the masterbuilder_april 2012_precast concrete special
DESCRIPTION
Contains three articles written by me:- Indian Mining Industry- Carbon fibres and their application in CE- Urban Rail Transport Trends & Policies_India(www.masterbuilder.co.in)TRANSCRIPT
82 The Masterbuilder - April 2012 • www.masterbuilder.co.in
Application of Laser Scanning in Mine Surveying
Laser scanners, often referred to as Lidars, have com-paratively only recently been introduced into the mine surveying kit. They bring to the table a host of addi-
tional capabilities as compared to their counterparts. Lidars are categorized into Aerial, Terrestrial and Underground.
Aerial scanners are intended for surveying of extended ar-eal and linear objects. In mining the new areas of designed mining companies, existing open-cast mining sites and collieries, oil and gas pipelines, roads, and power trans-mission lines are related to such objects. Formerly the simi-lar objects were mapped by aerial surveys and there was no other alternative for a long time.
Undoubtedly, laser ranging has a number of advantages over aerial surveys. The technology of field works is simple. After a short processing the coordinates of surveying points are got in a user’s coordinate system. The quality assess-ment of field works including a completeness of laser reflec-tion point clouds and orthophoto mosiacs is performed at the same aerial survey flight’s day that sufficiently reduces a scrap rate, preliminary ground survey works, and elimi-nates the necessity for ground control points. Only a lim-ited number of GPS-GLONASS base stations are required for laser scanning. Owing to a lidar’s navigation block, the direct geo-positioning method is used to provide with co-ordinates of laser ranging. The rate of cartographic works is 5-10 times higher than that of traditional technologies. The accuracy of laser scanning as proved from practice meets the requirements of “Guidelines for Mine Surveying Works”. In this case a survey team should strictly follow the standards for aerial surveys and calibration procedures outlined in corresponding manuals in detail.
The productivity of laser scanning is very high. It is real to survey in a day 500 km for linear object and 1,000 km for areal objects. Of vital difference of laser ranging is a pos-
sibility of night work. A key feature of this technology is its feasibility to survey a forested territory and simultaneously to generate a digital elevation model as well as to survey a territory with a slight relief and absence of marking situ-ation.
As an example we may take our project realized in Yaku-tia. The ore field with the area of 200 sq. km was taken as an object for laser scanning survey. The gold ore deposits have been mined there for a long time. In economic re-cession the gold was extracted with procedural violations of mining operations. Selective mining was practiced The mine surveying documentation was unconscientiously maintained and partially lost. At the moment of surveying the terrain had a lunar landscape with not clearly defined contours. Traditional surveying was practically unaccept-able. The work was started on August 10, and on Novem-ber 10 of the same year we handed over a final survey report together with accompanying digital topographical plans at scale 1: 2 000, orthophotos, digital terrain models, and 246 mine survey sheets. Our competitor participating in the same tender was ready to carry out the same volume of works for 18 months using classical aerial survey. Hav-ing got cartographic materials of good quality, the enter-prise has worked out a project and extracted about 3 tones of gold for the same months.
We have done a similar work at a gold ore deposit in Balei Zabaikalsky Region. The area of 50 sq. km was covered by aerial survey only for three flying hours. The office process-ing was finished in three weeks.
The economic efficiency of laser scanning is very high. The cost price is considerably lower than that of for aerial sur-vey. Why are the budget expenses drawn up according to traditional price lists? There is a simple explanation: they
Prof. Anatoly L. OkhotinPresident of the Baikal Union of Mining Surveyors, Director of “BaikalGeoService” Company, Head of the Department of Mine Surveying, Irkutsk State Technical University
Mining Lidars
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Figure 1: A survey team on board the aircraft
Figure 2: Orthophoto and map fragments
haven’t been drawn up for laser ranging yet. A profit is a good impetus to develop and implement the innovative technologies and to purchase the expensive equipment.
It should be mentioned that laser scanning has also dis-advantages as well. The process is strongly depended on weather conditions. It is influenced by precipitation and high humidity, low cloud and fumulus. There are some re-strictions on the flight height. The laser radiation is danger-ous for human eyes. However, all disadvantages should be related to the technology merits. Thereby, it should be also remembered that many of them are inhered aerial survey.
Land laser scanning has been successfully used on open-cast mining sites and collieries. Scanner positions (scan positions) are defined while the surveying is planning. In this case the scanner is set up either on a tripod or a ve-hicle (mounted on a mobile platform). There is no need in instrument centering of leveling. Its georeferencing is car-ried out on the targets located at a distance of 20-30 m apart of the scanner. The target positions are defined by a total station or GPS-GLONASS receiver. Laser scanner software is used for stitching of point clouds from differ-ent scan positions and creation a unified model. Terrestrial photography is performed
simultaneously along with laser scanning, which serves as a sketch and allows photographic interpretation in office.
As an example we can demonstrate the scanning of a Buryatiyan opencast
colliery occupied an area of 500 hectares. The team con-
sisting of 3 persons (a teacher and 2 students) has fin-ished field works for three working days. Office works have taken 2 weeks during which it was revealed that the team scanned and mapped 700 hectares instead of 500 hect-ares. In addition to traditional materials, 3D model of coal strip mine was added to DTM, a digital topographic plan and a metal-mounted board.
Once we have faced a problem to estimate load-bearing structure’s geometry of a new gold beneficiation plant’s in Bodaibo. The customer apprehensions were based on the fact that at building phases three different organizations were engaged in plant development. Besides, there were processing facilities on all the floors shading the build-ing structure that complicated essentially the execution of work.
Finally, the problem was solved. The scanning traverse was laid out round a building and continued through all the floors. It allowed the creation of a unified model where
Figure 3: Organization of field works
Figure 4: A topographic plan of opencast colliery compiled by scan data
the columns, beams and ties were distinguished on. This model was used as a basis to plot the measurement dia-grams with design deviations.
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Fortunately, all the drawings were correct enough and with-in specific tolerances.
We faced the same problem on the Beryozovsky hydro-electric power plant where it was necessary to estimate the
Figure 5: A view of the gold beneficiation plant
Figure 6: A stitched point cloud for the gold beneficiation plant
Figure 7: The distinguished load-bearing structures
structures geometry state of the highest in Russia industrial building 125 m in height.
The customer himself tried to solve this problem using re-
Figure 8: An approval drawing
Figure 9: A Shop at the Berezovsky hydroelectric power plant
Figure 10: 3D model of load-bearing structures
flectionless total stations for surveying purposes, but due to the complicated geometry of structures, their inacces-sibility and danger, he has given up this idea. We used RIEGL RIEGL LMS 420i and finished the whole volume of works for 16 days.
Mining Lidars
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Of great interest are mobile laser scanning systems. A scanner is set up on a moving platform (a car, a ship, a locomotive, etc.). The ready-made decisions are known in the world. They are used by such companies as IGI (Ger-many) and Optech StreetMapper(Canada). We have de-veloped our own version of the system. It is planned to be tested this year.
Laser scanners can be used in underground conditions as well. The main distinguishing feature here is the necessity to scan not only what is in front but next to and above. However, it does not influence on the laser scanning sur-vey technology and there is no need to make particular modifications in the technology. For example, we carried out the surveying of extraction chambers on the Tyretsky salt-mine.
Figure 11: A point cloud of for an extraction chamber
Figure 12: 3D model and vertical chamber sections
One of the key complicated problems in mine surveying is a regular vertical shaft alignment. In practice, misalign-ments of shaft guides are determined. A freelyhanging plumb is used as a vertical. Here should be mentioned that the alignment is complicated by the following factors: a vertical shaft depth, which is sometimes more than 1,000 m, airstream turbulence, atmospheric gas pollution, and high job hazard of working at heights in unconditioned situ-ation. The decision can be simplified if apply an automated station for the vertical shaft alignment, but in this case it provides with strictly limited volume of information that is not sufficient for estimation of shaft
conditions. From our point of view, application of laser scanning in a shaft would become the best solution of this problem. In-situ testing will be carried out in the nearest future.
Welfare and safety regulations in mining demand to carry
out regular scanning of capital closed work as well as its geometry, railtrack conditions and underground utilities.
Due to their long expansion and a large volume of works, the mine surveying service has not enough time for such works. We have developed a mobile scanning complex for automation of these works. We are planning to make its presentation soon.
Among other key complicated problems in mine survey-ing is a scanning of dangerous and inaccessible cavities. Such cavities are a lot of in chamber mining and rise driv-ing. At present, there are scanners able to scan the cavities at a distance. Among them we should mention Optech’s Cavity Monitoring System (CMS) provides fast, reliable and efficient scanning of underground cavities. By collecting thousands of measurements per minute, the data can be used to determine stope volumes, stope dilution, backfill volumes and create detailed drawings and data formats for use in any software workflow. With the recent intro-duction of the wireless feature, scanner operation can be controlled from a safe zone outside the cavity to enhance operator safety.
Practical applications of laser ranging and laser scanning in mine surveying given in this paper were carried out with the participation of the author.
An example of the underground chamber’s horizontal sectionson a pit of the “JSC Kola Mining & Metallurgical Company”
Mining Lidars
88 The Masterbuilder - April 2012 • www.masterbuilder.co.in
The use of Lattice Girders in the Construction of Tunnels
In recent years considerable advances have been made worldwide in the design and construction of sprayed concrete tunnel linings. Included in these advances has been the move away from traditional support using heavy rolled steel arches in linings, to lighter, more manageable lattice girders, steel mesh and/or steel fibres providing a continuous support. The development of lattice girders has provided engineers and contractors with greater options in design, increased flexibility and a more cost effective method of construction.
This paper outlines the development of lattice girders within the tunnelling industry, advantages of the product, other applications and specific project details from the United Kingdom and Australia.
History of Steel Support in Tunnels
At the turn of the 20th century steel supports were being used sporadically in tunnelling, usually patterned after the timber sets that were in widespread use at the time. In the late 1920’s and early 1930’s, steel began to be adopted more widely as its superiority was gradually recognised. By this time metallic linings such as cast iron were in virtually universal usage in soft ground tunnelling. With this major advance in lining an expansion of deep urban metro tun-nels had been seen, particularly in London, which contrib-uted generally to the increased use of steel.
The most common form of steel support used was rolled steel “I” or “H” section beams bent, welded and bolted to form an arch to fit the particular tunnel profile and secured in place by wood packing and wedges that span the re-maining gap formed by over break. Horizontal tie bars con-nect adjacent arches and wood or steel sheeting supports the exposed face behind. The completed arrangement is often referred to as “ribs and lagging” and is still in use today, particularly in the mining industry. In terms of con-struction the main disadvantage of this support method is the unwieldy nature of the heavy arch sections that often have to be manhandled in place.
From a design point of view the support system is es-sentially passive; i.e. ground movement must take place following installation for any load transfer to occur. If the arches are not correctly set or the blocking and lagging becomes loose or is insufficient, failure of the ground can occur locally placing severe loadings on individual sup-ports that then deflect.
Increasing emphasis on key elements of tunnel support such as: safety, speed, economy, performance and sur-face settlement led to major innovations during the late 1960’s and 1970’s with the development of tunnel boring machines with bolted, gasketted and fully grouted steel re-inforced precast concrete segments and the rapid spread of steel reinforced sprayed concrete or “shotcrete”. These methods offered systems that are quickly installed in full contact with the ground and are, therefore, able to provide early support that can restrict ground movement by struc-turally interacting with the surrounding ground to provide an active support system.
The first recorded application of shotcrete in tunnelling was in the USA during the early 1920s but it was not until the mid to late 1960s that it gained significant recognition following the development of new and innovative forms of tunnelling for the Snowy Mountains hydroelectric power scheme in
Komselis C.1, Blayney N.2, Hindle D.3
1Bekaert OneSteel Fibres Australasia (Brisbane Australia)2ROMTECH Ltd (Witham UK)3London Mining and Mineral Consultants Ltd (London UK)
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Australia and the New Austrian
Tunnelling Method in Europe. This followed pioneering work carried out in 1954 by a little known Austrian mining engineer, Anton Brunner, who patented a support system using the method.
Development of Shotcrete and Lattice Girders
Initially, shotcrete was applied either unreinforced, often in conjunction with rock bolting, or with more conventional steel arches embedded in a shotcrete lining. However, as the method was developed further, the main support was no longer provided by the steel arch sets but by the shot-crete shell in connection with the ground, which as a con-sequence required a thinner overall structural section than was possible with discrete arches. The steel arch became increasingly used only as a temporary supporting element to protect the face workers from unstable ground until the shotcrete was fully set. More suitable “V” section rolled steel arches became more popular for the thinner shotcrete lining and are still in sporadic use today, however, as is the case with conventional steel sections they are difficult to fully embed in the shotcrete shell without shadowing.
The problem of providing a steel arch that has both a low profile section and offers a minimal barrier to the place-ment of the shotcrete shell led to the development in the late 1970s of the lattice girder. Their ease of manufacture, transport, storage and handling underground ensured their rapid gain in popularity with tunnelling engineers and crews with additional benefits accruing to the design of the shotcrete lining itself. In tunnelling, the lattice girders retain many of the basic functions of conventional steel arch ribs requiring a degree of strength and rigidity that is efficiently provided by the 3dimensional steel lattice configuration.
