mix and improve fresh properties of concrete.

Measurement of workability is made by slump test and Vee-bee time test in order to have the correlation between the two and amount of compaction achieved is studied by measuring fresh density of concrete. It is found that superplasticizers become necessary with the reduction of water binder ratio and flyash and silica fume affect the fresh concrete in opposite ways. Also, the relation that exists between slump and Vee–bee time for normal concrete without superplasticizers does not remain valid for concrete having mineral admixtures and superplasticizers.

Introduction

The use of high range water reducers (superplasticizers), condensed silica fume and other fine mineral admixtures have lead to the production of high-strength concrete [1]. Mineral admixtures are used in order to increase strength and improve durability of concrete. Blast furnace slag, flyash and silica fume are some of the mineral admixtures used in varying proportions to achieve the desired results. The mineral admixtures also affect the properties in fresh state, which are directly related to development of strength and durability of hardened concrete. Economics (not always) and environmental considerations have also had a role in the growth of mineral admixture usage.

Much research has been conducted for improving both fresh and hardened properties by using various mineral admixtures. It is reported that fly ash contributes to increase flowability in the fresh state, a dense microstructure and develop higher mechanical properties at the later stage due to the pozzolanic reaction [2,3]. Silica fume, on the other hand, has very fine particles–average particle size is less than 1Fm, which decreases the flowability in fresh state of concrete although, provides a dense microstructure and improved mechanical properties at early stages due to fast pozzolanic reaction [4, 5]. Silica fume is considered to be most efficient in contributing towards both early and later age properties of concrete. However, in India, silica fume comes under the category of costly materials, whereas flyash is abundant in our country and its production is increasing day by day. In the study undertaken, silica fume and flyash are used in combination to see the effect on improvement in fresh properties.

It is widely known that better fluidity is achieved by addition of superplasticizer. The increase of superplasticizer in concrete began in 1960s and has proved to be a milestone in concrete technology and in the field of construction [6]. There is no doubt that the use of admixtures had a profound impact on the concrete practices in India during the last few years [7]. The superplasticizer is adsorbed on the cement particles, which deflocculates and separate, releasing trapped water from cement flocks [8]. Currently available superplasticizers are micro molecular organic agents which are often divided into four groups according to their chemical contents as sulphonate melamine formaldehyde, sulphonate napthalene formaldehyde, modified lignosulphonates and copolymers containing sulphonic and carboxyl groups [9]. The family of superplasticizers based on polycarboxylic products is more recent (1980s). These materials are of higher reactivity; they do not contain the sulphonic group and are totally ionized in alkaline environment. These do not have the side effect of delaying the curing of concrete [10]. In the present study, poly-carboxylic group based superplasticizer is used as a chemical admixture.

It is believed that admixtures mainly affect the flow behavior of cement paste and do not alter the behavior of aggregates. Therefore, in most of the studies on concrete rheology and selection of chemical admixtures, tests on cement pastes have been conducted [1, 3, 8]. The results are then related to concrete workability. Unfortunately, the relation between cement paste rheology and concrete rheology has never been completely established [11]. The main reason behind it is that cement rheology is typically measured under conditions that are never experienced by cement paste in concrete. The values that are usually reported in literature do not take into account the contribution of aggregates [12]. The aggregates act as heat sink and shear the cement paste during mixing process. Therefore, in order to predict concrete rheology accurately, the tests are directly conducted on concrete. For this, one of the most commonly used methods for measuring concrete workability, i.e. slump cone test, is used.

Slump cone test is the typically quantified field test for measuring concrete workability. However, in a survey conducted by National Ready Mix Concrete Association (NRMCA) and the National Institute of Standards and Technology (NIST) [13], it is determined that slump cone is not representative of the ease of handling high performance concrete in field, because in slump cone test, concrete does not undergo the same treatment as is met in the field. Therefore, along with the slump cone test, Vee–bee time is also noted, because in this test, concrete experience almost same vibrations as experienced in field.

The objective of the study is to look at the rheological characteristics of concrete which has silica fume and fly ash present either as binary or ternary combination with ordinary Portland cement. Secondly, the validity of existing relation between slump and Vee–bee time is checked for the mineral admixture concrete containing superplasticizers.

Materials

Cementitious materialASTM Type I Portland cement is used in this study. Its chemical composition is given in Table 1. The chemical and physical characteristics of two mineral admixtures silica fume and flyash can be seen in this table.

Aggregates

Crushed granite with a maximum nominal size of 10 mm was used as coarse aggregate and natural riverbed sand confirming to Zone II with a fineness modulus of 2.52 was used as fine aggregate. The properties of aggregates are listed in Table 2.

Superplasticizer

Poly-carboxylic group based superplasticizer, Structro 100 (a product of Fosroc chemicals), is used throughout the investigation. This group maintains the electrostatic charge on the cement particles and prevents flocculation by adsorption on the surface of cement particles [14]. It is a light yellow colored liquid complying with requirements of IS 9103 – 79, BS 5075 Part III and ASTM – C494 Type F. The specific gravity of superplasticizer is 1.2 and solid content is 40 percent by mass.

Mixture Details and Preparations

To explore the effect of superplasticizer, the rheological properties are studied for three water binder ratios: 0.25, 0.35 and 0.45. The three series obtained from three water binder ratios are designated as M1, M2 and M3 respectively for water binder ratios of 0.25, 0.35 and 0.45. The quantity of mineral admixtures is varied from 0 to 30 percent and is used either in a binary or a ternary combination. The mix designs used in the study are shown in Figure 1 and the mix details of specimens are listed in Table 3 and Table 4.

The mix preparation is very important because it influences its rheological behavior. The following procedure was adopted for mixing.

The cementitious materials (Portland cement, silica fume and flyash) were mixed together separately in a container. Coarse aggregates and the fine aggregates were mixed in a mixer rotated at slow speed of about 140 rev./min. for 1 minute. The cementitious material was then put in the mixing drum and the resultant mixture was dry mixed for one minute followed by addition of half of the total water content during the next one-minute mixing. The remaining water along with superplasticizer was then added and mixed at high speed of about 285 revolutions per minute for 1.5 minutes or till the uniform and homogeneous mix is achieved. (Superplasticizer was taken as percentage by mass of binder which included cement, silica fume

and flyash if any present. Water content of superplasticizer was taken into account when calculating the total water content of the mix [15].)

The prepared mix is used for obtaining slump and Vee–bee time. In all 24 mixes are prepared and three determinations of slump and Vee–bee time are made for each sample and the mean value is taken. It is worth mentioning at this stage that for the selected dose of superplasticizer, no segregation was observed at any stage.

Results and Discussions

For each of the mix, the superplasticizer dose is given step increments and the corresponding Vee–bee time and slump is noted. The saturation point is obtained from the slump verses superplasticizer dosage curves; and is taken as that value of superplasticizer beyond which it will not increase the slump with any further increase in dosage. (In other words, superplasticizer has no further plasticising effect). The results of these tests are presented in Figure 2 to 4 where slump is plotted against superplasticizer dosage and in Figure 5 where optimum superplasticizer dosage is plotted against water binder ratio. The nomenclature of mixes used is already presented in Table 4. The results are discussed as below.

Effect on Mineral Admixtures on Rheological Properties

The effect of the addition of a mineral admixture is detected by an increase in the slump or a reduction of water content or a reduction of superplasticizer dosage needed to obtain the same slump. The results are represented in Figure 2 to 4 in which the variation of slump is plotted as a function of superplasticizer dosage for three series of water binder ratios studied.

(a) OPC – SF System

For the same water binder ratio, with increase in silica fume content in concrete, the value of lump decreases and hence the optimum superplasticizer dosage increases, which can be attributed to high specific surface of silica fume with an average particle size of 0.1Fm. However, this is not the sole factor affecting the increase in superplasticizer demand for silica fume mixture. Long with the high specific surface area, the particles of silica fume are chemically highly reactive and have affinity for multilayer adsorption of superplasticizer molecules, which is also supported by other researchers [16, 17]. As a result, with increase in silica fume percentage, the quantity of superplasticizers in the concrete system decreases leading to steep increase in the superplasticizer dosage. The same type of behavior is observed for entire water binder range with an exception for water binder ratio of 0.25. At this ratio, with the addition of 5% silica fume, the optimum dosage of superplasticizer decreased by a small amount from 4% (for control mix) to 3.75%. This reverse trend can be explained by considering the dispersion action of flocculated cement particles by silica fume particles in combination with superplasticizer. Actually, the effectiveness of superplasticizer is enhanced in the presence of silica fume [18]. Similar observation is also made in some previous studies also [19, 20].

(b) OPC – FA System

The addition of flyash has just the opposite effect on the mix properties in terms of workability and optimum dosage of superplasticizer as compared to silica fume. With incorporation of flyash, the water demand and hence optimum percentage of superplasticizer required reduce as compared to the control mix without mineral admixtures for all water binder ratios studied. The reduction in water demand of concrete caused by the presence of flyash is ascribed to its

spherical shape, which reduces the frictional forces among the angular particles of OPC, called ball – bearing effect [21]. These spherical particles easily roll over one another, reducing inter-particle friction. The spherical shape also minimizes the particle’s surface to volume ratio, resulting in low fluid demands. Also, due to the electrical charges, the fine flyash particles become adsorbed on the surface of cement particles, which thus become deflocculated, reducing the water demand [22]. In other way, the effect of flyash can be considered similar to the action of superplasticizer

(c) PC–SF–FA System

From the above discussion, it can be stated that flyash act improves flowability and silica fume has a reverse effect, when added individually. Thus, it is thought that when used in combination, the beneficial effect of flyash on fluidity is used to compensate the loss of slump with silica fume addition. As expected, when the different combinations of silica fume and flyash are used, the slump values were higher and optimum superplasticizer dosage was lower in comparison with the corresponding mixes having only silica fume. The slump obtained increased with increase in flyash content in the mix and decreased with increase in silica fume content. For all the three water binder ratios, TC2 gave least superplasticizer dosage while MC3 gave maximum superplasticizer dosage. Thus, it can be said that the addition of flyash led to the production of economical mixes with greater workability.

Effect of Water Binder Ratio on Optimum Superplasticizer Dosage

Figure 5 shows the results of optimum superplasticizer dosage obtained for all mixes at various water binder ratios. From the figure, it is observed that as the water binder ratio decreases, the optimum dosage of superplasticizer increases. With the decrease in water binder ratio, more number of superplasticizer molecules are required for adsorption on the surface of cement and mineral admixture particles to increase the fluidity of the mix. The optimum dosage increases sharply as the water binder ratio is decreased from 0.35 to 0.25 as compared to the shift from 0.45 to 0.35. For example, in the control mix, the optimum superplasticizer dosage increased from 1.25% to 4% as the water binder ratio is decreased from 0.45 to 0.35. This is because at very low water – binder ratio, cement particles are very close and to overcome inter particle friction and inter particle forces of attraction, higher optimum dose of superplasticizer is required.

Relation Between Slump and Vee-bee Time

In order to formulate a relation between slump and Vee–bee time for mineral admixture concrete, Vee–bee time test is also conducted simultaneously to slump test. Figure 6 shows the graph for slump with Vee– bee time. In the graph, the doted line shows the approximate relationship between slump and Vee–bee time for the normal ordinary Portland cement concrete without using superplasticizers and the solid line is the best fit obtained for the test results in the present study. The marked shift of the present curve for concrete containing mineral admixtures and superplasticizers from the existing curve for normal concrete can be observed from the graph. For higher values of Vee–bee time, the amount of slump required is almost same from both the curves. However, when the Vee–bee time is lesser than 5 seconds, the difference in the values of slumps obtained from the two curves differ in the range of 20 to 50 mm. Since Vee–bee time is the representative of actual compaction in the field, it can be said that for equal compaction, the mixes with admixtures require 20 to 50 mm higher slump than the mix containing Portland cement only. This shift in the curve can be due to the effect of cohesive nature of the mix with silica fume, flyash and superplasticizers.

Effect of Mineral Admixtures on Fresh Density

In order to study the effect of mineral admixtures of superplasticizer on the degree of compaction achieved, fresh density of final mixes were also determined and the same is presented in Table 5. The fresh density of all the mixes lies in the similar range, although the mixes with flyash have a density somewhat higher than the other mixes which can again be due to ball bearing effect of flyash.

Conclusion

On the basis of the studies carried out, it can be concluded that in the binary system, silica fume increases the superplasticizer demand at a constant workability due to its high surface area and its strong affinity for multi— layer adsorption of superplasticizer molecules. Flyash addition, on

the other hand, decreases the water demand and hence optimum percentage of superplasticiser for constant workability due to its ball – bearing effect that reduces frictional forces among binder particles. Also, due to the electrical charges, the fine flyash particles become adsorbed on the surface of cement particles, which thus become deflocculated, reducing the water demand. Three-component system is much preferred for high performance concrete because in it, silica fume act as a filler and flyash controls rheology.

The existing relationship between slump and Vee –bee time changes with the addition of mineral admixtures and superplasticizer. For equal compaction, the mixes with admixtures require 20 to 50 mm higher slump than the mix containing Portland cement only.

Acknowledgments

This research is supported by the Department of Science and Technology Grant. The authors would like to acknowledge the authorities concerned for its assistance in carrying out the research. References

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22. Helmuth R. (1987); ‘Fly ash in cement and concrete,’ PCA, Skokie, Ill. pp 203.

Use of RECYCLED AGGREGATES In CONCRETE- A Paradigm ShiftS. K. Singh, Scientist, Structural Engineering Division, Central Building Research Institute, Roorkee and P. C. Sharma, Head ( Retd.), Material Sciences, SERC,(G) & Editor, New Building Materials & Construction World, New Delhi, Chairman, Indian Concrete Instt. UP Gaziabad Centre.

One of the major challenges of our present society is the protection of environment. Some of the important elements in this respect are the reduction of the consumption of energy and natural raw materials and consumption of waste materials. These topics are getting considerable attention under sustainable development nowadays. The use of recycled aggregates from construction and demolition wastes is showing prospective application in construction as alternative to primary (natural) aggregates. It conserves natural resources and reduces the space required for the landfill disposal.

This paper presents the experimental results of recycled coarse aggregate concrete and results are compared with the natural crushed aggregate concrete. The fine aggregate used in the concrete, i.e. recycled and conventional is 100 percent natural. The recycled aggregate are collected from four sources all demolished structures. For both types of concrete i.e. M-20 and M-25, w/c ratio, maximum size of aggregate and mix proportion are kept constant.

The development of compressive strength of recycled aggregate concrete at the age of 1,3,7,14,28, 56, and 90 days; the development of tensile & flexural strength at the age of 1,3,7,14 and static modulus of elasticity at the age of 28 days are investigated. The results shows the compressive, tensile and flexural strengths of recycled aggregate are on average 85% to 95% of

the natural aggregate concrete. The durability parameters are also investigated for recycled aggregate concrete and are found to be in good agreement with BIS specifications.