Two basic types of lattice girder have been developed and relate to the number of main support members present. The most commonly manufactured threechord lattice has an isosceles triangular section with a larger bar diameter (25 40mm) at the apex and two smaller diameter bars (20 32mm) at the base corners. The apex bar is separated from the sidebars by small diameter (10 12mm) sinusoi-dally bent bars that are welded at each node to the main bars. The entire fabrication is radiused about the apex of the triangle with the apex being located either on the inside or outside of the curve. The basal bars are separated and braced by straight 16mm diameter cross bars welded at the node locations of the “sinusoidals”. The much less fre-quently used fourchord lattice has four equally sized bars (20 40mm diameter) located at the corners of a square or rectangular section. Pairs of bars on each side of the rect-angle separated and braced by sinusoidal bars and the two pairs are crossbraced by 16mm diameter crossbars
in a similar manner to the 3chord lattice. The sinusoidally braced sides are normally radiused or form the upright member in a straight lattice girder. In both forms of lattice girder the ends are secured by steel plates either butt weld-ed to the main bar ends or fillet welded to form base plates or connections.
Function and Design of Lattice Girders
Lattice girders provide the following important functions in the tunnelling process:
- Emergency temporary support/restraint for unstable ground.
- Accurate template guide for profiling the tunnel excava-tion.
- Rigid fixing and support for steel fabric reinforcement to the intrados and extrados of the shotcrete lining shell.
- Cantilever fixing for spiling ahead of the advancing tun-nel face.
- Temporary support to the shotcrete as it is being ap-plied and until it gains sufficient strength to support it-self.
- Accurate guide to the thickness of the shotcrete appli-cation.
- Contribution to the overall structural steel reinforcement in the completed lining shell. In structures such as ring-beams the lattice girder may provide the primary stee reinforcement member.
- Accurate fixings for tunnel convergence monitoring sta-tions.
In determining the relational dimensions of the various bars that make up the lattice the following aspects of lattice de-sign must be considered:
- Required rigidity, capacity and moment characteris-tics.
- Avoidance of shotcrete voids (shadowing) for complete encapsulation.
- Required shotcrete lining thickness and cover to steel.
- Minimum required arch radius.
The optimisation of the lattice design is in part calculated but is largely based on experience. Consequently, manu-facturers usually offer a comprehensive range of lattice configurations that have known structural properties from which the tunnel designer can select the girder size that best suits the tunnel diameter and lining thickness. The separation distance between girders is usually equal to the advance length of each tunnel round and the girder may extend around the crown arch of the tunnel only or con-
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tinue below the springing line to the tunnel invert. Where full invert closure is critical to the tunnel construction the lattice girder may be completed across the tunnel invert.
In larger tunnels a sequential excavation sequence is used with the full tunnel profile being excavated in stages to form a top heading, bench and invert, sometimes involving single or multiple side drift excavations that are staggered longitudinally by several metres. The versatility of the lattice girder provides a particularly useful means of setting out and temporarily supporting complex excavation sequenc-es. For this the complete lattice girder arch is fabricated in manageable sections that can be bolted or pinned to-gether at the end plates to form full moment connections when fully encapsulated in shotcrete.
There is some debate as to whether to orientate the apex of a threechord lattice girder towards the intrados or extrados of the lining. Structurally there is little difference between the two options and there is some argument against in-trados orientation as the single bar may induce a crack to form in the unrestrained, exposed lining surface. On the other hand this orientation provides distinct advantages in terms of providing a more efficient load transfer between the formation and the girder. In addition, by placing the girder apex towards the intrados the insertion of a spile through the lattice located at the advancing tunnel face and cantilevering it against the preceding girder’s thickest bar is certainly simpler and more efficient.
Manufacture of Lattice Girders
To ensure that the lattice girders fulfil their design criteria to function in a safe and predictable manner, as a tempo-rary support and profile former, it is vital that the product is manufactured in a quality controlled factory environment.
Materials
Materials used in the manufacture process must comply with the relevant Engineers specification and the national standards of that country providing full traceability back to a reputable source. The material strength will determine the load carrying capacity of the section and the chemistry the ability to bend the section to the desired dimensions and its ability to be welded.
Manufacture
Personnel involved in the manufacturing process will have had to undertake an induction p eriod on the construction of lattice girders. A suitably recognised welding qualifica-tion with proven competence in welding are also required. The welding of the sections determine the performance and the tolerance of the sections. Purpose built jigs should be used for repetitive construction which will improve toler-
ances and manufacture speed. Individually made girders will require additional measurement checks for tolerance control.
Quality Control
Stringent documented procedures are enforced to ensure that each component is completed to specification. The documents cover issues of:
- traceability for each uniquely numbered girder from material source through to all stages of manufacture
- weld quality and strength through non destructive testing
- tolerance and dimensional check requirements de-pending on the complexity and number of sections.
Photo 1 Typical Lattice Girder Sections
Installation of Lattice Girders
The lattice girder is usually the first structural member to be installed in the tunnel excavation either as soon as the excavation round is completed or a “flash’” layer of shot-crete is applied to protect a potentially unstable profile. The girder is usually installed, often using a laser setting out guide, as close to the tunnel face as is practicable with-out interfering with the excavation of the next tunnel round. This is to enable the following shotcrete lining to be com-pleted as far forward as possible to minimise the length of unsupported ground and to provide a profiling guide for the next excavation advance. The girder is usually offered up in place manually, sometimes from mobile cradles, or it can be elevated and manoeuvred into place using spe-cial attachments fixed to the excavator. The girder can be temporarily held in place by propping or pinning it to the formation. Alternatively, it can be bolted to a girder that has been already secured.
Traditionally welded wire fabric (WWF) is placed behind the girder and tied in place lapping to the WWF from the previ-ous round. An initial layer of shotcrete is then applied to
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cover the WWF half encapsulating the lattice girder. The second, inner layer of WWF can then be fixed in place and the shotcrete layer completed to the required thickness. This staged installation improves the penetration of the shotcrete though and around the steel without overload-ing the girder with shotcrete that has not reached sufficient strength to begin to support itself. Also the initial shotcrete layer should not have cured sufficiently to prevent the sec-ond layer from bonding to it. In addition, the application of the second shotcrete layer normally falls short of the newly installed girder in order to stagger the construction joint be-tween rounds and reduce water penetration. The process also provides a safe working environment minimising the exposure of unsupported ground.
With the development of Fibre Reinforced Shotcrete (FRS) the need for two passes of plain shotcrete to fully encap-sulate the lattice girder and mesh has been eliminated. Full thickness single pass FRS can achieve full encapsulation and increase the speed of tunnel advance. The FRS is ta-pered towards the end of advance to offer protection and create a lapping section for the next section of spray. The use of single pass layers is discussed further in section 9.
Where the tunnel heading is constructed in side drifts a temporary lattice girder supports the section of the main arch in the crown and is removed as the second side drift is excavated. An easily dismantled pinning arrangement is normally incorporated in the design of this temporary connection whilst the permanent bolted connection with the adjacent girder in the crown arch must be protected from becoming encapsulated by shotcrete until the arch is completed. Where the lattice girder is extended down the sidewall of a bench, the design must incorporate some form of temporary longitudinal support to the base of the crown arch that is undermined by the bench excavation. Alternatively a horizontal lattice girder is placed below the crown arch girder that bridges the gap between the bench and the last installed sidewall girder. This configuration is termed a “wall plate” from the similar arrangement in tra-ditional “ribs and lagging” support and is a particularly ap-propriate use of a 4chord lattice girder. The base of the lattice girder arch is formed simply from a buttwelded steel plate that can be packed underneath to temporarily sup-port the structure until it is encapsulated in shotcrete.
The key to efficient construction of the lattice girder arch is the close consideration needed for the lattice girder geom-etry and connection detail in relation to the tunnel geometry and excavation sequence. Safety and handling consider-ations are also important, as it is often the ease of setting out and erecting the girder in a confined space and in a limited time period that dictates the accuracy and function-ality of the completed lining.
Special Support Systems and Applications
In addition to simple tunnelling applications, the versatility of the lattice girder/shotcrete lining system allows complex underground openings to be created even in soft ground. Multiple junctions into shafts, tunnels and caverns are pos-sible using specially fabricated lattice girders that form the opening “eye” or ring beam support. In addition, the “eye” beam can be further supported by the insertion of rock bolts and spiles through the lattice which when en-capsulated in shotcrete forms a structural connection and intimate bond with the ground. In addition to bored tunnel construction a lattice girder and reinforced shotcrete shell can be used in cut and cover tunnel construction. Here the lattice girder arches are fully erected in an open cutting and braced by the fixing WWF to the intrados and extrados sides. Rough, temporary shuttering panels are then fixed to the extrados and the sprayed concrete is applied from inside the structure. Once the shotcrete has cured suffi-ciently to be selfsupporting, the shuttering is removed and further sprayed concrete is applied to the exterior face. The resultant reinforced concrete shell can then be loaded with backfill once it is fully cured. This technique has also been used to construct portal canopies and even surface struc-tures and buildings, further demonstrating the versatility of the method in the forms and geometries of construction that are possible, which would require complex shuttering and casting using conventional construction techniques.
Tunnel refurbishment is an increasing field of application for lattice girder and shotcrete support. For example brick or masonry lined tunnels can be strengthened by the in-corporation of reinforced concrete arches formed by local removal of the existing lining to form a slot in which a lat-tice girder can be erected. Infilling with shotcrete provides a good structural bond with the adjacent lining improving its overall load carrying capacity through the formation of reinforced concrete arches.
Projects in the Uk
Channel Tunnel Rail Link, Contract 410, North Downs Tunnel
The tunnel, constructed by a joint venture of Miller Civil En-gineering, BetonUndMonierbau and Dumez GTM, is 3.2Km long with a tunnel face area of 165m2 is the largest tunnel ever constructed on the United Kingdom mainland.
With cost and time major factors the JV decided to proceed with a sprayed concrete primary lining comprising of lattice girders, 2 layers of steel mesh, spiles and rock bolts.
The advance rates varied between 1 metre and 2.2 metres providing a recorded daily advance of up to 14 metres.
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This allowed the cast concrete secondary lining to be started early, culminating in the concrete works being com-pleted within budget and ahead of schedule.
The lattice girders provided commencement profiles for the portals as indicted in Photo2. Photo 3 shows the tunnel heading being advanced with support from lattice girders and shotcrete.
Photo 2. North Downs Tunnel London Portal
Photo 3. North Downs Tunnel Country portal
Greenway Pumping Station, Storm Water Surge Shaft
Constructed during 1998 the 21 metre deep x 15 metre di-ameter surge shaft would traditionally have been construct-ed using a precast segment lining. An alternative solution was accepted by the client Thames Water, using a sprayed concrete lining, reinforced with lattice girders at 2 metre vertical centres and two layers of steel mesh. The support method selected proved to be successful with construc-tion being significantly faster and cheaper than traditional precast methods.
Heathrow Express rail link from London to all four Heath-row Airport terminals
With the main running tunnels constructed using a precast segmental lining, it was decided that the more complex ar-eas of the tunnelling works would be constructed using the NATM method of construction in London Clay. These areas included two Terminal 4 platforms, Terminal 4 crossover, Terminal 4 turnout, and a turnout for the future provision of a fifth terminal and various adits and escalator shafts.
Lattice girders used in the turnouts and crossover were a complete ring at 1 metre centres with two layers of steel mesh. The method of construction used either a single side drift and enlargement or two side drifts and enlargement.
The construction of the first turnout was complex requiring 75% of the lattice girder sections to having varying lengths and radii every metre. The lattice girder dimensions were rationalised on further elements to increase speed of in-stallation.
Recent Projects in Australia
Buranda Tunnel
The Buranda Tunnel forms part of the Water Street to O’Keefe Street section of the South East Transit Busway. The driven tunnel was 190 metres in length being ad-vanced in two stages, heading and bench, with a tunnel width varying between 19.2 metres and the southern portal and 12.6 metres an the northern portal. There were chal-lenges in driving the tunnel from the southern portal due to the rail freight line situated approximately 2.5 metres above the tunnel crown and the large tunnel width at this point, shown in photo 4. With the low overburden, canopy tubes were installed for the first 30 metres of the tunnel as tempo-rary support. Lattice girders and steel fibre reinforced shot-crete (SFRS) were then used as the primary support. The lattice girders were installed in 4 sections utilising bolted connections. Adjustable footing supports where used on the lattice to account for the varying profile of the excava-tion. Tie rods where used to transversely position the gird-ers. The tunnel was advanced in 1metre intervals from the
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Southern Portal.
Photo 4 – Buranda Tunnel Southern Portal
Modified lattice girders also proved to be useful as support to the water proofing membrane and to provide additional reinforcing for the final lining in the southern portal section, shown in figure 5.
Photo 5 – Membrane Support Lattice Girders
The northern portal lattice girders were installed at 1 me-tre intervals and increased to 1.5 metres as the ground conditions improved. The lattice girders were first installed and shotcreted in the top heading of the tunnel. Once the bench was excavated lattice girder legs were installed and shotcreted, extending the lattice girder to the invert level. Photo 6 indicates the connection point between heading and bench.
Vulture Street Tunnel
The Vulture Street Tunnel forms part of the City to Wool-loongabba section of the South East Transit Busway. The section comprises of a 410 metre driven tunnel with par-ticular interest to the 34 metre length of Y junction located in the tunnel. The purpose of the Y junction was to allow for future transport and dual use provisions. The Y junction provided complexities in both design and construction due to the width of the section and low overburden to the main road above. The section was advanced in multiple stages, with a central pillar to be removed once the two tunnel sec-tions were developed. The multiple advance, in 1 metre intervals, also allowed installation of rock bolts, cables, lat-
tice girders and SFRS. The lattice girders were installed in 5 sections with the bolted and sliding joints used to accom-modate excavation tolerances. The sliding joint was the central connecting point once the pillar was removed. The sliding connection was suggested by the consultant as a practical method of installing the lattice girders in difficult conditions within complex geometries. The lattice girder was modified further to allow the rock bolts to be installed through the top chord of the girder eliminating large cum-bersome bolt plates.
Future Potential of Lattice Girders and Shotcrete Sup-port
Significant moves towards single pass permanent shot-crete linings have been made in recent years following im-provements in the wetmix sprayed concreting process and shotcrete mix designs, where the placed concrete quality can equal or even exceed Castinsitu reinforced concrete in terms of strength, compaction permeability and finish. Al-though there has been a parallel development of the use of steel fibres in place of the WWF reinforcement particularly for crack control and fire protection purposes the need for lattice girders to provide profiling will still exist.
New uses of the method are being developed for a wide range of civil engineering applications but it is perhaps the mining industry, where the method first began, that is likely to provide the main future growth. To date lattice girder and shotcrete support has had limited use in this industry with a tendency to use more traditional methods such as ribs and lagging, but the benefits of safety, economy and speed will, in the long run, prove attractive.
References
- Baumann Th. & Betzle M. 1984. Investigation of the Perfor-mance of Lattice Girders in Tunnelling. Rock Mechanics and Rock Engineering 17, pp 6781.
- Baumeister A.E. & Ertel J. 1985. Lattice Girder Construction and Dimensioning. Tunnel 2/85, May 1985.
- Betzle M. 1987. Lattice girders giving arches a dig in the ribs. Tunnels & Tunnelling, November 1987.
Photo x – Northern Portal Buranda Tunnel
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When the 4th year studio project was awarded to the students (June 2011 – December 2011) of Bharath University,
Nandhinee, a 7th semester student adopted a very un-conventional approach. She decided to utilize ‘Straw Bale’ as a construction material sighting its various advantages for the given project. This ar-ticle explores the methods, advantages and dis-advantages involved with this type of construction and to spread awareness of such sustainable con-struction practices.
Ar. Nisha. KSchool of Architecture, Faculty at Bharath University
Sustainablity Building Material
Studio Project:
Design a Hospital with 100 beds.
Design Requirements:
- Emergency Department - Outpatient Department- Orthopedic Clinics - ENT - Diagnostic - Inpatient block - OT complex - Dietary unit- Laundry - Central Sterile Supply- Other ancillary departments
Chief Design Considerations:
- Calm and serene environment- Sterile atmosphere- Crowd control- Ease in maintenance- Suitable response to climatic conditions in
Chennai- Sustainable use of available indigenous construction
techniques and materials
Straw BaleA Sustainable Building Material
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Site given for the proposed hospital was odd shaped and was challenging. Nandhinee came up with solutions for the site layout and the internal planning with simple forms and horizontal development. Nandhinee’s initiative into learning ‘Intelligent Approach to Sustainable Architecture’ from all over the world narrowed on to ‘Straw Bale Construction’.
Straw bale construction technology
A brief introduction
People have used straw, reed or grass for construction throughout history in places which lack common construc-tion materials such as timber stone or brick. With straw often being an agricultural surplus by-product and, it’s in-expensive, and an easily renewable medium. Straw is an agricultural by-product; the dry stalks of cereal plants, after the grain and chaff have been removed. Straw makes up about half of the yield of cereal crops such as barley, oats, rice, rye and wheat. It has many uses, including fuel, live-stock bedding and fodder, thatching and basket-making. It is usually gathered and stored in a straw bale, which is a bundle of straw tightly bound with twine or wire.
In times gone by, it was regarded as a useful by-product of the harvest, but with the advent of the combine harvester, straw has become almost a nuisance to farmers.
Bales may be square, rectangular, or round, depending on the type of baler used. Properly built, straw bale structures are fire-resistant, waterproof and actually pest free, with su-per-insulated walls. Bales of recycled materials like paper, pasteboard, waxed cardboard, crushed plastics, whole tires and used carpeting have been used and are currently being explored for building.
Each year tons of straw remains as a by-product of the agri-cultural crops of wheat, rice, rye, corn and grass seed. This straw is generally left to compost in the fields or is burned. The burning of agricultural straw is a significant cause of air pollution and contributes to global warming. The table below indicates the quantity of agricultural byproduct avail-able for use with India being the second highest producer of rice straw and wheat straw in the world.
Rank Area Rice Production (Metric Tons)
1 China 197212010
2 India 120620000
3 Indonesia 66411500
4 Bangladesh 49355000
5 Viet Nam 39988900
6 Myanmar 33204500
7 Thailand 31597200
8 Philippines 15771700
9 Brazil 11308900
10 USA 11027000
11 Japan 10600000
12 Cambodia 8245320
13 Pakistan 7235000
14 Republic of Korea 5804000
15 Madagascar 4737970
16 Sri Lanka 4300620
17 Egypt 4329500
18 Nepal 4023820
19 Nigeria 3218760
20 Peru 2831370
Because of the large quantity of straw available, and their tensile qualities, many types of agricultural straw are ideal for a wide array of products including paper, building ma-terials, textiles and other fiber-based products. Where stor-age of agricultural straw was once a limiting factor in its supply, modern harvest methods support year-round stor-age thus facilitating a ready and available supply of the fibre.
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Why Straw as a building material?
Straw is appealing as a building material for several rea-sons.
- In areas of grain production, straw is inexpensive.
- The quality of lumber is dropping, prices are unpredict-able, and some suggest future supplies may be lim-ited.
- Straw is a secondary waste material from grain produc-tion; its embodied energy should be fairly low.
- In many areas straw is still burned in fields, producing significant air pollution. Regulations to ban straw burn-ing are being implemented both to reduce air pollution and to reduce the risk of accidents.
In some areas most straw is tilled back into the soil. While straw provides few nutrients to the soil, it does add organic matter and helps aerate the soil, there is a concern that the agricultural soils would suffer if all of the straw was harvested. However with careful management, intermittent harvesting of the straw could be done without harm.
There is also evidence that too much straw may not be good for soil. Straw is decomposed primarily by fungi and that too much straw in the soil will throw off the balance between soil bacteria and fungi, reducing soil fertility
Off-field utilization of paddy and wheat straws continues to get attention due to concerns regarding environmental impacts from open field burning of the crop residues for its disposal. The field baler is recent introduction in India for recovery of combine-harvested straw for its utilization either as animal feed or by paper and board making indus-tries. The knowledge of straw availability and losses are of paramount importance for determination of economics of the system.
Characteristics
The thick walls (typically 21 to 26 inches (530 mm) when plastered), result in deeper window and door “reveals”, similar to stone and adobe buildings. Since the bales are irregular and may be shaped easily, they are readily adapt-able to curved designs, and when plastered, tend toward a relaxed, imperfect texture and shape. If flat, straight walls are desired, this can be achieved, as well, by the applica-tion of more plaster. Straw is very low in embodied energy (Embodied energy is the total amount of energy which is consumed in the manufacture and transportation of a product, in this case, building materials.) compared to ce-ment , steel or wood.
Rank Area Wheat Production (Metric Tons)
1 China 115180303
2 India 80710000
3 USA 60102600
4 France 38207000
5 Russian Federation 41507600
6 Pakistan 23310800
7 Canada 23166800
8 Australia 22138000
9 Turkey 19660000
10 Argentina 14914500
11 Germany 24106700
12 Iran 15028800
13 Ukraine 16851300
14 United Kingdom 14878000
15 Kazakhstan 9638400
16 Egypt 7177400
17 Brazil 6036790
18 Poland 9487800
19 Uzbekistan 6730400
20 Italy 6900000
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Properties associated with straw bale when used as a building material can be listed as below:
- Acoustics- satisfactory sound insulation performance
- Insulation- A carefully constructed straw-bale building has excellent thermal performance because of their combination of the bales high insulative value and the thermal mass provided by the interiors thick plaster coating.
- Thermal mass- thermal mass on a diurnal cycle (Ther-mal mass reduces temperature swings due to daytime warming and night time cooling, by absorbing and then gradually releasing heat. This can result in a direct reduction in the need for fuel or electricity to regulate temperature, and indirectly in savings through lifestyle adjustments: occupants of a moderate environment, with only gradual temperature swings, are less likely to use artificial heating and cooling.) Straw bale construc-tion provides a means of thermally insulating buildings at low cost and low embodied energy. The good ther-mal performance of straw bales is due to the high pro-portion of entrapped air within the straws and the bale
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matrix. Straw bales around 450 mm thick will have a U-value of 0.13 W/m²•K.
- Passive solar- Passive temperature control refers to buildings designed to maximize the heating and cool-ing effects of the environment around them. They are called passive because there are none (or few) parts of the design that require energy to operate.
- Availability and cost- Straw is an agricultural waste product, a by-product of grain harvesting. Many differ-ent kinds of straw are baled and can be used for con-struction. Cost depends upon time of purchase(harvest months) of the bales, how far they need to be trans-ported, and type of bale - whether it’s wheat straw, flax straw, or rice straw. Different “waste” products have dif-ferent values for farmers and some are less usable than others for agricultural purposes.
- Types-Bales are rectangular compressed blocks of straw, bound by strings or wires. Straw bales come in all shapes and sizes. Rectangular bales are the only bales suitable for building.
- Resistance to pests- Straw bales are thick and dense enough to keep out many kinds of pests. As well, the outer layer of plaster makes them unattractive or im-penetrable to animals and insects. Finally, because straw contains little nutrient value to most animals and insects, it does not attract pests. Plastered surface with no openings prevent the structure from infestation.
- Resistance to fire- Although loose straw is quite flam-mable, once packed into a bale it is too dense to allow enough air for combustion. By analogy, it is easy to light
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a single piece of paper on fire, but difficult and time consuming to burn an entire phone book. In construc-tion it is critical to have, at a minimum, a plaster coat of plaster on all surfaces of the wall.
- Structural properties- Load-bearing straw-bale walls are typically used only in single-storey or occasionally double-storey structures. A basement is uncommon.
- Design and construction challenges- Straw-bale build-ings must be carefully designed to eliminate the pos-sibility of moisture entering the walls, especially from above. Successful designs often incorporate roof overhangs that are wider than normal and roof shapes and detailing that minimize the risk of water splashing against walls
- Structural Capabilities of Bale Walls-The bale assembly can do a number of things, depending upon the struc-tural design of the building:
- Holds itself up, be self-supporting and resist tipping.
- Keep out the wind; inhibiting air/moisture infiltration.
- Resist heat transfer (insulation)
- Reduce water intrusion and migration, store and trans-fer moisture within the wall.
- Keep the assembly from buckling, under a compres-sive load.
- Keep the assembly from deflecting in a strong wind.
- Keep the assembly from bursting apart in an earth-quake, when pushed and pulled from all directions.
- Hold the plaster at least while it’s curing.
- Keep the plaster from cracking after it is cured, from shrinkage or movement.
- Transfer and absorb loads to and from the plaster.
- Support the plaster skins from buckling.
- Support the roof load (compression).
- Reduce damage or failure from high winds (ductility).
- Reduce damage or failure from earthquakes (ductility).
- Stop bullets and/or flying debris.
Finishes
Straw-bale walls are most typically plastered on the outside with lime, clay, or cement and lime mix. Inside surfaces are typically lime, clay or plaster board (gypsum). Structural analysis has shown that the straw-bale/stucco assembly behaves much like a sandwich panel, with the rigid stucco skins initially bearing most of the load and adding consid-erable strength to the wall.
Disadvantages
- Requires technical know-how for electrical and plumb-
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ing installations and execution .Short term training workshops can create awareness to common man and labour regarding installation and maintenance of the structure. Job specific and site specific detail sugges-tions by the designers will increase the popularity of this material.
- Typical failure of straw-bale homes involves frame walls set against straw-bale walls without a mortar coat. A spark from an electrical short or an error by a plumb-er ignites the hair-like fuzz on the exposed bale. The flame spreads upward and sets the wood framing on fire causing the wood framing to burn. The typical fire results in little fire damage to bales, but extensive water damage due to the fire suppression activities.
Further innovations and improvements in this field of Sus-tainable construction practices is thus encouraged from the
future architects so as to make use of this abundantly avail-able resource efficiently in rural and suburban context.