Introduction

Any construction activity requires several materials such as concrete, steel, brick, stone, glass, clay, mud, wood, and so on. However, the cement concrete remains the main construction material used in construction industries. For its suitability and adaptability with respect to the changing environment, the concrete must be such that it can conserve resources, protect the environment, economize and lead to proper utilization of energy. To achieve this, major emphasis must be laid on the use of wastes and byproducts in cement and concrete used for new constructions. The utilization of recycled aggregate is particularly very promising as 75 per cent of concrete is made of aggregates. In that case, the aggregates considered are slag, power plant wastes, recycled concrete, mining and quarrying wastes, waste glass, incinerator residue, red mud, burnt clay, sawdust, combustor ash and foundry sand. The enormous quantities of demolished concrete are available at various construction sites, which are now posing a serious problem of disposal in urban areas. This can easily be recycled as aggregate and used in concrete. Research & Development activities have been taken up all over the world for proving its feasibility, economic viability and cost effectiveness.

An investigation conducted by the environmental resources ltd. (1979) for European Environmental commission (EEC) envisages that there will be enormous increase in the available quantities of construction and demolition concrete waste from 55 million tons in 1980 to 302 million tons by the year 2020 in the EEC member countries. As a whole, the safety and environment regulations are becoming stringent, demand for improvement in techniques & efficiency of the past demolition methods is getting pronounced. Special rules and regulations concerning the demolition have already been introduced in several countries like U.K., Holland and Japan.

The main reasons for increase of volume of demolition concrete / masonry waste are as follows:-

i. Many old buildings, concrete pavements, bridges and other structures have overcome their age and limit of use due to structural deterioration beyond repairs and need to be demolished;

ii. The structures, even adequate to use are under demolition because they are not serving the needs in present scenario;

iii. New construction for better economic growth;iv. Structures are turned into debris resulting from natural disasters like earthquake, cyclone

and floods etc.v. Creation of building waste resulting from manmade disaster/war.

In study conducted by authors for RCC buildings, the approximate percentage of various construction materials in demolition waste is presented in Fig. 1. This may vary depending upon the type of structure.

In many densely populated countries of Europe, where disposal of debris problem is becoming more and more difficult, the recycling of demolition waste has already been started. As per the survey conducted by European Demolition Association (EDA) in 1992, the several recycling plants were operational in European countries such as 60 in Belgium, 50 in France, 70 in the Netherlands, 120 in United Kingdom, 220 in Germany, 20 in Denmark and 43 in Italy. The recycling of construction & demolition waste becomes easy & economical, wherever combined project involving demolition and new construction are taken up simultaneously. The possible uses of construction and demolition wastes are given in Table 1.

Recycling and Reuse of Construction & Demolition Wastes in Concrete

The recycling and reuse of construction & demolition wastes seems feasible solution in rehabilitation and new constructions after the natural disaster or demolition of old structures. This becomes very important especially for those countries where national and local policies are stringent for disposal of construction and demolition wastes with guidance, penalties, levies etc. A typical lay out plan of recycling plant for construction waste has been shown in Figure. 2. The properties of recycled aggregate concrete obtained by various authors are given in Table2.

International Status

The extensive research on recycled concrete aggregate and recycled aggregate concrete (RAC) as started from year 1945 in various part of the world after second world war, but in a fragmented manner. First effort has been made by Nixon in 1977 who complied all the work on recycled

aggregate carried out between 1945-1977 and prepared a state-of-the-art report on it for RILEM technical committee 37-DRC. Nixon concluded that a number of researchers have examined the basic properties of concrete in which the aggregate is the product of crushing another concrete, where other concentrated on old laboratory specimens. However, a comprehensive state-of-the-artdocument on the recycled aggregate concrete has been presented by Hansen & others in 1992 in which detailed analysis of data has been made, leading towards preparation of guidelines for production and utilization of recycled aggregate concrete.

It has been estimated that approximately 180 million tones of construction & demolition waste are produced each year in European Union. In general, in EU, 500 Kg of construction rubble and demolition waste correspond annually to each citizen. Indicatively 10% of used aggregates in UK are RCA, whereas 78,000 tons of RCA were used in Holland in 1994. The Netherland produces about 14million tons of buildings and demolition wastes per annum in which about 8 million tons are recycled mainly for unbound road base courses.

The 285 million tons of per annum construction waste produced in Germany, out of which 77 million tons are demolition waste. Approximately 70% of it is recycled and reused in new construction work. It has been estimated that approximately 13 million tons of concrete is demolished in France every year whereas in Japan total quantity of concrete debris is in the tune of 10-15 million tons each year. The Hong Kong generates about 20 million tons demolition debris per year and facing serious problem for its disposal.

USA is utilizing approximately 2.7 billion tons of aggregate annually out of which 30-40% are used in road works and balance in structural concrete work. A recent report of Federal Highways Administration, USA refers to the relative experience from European data on the subject of concrete and asphalt pavement recycling as given in Table 3.The rapid development in research on the use of RCA for the production of new concrete has also led to the production of concrete of high strength/performance.

Indian Status

There is severe shortage of infrastructural facilities like houses, hospitals, roads etc. in India and large quantities of construction materials for creating these facilities are needed. The planning Commission allocated approximately 50% of capital outlay for infrastructure development in successive 10th & 11th five year plans. Rapid infrastructural development such highways, airports etc. and growing demand for housing has led to scarcity & rise in cost of construction

materials. Most of waste materials produced by demolished structures disposed off by dumping them as land fill. Dumping of wastes on land is causing shortage of dumping place in urban areas. Therefore, it is necessary to start recycling and re-use of demolition concrete waste to save environment, cost and energy.

Central Pollution Control Board has estimated current quantum of solid waste generation in India to the tune of 48 million tons per annum out of which, waste from construction industry only accounts for more than 25%. Management of such high quantum of waste puts enormous pressure on solid waste management system.

In view of significant role of recycled construction material and technology in the development of urban infrastructure, TIFAC has conducted a techno-market survey on ‘Utilization of Waste from Construction Industry’ targeting housing /building and road segment. The total quantum of waste from construction industry is estimated to be 12 to 14.7 million tons per annum out of which 7-8 million tons are concrete and brick waste. According to findings of survey, 70% of the respondent have given the reason for not adopting recycling of waste from Construction Industry is “Not aware of the recycling techniques” while remaining 30% have indicated that they are not even aware of recycling possibilities. Further, the user agencies/ industries pointed out that presently, the BIS and other codal provisions do not provide the specifications for use of recycled product in the construction activities.

In view of above, there is urgent need to take following measures:-

Sensitization/ dissemination/ capacity building towards utilization of construction & demolition waste.

Preparation and implementation of techno-legal regime including legislations, guidance, penalties etc. for disposal of building & construction waste.

Delineation of dumping areas for pre-selection, treatment, transport of RCA. National level support on research studies on RCA. Preparation of techno-financial regime, financial support for introducing RCA in

construction including assistance in transportation, establishing recycling plant etc. Preparation of data base on utilization of RCA. Formulation of guidelines, specifications and codal provisions. Preparation of list of experts available in this field who can provide knowhow and

technology on totality basis. Incentives on using recycled aggregate concrete-subsidy or tax exemptions.

Realising the future & national importance of recycled aggregate concrete in construction, SERC, Ghaziabad had taken up a pilot R&D project on Recycling and Reuse of Demolition and Construction Wastes in Concrete for Low Rise and Low Cost Buildings in mid nineties with the aim of developing techniques/ methodologies for use recycled aggregate concrete in construction. The experimental investigations were carried out in Mat Science laboratory and Institutes around Delhi/GBD to evaluate the mechanical properties and durability parameters of recycled aggregate concrete made with recycled coarse aggregate collected from different sources. Also, the suitability in construction of buildings has been studied.

The properties of RAC has been established and demonstrated through several experimental and field projects successfully. It has been concluded that RCA can be readily used in construction of low rise buildings, concrete paving blocks & tiles, flooring, retaining walls, approach lanes, sewerage structures, subbase course of pavement, drainage layer in highways, dry lean concrete(DLC) etc. in Indian scenario. Use of RCA will further ensure the sustainable development of society with savings in natural resources, materials and energy.

Experimental Investigations

In the present paper, an endeavor is made so as to compare some of the mechanical properties of recycled aggregate concrete (RAC) with the natural aggregate concrete (NAC). Since the enormous quantity of concrete is available for recycling from demolished concrete structures, field demolished concrete is used in the present study to produce the recycled aggregates. The concrete debris were collected from different (four) sources with the age ranging from 2 to 40 years old and broken into the pieces of approximately 80 mm size with the help of hammer & drilling machine. The foreign matters were sorted out from the pieces. Further, those pieces were crushed in a lab jaw crusher and mechanically sieved through sieve of 4.75 mm to remove the finer particles. The recycled coarse aggregates were washed to remove dirt, dust etc. and collected for use in concrete mix. The fine aggregate were separated out, and used for masonry mortar & lean concrete mixes, which is not part this reported study. But these were found to suit for normal brick masonary mortar and had normal setting and enough strength for masonary work.

Concrete Mixes

The two different mix proportions of characteristic strength of 20 N/ mm2 (M 20) and 25 N/mm2 (M 25) commonly used in construction of low rise buildings are obtained as per IS 10262 – 1982 or both recycled aggregate concrete and natural aggregate concrete. Due to the higher water absorption capacity of RCA as compared to natural aggregate, both the aggregates are maintained at saturated surface dry (SSD) conditions before mixing operations. The proportions of the ingredients constituting the concrete mixes are 1:1.5:2.9 and 1:1.2:2.4 with water cement ratio 0.50 & 0.45 respectively for M-20 & M-25 grade concrete. The ordinary Portland cement of 43 grade and natural fine aggregates (Haldwani sand) are used throughout the casting work. The maximum size of coarse aggregate used was 20 mm in both recycled and natural aggregate concrete.

The total two mixes were cast using natural aggregate and eight mixes were cast using four type of recycled aggregate concrete for M-20 & M-25. The development of compressive strength is monitored by testing the 150-mm cubes at 1, 3, 7, 14, 28, 56 and 90 days. In one set 39 cubes were cast for each mix. The cylinder strength and corresponding strain & modulus of elasticity were measured in standard cylinder of 150x300 mm size at the age of 28 days. The prism of size 150x150x700 mm and cylinder of size 150x300mm were cast from the same batches to measure Flexural strength and splitting tensile strength respectively. This paper reports the results of experimental investigations on recycled aggregate concrete.

Properties of Recycled Concrete Aggregate

Particle Size Distribution

The result of sieve analysis carried out as per IS 2386 for different types of crushed recycled concrete aggregate and natural aggregates. It is found that recycled coarse aggregate are reduced to various sizes during the process of crushing and sieving (by a sieve of 4.75mm), which gives best particle size distribution. The amount of fine particles (<4.75mm) after recycling of demolished were in the order of 5-20% depending upon the original grade of demolished concrete. The best quality natural aggregate can obtained by primary, secondary & tertiary crushing whereas the same can be obtained after primary & secondary crushing incase of recycled aggregate. The single crushing process is also effective in the case of recycled aggregate.

The particle shape analysis of recycled aggregate indicates similar particle shape of natural aggregate obtained from crushed rock. The recycled aggregate generally meets all the standard requirements of aggregate used in concrete.

Specific Gravity and Water Absorption

The specific gravity (saturated surface dry condition) of recycled concrete aggregate was found from 2.35 to 2.58 which are lower as compared to natural aggregates. Since the RCA from demolished concrete consist of crushed stone aggregate with old mortar adhering to it, the water absorption ranges from 3.05% to 7.40%, which is relatively higher than that of the natural aggregates. The Table 4 gives the details of properties of RCA & natural aggregates. In general, as the water absorption characteristics of recycled aggregates are higher, it is advisable to maintain saturated surface dry (SSD) conditions of aggregate before start of the mixing operations.

Bulk Density

The rodded & loose bulk density of recycled aggregate is lower than that of natural aggregate except recycled aggregate-RCA4, which is obtained from demolished newly constructed culvert. Recycled aggregate had passed through the sieve of 4.75mm due to which voids increased in rodded condition. The lower value of loose bulk density of recycled aggregate may be attributed to its higher porosity than that of natural aggregate.

Crushing and Impact Values

The recycled aggregate is relatively weaker than the natural aggregate against mechanical actions. As per IS 2386, the crushing and impact values for concrete wearing surfaces should not exceed 45% and 50% respectively. The crushing & impact values of recycled aggregate satisfy the BIS specifications except RCA2 type of recycled aggregate for impact value as originally it is low grade rubbles.

Compressive Strength

The average compressive strengths cubes cast are determined as per IS 516 using RCA and natural aggregate at the age 1, 3, 7, 14, 28, 56 and 90 days and reported in Table 5. The table 4 shows that the target cube strength was achieved at 28 days for all types of concrete. As expected, the compressive strength of RAC is lower than the conventional concrete made from similar mix proportions. The reduction in strength of RAC as compare to NAC is in order of 2- 14% and 7.5 to 16% for M-20 & M-25 concretes respectively. The amount of reduction in strength depends on parameters such as grade of demolished concrete, replacement ratio, w/c ratio, processing of recycled aggregate etc.

Splitting Tensile & Flexural Strength

The average splitting tensile and flexural of recycled aggregate are determined at the age 1, 3, 7, 14, & 28 days varies from 0.30 -3.1 MPa and 0.95- 7.2 MPa respectively. The reduction in splitting and flexural strength of RAC as compared to NAC is in order of 5-12% and 4 -15% respectively.

Modulus of Elasticity

The static modulus of elasticity of RAC has been reported in Table 4 and found lower than the AC. The reduction is up to 15% .The reason for the lower static modulus of elasticity of RCA is higher proportion of hardened cement paste. It is well establish that Ec depends on Ec value of coarse aggregate, w/c ratio & cement paste etc. The modulus of elasticity is critical parameter for designing the structures, hence more studies are needed.

Durability

The following parameters were studied to assess the influence of recycled aggregates on durability of concrete:

Carbonation

Freeze-Thaw Resistance

Carbonation

CO2 from the air penetrates into the concrete by diffusion process. The pores (pore size>100nm) in the concrete in which this transport process can take place are therefore particularly crucial for the rate of carbonation. The carbonation tests were carried out for 90 days on the specimens (150x150x150mm) of recycled aggregate concrete and natural aggregate concrete in carbonation chamber with relative humidity of 70% and 20% CO2 concentration. The carbonation depths of recycled aggregate concretes for different grade were found from 11.5 to 14mm as compared to 11mm depth for natural aggregate concrete. This increase in the carbonation depth of RAC as compared to NAC, attributed to porous recycled aggregate due to presence of old mortar attached to the crushed stone aggregate.

Freeze-Thaw Resistance

In the freeze-thaw resistance test (cube method), loss of mass of the concrete made with recycled aggregate was found sometimes above and below than that of concrete made with natural aggregate. The results were so close that no difference in freeze thaw resistance (after 100 cycles) could be found. The literature also found that the effect of cement mortar adhering to the original aggregate in RAC may not adversely affect the properties of RAC.