Picture Courtesywww.ko.wikipedia.org
Author’s Bio
After graduating from BMSCE- Bangalore in 2003, Nisha has worked as Designer Architect for 2.5 years at KOD Architects-Bangalore, to gain immense knowledge and work experience in the field of Architecture and Interior designing. Later she travelled in and around Ohio, United States and was exposed to Construction techniques and practices there. Back in In-dia being a Design guide (at School of Architecture, Bharath University) to enthusiastic students who constantly demand attention to develop their innovative ideas to be molded into reality, is a challenge which she had decided to take up. The author can be contacted at [email protected]
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Implementing Clean Hydro Electric Power Projects for Sustainable Development
Greenhouse Gases (GHGs) such as carbon di oxide, methane, nitrous oxide etc., are a major source of global warming for the deple-tion of the ozone layer environmental pollution and its protection is a matter of great concern for everyone. GHG concentration in the atmosphere has to be reduced; the Kyoto protocol came into force with itself a vast pool of Clean Development Mechanism (CDM). With an emphasis on India, the authors take a look at the current project profiles, the future areas of development, the challenges faced by the project owners, and the manner in which these problems can be addressed. In this back ground this paper argues for reduction in global warming by development of hydropower project BHAVANI BARRAGE (2*5MW) through CDM in meeting the future energy needs.
The world’s climate has always varied naturally, the vast majority of scientists now believe that rising concentrations of “Green House Gases” (GHGs)
in the earth’s atmosphere, resulting from economic and demographic growth over the past two centuries since the industrial revolution, are overriding this natural variability and leading to potentially irreversible climate change. Green House Gases (GHGs) – especially Carbon Dioxide, the most abundant from human sources – act like a blanket over the Earth’s surface, keeping it warmer.
Global Warming & Green House Effect
If heat sources are not controlled, it could alter temperatures, rainfall and sea levels of the earth. The buildup atmosphere of carbon dioxide and other gases such as methane, Chloro flouro carbons, nitrous oxide, ozone and others will continue to heat up the atmosphere, resulting into what is termed “Global Warming”.
Carbon dioxide [1] plays a critical role in controlling Earth’s Climate because as an aerosol it absorbs and reflects or scatters incoming radiation on the one hand, while absorbs and re-radiates outgoing infrared radiation on the other. This latter phenomenon results in what is popularly known as ‘Green House Effect’ – an analogy to what happens in a green house.
Such gases and their heating effects, have in fact survived life on earth as otherwise, temperature on earth could well have been below freezing, like that on mars. Excessive quantities of these gases may cause excessive heat, again to make life impossible like that on venus.
Effects of Global Warming
Following are some of the green house effect in the atmosphere, due to global warming
C.A. Murugesan1*, Dr. V.E. Nethaji Mariappan2, Dr. D. Joshua Amarnath3
1M.E. Student (Environmental Engineering) 2Scientist-D, Centre for Remote Sensing and Geoinformatics3Professor & HOD, Department of Chemical and Environmental Engineering,Sathyabama University
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- Rising of sea levels- Alteration of crop yields- Flood rising & increase in rainfall- Swamping cities and wet lands- Pests would proliferate- Diseases would spread
On an average of last 100 years, the earth’s temperature has gone up by 0.5ºC and water level of oceans has gone up by 10cm. It is estimated that if the present rate of heating continues then by the year 2030 – the earth’s temperature may go up by 1ºC and water level of oceans by 20cms, and by the year 2100 – the earth’s may go up by 4ºC and water level of oceans by 65cms.
Causes of Global Warming
Some of the causes of global warming are
- Population growth: The word population which is just over 5 billion is expected to double in the next 60 years. This increase will automatically result in more energy usage, driving more miles and producing more garbage.
- Emission of gases: Emission of gases in the due course of day to day human activities is another cause of global warming. Gases like carbon dioxide (due to burning of fossil fuels), chloro fluoro carbons (due to the refrigerator), methane (due to population growth) all are responsible for greenhouse effect in the atmosphere. The greenhouse shield of atmosphere is caused by the
following gases in the indicated proportion:
- Carbon dioxide (50%) Caused due to the burning of fossil fuels like coal, oil and natural gas. Emissions from incinerators, waste treatment plants, electric power plants, factories, house hold furnaces, car, buses etc.
- Methane (20%) Emitted into the atmosphere due to leaking of gas from natural gas walls, growing rice etc, Further, decomposition of organic matter especially in land fills also cause the gas emission.
- Chlorofluro Carbons (15%) This is used in refrigerators, car air conditioners, home insulation and a number of other products. These gases are man made incorporating modern technology.
- Nitrous Oxide (10%) This gas is released when coal and other fossil fuel are burned and is a product gasoline use and Nitrogen-based fertilizer emissions.
- Ozone (5%) This is a ground level gas-powered engine emission
- Cutting of forest trees: The indiscriminate cutting of rain forests contributes to global warming. In the process of photosynthesis, plants remove enormous quantity of carbon dioxide from the atmosphere every year (about 14 per cent of the total). As the rain forests are cut down, more and more carbon dioxide is left in the air. If adequate plans are not made to reforest before the rain forests are cut down – the result will be an ever grater build up of atmospheric carbon dioxide.
Clean Development Mechanism
The CDM allows Parties to implement projects that reduce emissions in the territories of non-Parties. The certified emission reductions – CERs – generated by such projects can be used by Parties to help meet their emissions targets, while the projects also help non-Parties to achieve sustainable development and contribute to the ultimate objective of the Convention.
The rulebook for the CDM set forth in the Marrakesh Accords focuses on projects that reduce emissions. Rules are being developed, however, for adoption at COP-9, for including afforestation and reforestation activities in the CDM for the first commitment period. Parties would be limited in how much they may use CERs from such sink projects towards their targets, up to 1% of the Party’s emissions in its base year, for each of the five years of the commitment period.
A CDM project might then involve, for example, a rural electrification project using solar panels, or the reforestation of degraded land. As with joint implementation projects, Parties are to refrain from using CERs generated through nuclear energy to meet their emissions targets.
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The CDM is expected to generate investment in developing countries, especially from the private sector, and promote the transfer of environmentally sound technologies in that direction. However, the finance and technology transfer commitments of Parties under the Convention and the Kyoto Protocol are separate and remain valid. Furthermore, public funding for CDM projects must not result in the diversion of official development assistance.
CDM projects must have the approval of all Parties involved, and this may be gained from Designated National Authorities (to be set up by each Party. Projects must lead to real, measurable and long-term benefits related to the mitigation of climate change, in the form of emission reductions or greenhouse gas removals that are additional to any that would have occurred without the project. The Protocol envisages a prompt start to the CDM, allowing CERs to accrue from projects from the year 2000 onwards. The election of the CDM executive board at COP-7, and the beginning of its work, has already put this prompt start into effect.
The 10-member Executive Board supervises the CDM, operating under the authority of the COP/MOP (a role being performed by the COP until the COP/MOP meets). Key initial tasks of the executive board are to develop simplified procedures to encourage small-scale projects, notably for renewable energy and energy efficiency activities, and to accredit independent organizations, known as Operational Entities, pending their formal designation by the COP or COP/MOP. These operational entities play an important role in the CDM project cycle, which is described below.
CDM projects must be based on a project-specific, transparent and conservative Baseline (the starting point for measuring emission reductions or removals), and must have in place a rigorous Monitoring Plan to collect accurate emissions data. The baseline and monitoring plan must be devised according to an approved methodology. If the project participants wish to use a new methodology, it must be authorized and registered by the executive board.
In order to implement a CDM project [3], the project participants must prepare a project design document, including a description of the baseline and monitoring plan to be used, an analysis of environmental impacts, comments received from local stakeholders and a description of the additional environmental benefits that the project will generate. An operational entity will then review the project design document and, after providing an opportunity for public comment, decide whether or not to validate it. If a project is duly validated, the operational entity will forward it to the executive board for formal registration. Unless a project participant or at least three executive board members request a review of the project, its registration
will be deemed final after eight weeks. Once a project is up and running, participants would monitor the project. They would prepare a monitoring report including an estimate of CERs generated by the project and would submit it for verification by an operational entity. (To avoid conflict of interest, this will usually be a different operational entity to that which validated the project design document.)
Following a detailed review of the project, which may include an on-site inspection, the operational entity will produce a verification report and, if all is well, it would then certify the CERs as legitimate. Unless a project participant or three executive board members request a review within 15 days, the executive board will issue the CERs and distribute them to project participants as requested. These six steps – validation, registration, monitoring, verification, certification and issuance – make up the CDM project cycle. Finally, the CERs generated by projects would be subject to a levy, termed the “share of the proceeds”. Two percent of the CERs of each project will be paid into a newly-created Adaptation Fund to help particularly vulnerable developing countries adapt to the adverse effects of climate change (projects in least developed countries are exempt from this part of the levy in order to promote the equitable distribution of projects). Another percentage, yet to be determined, is to cover the CDM’s administrative costs.
Objective of CDM
The CDM has the following three stated objectives:
- To assist Parties not included in Developing Countries in achieving Sustainable Development;
- To contribute to the ultimate objective of the Convention
Gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system); and to assist Parties Developed Countries in achieving compliance with their quantified emission limitation and reduction commitments under Article 3 of the Kyoto Protocol [5&6].
Project Monitoring
Monitoring describes the systematic surveillance of a project’s performance by measuring and recording performance-related indicators relevant to the project. The monitoring and verification protocol (MVP) should be described in the Project Design Document. The DOE would use the information and data collected through the MVP to verify a project’s emission reductions. The monitoring system should be capable of producing records on reported GHG emission reductions, social and environmental impacts and on project management, including monitoring, data collection and management systems.
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Verification, Certification and Issuance of CERs
Emission reductions must be verified and certified by a DOE before the executive board can issue CERs. The verification process involves periodic auditing of monitoring results, the assessment of achieved certified emission reductions and checking the Project’s continued compliance with all relevant criteria. The monitoring and verification process is therefore the basis for the production and delivery of CERs to the Project Operator (or other buyers) and for any related revenue stream that the operator expects to receive.
The audit process during verification is expected to be interactive, iterative and participatory. The DOEs may use spot checks of data measurement and collection systems and interviews with the key project participants to determine the credibility and accuracy of the reported performance. It is very important to understand the “Carbon Crediting Project Cycle” i.e. the process through which the project promoter has to undergo.
Emissions Trading (ET)
Through emissions trading, Parties may acquire assigned
amount units (AAUs) from other Parties that find it easier, relatively speaking, to meet their emissions targets. This enables Parties to utilize lower cost opportunities to curb emissions or increase removals, irrespective of where those opportunities exist, in order to reduce the overall cost of mitigating climate change. Similarly, parties may also acquire CERs (from CDM projects), ERUs (from joint implementation projects), or RMUs (from sink activities) from other Parties. In order to address the concern that some Parties could “over-sell” and then be unable to meet their own targets, each Party is required to hold a minimum level of AAUs, CERs, ERUs and/or RMUs. This is known as the commitment period reserve and cannot be traded.
It is calculated as 90% of the Party’s assigned amount, or as the amount of emissions reported in the Party’s most recent emission inventory (multiplied by five, for the five years of the commitment period), whichever is the lower figure. If an Party goes below its commitment period reserve, it is given 30 days to restore the reserve to its required level. (ERUs verified through the Article 6 supervisory committee, however, can be freely transferred, irrespective of the level of the commitment period reserve.)
Minimizing Impacts on Developing Countries
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The Protocol echoes the Convention in paying special attention to the concerns of developing countries, especially those particularly vulnerable either to the adverse impacts of climate change or to the implementation of response measures, along with the specific needs of least developed countries. The Protocol therefore commits Parties to strive to implement their emissions targets through policies that will minimize adverse impacts on developing countries.
The Marrakesh Accords require Parties to report on an annual basis on the actions they are taking to meet this commitment. The information reported may be considered by the facilitative branch of the Compliance Committee. Non-Parties, in turn, are invited to provide information on their specific needs and concerns. The Marrakesh Accords also call attention to certain actions that should be prioritized in order to minimize adverse impacts on developing countries. These include:
- Removal of subsidies for environmentally-unfriendly technologies;
- Development of non-energy uses of fossil fuels, advanced fossil-fuel technologies and carbon capture/storage technologies;
- Capacity building to improve efficiency; andAssisting developing countries that are highly dependent on fossil fuels to diversify their economies
In addition, as noted above in the discussion on the CDM, the Marrakesh Accords established an Adaptation Fund. The fund, which will be managed by the GEF is to be funded not only by the adaptation levy on CDM projects, but also by additional contributions from Parties.
The adaptation fund will finance concrete adaptation projects and programmes in developing countries, along with such activities as supporting capacity building. Parties that intend to ratify the Kyoto Protocol are required to report on their contributions to the fund on an annual basis, and these reports will be reviewed by the COP/MOP.
Hydro Electric Power Project by Tangedco/Tneb
The 10 MW (2×5 MW) Bhavani Barrage -1 Small Hydro power project is a run-of-the-river project proposed by Tamil Nadu Electricity Board (TNEB) in between Pillur dam and lower Bhavani dam. The purpose of the project activity is to generate hydro power by utilizing the potential energy available in the flows of the river Bhavani, which would generate energy of 16720000 kWh per annum on an average.
The proposed project activity utilises the tail waters let out from the existing Pillur Powerhouse (2×50 MW) for power generation. The maximum power discharge let out from Pillur power house is 173 cumecs. The Pillur reservoir receives
flows from Bhavani river, Kundah river and Niralapalam stream. The hydro potential due to the bed fall of 9m and discharge of 173 cumecs has not been exploited so far by any agency. Hence to harness the hydro potential TNEB has proposed the 10 MW hydro project.
Tamil Nadu Electricity Board (TNEB) owned by government of Tamil Nadu is responsible for power generation, transmission and distribution is exploring to tap power potential available in various forms of energy to improve access to the power in the rural areas. TNEB has proposed this project activity to improve availability of power and water in this remote region as well as to support environment by exploiting clean sources of energy [4]. The Bhavani reservoir improves ground water levels in the surrounding areas and thus ground water can be tapped by farmers for irrigation under bore wells by utilising electricity since it is not possible to arrange for conventional irrigation canal system under the foot hills of Western Ghats.
This project will ensure easy access to power and water to the surrounding rural people. The project operation would contribute to sustainable development, substitute fossil fuel generated power, reducing emissions of GHGs, contribute to economic development of area and reduce the dependence on fossil fuels. Thus, the power generation would be carried out in sustainable manner without causing any negative impact to the environment.
Hydro Power project activity’s contribution to sustainable development (EIA)
Project would contribute to the below indicators for sustainable development in the following manner:-- Social well-being- Economic well-being- Environmental well-being- Technological well-being
Social well-being
Presently some of the areas in the project region do not have access to the water for irrigation and drinking. Bhavani Barrage can ensure storage of water in the river which can be used by nearby farmers for irrigation and drinking purposes. Bhavani barrage also envisages improvement of ground water in the region. Power cuts are imposed in the rural areas due to power shortage. The power generated through proposed stations would reduce power cuts in the rural areas such as proposed project region. Further, it would contribute to the creation of employment opportunities to the local people during the construction (100 members) which is expected to last for about two years and operation phase (10 members) regular employment. Creation of these opportunities would partly prevent migration of rural population to urban areas [2].
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With availability of more stable power, there would be increase in the economic activities. The infrastructure in and around the project area would also improve due to project activity being set up in a rural area, which otherwise would not have taken place. With improved infrastructure, standard of living would also improve in the local area.
Economic well-being
The economy of the area is dependent on agricultural activities, in particular commercial crops and due to storage of water by building of pondage. Pondage is a small water storage facility to cater to the hourly demand for power generation. The ground water around the pondage is recharged so that the farmers could exploit the same to grow commercial crops in the lands surrounding the project. The project would greatly help the local farmers to supplement their water requirements for irrigation by installation of tube wells which could be operated by power generated from the project activity.
Project proponents would mobilise investment in the region to an extent of about ` 643.00 millions which would not have occurred in the absence of the project activity. This is a significant investment in the region. The available power can also be utilized for growth of small industries which will boost the general economy of the area.
As per CEA, Power Scenario at a Glance for Tamilnadu, September, 2008 (page 47), there is power deficit of 11.5% in the State of Tamil Nadu during the period 2005-06. This causes power instability in the grid leading to power cuts in the rural areas. The power generation from the project area would stabilize the grid as well as quality of power in the local area. With rising hydro power generation and improving efficiencies in distribution of electricity, the project activity would be offering energy at stable prices for economic development in the remote rural areas.
The present project activity will be operated as peak load station. The storage envisaged for the project is on daily basis. The storage is utilized to run the project to the rated capacity during peak load hours in the morning and evening. Thus the project activity would result in stabilization of the local area during peak time. The project would attract investments for developing agro based industries in the region.
Environmental well-being
The proposed project activity is small hydro project which utilise the discharges available from Pillur power house. The project is environment friendly with least disturbance to natural eco-system and the submergence of fertile lands will be avoided by construction of afflux bunds (these are flood banks at higher elevation to prevent
submergence of near by agricultural lands during larger inflows at the location). For the purpose of encouraging the agricultural production in the region optimal submergence is maintained. During construction phase pollution levels such as particulate matter, SO2, NOx and CO would likely to increase to a negligible extent and would not have any effect on the environment. The energy generated from the project activity would be clean energy as it is a small hydro based project, which would not involve any GHG emissions and in the process support the global climate change programme. Thus, the project activity would not cause any negative impact on the environment.
Technological well-being
The project activity is the first small hydro project in Coimbatore district of Tamilnadu. The project would utilise environmentally safe and sound technology in small-scale hydro-electric power sector. Further the project would demonstrate the feasibility of harnessing water discharges in the river under low head and encourage setting of similar projects in future. The above benefits due to the project activity would ensure the project contributes to the sustainable development of the region.
Study area
The project Bhavani Barrage-1 is located on the barrage developed across river Bhavani, a tributary of river Cauvery. The project is situated at Samayapuram village, Mettupalayam Taluk and Coimbatore Dist. The location is a distance of 6Km from Metttupalaym and 42km from Coimbatore. The nearest railhead is Mettupalayam (6km). The nearest airport is Coimbatore. The project is located between Latitude 11”17’37” N and Longitude 76”53’38”E.
Physical location of the project is marked in the maps below.
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Application of environmentally sound and safe technology
The technology of power generation process using hydro resources is by conversion of the energy available in the water flow into mechanical energy using hydro turbines and then to electrical energy using alternators. The generated power will be transformed to match the voltage of nearest grid substation for proper interconnection and smooth evacuation of power. In this process there would be no greenhouse gas emissions or burning of any fossil fuels. Thus, electricity would be generated through sustainable means without causing any negative impact on the environment. Therefore, the technology is environmentally safe and sound.
Technical Details
Hydrology Particulars
Design Discharge : 85m3/s (per machine)Gross Head : 9 mDesign Head : 7 m
Energy Production
Expected annual generation : 16720000 kWh (units)Auxiliary consumption : 167200 kWh (units)Expected annual export : 16552800 kWh (units)
Plant Equipment Details
Hydro Turbine : Bulb TypeNo. of generating units : 2Generator type : SynchronousCapacity of each generating unit: 5MWGeneration voltage : 6.6 kV, 3 phaseGrid transmission voltage : 22 kVFrequency : 50 HzDiesel Generator Set : 2 x 63.5 kVA
Description of the project boundary
The project boundary of the project activity will consist of diversion structure, penstock, powerhouse, DG Set, tail race channel and the transmission system till switch yard. The figure depicting the project boundary is furnished below:
Emission reductions
Explanation of methodological choices
The project activity is generation of electricity using hydro potential and exporting the same to the grid system, which is also fed by other fuel sources such as fossil and non-fossil types. Emission reductions due to the project activity are considered to be equivalent to the emissions avoided in the baseline scenario by displacing the grid electricity. Emission reductions are related to the electricity exported by the project and the actual generation mix in the grid system.
The key plan indicating location and distances on the Pillur dam and lower Bhavani dam
CONSTRUCTION PHOTOS OF BHAVANI BHARRAGE (Near Mettupalayam at Coimbatore, Tamil Nadu
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Baseline
The baseline emissions are calculated based on the net energy supplied to the grid (in kWh/year), and an emission factor for the displaced grid electricity (in kg CO2 e/kWh).
BEy = EGy * EFy
where
EGy = the net electricity exported to the grid system during the year y
EFy = the emission factor of the grid to which the project exports electricity
The emission factor of the grid for the ex-ante approach is calculated in the following way
In accordance with the “Tool to calculate the emission factor for an electricity system,” the grid emission factor is calculated using Combined Margin (CM), comprised of an Operating Margin (OM) emission
factor and a Build Margin (BM) emission factor. The following procedure was adopted for estimating the grid electricity emission factor:
Step 1. Identify the relevant electric power system.
Step 2. Select on operating margin (OM) method.
Step 3. Calculate the operating margin emission factor according to the selected method.
Step 4. Identify the cohort of power units to be included in the build margin (BM).
Step 5. Calculate the build margin emission factor.
Step 6 Calculate the combined margin (CM) emission factor.
Step 1 – Identify the relevant electric power system
The CEA of the host country has published a delineation of the project electricity system and connected electricity systems. According to data published by the CEA of India the project activity falls under southern regional grid.
Step 2 – Select an operating margin (OM) method
The approved methodological tool recommends the use of one of the following for the calculation of the operating margin emission factor (EF grid,OM,y):
a) Simple OM, orb) Simple adjusted OM; orc) Dispatch data analysis OM; ord) Average OM.
The methodological tool recommends the use of dispatch
data analysis as the first methodological choice.
However, in India availability of accurate data on grid system dispatch order for each power plant in the system and the amount of power dispatched from all plants in the system during each hour is practically not possible. Also, still the merit order dispatch system has not become applicable and is unlikely to be so during the crediting period.
Step 3 – Calculate the operating margin emission factor according to the selected method.
a) Simple OM
In the Simple OM method, the emission factor is calculated as generation weighted average CO2 emissions per unit net electricity generation (tCO2/MWh) of all generating sources serving the system, not including low-operating cost and must-run power plants. Simple OM can be calculated using any of the three available methods. Option A has been selected where the data on fuel consumption and net electricity generation of each power plant/ unit is available. The CEA baseline is derived using the following formulae to calculate simple OM
(1)
Where:
EFgrid,OM,Simple,y is simple operating margin CO2 emission
factor in year y (tCO2/MWh)
FCi,m,y is amount of fossil fuel type i consumed by power
plant / unit m in year y (mass or Volume unit)
NCVi,y is net calorific value (energy content) of fossil fuel
type i in year y (GJ / mass or volume unit)
EFco2,l,y is CO2 emission factor of fossil fuel type i in year y
(tCO2/GJ)
EGm,y is net electricity generated and delivered to the grid
by power plant / uit m in year y (MWh)
m is all power plants / unit serving the grid in year y except low-cost / must-run power plants / units
i is all fossil fuel types combusted in power plant / unit m in year y
y is either the three most recent years for which data is available at the time of submission of the CDM-PDD to the DOE for validation (ex-ante)
Step 4 – Identify the cohort of power units to be included in the build margin
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The tool to calculate the emission factor for an electricity system offers two options for determination of build margin emission factor: ex ante and ex post determination of the Build Margin (BM). Option 1 is selected wherein the build margin emission factor is calculated ex- ante based on most recent information available on plants already built for sample group m in southern region. This simplifies the monitoring procedures, but also offers a conservative approach of BM calculation. The sample group m shall be the one having higher power generation between (a) five power plants that have been built most recently and(b) the capacity additions in the electricity system that comprises 20% of the system generation built most recently. It is found that the option (b) has higher generation compared to option (a). Hence option (b) is selected.
Step 5 – Calculate the build margin emission factor
The build margin emissions factor is the generation of weighted average emission factor (tCO2/MWh) of all power units m during the most recent year y for which power generation data is available, calculated as follows:
Where:
EFgrid,BM,y - Build margin CO2 emission factor in year y
(tCO2/MWh)EGm,y - Net quantity of electricity generated anddelivered to the grid by power unit m in year y (MWh)EFEL,m,y - CO2 emission factor of power unit m in year y(tCO2/MWh)m - Power units include in the build marginy - Most recent historical year for which power generation data is available
Step 6 – Calculation of the baseline emission factor (Combined Margin)
The baseline emission factor in year y is calculated as the simple average of the OM and BM emission factors, i.e. OM and BM are each weighted with 50% for the first crediting period. As noted above, the resulting Combined Margin is fixed ex ante for the duration of the crediting period:
EFgrid,CM,y = Wom EFgrid,OM,y + WBM EFgrid,BM,y
Where:
EFgrid,BM,y - Build margin CO2 emission factor in year y
(tCO2/MWh)
EFgrid,OM,y - Operating margin CO2 emission factor in year
y (tCO2/MWh)Wom - Weighting of operating margin emissions factor (%)WBM - Weighting of build margin emissions factor (%)
As the proposed project activity is Hydro, the weighting of operating margin emission factor and weighting of build margin emission factor is considered as 0.5 and 0.5 respectively and calculated combined margin as under.
Combined Margin (CM) Simple average of OM and BM
0.854239642 kg CO2 e / kWh
Project emissions
As part of the project activity a backup diesel generator (2 x 63.5 kVA) to meet the emergency requirements of power house will be installed. Emissions resulting from usage of diesel backup generator will be accounted as project emissions based on the following equation as provided in the approved consolidated methodology.
PE diesel,y = Fd,y * Density * NCV * EFCO2 * OXID / 10^6
Where Fd,y is the quantity of diesel used during the year (Ltrs)
Density of diesel (0.845 kg/Ltr. as per Society of Indian Automobile Mfgs.
http://www.siamindia.com/scripts/Diesel.aspx)
NCV is the calorific value of diesel (43 TJ/Gg as per IPCC 2006 default value)
EFCO2 is the CO2 emission factor of Diesel (74.1 t CO2/TJ
as per IPCC 2006)
OXID is the oxidation factor (1 as per IPCC 2006 default value)
Barriers in executing the Hydro Project
Hydrology risks:
The monsoon characteristics of the Kerala play a dominant role in determining water availability for the project, as the catchment area lies in the Kerala. The Bhavani Barrage-1 small Hydel Scheme is a run-ofriver project. As such, it is dependent upon the availability of water in the river – which is controlled by the Pillur power house located in Kerala, upstream – for generating electricity. The existing Pillur Power House (2x50MW) is a peak load station. As such, the hydrological studies of water availability for this project is determined exclusively by the discharges into the river from the Pillur power house. Gauged data for the flow at the tailrace of the existing Pillur powerhouse (2 x 50 MW) is available for the years 1999-2000, which is considered for estimating the power potential at the
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proposed project site.