Obstacles in Use of RCA & RAC

The acceptability of recycled aggregate is impeded for structural applications due to the technical problems associated with it such as weak interfacial transition zones between cement paste and aggregate, porosity and transverse cracks within demolished concrete, high level of sulphate and chloride contents, impurity, cement remains, poor grading, and large variation in quality.

Although, it is environmentally & economically beneficial to use RCA in construction, however the current legislation and experience are not adequate to support and encourage recycling of construction & demolished waste in India. Lack of awareness, guidelines, specifications, standards, data base of utilization of RCA in concrete and lack of confidence in engineers, researchers and user agencies is major cause for poor utilization of RCA in construction. If the Govt wishes these obstacles can easily be removed.

Conclusion

Recycling and reuse of building wastes have been found to be an appropriate solution to the problems of dumping hundred of thousands tons of debris accompanied with shortage of natural aggregates. The use of recycled aggregates in concrete prove to be a valuable building materials in technical, environment and economical respect

Recycled aggregate posses relatively lower bulk density, crushing and impact values and higher water absorption as compared to natural aggregate. The compressive strength of recycled aggregate concrete is relatively lower up to 15% than natural aggregate concrete. The variation also depends on the original concrete from which the aggregates have been obtained. The durability parameters studied at SERC(G) confirms suitability of RCA & RAC in making durable concrete structures of selected types.

There are several reliable applications for using recycled coarse aggregate in construction. However, more research and initiation of pilot project for application of RCA is needed for modifying our design codes, specifications and procedure for use of recycled aggregate concrete. The subject of use of RCA in construction works in India should be given impetus, because of big infrastructural projects are being commissioned including Common Wealth Games in 2010.

References

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8. Gottfredsen, F.R. and Thogerson,F. (1994), “Recycling of Concrete in Aggressive Environment,” Demolition and Reuse of Concrete and Masonry; Rilem Proceeding 23, E & FN Spon, pp. 309-317.

9. Hansen, T.C. (1986) “Recycled Aggregate and Recycled Aggregate Concrete, Seocnd state of Art Report, Development 1945–1985,” Rilem TC-DRC, Material & Structure, Vol. 19, No. III. pp. 201- 248.

10. Hendricks, Ch.F. (1996), “Recycling and Reuse as a Basis of Sustainable Development in Construction Industry,” Concrete for Environment, Enhancement and Protection, E&FN Spon, pp. 43-54.

11. Kikuchi, M. and Yasunaga, A. (1994), “The Total Evaluation of Recycled Aggregate and Recycled Concrete” Demolition and Reuse of Concrete and Masonry, Rilem Proceedings 23, E&FN Spon, pp. 367-377.

12. Lauritzen, E.K. (1994), “Introduction,” Disaster Planning, Structural Assessment, Demolition and Recycling, Rilem Report No. 9, E&FN Spon pp.1 –10.

13. Mc Laughliu, J. (1993), “A Review of the Prospect for Greater Use of Recycled and Secondary Aggregate in Concrete,” Concrete, The Concrete Society Journal, Vol. 27, NO. 6,pp. 16-18.

14. Merlet, J.D. and Pimienta, P. (1994), “Mechanical and Physico- Chemical Properties of Concrete Produced with Coarse and Fine Recycled Concrete Aggregates,” Demolition and Reuse of Concrete and Masonry, Rilem Proceeding 23, E&FN Spon, pp. 343-353.

15. Nikon, P.J. (1986), “Recycled Concrete an Aggregate for Concrete–a Review,” Rilem TC-37, DRC, Materials Structures, Vol. 19, No. 111.

16. Pauw, C.D. (1994), “Reuse of Building Materials and Disposal of Structural Waste Material,” Disaster Planning, Structural Assessment, Demolition and Recycling, Rilem Report 9, E&FN Spon, pp. 133-159.

17. RILEM TC 121 DRG Recommendation (1994), “Specification for Concrete with Recycled Aggregates,” Materials and Structure, Vo. 27, No. 173, pp. 557- 559.

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29. An Application of Operating Characteristic Curves in Online Strength Monitoring of Ready Mixed Concrete30. Debasis Sarkar, Lecturer CPM, CEPT University Navrangpura, Ahmedabad, Dr. Manish

Thaker, Lecturer in Statistics, Statistics Department M.G. Science Institute, Navrangpura, Ahmadabad.

31. The operating characteristic (OC) curves measure the performance of a sampling plan. This can be used to find out the producer’s risk and consumer’s risk. In context to commercial ready mixed concrete plants, the producer’s risk is associated with the risk of a good quality concrete being rejected by the client and the consumer’s risk is associated with the risk of accepting a poor quality concrete. Online quality monitoring deals with the monitoring techniques applied during the production of the ready mixed concrete in the RMC plants. 28 days cube compressive strength data of concrete grades have been collected from RMC plants in and around Ahmedabad and attempts have been made to investigate the producer’s risk and consumer’s riskwhich would enable RMC producers and consumers to assure quality levels.

32. Quality monitoring of ready mixed concrete (RMC) has to be carried out throughout its production process. There are various techniques for online monitoring of the concrete , namely (i) Control charts–Cusum Control charts, Schewart Control charts, EWMA Control charts (ii) Acceptance Sampling (iii) British Ready Mixed Concrete Association (BRMCA) concrete control system. However, if the RMC producers in our country adopts any of the above monitoring techniques, it would enable them to produce and sell quality product at reasonable prices. The operating characteristic curves which measure the performance of a sampling plan can be utilized for finding out the producer’s risk (associated with the risk of a good quality concrete being rejected by the client) and consumer’s risk (associated with the risk of accepting a poor quality concrete). In this paper, an attempt has been made to plot the OC curves for various grades of concrete collected from the RMC plants in and around Ahmedabad. The collected 28 days cube strength data for the various concrete grades produced by these RMC plants have been utilized in finding out the producer’s risk, average quality level, consumer’s risk and limiting quality level from the OC curves plotted. These parameters would provide adequate information about assuring the quality levels for RMC producers and consumers.

33. Acceptance Sampling Plan34. Acceptance sampling is the type of inspection procedure employed when inspection is for the

purpose of acceptance or rejection of a product based on adherence to a standard.35. In acceptance sampling plans by attribute, a product item is classified as conforming or not, but

the degree of conformance is not specified. In acceptance sampling plans by variable the quality characteristic is expressed as a numerical value.

36. Acceptance sampling can be used as a form of product inspection between companies and their customers, or between departments or divisions within the same company. It does not control or improve the quality level of the process. It is merely a method for determining the disposition of the lot. Acceptance sampling procedures will accept some lots and reject others, even though they are of same quality. There is a risk of rejecting “good” lots or accepting “poor“lots, identified as producer’s risk and consumer’s risk respectively.

37. Operating Characteristic Curve38. Operating characteristic (OC) curve measures the performance of an accepting sampling plan.

This curve plots the probability of accepting the lot versus proportion non–conforming. Thus, the OC curve displays the discriminatory power of the sampling plan and it shows the probability that a lot submitted with a certain fraction defective will be either accepted or rejected.

39. If the producer designs for 97.5% of concrete to be above the specified strength when the theoretical basis of the compliance rules is 95%, the risk of failing on any normal size of contract is acceptably low. By this approach the ready mixed concrete industry runs less risk problems with each individual small contract and also safeguards larger contracts and its overall production Thus the conventional way by which probabilities may be judged by both producer and consumer is through the use of the operating characteristic curve (Figure. 1.0). For any compliance clause, the producer may assess the risk of having complying concrete rejected (producer’s risk) and the consumer can assess the risk of accepting non-complying concrete (consumer’s risk). In the theoretical diagram (Figure. 1.0a), the producer’s risk and consumer’s risk are both nil. In practice, diagrams are usually of the form of Figure 1.0b where both run some risk.

40. Key elements of an OC curve are described as follows. 41. Producer’s risk () : This is a risk associated with rejecting a lot of “good” quality. It is generally

denoted by “” Since á is expressed in terms of the probability of non-acceptance, it cannot be located on an OC curve unless it is specified in terms of probability of acceptance. This conversion is given below :

42. Probability of acceptance (Pa) = 1 - 43. Acceptable quality level (AQL): This is a numerical definition of a good lot, associated with the

producer’s risk a. Thus AQL is a percent defective that is the base line requirement for the quality

of the producer’s product. The producer would prefer the sampling plan to have a high probability of accepting a lot that has a defect level less than or equal to the AQL.

44. Consumer’s risk (): This is a risk associated with accepting a lot of “poor” quality. It is generally denoted by “)”

45. Limiting quality level (LQL): This is a numerical definition of a poor lot associated with consumer’s risk. Thus the LQL is a designated high defect level that would be unacceptable to the consumer. The consumer would prefer the sampling plan to have a low probability of accepting a lot with a defect level as high as the LQL.

46. To construct an OC curve, we assume that the process produces a stream of lots and the lot size is large and the probability of non– conforming item is small. Poisson distribution can be used to find out the probability of “x” non– conforming items in the sample (n). This probability is given by:

47. P(x)= e-λ λx / x!48. where x49.  50.  = 0,1,2, ….. ∞ 51. λ = Average number of non– conforming items in the sample. 52. = np (where n = sample size and p = proportion of non– conforming items) 53. The probability of lot acceptance can be found out by using : 54. Pa = c) , here ‘c’ denotes the acceptance number.P (x ≤

55.

56.57. OC curve has the following properties:58. (i) OC curve in general is continuous in nature 59. (ii) Pa = 1 ie. lots with no defectives must always be acceptedWhen P = 0 ⇒ 60. (iii) Pa = 0, ie. lots with all defectives must always beWhen P = 1, ⇒ rejected. Probability of

acceptance of lots with few defectives can be identified from the OC curve plotted for that particular case.

61. All OC curves passes through points (0,1) and (1,0) 62. (iv) As P increases Pa decreases and vice- versa

63. (v) The points on the OC curve are ( LQL, 1-) , ( AQL, ) where a denotes producer’s risk. and b denotes consumer’s risk.

64. The above parameters can also be represented by the decision table Figure 3.0. 65. Conformity criteria for compressive strength (EN 206- 1:2000) 66. Conformity assessment shall be made on test results taken during a n assessment period that

shall not exceed the last twelve months . Conformity is confirmed if both the criteria given in Table 1.0 for either initial or continuous production are satisfied.

67. Current compliance rules of BS 5328 68. The current strength compliance rules of BS 5328 are: 69. (i) No result for a batch shall be less than the specified strength less 3 Mpa. 70. (ii) No mean of four consecutive results shall be less than the specified strength plus 3 Mpa. 71. Where a result is the mean for a pair of cubes from a single batch and tested at 28 days.

72. Data Collection and Case Study73. Data in form of 28 days cube compressive strength results available for a number of grades of

concrete, the corresponding mix proportions followed by the RMC plant operating around Ahmedabad were collected. About twenty samples for grade of concrete M20, and about thirty samples for grades M25, M30, M40 have been considered for the analysis of OC curves.

74. In this paper, an attempt has been made to apply clause BS 5328(ii) to the 28 days compressive strength results for M20 grade concrete, which has resulted in identification of the batches in

which “good” quality concrete is being rejected by the client and the batches in which “poor” quality concrete is being accepted by the consumer. The details of this case is demonstrated in Table 3.0. A similar attempt will be made in future papers to understand the application of conformity criteria as per Table 1.0, Criterion 1 (EN- 206-1:2000).

75. For the batch 18-21, result no. 18 (23.20 Mpa), result no. 20 (24.50 Mpa) and result no. 21 (22.45 Mpa) inspite of being above the specified strength (20 Mpa) are being rejected during sampling as the other faulty results of the batch like result no. 19 (18.95 Mpa) together with the good results fail to satisfy the acceptance criteria. Inspite of these strengths being more than the specified strength, but during sampling these three results are being rejected. Thereby the producer has a risk of about three good samples out of a lot of thirty being rejected by the client along with poor quality samples. Thus the producer’s risk can be quantified as 10%. A similar logic can be applied for quantifying consumer’s risk. In the above case one poor sample i.e. result no. 19 (18.95 Mpa) is being accepted by the consumer along with other good samples for a lot of thirty. Thus the consumer’s risk can be quantified as 3.3%.

76. Similar analysis has been carried out for concrete grades M25, M30, and M40. A comparative statement showing the results of the analysis is represented here.

77. From the above Table, it has been observed that the producers risk for the RMC plant under study ranges between to 16.66% and the consumers risk is about 3.3%. An Operating characteristic curve (OC) has been plotted from the data available pertaining to the probability of acceptance (Pa) and proportion nonconforming (p). Poisson Distribution is applied for plotting the OC curve for a reasonably large sample size (n) and small proportion nonconforming (p). The acc eptance number (c) is assumed to be 0 for this analysis. Lot acceptance probabilities for different values of proportion nonconforming for sampling plan n = 30, and c = 0 is analyzed in the Table 5.

78. A plot of Probability of acceptance (Pa) versus Proportion nonconforming (p) for the values obtained from Table 5.0 for concrete grade M20 produces the graph as represented in Figure. 4.0. The values of the producers risk and consumers risk obtained from the analysis of Table 3.0 are represented as comparative statement in Table 4.0. These values of the producers risk and consumers risk can be superimposed on the OC curve to find out the Acceptable quality level (AQL) and Limiting quality level (LQL) for the concrete grade under analysis (M20). Similar analysis can be carried out for concrete grades M25, M30, and M40.

79. As per Figure. 4.0 and Table 4.0 for M20 grade concrete lot with 10% defectives will be accepted only 3.3 % of times by the consumer (with probability of 0.033). With á = 0.1 the value of AQL obtained from the = 0.90 lot withFigure. 4.0 is 0.0041. Thus with AQL = 0.76% and 1- 0.76% defectives should then be accepted only 90% of times. Similar analysis for concrete grades M25, M30, and M40 is represented in the Table 6.0.

80. As per Table 6.0 analysis of grade of concrete M25 resulted in an ideal situation where the producers risk and consumers risk is zero. Thus the probability of acceptance is 100%. For M30 grade lot with 16.6% =defectives will be accepted only 3.3% times by the consumer. For 0.166 (16.6%) the AQL obtained from graph (Figure. 4.0) is 0.0076. Thus = 0.834, which indicates that lot with 0.76% defectives should then1- be accepted only 83.4% of times. Analysis of M40 concrete projects a case where a lot with 0.41% defectives had 90% probability of acceptance. The consumers risk ) being nil, the LQL tends to 1.00. Thus a lot with 100% defectives has 0% probability of acceptance. The above analysis of various grades of concrete indicate that the RMC plant operators have a general tendency to reduce the producers risk by following a mix design where the Target Mean Strength (TMS) is obtained from the where fck is 28 days characteristic compressiveequation TMS = fck + 2 strength and = Plant Standard Deviation. The design margin for a confidence level of 97.5%. However, too m u c his kept as 2

overconservative mix design will lead to an uneconomical mix thereby resulting in higher price per cum. of concrete for various grades. A representation of the OC curve by considering the parameters Probability of acceptance Vs Percent within limits (%) is shown in Figure. 5.0. As per Figure. 5.0 and as per analysis (Table: 7.0) it is suggested that the RMC producers can fix up the ) within 5% and LQL =AQL = 97.5% which will keep the producers risk ( 0% which will also restrict the consumers risk () within 5%. Thus fixing up the AQL and LQL to the above mentioned limits will develop a practical situation where the chances of a good concrete being rejected by the client and chances of the consumer to accept bad concrete are both restricted to 5%. This criteria can be taken care of during the mix design of the concrete grades.