The proposed project is also a peak load station. The net flows after meeting the irrigation requirements, at the proposed project site are available for power generation. As the project activity is depending on the flows from the catchments area located in the State of Kerala, any change in the governmental policies or any other barrage upstream will weaken the project and result in a significant risk of seasonal disruptions in power generation from the plant.
Policy related barrier
The power generated by this project will be fed to the grid, which meets the power requirement of various classes of consumers in Tamil Nadu. All the consumers do not pay the same tariff for the power consumed. As per the policy of Govt. of Tamil Nadu, the power is being supplied to different consumers at different tariff. TNEB, being a State Utility has certain social obligations to fulfil. For example, it has to supply power to agriculture at a highly subsidised rate and charge low tariff to domestic consumers.
Likewise, it has also certain restrictions. For instance, it cannot sell power outside the State and earn a higher tariff. As of 2004-05, TNEB was supplying power to agricultural sector at ` 0.22 per kWh; domestic consumers at ` 2.16 per kWh; public utilities at ` 3.42/kWh and commercial organizations at ` 5.79/kWh. In the ultimate analysis, what accrues to TNEB is the weighted average of all these prices, which worked out to ` 3.37 per kWh2. The tariff to agriculture and domestic consumers, being a political issue and TNEB being a State Government utility, it has little choice but to follow the policy set by the Government in power.
The weighted average tariff accruing to TNEB, therefore, remains lower. It is well high impossible to predict what type of concessions in tariff the Government would offer, as it depends upon political expediency. This is a major barrier for the project. However, this gap (between weighted average tariff accruing to TNEB and cost of generation) has to be narrowed down to improve the loan serviceability of the project, as this project is fully funded by debt. It is here that CDM revenues would of great help. CDM benefits would enable the project to bridge the gap between accruing tariff and expended cost in generation.
This, in turn will set in motion a demonstration effect, in that it would encourage undertaking such new projects, which would go a long way in not only augmenting the power supply, but also contributing to the development of hinterlands, improving the overall living standards of the people and bringing the hitherto neglected rural areas into the main stream of economic activity. This will also prevent the rural exodus, as the availability of power at subsidised rate (facilitated by CDM revenue) will render agriculture a profitability activity, besides contributing to rural health and education.
Probability of cost escalation
The project proponent has to invest around Rs 643 millions for the establishment of the 10 MW Bhavani Barrage-1 small hydroelectric projects. Since the project estimates were made in 2004 and since then the price of cement and steel have gone up, the possibility of an escalation in the investment is likely to be high.
Estimated amount of emission reductions over the chosen crediting period
The crediting period chosen for the proposed project activity is 10 years and the crediting period commences from the date of registration of the project activity. Year wise estimation of emission reductions as well as total emission reductions during the crediting period is shown in the following tabular form.
In the above table, the year 2009 corresponds to the period starting from 1/07/2009 to 30/06/2010 or from the date of registration to the successive 365 days which ever occurs later. Similar interpretation shall apply for the remaining years.
Conclusion
“Green House Gases” (GHGs) in the earth’s atmosphere, resulting from atmosphere of carbon dioxide and other gases such as methane, Chloro flouro carbons, nitrous oxide, ozone and others will continue to heat up the atmosphere, resulting into what is termed “Global
Years Estimation of annual emission reductions in tonnes of CO2e
2009 14140
2010 14140
2011 14140
2012 14140
2013 14140
2014 14140
2015 14140
2016 14140
2017 14140
2018 14140
Total estimated reductions (tones of CO2e)
141400
Total number of crediting years 10
Annual average of the estimatedreductions over the crediting period (t CO2e)
14140
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Warming”. Effects of Global warming results in possible: Rising of sea levels, Alteration of crop yields, Flood rising & increase in rainfall, Swamping cities and wet lands, Pests would proliferate, Diseases would spread and etc., Clean Development Mechanism aims at projects that reduce emissions. Rules are being developed for including afforestation and reforestation activities in the CDM for the first commitment period. Parties would be limited in how much they may use CERs from such sink projects towards their targets, up to 1% of the Party’s emissions in its base year, for each of the five years of the commitment period.
Hydro Electric Power Project by TANGEDCO/TNEB of 10 MW (2×5 MW) Bhavani Barrage -1 Small Hydro power project is a run-of-the-river project proposed by Tamil Nadu Electricity Board (TNEB) in between Pillur dam and lower Bhavani dam is one of the novel way . The purpose of the project activity is to generate hydro power by utilizing the potential energy available in the flows of the river Bhavani, which would generate green and clean energy of 16.72 MU per annum on an average for sustainable development.
References
1. Carbon dioxide Information Analysis Centre (CDIAC) (2004), World CO Emissions, Washington DC.
2. Fenhann Jorgen (2006), UNEP Collaborating Center on Energy and Environment, Roskildle (www.cd4cdm.org/publications/cdmpipeline.xls)
3 Institute for Global Environmetal Strategies (IGES) (2005), CDM Country Guide for India, Tokya (www.iges.or.jp/en/cdm/pdf/countryguide/india.pdf).
4 Ministry of Environment and Forest (MOEF) (2004), India’s Initial National Communication to the UNFCCC, NewDelhi (http://unfccc.int/resource/docs/natc/indnc1.pdf).
5. United Nations Framework Convention on Climate Change (UNFCCC)(1997), The Kyoto Protocol, Geneva (http://unfccc,int/resource/docs/convkp/kpng.pdf)
6. United Nations Framework Convention on Climate Change (UNFCCC)(2006), A summary of the Kyoto Protocol, Geneva (http://unfccc,int/essential_background/feeling_the_heat/items/2879.php)
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Aerogel: A Revolutionary Advancement in Thermal Technology
Aerogel is a solid with the lowest known density. It stands up to 2000 time’s greater load than its own weight. It has extremely low thermal conductivity; the
material is very suitable to limit the heat losses of buildings. Its melting temperature is 1200°C. Well dampens vibration and sound. Aero gel is the only compound with significant thermal insulation capabilities, which is currently well clear. For these reasons, it is possible to consider aerogel as a material of the future not only in construction, and therefore it is necessary to deal with this interesting material now.
Sonjoy DebB.Tech.’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
The future use of aerogels as filler in thermal insulating glazing will cause a significant reduction of heat transfer coefficient in lightweight claddings, this follows from the results of our calculations. The bluish color of aerogels is not always a bad thing.It is known also as frozen smoke or air-glass (See Figure1). Microscopically consists of nano-meter sized particles 1-10 nm diameter, whichstick together and form chains. These particles have so many points of contact that a stable three dimensional network is established in which the distance between the chains
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(the diameter of porechannels) is typically 10-100 nm i.e. 1/10,000th the diameter of human hair (See Figure 2).
Many people assume that aerogels are recent products of modern technology. In reality, the first aerogels were prepared in 1931 by Steven. S. Kistler. If the wet gel is simply allowed to dry on its own, the gel would shrink, often to a fraction of its original size, since the liquid-vapor interface of the evaporating liquid exerted strong surface tension forces that collapsed the pore structure. If a liquid is held under pressure always greater than the vapor pressure, and the temperature is raised, it will be transformed at a critical temperature into a gas without two phases having been present at any time.They had been largely forgotten when, in the late 1970s, the French government was seeking a method for storing oxygen and rocket fuels in porous materials. This directly led to the major advances in aerogel science, namely the application of sol-gel chemistry to silica aerogel preparation. The reaction of a metal alkoxide with water a metal hydroxide is formed and then a condensation reaction occurs between each two metal hydroxides. The molecular weight of the produced oxide species continuously increases and as they grow they begin to link together and making an alcogel. The drying alcogel under supercritical alcohol conditions produces aerogel.
Figure 1: Aerogel as frozen smoke
Different products of aerogel could be: powders, monolithic, flexible blankets and clamshell preformedinsulation. The disposal of silica aerogels is perfectly natural. In the environment, they quickly crush into a fine powder that is essentially identical to one of the most common substances on earth, namely, sand. Additionally silica aerogels are completely non-toxic and non-flammable.
Properties
Microstructure:
Silica aerogels contain primary particles of 1-10 nm
Figure 2: Pore of Aerogel
diameters. Silica particles of such a small size have an extraordinarily large surface-to-volume ration as 2*109 m-1 and a corresponding high specificsurface area of 900 m2/gr. It is not surprising, therefore, that the chemistry of the interior surface of anaerogel plays a dominant role in its chemical and physical behavior. It is this property that makes aerogels attractive materials for use as a catalyst substrate, and adsorbent. Most of the properties listed here are significantly affected by the conditions used to prepare the aerogel and any subsequent post-processing.
Thermal property
A single one-inch thick window pane of silica aerogel is equivalent to the insulation provided by 20 window panes of glass. Window heat loss accounts for up to 30 % of energy lost from home, but a well designed aerogel window could lower the needed heating and cooling cost by comparable figure. At higher temperature mostly above 200oC, radiative transport becomes the dominant mode of thermal conduction, and must be dealt with. If silica aerogels are to be used at temperature above 200oC, this mode of energy transport must be suppressed. This is accomplished by adding an additional component to the aerogel, either before or after supercritical drying. One of the most promising additives is elemental carbon, which is an effective absorber of infrared radiation and actually can also increase the mechanical strength of the aerogel. Generally these additives are with dimensions on the order of nanometers so the product can also legitimately be classified as nano-composite.
Transparency property
Most of the pores in aerogel are too small to scatter visible light, but once in a while a few of the poreare larger. The lager pores scatter light as it passes through aerogel and this creates the hazy appearance. Aerogel produced on earth is cloudy, but scientist hope to produce a transparent
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variety inspace that could lead to advances such as super insulating windows and extraordinary high speed computers.
Applications
- Aerogel has found applications in following areas- Space technology- Kinetic energy absorber- Microelectronics- Transparent metals- Drug Delivery System- Thermal Insulation
Difficulties with using Aerogels
What prevents it from being used on an industrial scale is the fact that contact with water turnsit back into a gel. It should also be said that at present Aerogel is not perfectly clear, but occurs with a blue tinge, which currently limits the use of it in transparent plastics. However, it has found application in construction work, where the bluish color is not an inhibiting factor; it may even bedesirable, for example, in indoor swimming pools (See Figure 3).Despite theoretical expectations, Aerogel with precisely defined properties has never been successfully produced in a space with gravity. As a result, it seemed almost impossible to set up the mass production of aerogel for engineering use. One special difficulty was maintaining the size of the pores in the material, as well as the dimensions and proportions of the solid parts. Research has found that this problem can be solved by manufacturing it in a state of weightlessness. The aim of the research is not to produce a unique material
Figure 3: Indoor swimming pool Cow Mountain
in a state of weightlessness because in the current stateof astronautics, production would not be cost effective. Space travel, however, should provide knowledge about which factors and means will affect the particle size of Aerogel, its internal structure and visual properties. Once it succeeds, the world will have a completely new product thatopens up unheard of possibilities. That is why the attempt is being made here to find other options through research in space and bring knowledge to industry that would enable an economical meansfor producing it here on Earth. The first experiments in zero gravity were conducted in 1996, using starfire rockets on short, suborbital flights. It showed that the material has a four to five-time better parameters than when producing it on the ground.
Prospects for use in Civil Engineering
Since Aerogel has excellent thermal insulating properties (minimal heat loss), no critical surface temperature is reached, nor does any condensation of water vapor occur, at standard borderline conditions. For these reasons, no misting or fogging of window components occurs.Unlike gas-filled or vacuum-sealed windows, windows filled with Aerogel do not lose their thermal resistance over time. For gas-filled windows, there is a gradual reduction in thermal resistance due to the long-term leakage of inert gas. The performance of double glazing with inert gas has reached the limit of its ability today, while Aerogel has the potential to achieve even better properties in the future. With the gradual development of manufacturing processes, the price of Aerogel is becoming acceptable. We can recall a time when silicone sealants started off asan extremely expensive and rare material.So far, the most promising use of Aerogel appears to be as thermal insulation. The material has significantly better insulating properties than glass, yet weighs only a thousandth of the mass.This opens up possibilities of using it for extra light shells, which will significantly reduce the static load on the loadbearing
Figure 4: Solar Decathlon House
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system of the building.Aerogel is the only material that, in addition to significant thermal insulation capability,ranges from translucent to clear. The use of Aerogel is mainly for the transparent shells of buildings and, with improvement in the transparency of the material, in future glazing. Transparent siliconAerogel would be very good as thermal insulation for windows, where it would significantly limitheat loss from buildings. The Swedish company Airglass is developing new material for glazing windows, made out of a layer of Aerogel vacuum-sealed between two glass panes. Production is stillin the pilot plant phase, with a monthly production of 3 – 6 m2, which is now used only for testing.Commercially, Aerogel was used in granular form toimprove the insulation properties of roof windows. The first test use of Aerogel as an insulator is inthe Georgia Institute of Technology’s Solar Decathlon House (See Figure 4), where it is used asa semi-transparent roof.
Unique is the ability of the translucent layers of Aerogel to capture daylight in poor lightconditions and disperse it in the space where it does not directly impact.
It is an ideal solution for illuminating museums, galleries, libraries and sports halls. This knowledge is already being used byseveral manufacturers of flat or celled lighting panels made of polycarbonate filled with Aerogel. Inaddition, the glass absorbs heat radiation. It absorbs up to 27% of thermal radiation for a 13 mm thicklayer of Aerogel.
One more use of aerogel is in insulation of Cryogenic Piping (See Figure 5).
like Aerogels. But their time is probably only just now coming and will depend on how quickly and with what means the technology for their manufacture will be developed. One NASA scientist even predicted that Aerogel will soon be an ordinary material in our surroundings and will be used like plastics are today. Aerogel will certainly become a part of our homes just as Goretex is a part of our clothing or vapor permeable and diffusion-open foil is. The price of some components of Aerogels is now approaching affordable levels mainly due to the price / performance ratio. So it seems that the main advantage of Aerogels istheir thermal insulating properties. Spreading the use of Aerogels as thermal insulation will reduceenergy consumption, thereby reducing greenhouse gas emissions and the pollution of the earth.Thanks to its excellent properties and environmental harmlessness, Aerogels will likely find a place in the market in the foreseeable future.In conclusion, it can be said that Aerogels have certainly surprised us in many aspects and it has gradually begun to fulfill a number of expectations, certainly for a material that is destined to become one of the most important materials of the 21st century.
Reference
- CSN EN 1279-3 – Glass in construction – Insulating glass – Part 3: Long-term test methods and requirements for gas leakage rates and for gas concentration tolerances (2003)
- CSN EN ISO 13947 – Thermal performance of lightweight shells – Calculation of the heat transfer coefficient (2007)
- CSN 73 0540-2 – Thermal protection of buildings – Part 2: Requirements (2007)
- http://www.enviweb.cz/clanek/staveni/84815/Aerogel-izolacni-material-budoucnosti
- http://www.aldebaran.cz/bulletin/2003_40_aer.html
- http://cs.wikipedia.org/wiki/Aerogel
- http://www.mmspektrum.com/clanek/Aerogel-se-vratil-z-kosmu-a-miri-do-izolaci
- http://stavba.tzb-info.cz/okna-dvere/6425-koncentrace-inertniho-plynu-ve-vztahu-k-ug
- http://brno.tucnacek.cz/2005050601
- http://solar.gatech.edu/home.php
- Nano/Meso Porous Silica Aerogel, The state of the art and possible new applications, T. Faez, M.S. Yaghmaee, S. Sarkar, Research Center for Science & Technology in Medicine,Tehran University of Medical Sciences, Tehran, Iran
- Source: stardust.jpl.nasa.gov/photo/aerogelbrick.jpg
- http://www.aerogel.com/Aspen_Aerogels_Spaceloft.pdf
- AEROGEL – MATERIAL OF THE FUTURE FOR CIVIL ENGINEERING, No. 2, 2011, Vol. XI, Civil Engineering Series, paper #26, Technical University of Ostrava
Figure 5: Use of Aerogel in Cryogenic Piping
Advantage achieved through use of aerogel for insulating Cryogenic Piping are as below
- Installation speed and flexibility of the industrial model- Space savings reduces infrastructure (pipe racks,
pads)- Logistics and inventory management savings- Fire protection enhances facility reliability, lowers costs- It’s physical and thermal durability lower maintenance
costs and improve reliability
Conclusion
There are not too many similar multi-application materials
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Geopolymer Concrete
Literature Survey on Geopolymer Concretes and a Research Plan in Indian Context
A comprehensive literature survey on various aspects of Geopolymer Concretes (GPCs) has been provided in this paper to under-stand the nature of GPCs from engineering applications point of view so that a rational technical plan for development of GPCs with given aluminosilicate sources (such as fly ash, blast furnace slag powder etc) can be formulated. The literature survey indicates that ‘geopolymer’ (GP) is only one of the many names used for describing the binder formed with alumino-silicate gel structure. Com-paratively, more papers are published on science of geopolymerisation where often paste is utilised. Concretes and mortars based on GPs are also reported, but, lesser in numbers. The science of GP has not yet reached the stage where GPC mix can be made by user by just adding water as it has happened in case of Portland cement technology. This requires the actual engineer on site to be aware of chemical nature of the GP binding action involved. However, enough qualitative information is available on the mechanical strength so that GPC mixes can be developed to achieve the desired level of strength for use in structures. The second part of this paper would concentrate on the typical research plan to develop engineering properties of GPCs based on the information available in the literature.
Portland Cement Based Concretes
Cement concrete is often considered as an artificial stone which is made by mixing Portland cement (P-C), water, sand, and crushed stone aggregate to produce a mouldable mixture. This concrete, during the last century, has developed into the most important building material in the world; the beginning was made by August Perret, in 1902, by designing and building an apartment building in Paris employing “a system for reinforced concrete” (columns, beams, and slabs, but with no load-bearing walls) [URLa]. Concrete is, now, an essential product used in a variety of constructions including infrastructure and industrial sectors. This is partly due to the fact that concrete is produced from natural materials available in all parts of the globe, and partly due to the fact that concrete is a versatile material, giving architectural freedom. Concrete is used more than any other man-made material in the world [Bjorn Lomborg, 2001]. More than a ton of concrete is produced every year for each human on the earth planet, making the concrete as the second most widely consumed substance on the earth after water [Sara Hart, 2008]. But,
the environmental aspects of concrete are now being discussed with a view to develop an eco-friendly material of construction. In this regard, it would be interesting to note that the ‘embodied carbon dioxide’ (ECO2) of a tonne of concrete was reported to be in the range of 75–176 kg CO2/tonne, depending upon the type and method of mix design [URLb]. The ‘embodied energy’ (EE) content of concrete is also very high which could vary from 400 to 600 kWH/m3 of concrete. Therefore, there is an urgent need for making the concretes more eco-friendly so that both ECO2 and EE of concrete are reduced. It has been well established that any developmental activities aimed towards improvement of quality of life of human beings involves always a large amount of construction activities which in turn require production of varieties of concretes. Therefore, development of concretes with more eco-friendly characteristics has become tasks of many scientists all over the world.
Need for Alternate Concretes
Continuous technological upgrading and assimilation of
Rajamane N. P.1, Nataraja M. C.2, Lakshmanan N 3, and Ambily P S4
1Head, CACR, SRM University, 2Professor, Dept. of Civil Engg, SJCE,3Former Director, CSIR-SERC, 4Scientist, CSIR-SERC
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latest technology has been going on in the cement industry. Presently, 93% of the total capacity in the industry in India is based on modern and environment-friendly dry process technology and only 7% of the capacity is based on old wet and semi-dry process technology. There is a scope for waste heat recovery in cement plants and thereby reduction in emission level.
The cement production is highly energy intensive next only to steel and aluminium (also consumes significant amount of non-renewable natural resources such as lime stone deposits, coal, etc.). The ‘EE’ of P-C being about 1.3 kWh / kg, is a very high quantity. A tonne of P-C production involves emission of about a tonne of CO2, which is a greenhouse gas causing global warming. More than 7% of world CO2 production is attributed towards production of P-C. Moreover, among the greenhouse gases, CO2 contributes about 65% of global warming [McCaffery, 2002]. Therefore, the Portland cement industry does not fit the contemporary desirable picture of a sustainable industry. There is an urgent need to find an alternate to P-C in order to make the construction industry eco-friendly. However, the new binder material should also possess satisfactory strength and durability characteristics which are comparable, preferably superior to those ‘conventional concretes’ (CCs) based on P-C.
Geopolymer as Alternate to Portland Cement
A new binder material, known as ‘geopolymer’ was first introduced by Davidovits in 1978 to describe a family of mineral binders with chemical composition similar to zeolites but with an amorphous microstructure [Davidovits, 1994]. He utilised silica (SiO2) and alumina (Al2O3) available in the specially processed clay (metakaolin) to get inorganic polymeric system of alumino-silicates. Unlike ordinary Portlandcement (P-C), geopolymers do not need calcium-silicate-hydrate(C-S-H) gel for matrix formation and strength, but utilise the polycondensation of silica and alumina precursors to achieve required strength level. Two main constituents of geopolymers are: geopolymer source materials (GSMs) and alkaline activator liquids. The GSMs should be alumino-silicate based and rich in both silicon (Si) and aluminium (Al) and thus, by-product materials such as fly ash, silica fume, slag, rice-husk ash, red mud, etc. can form GSMs.
Recently, Rangan and Hardijto, [2005] exploited silica and alumina of fly ash to produce three-dimensional polymeric chain and ring structure consisting of Si-O-. Geopolymers are unique in comparison to other aluminosilicate materials (e.g. aluminosilicate gels, glasses, and zeolites). The concentration of solids during geopolymerisation reactions is higher than that in aluminosilicate gel or zeolite synthesis
[Rangan, 2005; Rajamane, 2011a and 2011b, Sindhunata, 2006]. Al-O bonds of geopolymeric binder are useful to prepare structural grade concretes.
From above, it is clear that any of the minerals containing reactive oxides of silicon and aluminium can be activated by suitably formulated highly alkaline liquid to obtain inorganic polymeric binding material [Sindhunata, 2006]. Preliminary studies in this regard, were carried out at SERC in early 2000s; both fly ash and Ground Granulated Blast Furnace Slag (GGBS), (either individually or combined in certain proportions) from indigenous sources were found to be suitable to produce geopolymeric systems to achieve sufficient strength levels in geopolymer concretes (GPCs) [Rajamane and Sabitha, 2005]. It was observed that the activation of FA and GGBS involved use of hydroxides and silicates of alkali (such as sodium, potassium) which are commonly available in India; the processing conditions for GPCs were almost similar to Conventional Concretes (CCs) except that during mixing operations of GPCs, instead of water, a premixed alkaline solution, known as ‘Alkaline Activator Solution’ (AAS), was added. Following materials were used to produce GPCs [Rajamane, 2009a]:
- Fly ash,
- Ground Granulated Blast Furnace Slag(GGBS),
- Fine aggregates (in the form of river sand),
- Coarse aggregates (in the form of crushed granite stone),
- Alkaline Activator Solution (AAS):
(It is a combination of solutions of alkali silicates and hydroxides, besides distilled water. The role of AAS is to activate the GSMs, containing Si and Al, such as FA and GGBS).
Besides above mentioned materials, synthetically produced alumina and silica, metakaolin, rice husk ash, silica fume, etc. can also be used appropriately keeping considering that both aluminium and silicon elements are required beside small amount of alkali elements (such as Sodium, Potassium, etc.) to form alumino-silicate geopolymers. GPCs, being a new class of materials (with complete absence of Portland cement), conventional concrete mix design approaches are not generally directly applicable. The formulation of GPC mixtures requires systematic numerous investigations on the materials available [Rajamane, 2005]. However, basic concepts related to particle packing, rheology of fresh mixes, etc, can be judiciously utilised in developing GPC mixes which require AAS consisting of hydroxides and silicates of sodium whose concentration plays a major role in determining the geo polymerization ratio of alumina and silica of ‘geo polymeric source material’
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(GSM) such as fly ash, metakaolin, GGBS etc., of GPC.
Desirable Properties of GPCs
It was recognised that any new binder material to be developed for use in concretes should be eco-friendly and it would be acceptable if it has following characteristics:
- It should be preferably produced from widely available waste by-products from industries
- ‘Internal Energy Content’ (Embodied Energy) should be less
- Chemical activators for generating binding system should be commonly available
- The new binder based concretes should be similar or superior to that of P-C based concretes in respect of :–
- processing conditions for production of fresh mixes
- time required for demoulding or formwork removal
- curing regimes and periods
- rate of strength developments with age
- mechanical properties such as
- compressive strength
- tensile strength
- flexural strength
- modulus of elasticity
- durability related properties such as
- protection to embedded steel reinforcement
- diffusion of
- chloride ions
- moisture/water, etc
- resistance against attack by
- sulphates
- acidic solutions, etc
- cost per unit volume
- long term chemical stability of the binding system formed
- capable of accepting common filler aggregate systems such as sand, crushed natural stones, etc
Literature Review
Origin of Term ‘Geopolymer’
The term ‘‘geopolymers’’ was first introduced to the world by Davidovits of France resulting in a new field of research and technology. Davidovits explained that geosynthesis is the science of manufacturing artificial rock at a temperature below 100°C in order to obtain natural characteristics (hardness, longevity and heat stability) of
rock. Geopolymers can be thus viewed as mineral polymers resulting from geochemistry or geosynthesis. However for the purpose of this literature survey, geopolymer (GP) means any aluminosilicate based binder.