81. Limitations82. The major disadvantage of this type of OC curve is that there are no provisions for the AQL and

LQL in the input parameters of the Poisson Distribution. In this case, the only input parameters are the sample size (n), acceptance number (c), and proportion nonconforming (p). Also it is not reasonable to assume that the distribution will be the same for any given RMC plant. But when AQL and LQL are specified, it is reasonable to assume that the distribution will vary based on these two limits. In addition to this weakness, it is not reasonable to assume a proportion of nonconforming units of any lot for given RMC producer is unique throughout its production

process. In real practice each lot might have different proportion of nonconforming items, which is timated from each lot data.

83. Conclusions84. The OC curve is a widely accepted tool to quantify the producers risk and consumers risk.

Sampling plans with large sample sizes are better able to discriminate between acceptable and unacceptable quality. Therefore, fewer lots of unacceptable quality are accepted and fewer lots of acceptable quality are rejected. As per the analysis done in this paper, it is suggested that the RMC producers ideally maintains the Acceptable quality level (AQL) = 97.5% and Limiting quality level (LQL) = 0%, which will restrict the producers risk and consumers risk to 5%. This will also

ensure that unnecessarily there is no requirement of over conservative mix design for the concrete grades which will definitely make the grade prices more economical. Thus a good compliance scheme should ensure that the producers risk and consumers risk are at an acceptable level and are properly distributed between the producer and

the consumer. Here lies the importance of OC curves which can be used to evaluate the desired compliance scheme.

85. References86. 1. Neville, A.M. and Brooks, J.J. Concrete Technology ELBS with Longman, 1994. 87. 2. Dewar, J.D. and Anderson, R. Manual of Ready Mixed Concrete Blakie and Son Ltd, Glasgow

and London, 1988.88. 3. Miller, I and Freund, J.E. Probability and Statistics for Engineers Pearson Education

(Singapore) Pte. Ltd., 2001.89. 4. Mitra, A. Fundamentals of quality control and improvement Pearson Education (Singapore)

Pte. Ltd.,2004.90. 5. Montgomery, D.C. Introduction to Statistical Quality Control John Wiley & Sons, Inc. 1985.

Indian standard code of practice for plain and reinforced concrete, IS 456: 2000, Bureau of Indian Standard, New Delhi.

Effects of Addition of More than two Chemical Admixtures on the properties of Retempered Concrete

D.K. Kulkarni, Assistant Professor, Civil Engineering Department, Rajarambapu Institute of Technology Rajaramnagar Islampur, Maharastra. Dr. K.B. Prakash, Professor Civil Engineering Department, K.L.E

Society’s College of Engineering and Technology, Belgaum.

In situations like delivery of concrete from central mixing plant, in road construction, in constructing lengthy tunnels, in transportation of concrete by manual labor, in hilly terrain long hauling of concrete is required. Loss of workability and undue stiffening of concrete may take place at the time of placing on actual work site. In such situations engineers at site, many a time reject the concrete partially set and unduly stiffened due to the time elapsed between mixing and placing. Mixed concrete is a costly material

and it cannot be wasted without any regard to cost. It is required to see whether such a stiffened concrete could be used on work without undue harm with use of combinations of admixtures. The process of remixing of concrete, if necessary, with addition of just the required quantity of water is known as ‘retempering of concrete’. Sometimes, a small quantity of extra cement is also added while retempering.

In the sites sometimes the concrete has to wait for some time to enter in the formwork after it is mixed. This may be due to some break down in the conveyance or quarrel between the labors. In such situations the concrete looses its plasticity. But since the quantity is enormous, such concrete cannot be wasted. In such situations addition of small quantity of cement and water along with combinations of admixtures can bring back the plasticity to concrete. Thus retempering becomes important in such odd situations.

In this paper an attempt is made to study the strength characteristics of concrete containing combination of admixtures at retempering time of 15 min upto 90 min. The combinations of admixture studied in this experimentation is

Superplasticiser + Air Entraining Agent + Water proofing compound (S+AEA+W).

The tests were conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength and impact strength for different retempering times.

Introduction

One of the adverse effects of hot weather concreting is loss of slump. Delay in the delivery of ready mixed concrete has the same result and leads many people in the concrete industry to regain the original slump by adding water, a process known as ‘retempering’1.

Ready-mixed (RMC) concrete, which is mixed at the plant, using a normal, well-designed concrete mix, should arrive at its destination with sufficient workability to enable it to be properly placed and fully compacted. In such circumstances, where there is a significant period of time between mixing and placing the concrete, there will be a noticeable reduction in the workability of the fresh concrete. If for any reason, the placement of the concrete is unduly delayed, then it may stiffen to an unacceptable degree and site staff would normally insist on the rejection of a batch or otherwise good concrete on the grounds of insufficient workability. If not rejected, excessive vibration would be needed to attempt to fully compact the concrete, with the risk of incomplete compaction, expensive repair, or, at worst, removal of the hardened concrete.

If abnormal slump loss in anticipated or if transport times are significant, then the intelligent use of admixtures can alleviate the potential workability difficulties, although at additional cost, and this practice is common place 2, 3, 4. However, in cases where unforeseen delay or some other cause has lead unexpectedly to poor workability, retempering of the concrete by water, while normally considered to be bad practice, may, in reality, be contemplated as a possible course of action. The increase in the water content of the concrete immediately prior to discharge will improve the consistency, but it is widely held that there must be a subsequent increase in the water/ cement (w/c) ratio which will be detrimental to the hardened concrete5, 6.

Adding water to a plastic mix to increase slump is an extremely common practice, even though it is not recommended because it increases the porosity of concrete. Concrete often arrives on site more than half an hour after initial mixing. Placement operations can take anywhere from 10 to 60 minutes, depending on the field conditions and the size of the load. When the slump decreases to an unacceptable level during the operations, water is added to the mix and, very often, experienced field inspectors will tolerate what can be termed ‘reasonable’ retempering, i.e., enough to increase slump by 50 or 60 mm7.

Research Significance

In the circumstances like breakdown of any concreting equipment or quarrels between the labors or suddenly erupted strikes on the site may put the green concrete into difficult situation. In such above situations the concrete which is already mixed may have to wait for a longer time before entering into the formwork. This causes the loss of plasticity and if such concrete is used, the strength and other characteristics of concrete are affected. Such concrete has to be either discarded or used with little addition of extra water and cement so that a part of plasticity is regained, and such concrete is called retempered concrete. Probably use of some admixtures may induce some good qualities to such retempered concrete. Therefore, it is essential to study the characteristic properties of retempered concrete containing combination of admixtures.

Experimental Programme

The main aim of this experimentation work is to find the effect of addition of more than two admixtures on the properties of retempered concrete. The combination of admixtures selected for the study on concrete is Superplasticiser + Air Entraining Agent + Water proofing compound (S+AEA+W)

Ordinary Portland Cement and locally available sand and aggregates were used in the experimentation. The specific gravity of fine and coarse aggregate was 2.66 and 2.51 respectively. The experiments were conducted on a mix proportion of 1: 1.26:2.1 with w/c = 0.41 which corresponds to M20 grade of concrete. The admixtures and their dosages used in the experimentation are shown in Table 1.

After thoroughly mixing all the ingredients in dry state, the required quantity of water was added in the mix and thoroughly mixed. At this stage the different admixtures like superplasticiser, air entraining agent and water proofing compounds were added and a homogeneous concrete mix was obtained. This concrete mix was covered with gunny bags for 15 minutes. The time was reckoned, the moment the water was added to the concrete mix. After 15 minutes the mix was poured into the moulds and the specimens were cast with sufficient compaction through vibration. This forms retempered concrete for 15 minutes. Similarly, the specimens were prepared with retempered concrete with a retempering time of 30 minutes, 45 minutes, 60 minutes, 75 minutes and 90 minutes.

Another set of retempered concrete specimens were cast by adding 5% extra cement and the required extra amount of water to balance a w/c ratio of 0.41. All the specimens were demoulded after 12 hours of their casting and were transferred to curing tank to cure them for 28 days. After 28 days of curing the specimens were tested for their compressive strength, tensile strength, flexural strength and impact strength as per IS specifications.

For compressive strength test, the cubes of dimensions 150 X 150 X 150 mm were cast and were tested under compression testing machine as per I S 516-1959.8 For tensile strength test, the cylinders of diameter 100 mm and length 200 mm were cast and were tested under compressive testing machine as per I S 5816- 1999.9 For flexural strength test the beams of dimensions 100 X 100 X 500 mm were cast and were tested on an effective span of 400 mm with two point loading as per I S 516-1959.8 For impact test four different test methods are referred in the literature.10 Drop weight method being the simple method, was adopted to find the impact energy. Impact strength specimens were of dimensions 250 X 250 X 30 mm. A steel ball weighing 12.6 N was dropped from a height of 1 m on the centre point, which was kept on the floor. Number of blows required to cause first crack and final failure were noted down. From these number of blows, the impact energy was calculated as under.

Impact energy = w h N (N-m)

Where w = Weight of steel ball = 12.6 N

h = Height of drop = 1 m

N = Number of blows required for first crack or final failure as the case may be.

Test Results

Table 2 gives the compressive strength test results of retempered concrete. It also gives percentage increase or decrease of compressive strength w.r.t. reference mix. Table 3 gives the tensile strength test results of retempered concrete. It also gives percentage increase or decrease of tensile strength w.r.t. reference mix. Table 4 gives the flexural strength test results of retempered concrete. It also gives percentage increase or decrease of flexural strength w.r.t. reference mix. Table 5 gives the impact strength test results of retempered concrete. It also gives percentage increase or decrease of impact strength w.r.t. reference mix.

The variation of these strengths are depicted in the form of graphs as shown in Figure.1, 2, 3 and 4.

Discussion of Test Results

1. It has been observed that the concrete without any admixture shows maximum compressive strength, tensile strength, flexural strength and impact strength at a retempering time of 60 minutes. It is true for both concretes which are produced by adding 5% extra cement and water and concrete without adding 5% extra cement and water.

This may be due to the fact that the evaporated water up to Figure 3: Variation of Flexural Strength w.r.t. Retempring Times Figure 4: Variation of Impact Strength w.r.t. Different Retempring Times 60 minute may bring down the w/ c ratio resulting in an enhanced strength.

Thus it can be concluded that the concrete without any admixture show maximum strengths at a retempering time of 60 minutes.

2. It has been observed that the concrete produced with addition of 5% extra cement and water show higher compressive strength, tensile strength, flexural strength and impact strength as compared to concrete produced without 5% extra cement and water. This is true for all the retempering times from 15minutes to 90 minutes.

Obviously this may be due to the fact of presence of 5% extra cement.

Thus it can be concluded that the concrete produced with addition of 5% extra cement and water yields more strength, for all the retempering times up to 90 minutes.

3. It has been observed that the concrete with the combination of admixture (S+AEA+W) shows maximum compressive strength, tensile strength, flexural strength and impact strength at a retempering time of 45 minutes. It is true for both the concretes which are produced by adding 5% extra cement and water and concrete without adding 5% extra cement and water.

This may be due to the fact that the evaporated water up to 45 minute may bring down the w/c ratio resulting in an enhanced strength.

Thus it can be concluded that the concrete with the combination of admixture (S+AEA+W) shows maximum strengths at a retempering time of 45 minute.

4. It has been observed that the concrete produced with addition of 5% extra cement and water show higher compressive strength, tensile strength, flexural strength and impact strength as compared to concrete produced without 5% extra cement and water, when the combination of admixture (S+AEA+W) is used. This is true for the retempering times from 15minutes to 90 minutes.

Obviously this may be due to the fact of presence of 5% extra cement.

Thus it can be concluded that the concrete produced with addition of 5% extra cement and water and with combination of admixture (S+AEA+W) yields more strengths for all the retempering times up to 90 minutes.

5. It has been observed that the compressive strength, tensile strength, flexural strength and impact strength of concrete produced with the combination of admixture (S+AEA+W) is higher than that without any admixture. This is true for all the retempering times and also it is true for the concrete produced by addition of 5% extra cement and water and concrete without 5% extra cement and water.

This may be due to the fact that the addition of combination of admixture (S+AEA+W) induce more workability which will facilitate for full compaction and in turn this results in higher strengths.

Thus it can be concluded that the concrete produced with the combination of admixture (S+AEA+W) show higher strengths than that of without admixtures for all the retempering times.

Conclusions

The concrete without any admixture show maximum strengths at a retempering time of 60 minutes.

The concrete produced with addition of 5% extra cement and water yields more strength, for all the retempering times up to 90 minutes. The concrete with the combination of admixture (S+AEA+W) shows maximum strengths at a retempering time of 45 minute. The concrete produced with addition of 5% extra cement and water and with combination of admixture (S+AEA+W) yields more strengths for all the retempering times up to 90 minutes. The concrete produced with the combination of admixture (S+AEA+W) show higher strengths than that of without admixtures for all the retempering times. Thus instead of wasting the bulk concrete, the retempering can be recommended either with the use of combination of admixture (S+AEA+W) or without admixture.

Acknowledgments

The authors would like to thank Dr. (Mrs) S. S. Kulkarni, Principal, RIT, Sakharale and Dr. S.C.Pilli, Principal, KLESCET, Belgaum for giving all the encouragement needed which kept our enthusiasm alive. Thanks are also due to the management authorities and others who constantly boosted our morale by giving us all the help required. Thanks are also due to authorities of MBT Pvt.Ltd(Degussa) Mumbai for supplying the required admixtures.

Reference

M A A l Kubaisy and A S K Palanjian, “Retempering studies of concrete in hot weather,” Proceedings of colloquium organized on behalf of the coordinating committee for concrete technology of RILEM, Oct 3-5, 1990, pp.83-91. Previte R W, “Concrete slump loss,” ACI Journal, Aug-1977, pp. 361-367. Mayer L M and Perenchio W F, “Theory of concrete slump lossas related to the use of chemical admixtures,” Concrete International, Jan-1979, pp. 36- 43. Erlin B and Hime W G, “Concrete slump loss and field example of placement problems,” Concrete international, Jan1979, pp. 48- 51. Gonnerman H F and Woodworth P M, “Tests on retempered concrete, ACI Journal, 1929, pp. 25. R P West, “Concrete Retempering without strength loss,” Proceedings of colloquium organized on behalf of the coordinating committee for concrete technology of RILEM, Oct 3-5, 1990, pp.134-141. Michel Pigeon, Francois Saucier, and Patrick Plante, “Air-void stability, part IV: Retempering,” ACI Materials Journal, May-June 1990, pp.252-259. I S : 516-1959 “Methods of tests for strength of concrete,” Bureau of Indian Standards, New-Delhi. I S : 5816-1999 “Splitting tensile strength of concrete method of test,” Bureau of Indian Standards, New-Delhi. Balsubramanain, K. et al, “Impact resistance of steel fiber reinforced concrete,” The Indian concrete Journal, May 1996, (pp 257-262).

Cements and Concrete Mixtures for Sustainability

Mehta, P. Kumar, University of California, Berkeley, U.S.A.

The climate changes, due to man-made global warming triggered by steeply rising volume of greenhouse gases, composed mostly of carbon-dioxide, is a very serious issue that is being addressed worldwide by every major sector of economy. There is a general acceptance of the view that firm measures must be taken without delay to bring down the global carbon emissions to the 1990 level or less during the next 15 years.

The focus of this paper is on portland-cement concrete, which is the most widely used manufactured product in the world today. Cement production is not only energy-intensive but also responsible for direct release of nearly 0.9 tonne carbon-dioxide for each tonne of portland clinker, which is the principal

component of modern cements. Fifteen years ago, in 1990, the world production of cement was slightly more than 1 billion tonnes. In 2005, it already crossed 2 billion tonnes which means that direct CO2 emissions from the portland clinker production have nearly doubled. Fifteen years from now, with businessas- usual, the estimated cement requirement would be 3.5 bilion tonnes, and direct CO2 emissions from cement kilns would triple the 1990 level. Thus, the challenge before the global construction industry is how to meet the buildings and infrastructure needs of rapidly growing economies of the world, and at the same time, cutting down the CO2 emissions attributable to cement consumption to the 1990 level, in conformity with other sectors of economy.

Different options for consideration of the construction industry are presented in this paper.

The production and use of blended Portland cements containing large proportion of complementary cementing materials, such as coal fly ash and granulated blast-furnace slag provide an excellent strategy for immediate and substantial reduction of direct CO2 emissions associated with the manufacture of portland-cement clinker. Both EU and North American cement standards now permit more than 50 % clinker replacement in composite cements. Furthermore, the use of composite cements and concrete mixtures containing large addition of complementary cementing materials would yield crackresisting structural elements of radically enhanced durability. High-volume fly ash concrete applications for recently built structures in North America are cited as typical examples of possible CO2 reduction.

Sustainability–an Introduction

During the 1990s, it became abundantly clear that industrialization of the world is happening at an unsustainable speed. Among the major sustainability issues of public concern are high rates of consumption of energy and materials, short service life of manufactured products, and lack of space for safe disposal of huge volumes of solid, liquid, and gaseous wastes generated by human activities. Global warming, the cumulative effect of these problems, has emerged today as the most serious sustainability issue of the 21st century.

The term, global warming, refers to the greenhouse-gas effect leading to a steady increase in the earth’s surface temperature since 1950s. According to a World Watch Institute report, twenty-four of the last 27 years have been the warmest on record. Weather scientists around the world have concluded that a linear relationship exists between the earth’s surface temperature and the atmospheric concentration of CO2, which makes up 85 % of the greenhouse gases. The current CO2 concentration, about 380 ppm (mg/L) in 2005, is the highest in recorded history (Figure 1). With business as usual, it is projected to increase at an exponential rate. In 2006, the annual global CO2 output reached a staggering 30 billion tonnes.

Evidence of global warming is not confined to temperature measurements. The following list includes some of the observable effects of the phenomenon:

A sharp increase in the melting rates of glaciers, polar caps, and ice sheets. Rising ocean levels–a potential threat to coastal populations. Unusual increase in frequency and intensity of rainstorms, flash floods, cyclones, hurricanes, heat waves, droughts, and wild fires. Adverse impact on current sources of agriculture and water. Disruption of the earth’s carbon cycle due to changes in the botanical species on land and oceans.

In a series of reports, issued earlier this year by the United Nations Intergovernmental Panel on Climate Change, leading weather scientists of the world have unequivocally stated that global warming is occurring, and that it has been triggered by human activities. They have warned about devastating consequences of global warming if immediate action is not taken by national and industry leaders to reduce the carbon dioxide emissions to the 1990 level or less.

Although climate change is a global phenomenon, it has to be tackled in every country individually by each of the major CO2 emitting sectors of economy, such as power generation, transportation, and energy consumption associated with the use of buildings, and manufacture of structural materials like concrete and steel. According to Kyoto Protocol, proposed in 1990 and signed in 2005 by 141 countries, the signatories agreed to stabilize the greenhouse gas emissions by 2012 to 6 % below the 1990 level. The two largest polluting countries, the U.S. and China, which are responsible for nearly half of the global CO2 emissions, have yet to show a willingness to commit to any specific goals. However, in 2005, many multinational corporations, State governments in the U.S., and over 400 mayors representing 60 million Americans have signed on to programs that intend to meet or beat the Kyoto targets by 2020. In September 2006, the State of California approved the Global Warming Solutions Act according to which, by 2020, California’s CO2 emissions would be reduced to the 1990 level.

Concrete Industry’s Environmental Impact

The subject of environmental impact of the concrete industry is covered by numerous publications across the world including those listed in References (1-6). The embodied energy content, i.e., the sum total of energy required to extract raw materials, manufacture, transport, and install building elements is only 1.3 MJ/kg for 30 MPa concrete, compared to 9 MJ/kg for recycled steel and 32 MJ/kg for new steel. However, being the largest manufactured product consumed in the world, quantitatively concrete represents considerable embodied energy.

Worldwide today, approx. 17,000 million tonnes of concrete is being produced annually. Besides natural resources, such as aggregates and water, the concrete industry is a large consumer of cement– a manufactured product directly responsible for high CO2 emissions. In 2005, according to Cembureau, the global cement consumption was 2,270 million tonnes. Therefore, carbon footprints of the global cement industry are very significant considering the amount of fossil fuels and electrical power consumed for crushing, grinding and transport of materials, and for the 1400 to 1500°C burning operation to make portland clinker –the principal ingredient of hydraulic cements. The scope of this paper is limited to direct CO2 emissions, of which approx. 6.3%2 of the global emissions are attributable to portland clinker manufacture.

Co2 Emissions from Cement Kilns

Typically, ordinary portland cement is composed of 95 % clinker and 5 % gypsum, which is a complementary cementing material (CCM) because it enhances the cement performance by improving the setting and hardening characteristics of the product. Depending on the carbon content of fossil fuels used for clinkering, 0.9 to 1.0 tonnes of CO2 is directly released from cement kilns during the manufacture of clinker. In addition to gypsum, sometimes other mineral additives, commonly known as supplementary cementing materials (e.g., coal fly ash, granulated blast-furnace slag, natural and calcined pozzolans, pulverized limestone, and silica fume) can either be interground with clinker and gypsum or added directly during the concrete mixing operation. Large quantities of these materials are available as industrial by-products. As discussed in this paper, when properly used, the mineral additives have the ability to

enhance considerably the workability and durability of concrete. Therefore, these additives too are treated as complementary cementing materials (CCM) in this paper.

Global statistics for 1990 and 2005 on cement production, CCM consumption, and direct CO2 emission attributable to Portland clinker manufacture, are presented in Table 1. According to the U.S. Geological Survey records, the world consumption of cement in 1990 was 1,044 million tonnes. From the fragmentary information available it is estimated that, globally, the average clinker factor of cement (units of clinker per unit of cement) in 1990 was 0.9, which means that 940 million tonnes of clinker and 104 million tonnes of CCM were used. Assuming the average CO emission rate as 1.0 tonne CO2/ tonne clinker, in 1990 the direct CO2 emission from clinker production were 940 million tonnes.

In 2005, due to a gradual increase in the use of CCM, it is estimated that 370 million tones of CCM were incorporated into 2,270 million tonnes of cement. This gives a clinker factor of 0.84. Also, in 2005, due to increase in the use of alternate, low-carbon, fuels for burning clinker, the average CO2emission rate dropped to 0.9 tonne per tonne of clinker. This means that, in 2005, 1,900 million tonnes of clinker was produced, with 1,700 million tonnes of direct CO2 release to the environment. In conclusion, the global cement industry has almost doubled its annual rate of direct CO2 emissions during the last 15 years.

Reducing the Co2 Emissions

Comparing the 1990 and 2005 global CO2 emissions directly attributable to clinker production (Table 1), the magnitude of the problem becomes at once clear. Not only the annual rate of cement consumption in the world has nearly doubled during the last 15 years but also, at the current rate of economic growth in many developing countries, by the end of the next 15 years the cement requirement is expected to go up to about 3,500 million tonnes a year. Assuming that during the same period the use of CCM increases from 15 to 20 % of the total cement, the global clinker production and CO2 emission in 2020 would amount to 2,800 million tonnes, and 2,520 million tonnes, respectively. To bring down the CO2 emission from 2,520 to 940 million tonnes (the 1990 level) involves nearly a two-third reduction in clinker requirement, which is unlikely barring a global catastrophe.

In the portland clinker manufacturing process, direct release of CO2 occurs from two sources, namely the decomposition of calcium carbonate (the principal raw material) and the combustion of fossil fuels. The former accounts for about 0.6 kg CO2/kg clinker and the latter 0.25-0.35 kg CO2/kg clinker (depending on the carbon content of the fossil fuel); the global average being 0.9 kg CO2/kg clinker. Alternate sources of energy other than fossil fuels are being sought but, at present, they are too expensive. Also, there are some cements that do not require calcium carbonate as a raw material (e.g., magnesium phosphate cements) but they are neither economical nor technically feasible for large-scale production. Obviously, it will not be possible to achieve any drastic cuts in CO2 emission as long as technical and economic reasons favor the use of portland clinker as the major component of hydraulic cements.

The golden rule or mantra for successful resolution of all sustainability issues is, “Consume less, and think more.” Based on this mantra, the author proposes the following three tools, the simultaneous use of which would enable the cement industry to reduce greatly the direct CO2 emission attributable to clinker production:

1. Reduce the consumption of concrete: Architects and structural designers must develop innovative designs that minimize the consumption of concrete. Service life of repairable structures should be extended as far as possible by the use of proper materials and methods of repair. Low-priority projects should be postponed or even canceled when possible. Foundations, massive columns and beams of concrete, and pre-cast building components that can be assembled or disassembled as needed, should be made with highly durable concrete mixtures described in this paper.

2. Reduce the cementing materials in concrete mixtures: Mix design procedures that involve prescriptive codes (e.g., minimum cement content, maximum w/cm, and much higher than needed

strength) lead to considerable waste of cement, besides adversely affecting the durability of concrete. Such prescriptive codes have outlived their usefulness and must be replaced with performance-based specifications that promote durability and sustainability. For example, to achieve durability, it is not the w/c but the cement paste content which should be minimized through optimum aggregate grading, use of plasticizing admixtures, and specifying 56 or 91-day strength for the structural components that do not have to meet a minimum 28-day strength requirement.

3. Reduce the clinker factor of cement: Every tonne of clinker saved would reduce the direct CO2 release from cement kilns by an equivalent amount. Furthermore, as explained below, concrete products made with cements of low clinker factor are expected to be much more durable when compared to ordinary portland cement products.

Imagine if it were possible to enhance the durability of most cement-based products by factor 10 or more, without using any expensive technology and materials! Unquestionably, in the long term, this would serve as an excellent strategy for minimizing the wasteful consumption of cement and other concrete ingredients for general construction.

Published literature contains numerous reports showing that high-early strength concrete mixtures used in modern, high-speed, construction often suffer from lack of durability because they are usually made with high content of a cementing material and a high clinker factor of cement. The hardened product contains a heterogeneous cement paste, with weak interfacial bonding, and is vulnerable to cracking from excessive thermal shrinkage and drying shrinkage. According to Reinhardt (7), to minimize the shrinkage, volume of the paste (cement plus mixing water) in concrete should not exceed 290 L/ M3. High-volume fly ash concrete mixtures, described in this paper are made with cements of low clinker factor (0.4 – 0.5), and less than 290 L/m3 cement paste content. Therefore, they can be used for making relatively crack-free products of excellent durability without any added cost.

Options

As shown in Table 1, compared to the base year 1990, global carbon emissions direct from Portland clinker production have already doubled in the past 15 years. If no serious measures are put into place quickly by the world’s construction industry, i.e. with business-as-usual it is estimated that the rate of direct carbon emissions from cement kilns will almost triple in the next 15 years (Table 2, Option 1). Table 2 also includes data on two other options, an easy option (Option 2) and a challenging but preferable option (Option 3). Note that Option 1 (business-as-usual) data will be used as a reference point for both Options 2 and 3, that are discussed next.

According to Option 2, by 2020, if the global concrete construction industry is able to reduce the concrete consumption by 20 % (compared to Option 1) and at the same time increase the CCM utilization to 30 % of the total cement, these steps will have the effect of reducing the direct CO2 emissions from cement kilns to 1,760 million tonnes. This is nearly twice as much as the 1990 emissions rate of 940 million tonnes.

According to Option 3, in 2020, the total cementing material (2,100 tonnes) would comprise 1050 million tonnes of portland clinker and the same amount of complementary cementing materials. In Table 3, estimates of different types and amounts of complementary cementing materials that would be available for use in 2020 are given. Note that coal fly ash is expected to make up 760 million tonnes or nearly threefourths of the total CCM. Would such a large quantity of fly ash be available in 2020? It is difficult to provide a definite answer, but let us examine the assumptions under which this is possible.

In the foreseeable future, fossil fuels will continue to remain the primary source of power generation, and due to the low cost of coal, expansion of the coal-fired power industry will continue in major coalproducing countries such as China, India, and the United States. According to one estimate, approximately 1200 million tones of fly ash would be available in 2020. It would indeed be a formidable job to ensure that nearly two-thirds of the fly ash produced by coal-fired power plants is suitable for use as a complementary cementing material. This goal can be accomplished, provided the key players, i.e., the producers of fly ash, the consumers of cement and concrete, and individuals or organizations responsible for specifications work together to overcome the problems, discussed below.

The power sector of the global economy is the largest single source of carbon emissions in the world. It is estimated that about 7 billion tonnes a year of CO2 is being released today from the combustion of all fossil fuels, and that the coalfired power plants alone generate 2 billion tonnes of CO2. Besides carbon emissions, according to Malhotra (5), coal combustion in 2005 generated approximately 900 million tonnes of solid by-products including 600 million tonnes of fly ash. Due to rapidly changing rates of fly ash production and use in the two large economies of the world, China and India, which meet threequarters of their electrical power requirement from coal-fired furnaces, accurate data on today’s global rates of flyash production and utilization are not available. However, a rough estimate shows that the current rate of fly ash production is approximately 750 million tonnes/ year, and that nearly 140 million tonnes/year is being consumed as an ingredient of blended cements and concrete mixtures. The remaining fly ash either ends up in low-value applications, such as road sub-bases and embankments, or is disposed to landfills and ponds.

When used as a complementary cementing material, each tonne of fly ash can replace a tonne of portland clinker. Diverting fly ash from the waste stream and using it to reduce direct carbon emissions from the cement industry is like killing two birds with one stone. Therefore, increasing the utilization of most of the available fly ash as a complementary cementing material is, unquestionably, the most powerful tool for reducing the environmental impact of two major sectors of our industrial economy, namely the cement industry and the coal-fired power industry.

In spite of proven technical, economic, and ecological benefits from the incorporation of high volumes of fly ash in cements and concrete mixtures, why does the fly ash utilization rate as a complementary cementing material remain so low? Obsolete prescriptive codes, lack of state-ofthe- art information to architects and structural designers, and lax quality control in power plants are among some of the reasons. Also, all of the currently produced fly ash is not suitable for use as a complementary cementing material, however cost-effective methods are available to beneficiate the material that does not to meet

the minimum fineness and maximum carbon content requirements–the two important parameters by which the flyash suitability is judged by the cement and concrete industries (5).

Sustainable Cements

Sustainable, portland-clinker based cements can be made with 0.5 or even lower clinker factor using a high volume of granulated blast furnace slag (gbfs), or coal fly ash (ASTM Class F or C), or a combination of both. Natural or calcined pozzolans, in combination with fly ash and/or gbfs, may also be used. Compared to portland cement, the high-volume flyash and slag cements are somewhat slower in setting and hardening, but they are more suitable for producing highly durable concrete products. Unfortunately, worldwide, the conventional concrete construction practice is dominated by prescriptive specifications that do not permit the use of high volume of mineral additives.

Cement containing a high volume of complementary cementing materials can now be manufactured in accordance with ASTM C 1157–a new standard specification for hydraulic cements, which is performance-based. However, in North America significant amount of blended portland cements are not produced, because it is customary to add mineral admixtures at the readymixed concrete plants. According to American Coal Ash Association, at present about 14 million of the available 70 million tonnes/year fly ash is being used as a complementary cementing material in concrete mixtures. Reliable estimates are not available from China and India, however, it is reported that significant quantities of blended cements containing 20- 30 % flyash, are being manufactured in these countries.

The European Cement Specification EN 197/1, issued in 2002, contains 26 types of blended portland cements including three cement types that have clinker factors ranging between 0.35 and 0.64. Type III-A Cement covers slag cements with 36-65 % gbfs; Type IV-B Cement covers pozzolan cements with 36-55 % pozzolans including fly ash, natural or calcined pozzolanic minerals, and silica fume; Type V-A Cement covers composite cements containing 18-30 % gbfs plus 18-30% pozzolans. According to Cembureau statistics for 2005, the consumption of ordinary portland cement in the European Union countries has dropped to 30 % of the total cement produced, whereas blended portland cements containing up to 25% CCM have captured 57% of the market share, and blended cements with more than 25% CCM are approaching 10% of the total cement consumption.

Sustainable Concrete Mixtures

For reducing direct carbon emissions attributable to Portland clinker production, the emerging technology of high-volume flyash (HVFA) concrete is an excellent example showing how highly durable and sustainable concrete mixtures, with clinker factor of 0.5 or less, can be produced by using ordinary coal fly ash (ASTM Class F or Class C), which are available in most parts of the world in large amounts. The composition and characteristics of HVFA concrete are discussed in many publications and are briefly described below. Note that concrete mixtures with similar properties can be produced by using a high volume of granulated blast-furnace slag or a combination of flyash and slag, with or without other mineral admixtures.

The cementing material in HVFA concrete is composed of ordinary portland cement together with at least 50% flyash by mass of the total cementing material. The mix has a low water content (100- 130 kg/m3), and a low content of cementing materials (e.g. 300 kg/ m 3 for ordinary strength and max. 400 kg/m3 for high-strength). The plasticizing action of the high volume of flyash imparts excellent workability even at w/cem of the order of 0.4. However, chemical plasticizers are often used, when lower w/cem are required.

Occasionally, an air-entraining admixture is also included in the mix when protection against frost action is sought.

Compared to portland-cement concrete, the HVFA concrete mixtures designed to achieve the same 28-d strength exhibit superior workability without segregating even at slump values of 200-250 mm. Typically, the concrete is slow in setting and hardening, i.e. develop slightly lower strength at 3 and 7-d, similar strength at 28-d, and much higher strength at 90-d and 1-year. The pozzolanic reaction leading to complete removal of calcium hydroxide from cement hydration products enables the HVFA concrete to become highly resistant to alkaliaggregate reaction, sulfate and other chemical attacks, and reinforcement corrosion (due to very low electric conductivity). Furthermore, the HVFA concrete mixtures are much less vulnerable to cracking from both the thermal shrinkage (less heat of hydration), and the drying shrinkage (less volume of cement paste). Therefore, in addition to very low clinker factor, the ability of HVFA concrete to enhance the durability by factor 5 to10 makes it a highly suitable material for construction of sustainable structures in the future. The author has been involved with many field applications of HVFA concrete that are described in earlier publications (8-11). Three recently built structures in the U.S., with large reduction in CO2-emissions resulting from the use of HVFA concrete, are described below.

A Hindu Temple, built with concrete members designed to endure for 1,000 years or more, was constructed in Chicago in 2003 (Figure 2). The superstructure of the temple is composed of some 40,000 individual segments of intricately carved white marble (Figure 2). Unreinforced monolith slabs are a part of the foundation, supported by 250 drilled piers, 9 m high and 1 m diameter. All structural elements were made with, cast-inplace, HVFA concrete containing 105 kg/m3 ASTM Type I portland cement and 195 kg/m3 Class C flyash, 2 L/ m3 polycar– boxylate superplasticizer, and 100 kg/m3 water. Note that the total cementing material was 300 kg/m3, the clinker factor was only 0.33, and the w/cem was also 0.33. The fresh mix had 150-200 mm slump and showed excellent pumpability, which made it possible to place and finish 400 m 3 Concrete for the main prayer-hall slab (22 by 18 by 1 m), in less than 5 hours. Typical compressive strength values at 3-d, 7-d, 56-d, and 1-y were 10 MPa, 27 MPa, 48 MPa, and 60 MPa, respectively. No structural cracks in any concrete member were reported. Also, the chloride penetration permeability, which is an excellent index of long term durability of concrete, was surprisingly low (< 200 coulombs) in 1-year old core samples. A conventional concrete mix would have required 400 kg/m3portland cement to achieve similar3 highstrength. The use of 3,000 m 3 HVFA concrete mix resulted in 900 tonnes of portland cement saving, which corresponds to about 800 tonnes of CO2 emissions reduction.

The Utah State Capitol Building, Salt Lake City, underwent seismic rehabilitation in 2006 (Figure 3a). Due to heavily congested reinforcement in the foundations, floor beams, and shear walls, a nearly self-consolidating mix containing 160 kg/m3 ordinary portland cement, 200 kg/m3 ASTM Class F flyash, 138 kg/m3 water, and 1 L/m3 superplasticizer was used. The clinker factor of this mix was 0.44, and the w/cem was 0.38. The specified slump and 28-d compressive strength were 150 mm and 27 MPa, respectively. The field concrete showed an average of 225 mm slump and 34 MPa strength. It is estimated that this 4,500m3 HVFA concrete job, enabled 900 tonnes of reduction in CO2 emissions attributable to clinker saving.

The CITRIS Building at the University of California at Berkeley contains 10,700m3 HVFA concrete – the largest volume ever used for construction of a single building. For foundations and mats, a concrete mix containing 160 kgm3 of ASTM Type II portland cement, 160 kg of Class F fly ash, and 123 kg/m3 water (0.37 w/cem) was used. For heavily reinforced columns, walls, beams, girders and slabs, a concrete mix containing 200 kg/m3 ASTM Type II portland cement, 200 kg/m3 Class F flyash, and 140 kg/ m3 water (0.35 w/cem) was used. In both cases the clinker factor is 0.50. The specified compressive strength was 27 MPa @ 28-d for all structural members except the foundations and mats which were designed for a specified strength of minimum 27 MPa @ 56-d. Note that the concrete used for reinforced columns achieved 20 MPa strength @ 7-d, and nearly 40 MPa @ 56-d. It is estimated that the choice of HVFA concrete as a structural material for the CITRIS Building resulted in a reduction of 1950 tonnes of direct CO2 Emissions attributable to the low clinker factor of the cementing material.

Economic and Technical Barriers

For utilization of high proportions of complementary cementing materials in general construction, human perception appears to be a far more formidable barrier than actual economic and technical barrier. According to Meryman and Silman (12):

Sometimes, there is a perception that a “green” material or practice is more costly, but on further examination, it proves no to be so; often it is just a matter of getting on the other side of the learning curve. We must clarify the difference between life cycle cost and first cost, since many sustainable products have better life cycle performance. We need to define the term ‘economic’ and include the collateral cost of using non-sustainable practices.

The use of sustainable cements and concrete mixtures, described in this paper, would undoubtedly produce structural members of high durability. However, a statistical life-cycle analysis is not possible because there are no reliable laboratory tests for quantitative assessment of longterm durability of field structures. Other major barriers are lack of codes of recommended practice and unwillingness of structural designers and engineers to be among the first to champion the use of new materials. Again, according to Meryman and Silman (12):

How can an underused material or method become tried, trusted and ultimately the standard? These materials and methods need advocates. As technical professionals, structural engineers can use specifications to communicate a commitment to and confidence in more sustainable choices. By taking responsibility for those practices, we become their advocates.

From my own personal experience, I confirm the observations of Meryman and Silman. I have come to the conclusion that it is the hand that writes the specifications which holds the power of leading the concrete construction industry to an era of sustainability. Codes of recommended practice advocated by organizations, such as American Concrete Institute and U.S. Green Building Council, can play an important part in accelerating the sustainability of the concrete industry. For instance, the USGBC point-rating system for new construction has already become a powerful driving force for sustainable building designs. The rating system awards sufficient points for buildings that would consume less energy in their use. A similar emphasis is needed in favor of sustainable materials that produce less CO2 during their manufacture. By suitably amending the rating system so that some points based on CO2 emissions reduction are directly assigned for the use of sustainable materials in new construction, the USGBC can help sustainability of the cement and concrete industries.

Concluding Remarks

The high carbon dioxide emission rate of today’s industrialized society has triggered climate change that is potentially devastating to life on the planet earth. To meet the global concrete demand, which was 17 billion tonnes in 2005, two billion tonnes of CO2 were directly released to the atmosphere from the manufacturing process of portlandcement clinker, which is the major component of modern hydraulic cements. With business-as-usual, the direct CO2 emissions from portland clinker production, in the year 2020, would triple the 1990 level unless immediate steps are taken to bring down the emissions by making significant reductions in the: (a) global concrete consumption, (b) volume of cement paste in concrete, and (c) proportion of portland clinker in cement.

Examples of recently built structures prove that by using high volume of coal flyash and other industrial wastes as complementary cementing materials with portland clinker, we can produce low cost, highly durable, and sustainable cements and concrete mixtures that would significantly reduce both the carbon

footprints of the cement industry and the environmental impact of the coal-fired power generation industry.

It seems that the game of unrestricted growth, in a finite planet, by reckless use of energy and materials, is over. Most sectors of the global economy have already initiated action plans to bring down their share of carbon emission to the 1990 level or less, by the year 2020. The construction industry is already pursuing the goal of designing and constructing sustainable buildings that consume less energy and resources to maintain. Now, all segments of the construction industry–owners, designers, contractors, and cement and concrete manufacturers–will have to join the new game of building sustainable structures using only sustainable materials.

We have the tools to win this game. What is needed now is the will and the individual initiative. To paraphrase John F. Kennedy, “Ask not what others can do. Ask what you can do to promote the use of sustainable construction materials.”

Acknowledgement

The author would like to thank Mason Walters of Forell Elsesser Engineers, San Francisco, for the photographs in Figures 3 and 4.

References

P.K. Mehta, and P.J.M. Monteiro, “Concrete: Microstructure, Properties, and Materials," McGraw-Hill, New York, 2006 ACI Board Advisory Committee on Sustainable Development, “White Paper on Sustainable Development,” Concrete International, American Concrete Institute, Vol. 27 No. 2, 2005, pp. 19-21 The Concrete Center of U.K., “Sustainable Concrete,” www.concretecenter.com, 2007, 18 pages World Business Council for Sustainable Development, “The Cement Sustainability Initiative,”www.wbcsdcement.org, Geneva, Switzerland, 2007 V.M. Malhotra, “Reducing CO2 Emissions,” Concrete International, American Concrete Institute, Vol. 28 No. 9, 2006, pp. 42-45 P.K. Mehta, “Greening of the Concrete Industry for Sustainable Development,” ibid., Vol. 24 No.7, 2002, pp. 23-28 H.W. Reinhardt, “New German Guideline for Design of Concrete Structures for Containment of Hazardous Materials,” Otto Graf Journal, FMPA, Univ. of Stuttgard, Germany, Vol. 17, 2006, pp. 9-17 P.K. Mehta and W.S. Langley, “Monolith Foundation Built to Last a 1,000 Years,” Concrete International, American Concrete Institute, Vol. 22 No. 7, July 2000, pp. 27-32 D. Manmohan and P.K. Mehta, “Heavily Reinforced Shear Walls and Reinforced Foundations Built with Green Concrete,” ibid., Vol. 24 No. 8, 2002, pp. 64-70

P.K. Mehta and D. Manmohan, “Sustainable, High-Performance Concrete Structures,” ibid., Vol. 28 No. 7, 2006, pp. 37-42  

V.M. Malhotra and P.K. Mehta, “High-Performance, High-Volume Flyash Concrete,” Supplementary Cementing Materials for Sustainable Development, Ottawa, Canada, 2002

H. Meryman and R. Silman, “Sustainable Engineering–Using Specifications to Make it Happen,” Structural Engineering International, Vol. 14 No. 3, Aug. 2004, pp 216-219.

Acknowledgement

The article has been reproduced from the SEWC’07 proceeding with the kind permission from the SEWC organisers.

High–Performance Fiber Reinforced Concrete under Compression and Flexure

P. Ramadoss, Research Scholar and K. Nagamani, Professor, Structural Engineering Division CEG, Anna University, Chennai. 

Mechanical properties of fiber reinforcement concrete are needed to use the FRC in various structural applications. This paper presents the experimental investigation carried out to study the behavior of high performance fiber reinforced concrete (HPFRC) under compression and flexure with compressive strength ranging from 60 MPa to 86 MPa and flexural strength ranging from 6 MPa to 10 MPa. Steel fiber volume fraction ranges from zero to1.5 percent (39, 78 and 117.5kg) with aspect ratio of 80 were used. The influence of fiber content on the compressive strength and flexural strength with w/cm ratios ranging from 0.40 to 0.25 is presented. Equations are proposed using regression analysis to predict the strength of HPFRC effecting the fiber addition in terms of fiber reinforcing index. Strength comparison analysis was carried out to validate the empirical equation and the maximum absolute variation obtained was 4 percent. Addition of 1.5 % volume of crimped steel fiber resulted in 10% increase in the compressive strength while flexural strength increased by 37% and the RCSR value evaluated is between 0.107 and 0.149.

Introduction

In last 3 decades, numerous research and development studies have been taken up in the field of steel fiber reinforced concrete (SFRC) for understanding the behavior of sound composites. SFRC has been used in several areas of infrastructure and industrial applications including highway and airport pavements, bridge decks, sky scrappers, hydraulic structures, industrial floors, tunnel lining, etc. [1, 2]. As noted by ACI Committee 544, the composite has great potential for many use in civil Engg., applications, especially in the area of structural elements.

High performance concrete (HPC) is defined according to ACI 363-1992 [3] as concrete, which meets special performance and uniformity requirements that can’t always be achieved by using only the conventional materials and normal mixing, placing and curing practice. Typical high performance requirements specify high strength (above 41 MPa), enhanced impermeability (permeability less than 10-10 m/s), and high tensile strength (above 4 MPa) and other special requirements. HPC is achieved by using super plasticizen (SP) to reduce water/cm ratio and by using SCM (supplementary cementing material) as silica fume having pozzolanic reaction and filler effect, which usually combines high strength with high durability [4].

High-performance/highstrength concrete leads to the design of smaller sections and reduces the dead weight, allowing longer spans and more useful area of structures. Reduction in mass is also important for economical design of seismic resistant structures. is responsible for the enhancement of strength and durability of the concrete [4]. Strength, ductility and durability are the important factors to be considered in the design of earthquake resistant R.C. structures. Due to the inherent brittleness of HPC/ HSC, it lowers its post-peak portion of the stressstrain diagram almost vanishes or descends steeply [5]. This inverse relation between the strength and ductility is a serious drawback for the use of HPC/HSC and a compromise to this drawback can be obtained by the addition of discontinuous short steel fibers in to the concrete. When concrete cracks, the randomly oriented fibers arrest a micro cracking mechanism and limit the crack propagation, thus improving the strength and ductility thereby enhances the durability of structural elements. Wafa and Ashour [6] have reported that addition of steel fibers in to HSC changes its brittle mode of failure in to a more ductile one and results in a small increase in compressive strength and more increase in tensile strengths compared to plain concrete. Fibers with 1 percent volume fraction and aspect ratio of 75 provide maximum stiffness to concrete and results in maximum increase in compressive strength [7]. Although a number of researchers investigated the effect of inclusion of discrete steel fibers on the compressive strength [8, 9, 10], research on HPC where fibers play a major role is lacking. The main objective of this paper is to study the influence of crimped steel fibers on the compressive strength and flexural strength of high performance- fiber reinforced concrete with varying w/cm ratios and 10% silica fumereplacement.

Research Significance

High performance concrete with and without fibers possesses mechanical properties that are significantly different from normal strength concrete materials. This paper presents an extensive experimental investigation on the mechanical properties of HPFRC with w/cm ratios of 0.40, 0.35, 0.30 and 0.25 and studies the effects of inclusion of fiber contents on improving these properties.

Experimental Program

Four basic mixes for plain concrete designated as FC1-0.0, FC2-0.0, FC3-0.0 and FC4-0.0 according to the w/cm ratios of 0.4, 0.35, 0.30 and 0.25 were selected.

Materials and Mixture Proportions

Ordinary portland cement-53 grade satisfying the requirements of IS: 12269–1987 [11] and silica fume supplied by Elkem India Ltd. having specific surface area of 23,000 m2 /kg, a specific gravity of 2.35 and contained 88.7% of SiO2 were used in the ratio of 9:1 (1:1 partial replacement of cement) in all the mixes. Fine aggregate of locally available river sand conforming to IS: 383-1970 [12] with fineness modulus of 2.55 and a specific gravity of 2.63 and coarse aggregate of blue granite crushed aggregates conforming to IS: 383- 1970 with 12.5 mm maximum size and fineness modulus of 6.4, a specific gravity of 2.70 were used.

Fibers used are crimped steel fibers of diameter =0.45 mm and length = 36 mm, giving an aspect ratio of 80. The properties of fibers used are given in Table 1. Mixture proportions used in the test programme are summarized in Table 2 [13, 14, 15]. For each water to cemetitious material ratio three fiborus concrete mixes were prepared with fiber volume fractions, Vf of 0.5, 1.0 and 1.5 percent (39, 78 and 117.5kg/m3). Due to the inclusion of the fibers some minor adjustments in terms different ingredients had to be made as shown in Table 2. A naphthalene sulphonated formaldehyde (NSF) as HRWR admixture (super-plasticizer) conforming to ASTM Type F with dosage range of 1.75% to 2.75% by weight of cementitious materials has been used to obtain the adequate workability of plain and fiber reinforced concrete. Dosage of super plasticizer was arrived based on the workability tests conducted on trial mixes.

Mixing and Curing

Silica fume was mixed with cement uniformly and thoroughly till homogeneity is attained. Concrete was mixed using a tilting type mixer and specimens were cast using steel moulds, compacted with table

vibrator. For each mix at least three 150 mm cubes and three 100 x 100 x 500 mm prisms were produced. Specimens were demolded 24 hours after casting and water cured at 27± 2oC until the age of testing at 28 days. All the specimens were cured in the same water tank to maintain uniform curing.

Results and Discussions

The results of this investigation are applicable to the material and the type of fibers used.

Compressive Strength

Compressive strength tests were carried out according to IS: 516- 1979 [16] standards using 150 mm cubes loaded uniaxially. The tests were done in a servo–controlled compressive testing machine by applying load at the rate of 14 MPa/min. Minimum of three specimens were tested to assess the average strength.

Behavior under Compression

Under uniaxial compressive loading, extensive crack was produced in the concrete during pre-peak stage and then failed suddenly at peak load. Figure 1(a) reveals the failure mechanism of Silica fume concrete and indicates that it increases its compressive strength and makes it more brittle, fails violently and suddenly. It is observed from Figure 1(b) that when fibers in discrete form present in the concrete, propagation of crack is restrained which is due to the bonding of fibers in to the concrete and it changes its brittle mode of failure in to a more ductile one and improves the post cracking load and energy absorption capacity.

Test results show that addition of fibers has a moderate effect on the improvement of compressive strength values. Table 3 and Figure 2 show that the addition of fiber volume fraction from 0 to 1.5% increases the compressive strength by about 10 percent compared to zero 20 percent given in the literature [5, 7, 9, 10, 17] for normal strength concrete. It is observed from the test results that for 1.0 percent fiber content, the increase in strength is about 10 percent but beyond 1.0% fiber content, there is

only marginal increase in strength. Based on the test results, using linear regression analysis, the compressive strength of HPFRC may be estimated in terms of compressive strength of plain concrete, fc and Reinforcing index, RI and volume fraction, Vf respectively, as follows:

fcf = fc+ 1.84 (RI)

fcf = fc = + 4.74 (Vf )                         (2)

Where fcf = compressive strength of fiber reinforced concrete, MPa; fc = compressive strength of plain concrete, MPa;

RI= steel fiber reinforcing index; Vf = volume fraction of steel fiber, percent

RI = wf * (l/d); average density of HPFRC = 2415 kg/ m3

Weight fraction (wf) = (density of fiber/ density of fibrous concrete) * Vf

Aspect ratio (l/d) = length of fiber/diameter of fiber.

The effect of fibers on 28-day compressive strength of concrete may be evaluated form Figure 3. In this figure, the cube compressive strength of all fibrous concrete irrespective of Vf, is plotted against the plain concrete. The least square line obtained using linear regression analysis with correlation coefficient, r =0.98 is given by:

fcf = 1.087                         (3)

The predicted values as obtained by equation (1) and the equations proposed by the researchers [5,7,17] were compared with the experimental values, are presented in Table 4. The proposed equation (1) evaluates the compressive strengths with absolute variations less than 4%. Figure 4 (Bar chart) shows the improvement of compressive strength on the effect of addition of fiber content for different w/cm ratios. It may be seen from the Figure 8 that the predicted compressive strength by the equation (1) and different researchers are having a good correlation with experimental values.

Flexural Strength (Modulus of Rupture)

The flexural strength (Modulus of rupture) tests were conducted as per the specification of ASTM C 78- 94 [18] using 100 x 100 x 500 mm prisms under third point loading on a simply supported span of 400 mm. The tests were conducted in a 100 kN closed loop hydraulically operated Universal testing machine. The load was applied at the rate of 0.1 mm/ min. Minimum of three specimens were tested to compute the average strength.

Table 3 and Figure 5 present the variation of the modulus of rupture f rf, on the effect of fiber content. Figure 6 shows the variation of the modulus of rupture fcf as a function of the compressive strength fcf of the HPFRC. The equation obtained for predicting the modulus of rupture, frf of fiber reinforced concrete using non– linear regression analysis, as follows:

frf = 0.019fcf 1.425, MPa                        (4)

The Cement and Concrete association of Australia adopts the following relationship for FRC, in its publication Industrial pavement-Guidelines for design, construction and specification.

frf = 0.70 √fc,' MPa which yields lower value compared to the proposed equation (4).

Figure 7 shows the variation of the modulus of rupture, fr as a function of compressive strength √fc of the HPC (plain concrete). The following equation is provided using regression analysis for the test results.

fr = 0.861, √fc MPa                             (5)

This equation (4) yields the values less than that obtained by Wafa and Ashour [6] of 1.03’ √ fc for HSC ACI 363-1992 [3] proposed the equation fr = 0.94 (ƒ’c)0.5 for concrete strength range of 21 MPa < ƒ’c < 83 MPa which yields higher values to the predicted equation (4).

Rashid et al. 2002 have proposed the equation which was obtained from the correlation of data by least square regression analysis (correlation coefficient = 0.94) as = 0.42 (ƒ’c) 0.68 for 5 MPa < ƒ’c < 120 MPa.

It is observed from the test results (Table 3) that there is a significant improvement in flexural strength in increasing the steel fiber content from 0.0 to 1.5 percent for all the mixes, varying from 16 to 38 percent of plain concrete. The increase in strength of 38 percent for 1.5% fiber content and 29 percent for 1.0% fiber content reveal that toughness will be much more than that of plain concrete. Using the tested results presented in Table 4, by performing linear regression analysis, the peak values of modulus of rupture of HP-FRC may be expressed as a function of RI and Vf respectively as follows.

frf = fr +0.665(RI)                        (6)

frf = fr + 1.695 ( Vf)                     (7)

where frf= modulus of rupture of HPFRC, MPa

frf = modulus of rupture of concrete, MPa

RCSR values obtained, varied in the range of 0.107–0.149.

Where, RCSR is the ratio of rupture strength to compressive strength.

Conclusion

Based on the test results of the experimental investigation using crimped steel fiber reinforcement with an aspect ratio of 80, the following observations can be drawn:

High performance concrete (silica fume concrete) is a highly brittle material and fails suddenly. The brittle mode of failure is changed by addition of steel fibers in to HPC, in to a more ductile one. It was observed that SFRC improves the concrete ductility, its post-cracking load carrying capacity. Fiber volume fraction up to 1.0 percent (RI= 2.58) is effective in increasing compressive strength which is by about 10% of plain concrete (silica fume concrete). The addition of steel fibers by 1.50 percent volume fraction results in an increase of about 10 percent in the compressive strength, and results in an increase of 38 percent in the flexural strength compared to no fiber matrix. The improvement in flexural strength varying from 16 to 37 percent of plain concrete. Empirical equations that predict the influence of fiber contents in terms of fiber reinforcing index on mechanical properties of HPFRC are presented.

The tensile strength as measured by modulus of rupture of HPC (plain concrete) is closely estimated by the equation fr= 0.861, fc, MPa The validity of the proposed expressions is limited to the type of fiber used up to 1.5 % volume fraction (RI= 3.88). The high-performance fiber reinforced concretes obtained have higher RCSR values, which varied in the range of 0.107–0.149.

Notations

HPC= high performance concrete

HPFRC= high performance fiber reinforced concrete

fc = compressive strength of plain concrete, MPa

fcf = compressive strength of HPFRC concrete, MPa

fr = modulus of rupture plain concrete, MPa

frf = modulus of rupture of HPFRC concrete, MPa

Vf = volume fraction of steel fiber, percent

l/d= aspect ratio

RI= fiber reinforcing index.

References

Balaguru, N., and Shah, S.P., Fiber reinforced concrete composites, McGraw Hill international edition, 1992, p.179- 214. ACI Committee 544, “State-ofthe- art report on fiber reinforced concrete,” ACI 544.1R- 82, American Concrete Institute, Detroit. ACI Committee 363, State-ofthe- art report on High strength concrete, ACI 363R- 92, American Concrete Institute, 1992. Attcin, P.C., High performance concrete, 1st edition, E& FN, Spon, London., 1998. Samer Ezeldin, A,. Balaguru, P.N., “Normal and high strength fiber reinforced concrete under compression,” ASCE, Journal of Mate. in Civil Eng., 4(4) (1992), p. 415- 429. Wafa, F.F. and Ashour, S.A. (1992), “Mechanical properties of high strength fiber reinforced concrete,” ACI Materials Journal, 89(5) (1992) p. 445- 455. Saluja, S.K., Sarma, M.S., Singh, A.P., and Kumar. S., “Compressive strength of fibrous concrete,” Indian Concrete Journal, 66(2) (1992), p. 99–102. Hsu, L.S., and Hsu, C.T.T., “Stress–strain behavior of steel fiber reinforced high strength concrete under compression,” ACI Materials Journal, 91(4) (1994) p. 448-457. Mansur, M.A., Chin, M.S., and Wee, Y.H., “Stress–strain relationship of high strength fiber concrete in compression,” ASCEJournal of mate. Civil. eng, 13(1) (1999), p. 21- 29. IS: 12269-1987, Specification for 53-grade OPC, Bureau of Indian standards, New Delhi, India. IS: 383-1970, Specification for coarse and fine aggregates from natural sources for concrete, Bureau of Indian standards, New Delhi, India. ACI 211.4R-93., Guide for selecting proportions for High strength concrete with Portland cement and Flyash, A C I Manual of concrete practice 1999. IS: 10262-1992, Recommended guide lines for concrete mix design, Bureau of Indian standards, New Delhi, India.

ACI Committee 544, “Guide for specifying, mixing, placing and finishing steel fiber reinforced concrete,” ACI materials Journal, 90(1) (1993), p. 94-101. IS: 516-1979, Indian standard methods of tests for strength of concrete, BIS 2002 Bureau of Indian Standards, New Delhi, India. Nataraja, M.C., Dhang, N., and Gupta, A.P., “Steel fiber reinforced concrete in compression,” ICJ, (July 1998), p. 353- 356. Standard test method for flexural strength of fiber reinforced concrete, ASTM C78-1994, Annual book of ASTM standards, American society for testing and materials, USA. Bayasi, Z, and Soroushan, P., “Effect of steel fiber reinforcement on fresh mix proportions of concrete,” ACI materials journal, 89(4)(1992), p. 369-374. Bhattacharya, G.K., Johnson, R.A. Statistical Concepts and Methods, Wiley, New York, 1977.

Effect of Fineness of Sand on the Cost and Properties of Concrete

Prashant Agrawal, QC Manager, HCC Ltd. Dr. Y.P. Gupta, Materials Consultant, BCEOM-LASA JV, Suryakanta Bal, QC Engineer, HCC Ltd. Allahabad Bypass Project, Allahabad, UP.

The grading and maximum size of aggregates is important parameters in any concrete mix. They affect relative proportions in mix, workability, economy, porosity and shrinkage of concrete etc. Experience has shown that very fine sands or very coarse sands are objectionable – the former is uneconomical, the latter gives harsh unworkable mixes. Thus the object in this paper is to find the best fineness modulus of sand to get the optimum grading of combined aggregate (all-in-aggregate), which is most suitable, and for economy. In general, the grading of aggregates, which do not have a deficiency or excess of any size of aggregate and give a smooth grading curve, produce the most suitable concrete mix. Further a cohesive mix is also desired for the pumped concrete produced by RMC Plant. In the present investigations, effect of the grading of river sand particles has been investigated for a good Concrete mix. Sand has been sorted in three categories i.e. Fine, Medium, and Coarse. These were mixed with coarse aggregate in different proportions so as to keep the combined Fineness Modulus (all-inaggregate) more or less the same. Various proportions of such aggregate are mixed in preparing M 30 grade of Concrete mix. Effect

is studied on concrete workability, cube strength, flexural strength and permeability. The results indicate that with the change in fineness of sand, workability gets affected. The details of findings and its effect on compressive and flexural strength and permeability, influencing durability are reported in this paper.

Introduction

Fineness Modulus is a term used as an index to the fineness or coarseness of aggregate. This is the summation of cumulative percentage of materials retained on the standard sieves divided by 100. It is well–known that aggregate plays an important role in achieving the desired properties of concrete. Though, aggregate constitute 80 to 90% of the total volume of concrete, yet very littleattention is given in controlling the grading and surface texture of aggregate to optimize the properties of concrete. Improper blend of aggregate influences the cement and water demand for a given concrete mix and affects workability, compactibility, and cohesion characteristics of pumpable concrete mix. It also influences the compressive strength, flexural strength and other properties like permeability & durability of concrete.

Review of Provisions in Different Specifications

IS 383: “Specifications for Coarse and Fine Aggregates from Natural Sources for Concrete.” This publication deals with specifications for Coarse and Fine aggregates from natural sources for Concrete. These specifications do not specify any limit for fineness modulus to be used in concrete. It divides the sand in four zones i.e. from Zone I to Zone IV. Zone I–Sand being very coarse and Zone 4 sand is very fine. It is generally recommended by code to use sands of zones I to Zone III for Structural concrete works.

AASTHO Designation: M6-93- “Standard Specification for Fine Aggregate For Portland Cement Concrete”- It indicates that the fineness modulus of sand will not be less than 2.3 and nor more than 3.1. Further, fine aggregate failing to meet the fineness modulus requirement as above may be accepted, provided concrete made with similar fine aggregate from the same source has an acceptable performance record in similar concrete construction; or in absence of a demonstrable service record, provided, it is demonstrated that concrete of the class specified, made with the fine aggregate under consideration, will have relevant properties at least equal to those of concrete made with the same ingredients, with the exception that a reference fine aggregate be used which is selected from a source having an acceptable performance record in similar concrete construction.

ASTM Designation: C33-93- standard specification for concrete aggregates”–The fine aggregate shall have not more than 45% passing any sieve and retained on the next consecutive sieve and its fineness modulus will not be less than 2.3 and not more than 3.1. Rest is the same as for AASTHO M6-93.

U.S.B.R: The code has specified that the fineness modulus of sand shall not be less than 2.50 and not more 3.0.

Experimental Investigation

In the present investigations, the effect of Fineness Modulus of sand has been investigated. The Fine aggregate (Sand) taken is Yamuna river sand and coarse aggregate taken is Dolomite limestone in crushed form. It has been sorted in several categories starting the Fineness Modulus (FM) of sand from 2.0 to 3.0. These were mixed in different proportions to get a consistent combined FM. The combined FM is determined like All-in-aggregate FM. In the present study we have selected M30 Grade of concrete mix. To find out the effect of fineness modulus (FM) of sand on concrete, sand of different FM from 2.0 to 3.0 is chosen. Two sizes of coarse aggregate particles: i.e. 20 & 10 mm, which are generally used in standard concrete mix, were chosen for the investigations.

Concrete Mix Selected:

Concrete Grade : M30

Water-Cement Ratio : 0.45

Cement: OPC 53 grade (350 Kg)

Aggregate to Cement Ratio : 5.52

Admixture: Super plasticizer (as required)

Fine Aggregate : Yamuna River Sand (Average 777Kg)

Coarse Aggregate: Dolomite; Crushed Stone (Average 1155Kg)

The material properties are given in Table 1.

When we choose very fine sand (i.e. FM 2.0), and very coarse sand (i.e. of FM 3.0), and if the proportion of sand is fixed in the mix then due to poor all-inaggregate grading, the mix may become very harsh or

not give correct results. So in present study proportions of coarse aggregate and fine aggregate, are slightly adjusted in the mixes to keep allin- aggregate grading within envelope of desired all-inaggregate grading given in IS: 383. The FM of combined mix is kept in range of 4.94 to 4.97 as seen from Table 2.

In this study water-cement ratio (W/C) of mix is kept constant for all the trial mixes with sand of different fineness modulus. Workability of mix is also fixed in range of 45 to 55 mm slump. Since mix with so different fineness modulus of sand, will result in different water demands, so watercement ratio is kept constant and to adjust workability slight adjustments in admixture dosages has been made. Various proportions of such ingredients are mixed in laboratory mixer of 0.1  capacity for preparing M30 grade of Concrete mix. Cubes (150 x 150 x 150 mm size), cylinders (150 Ö x 150 mm height) and beams (150 x 150 x 700 mm length) are cast. Effect of varying FM of sand is studied on concrete density, workability, compressive strength, flexural strength and permeability.

Observations & Discussion of Results

Table 3 gives the total observations recorded during the experimental investigations. Effect on Workability, Density, Strength, and Permeability due to variations in FM of sand is discussed here.

A. Workability of Concrete Mix: The workability of concrete mix was measured with the help of 300 mm standard size slump cone. A little amount of admixture dose was added to concrete mix. Each time concrete mix was examined for the behavior in slump, segregation and Bleeding etc. The slump observed was about 50 mm in all cases. No segregation or bleeding was observed in the mix.

Figure 1, shows the type of slump observed. The results indicate that with the increase in fineness modulus of sand, water demand in the mix got affected consequently workability gets affected. Since water-cement ratio is kept constant, so to keep workability in the same range of 50 mm, admixture dosages were varied. The admixture dosages reduced considerably as fineness of sand increases as shown in Figure 2. The Figure 2 shows that:

Admixture Dosage reduced from 1.0 percent to 0.2 percent as sand fineness modulus increases from 2.0 to 3.0.

For Every 5% increase in FM of sand, admixture dosage reduced by 0.1%.

Effect of Fineness of Sand on Density of Concrete

After measuring the slump, several 150 mm cubes were filled. These were cured in water tank for 28 days. After curing, each cube was weighed using electronic balance and density of concrete was calculated. The variation of density with FM of sand is shown in Figure 3 for different cases. From this figure, it is evident that there is slight increase in density i.e. 0.80 to 1.20 percent, when fineness modulus increases from 2.0 to 3.0.

Effect of Fineness of Sand on Compressive Strength of Concrete

Cubes of 150 mm were tested for compressive strength at 7 and 28 days. This compressive strength is given in table 3 for varying FM of sand. The variation is shown in Figure 4. The figure indicates that:

As fineness modulus of sand changes from 2.0 to 2.5 there is an increase in compressive strength from 43.07 to 49.00 MPa. i.e. strength increases by 14%. On the other hand by increasing Fineness Modulus from 2.5 to 3, Compressive strength increases from 49.00 to 56.83 MPa resulting in 16% increase in strength. For Every 0.1 increase in FM of sand from 2.0 to 3.0, 28 days Compressive Strength increases by 2.5 to 3.0%. 7 days compressive strength also increases in the similar proportion. There is faster increase in strength towards coarser side of sand.

Effect of Fineness of Sand on Flexural Strength of Concrete

Flexural strength is calculated from 28 days testing of beam of size 150x150x700 mm by using following formula.

Flex. Strength = P x 1000 x L / (b x d x d), for a > 200 mm but less than 200 mm

= P x (3000 x a) / (b x d x d), for a > 170 mm but less than 200 mm

= Result is discarded when a > 170 mm

Where,

b = width of sample beam (150 mm).

d = depth of sample at the point of failure (1500 mm).

a = distance between the line of fracture and the nearest support (recorded for each sample after test).

P = failure Load.

L = total support length of specimen (600 mm).

The variation of flexural strength with respect to different parameters is also given in Figure 4. This figure indicates the following:

As fineness modulusincreases from 2.0 to 2.5 there is an increase in 28 days flexural strength from 3.82 to 4.25 MPa i.e. strength increases by 11.25%. On the other hand by increasing Fineness Modulus from 2.5 to 3, the strength increases from 4.25 to 4.81 MPa resulting in 13.1% increase in strength. For Every 0.1 increase in FM of sand from 2.0 to 3.0, Flexural Strength increases by 2.1 to .5% The increase in strength is more towards coarser side of sand.

Effect of Fineness of Sand on Permeability of Concrete

Permeability of concrete is determined by using cylinder specimen having 150 mm diameter and 160 mm height. They were applied water pressure of 7 Kg/ cm2 for 96 hours in the Permeability Apparatus shown in Figure 5.

Immediately after 96 hours cylinders were split under line load test. The depth of penetration of water in cylinder was measured as well as volume of water lost is recorded.

The results are interpreted as:

1. Average depth of water penetration in cylinder is

2. Coefficient of permeability is calculated as volume of water lost divided by volume of concrete penetrated with water i.e. Permeability coefficient = vol. of water lost / (Area of cylinder x Average depth of concrete having effect of penetration of water).

The Permeability Coefficient of concrete Vs FM of sand is plotted in figures 6. It is seen from figure that permeability coefficient is more or less constant with respect to fineness of sand. Thus FM of sand has very little impact on Permeability Coefficient of Concrete and the value remains more or less constant.

Failure Pattern of Beams & Cubes

1. It is generally seen that the failure occurs at the interface of aggregate and mortar.

2. In flaky aggregate, some voids are observed at the interface of concrete and mortar. Elongated aggregate pieces are broken.

3. Mortar matrix isgenerally crushed.

Cost Benefit Ratio

Cost of Concrete mix per  is calculated on the basis of unit cost of each ingredient material in the mix. The following market rates have been taken for Cement, Sand, Coarse aggregate, Admixture and a nominal cost for water. No labor cost has been added in the calculation as it will remain constant.

Cement: Rs. 4.25 per Kg

Sand*: Rs. 0.30 to Rs. 0.32 per Kg (depending upon Fineness of Sand)

Coarse Aggregate: Rs. 0.75 per Kg

Admixture: Rs. 40.00 per Kg

Water: Rs. 0.10 per Kg.

* Rate of Sand for FM 2.0 to 2.3 is Rs. 0.30 per Kg, for FM 2.4 to 2.7 is Rs. 0.31 per Kg, and for F. M. 2.8 to 3.0 Rs. 0.32 per Kg. The variation of rate of sand depends on market which can have much more difference.

The quantities of ingredients for one  of concrete are given in table 4 (a). Cost of concrete is calculated by taking above rates and quantities given in table 4 (a). On the basis of cost calculated for concrete and the corresponding 28 day compressive strength, the cost benefit ratio is calculated as follows. This is given in Table 4 (b)

a) Cost of concrete is calculated in terms of quantity of material used & market rates as given above.

b) Cost Benefit Ratio is calculated as:

C/B ratio = Total cost of Concrete/28 days Compressive Strength

A curve has been plotted between FM of sand Vs C/B Ratio as shown in Figure 6. From this graph, it is seen that C/B ratio reduces considerably as the FM of sand increases. From FM varying from 2.0 to 3.0 the C/B ratio reduces by 71%. Thus, it is advisable to use coarser sand in Concrete.

Conclusion

Fineness Modulus of Sand affects Compressive and flexural strength of Concrete. Sand, with higher FM, results in higher strength of concrete. It is evident by cost benefit ratio that overall concrete mix is becoming economical if we use sand with higher FM. The results indicate that with the increase in FM, workability gets affected considerably. The cement demand also gets modified. Some of the observations are given below:

Fineness Modulus has larger impact on 28 days Compressive & Flexural Strength. Fineness Modulus has very little impact on Permeability of Concrete. Permeability coefficientis changed by about 2% for FM from 2.0 to 3.0. Fineness Modulus also affects the density of concrete. It increases by about 2.3% as the FM increases from 2.0 to 3.0. Optimum value of density and other parameters are obtained when FM is 2.8. The optimum value of strength can be taken when workability of concrete is also good. It is obtained when Fineness Modulus is about 2.7. The net cost of Concrete reduces when FM of sand increases. It reduces by about 6.5% for an increase of FM from 2.0 to 3.0. As the fineness modulus of Sand increases, the Cost/Benefit Ratio reduce by a very large factor. This is 29% when FM changes from 2.0 to 3.0. That means we can get large advantage by using concrete having Coarse Sand. A well adjusted grading (all-inaggregate) of concrete mix is also suitable for pumped concrete produced through RMC Plant. This is achieved by using a sand having FM of around 2.5.

References

NEVILLE, A.M, ‘Properties of concrete’, IV edition, Pearson Education Pvt. Ltd. 2005. MEHTA, P.K, PAULO J.M. MONTEIVO, ‘Concrete microstructure, properties and materials’, ICI, 1999. IS: 383-1970; ‘Specifications for coarse and fine aggregate from natural sources for concrete’, BIS, New Delhi. SP: 23-2001; ‘Handbook on concrete mixes based on Indian standards,' BIS, New Delhi. IRC 2001: ‘Specifications for Road and Bridge Works,' Indian Road Congress, New Delhi. AASHTO Designation: M6 – 93, ‘Standard Specification for Fine Aggregate for Portland CementConcrete. ASTM Designation: C33 – 93, Standard Specification for Concrete Aggregates. BS: 812, ‘British Standard for size and shape of aggregates’

Acknowledgment

The work has been carried out in M/ S HCC Ltd. Site Laboratory at Allahabad. The authors are thankful to them and QC staff of M/S BCEOM and HCC for their help.