History of Geopolymers
Davidovits coined the term geopolymer in 1978 to represent a broad range of materials characterised by chains or networks of inorganic molecules [Davidovits, 1979, 1993, 2008], and explained in many of his publications about the possibility of GPs being used by Egyptians construction of pyramids, based on microscopy, IR and NMR spectroscopy of sparse specimens from ancient Egyptian constructions [Davidovits and Morris, 1988; Davidovits, 1999]. Demortier observed the noticeable differences in porosities in the top and bottom sections of pyramid blocks which were also subjected to X-ray and NMR analyses to conclude that pyramids could be made from ‘concreting’ operations [Demortier, 2004]. Use of slurry to form bearing courses of horizontal joints and vertical joints between the blocks including presence of hair in the joints of pyramids did indicate the possibility of ‘concrete’ like technology for pyramid constructions [Škvára et al, 2008].
But, the actual modern alumino-silicate based work could be traced to 1930s when alkali oxides were used for reaction with slags to test their suitability for use in Portland cement. A rapid hardening binder by slag activation was reported in 1940 by Belgian scientist [Purdon, 1940]. US Army used, in 1950s, NaCl and NaOH to activate slag to produce binder for use in Military applications [Malone et al, 1986]. Glukhovsky in 1965 observed that alumino-silicate hydrates as solid binder products are formed during alkali activation of slag and these are also noticed during alkali treatment of rock and clay minerals, prompting him to call the binder as ‘soil cements’ and concrete as ‘soil silicate concretes’ [Glukhovsky, 1965]. In 1974, Davidovits and Legrand filed a patent on ‘Siliface process’ which involved use of NaOH, Quartz, kaolinite, and water. It is interesting to note here that alkali activated slag (AAS) would basically consist of silicon element in mainly one dimensional chains whereas, GPs would have a 3-dimensional alkali-alumino-silicate framework [Duxon et al, 2007].
However, after Davidovits (1991) described his new breed of aluminosilicate binders (synthesised by activating calcined kaolinitic clay with sodium silicate solution at low temperature) as ‘geopolymer’ for the first time, the real impetus to the field of GP technology started. The ‘Geopolymer’ was an aluminosilicate gel, where the silicon and aluminium are tetrahedrally-bonded through sharing oxygen atoms forming the basic monomer unit is a sialate (O-Si-O--Al-O) carrying excess negative charge which occurs when the Al3+(of the source material such as clay)
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is substituted by Si4+.The polysialate structure is charge-balanced by alkali metal cations (K+ or Na+).
The field of geopolymers saw major contributions from authors such as Alonso (2001), Bakharev (2005), Sanjayan (1999), Bankowski (2004), Cheng (2003), Duxson (2005), Fernandez-Jimenez (2006a), Iler (1979), Katz (1998), Khalil (1994), Kriven (2003), Krivenko (2002), Lee and Van Deventer (2002a), Li (2006), Palomo (2004), Phair (2001), Provis (2005b), Shi (1996), Sindhunata (2006), Xu (2004), Talling (1989), Van Jaarsveld (2000), Wang (1995), Xu (2000b), Yip (2003), Hardjito, Wallah, Sumajouw, Rangan (2001). Meantime, a few books having comprehensive information on geopolymer based on the vast literature were also published [Davidovits, 2011; Shi, 2006; Provis, 2009].
Though the term, ‘geopolymer’ has become now more common to represent the synthetic alkali aluminosilicate material (produced by reaction of a solid aluminosilicate with a highly concentrated aqueous alkali hydroxide or silicate solution), it is worthwhile to note that the following nomenclatures are also reported to describe similar materials:
- Inorganic polymer [Van Wazer, 1970]
- Low-temperature aluminosilicate glass[Rahier, 1996]
- Alkali-activated cement [Roy, 1999; Palomo, 2003; ]
- Alkali-activated binders [Torgal, Gomes, and Jalali, 2008]
- Geocement [Krivenko, 1994]
- Alkali-bonded ceramic [Mallicoat, 2005]
- Inorganic polymer concrete [Sofi, 2006]
- Hydroceramic [Bao, 2005]
- Mineral Polymers[Davidovits, 1980]
- Inorganic polymer glasses[Rahier, 2003]
- Alkali ash material[Rostami, 2003]
- Soil cements [Glukhovsky, 1965]
- Alkali Activated Binder [Provis and Deventer, 2009],
It is seen that GP is a versatile binder being studied by scientists of various backgrounds and expertise, but, having good potential to become eco-friendly alternate to P-C for use in civil engineering applications. But, to understand various aspects of this new material from, it is necessary to consider the above nomenclatures also so that the information available in various forums is readily utilised. In the present paper, more widely used term, ‘geopolymer’ is adopted for presentation of data and discussions.
Basics of Typical Geopolymer Concretes
Major ingredients of geopolymer concretes (GPCs) having
geopolymer (GP) as the binder, are:
- Geopolymeric source materials (GSMs) such as fly ash, GGBS, etc
- Aggregate system consisting of fine and coarse aggregates
- Alkaline Activator Solution (AAS)
It is seen that GPCs are almost similar to conventional concretes (CCs) (which are P-C based), consisting of binder made from fine powdery materials, bulk volume filling granular particles made of aggregates, and liquid component of the mix made of alkaline chemicals. Thus, the powdery P-C of CCs is replaced by mineral materials (usually referred as mineral or pozzolanic admixtures in CC technology), and liquid component of water of CCs is replaced by viscous, alkaline activator solution made of hydroxides and silicates of alkali metals such as sodium and potassium. The aggregate filler component of CCs is retained in GPCs.
Besides above mentioned materials, synthetically produced alumina and silica, metakaolin, rice husk ash, silica fume, etc. can also be used appropriately keeping in view that both aluminium (Al) and silicon (Si) elements are both required beside small amount of alkali elements such as Sodium (Na), Potassium (K), etc to form alumino-silicate geopolymers.
GPCs being a new class of materials (with complete absence of Portland cement), traditional CC mix design approaches cannot generally be directly applied. The formulation of the GPC mixtures requires systematic numerous investigations on the materials available [Rajamane, 2005]. However, basic concepts related to particle packing, rheology of fresh mixes, etc can be judiciously utilised in developing GPC mixes.
To prepare a typical AAS consisting of hydroxide and silicate of sodium, Sodium Hydroxide flakes (SHf), a highly hygroscopic granular material are first dissolved carefully in Distilled Water (DW), to get sodium hydroxide solution (SHS). After allowing the SHS to cool to room temperature, Sodium Silicate Solution (SSS) is added to it, the resulting liquid is termed as AAS which is used to prepare the GPC mix. Mixing of GSMs, aggregates, and AAS is done using the conventional tools (such as mixer machine) adopted for producing of CCs, however, with due considerations for viscosity and chemical nature of the AAS. The rates of hardening and chemical reactions in GPCs are quite different from the concretes based on Portland cement. Handling of some ingredients, especially, the constituents of AAS would require specific precautions. It is to be noted here the exactly required liquid component of GPCs is
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not readily available in the market, unlike water in case of conventional concrete (CC), and it has to be prepared carefully much before the actual mixing of GPCs is started. It may be worth noting here that specifically formulated chemical additives to function reliably as setting time changing admixtures such as retarders, accelerators, etc are not yet readily available for geopolymers. Hence, field adjustments for changed ambient temperature conditions and changes in properties of ingredients would not be easy, even though conventional methods of mixing, compaction, moulding, and demoulding can be still adopted for GPCs also. However, for some mixes, ambient conditions may not be adequate for demoulding within 24 hours of casting and some higher temperature exposure may have to be created for effecting the setting of the GPC mixes.
Many of the geopolymeric systems reported in the literature are involving use of high temperature curing in the form of storing the moulds containing GPC mixes in hot air oven or steam chambers [Rangan, 2005]. However, the works at CSIR-SERC has shown that it is now possible to formulate the GPC mixes for self-curing so that ambient conditions would be sufficient for setting as well as for gaining mechanical strengths [Rajamane, 2009b].Curing of GPCs may involve application of steam and hot air, in contrast to water curing of CCs. With special formulations, GPCs can get cured at ambient conditions after demoulding, thereby they can be considered as self-curing. For visual inspection, GPCs and CCs would look similar, but, chemical natures of microstructures are quite different.
Literature on Geopolymer Science
Ingredients of GP and Geopolymeric Source Materials
Geopolymer concretes (GPCs) have geopolymer (GP) as the binder to bind the aggregate system consisting of fine and coarse aggregates. Two main ingredients required for creation of geopolymer binders are:
- Geopolymeric source materials (GSMs) rich in silica and alumina, which could be natural minerals (such as kaolinite, clays, etc) or industrial by-products (such as fly ash, silica fume, slag, rice-husk ash etc).
- Alkaline Activator Solution (AAS) based on alkali metals (commonly Sodium or Potassium) based. The most common AAS is a combination of alkali hydroxide (NaOH, KOH) and alkali silicate (Sodium or potassium silicate).
Geopolymers made from calcined source materials, such as metakaolin (calcined kaolin), fly ash, slag etc., yield higher compressive strength when compared to those synthesised from non-calcined materials, such as kaolin clay. The source material used for geopolymerisation can
be a single material or a combination of several types of materials (Xu & van Deventer 2002).
Geopolymerisation Reactions
The mechanism of geopolymerisation may be considered to occur in three stages (Xu & van Deventer 2000) :
- dissolution,- transportation or orientation, and - polycondensation
The reactions of geopolymerisation take place through a series of exothermic processes (Palomo, Grutzeck & Blanco 1999; Davidovits 1999).
Cheng and Chiu (2003) had observed that unlike conventional organic polymers, glass, ceramic, or cement, the geopolymers are formed at low temperatures and they are non-combustible, heat-resistant, and fire/acid resistant. It was recognised that three sources essentially are needed for synthesis of geopolymer: (i) raw materials (such as fly ash, GGBS, MK, etc), (ii) inactive filler (such as sand and crushed granite aggregate), and (iii) geopolymer liquor (Alkali Activator Solution (AAS). Raw materials (or geopolymer source materials) can be industrial wastes, such as fly ash, blast furnace slag, red mud, waste glasses, or some natural minerals and rocks. The active powdery fine material, containing mainly geo-synthesis supporting Al+3 ions, can be kaolinite or metakaolinite. Geopolymer liquor (AAS) consists of sodium silicate solution acting as binder, and alkali hydroxide solution for the dissolution of raw materials. The authors noted that the chemical process to form geopolymers involves two steps: (i) dissolution of raw materials in alkaline solution to form Si and Al gel on the materials’ surface, (ii) polycondensation to form networked polymeric oxide structures. However, the exact mechanism of geopolymer setting and how hardening occurs was felt to be still not fully understood
Xu and Van Deventer (2000) investigated the geopolymerisation of 15 natural Al Si minerals. It was found that the minerals with a higher extent of dissolution demonstrated better compressive strength after polymerisation. The percentage of calcium oxide (CaO), potassium oxide (K2O), the molar ratio of Si-Al in the source material, the type of alkali and the molar ratio of Si/Al in the solution during dissolution had significant effect on the compressive strength.
In the synthesis of geopolymers, there are essentially two types of raw materials, the aluminosilicate-containing solids and alkali-silicate solutions. The aluminosilicate solids function as sols in the alkali-silicate liquid medium. The sol-liquid combination will turn into a sol-gel matrix, as is usually done in the sol-gel methodology. The aluminosilicate
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sources include the commonly used kaolinite, especially, calcined kaolinite, or metakaolinite (Barbosa et al., 2000; Davidovits, 1991; 1999; Davidovits and Davidovics, 1998; Palomo and Glasser, 1992; Rahier et al., 1996a; b; 1997) and other natural aluminosilicate minerals (Xu and Van Deventer, 2000b; 2002a) and industrial waste-based materials, such as GGBS (Cheng and Chiu, 2003; Yip and Van Deventer, 2003) and FA(Lee and Van Deventer, 2002a; b; Palomo et al., 1999b; Phair and Van Deventer, 2001).
Van Jaarsveld, Van Deventer and Lukey (2002) studied the interrelationship of parameters that affected the properties of FA-based geopolymer and reported that the properties of geopolymer were influenced by the incomplete dissolution of the materials involved in geopolymerisation. The water content, curing time and curing temperature affected the properties of geopolymer; specifically the curing condition and calcining temperature influenced the compressive strength. When the samples were cured at 700C for 24 hours a substantial increase in the compressive strength was observed. Curing for a longer period of time reduced the compressive strength.
Wang Bao-min and Wang Li-jiu (2005) studied the applications of geopolymeric activation techniques of FA in conventional cement concretes. The research showed that when weight of FA reaches 20%-80% of 32.5 grade cement, M40 concrete with satisfactory properties can be prepared through using activating techniques such as adding some high-efficiency FA activating admixture.
[3] Fresh Geopolymer concrete mixes
Hardjito et al, (2002) observed that fresh geopolymer concrete is highly viscous, and cohesive with low workability when the calcined kaolin was the source material.
Structural Usages
Davidovits and Sawyer (1985) used ground blast furnace slag to produce geopolymer binders. This type of binders patented in the USA under the title ‘Early High-Strength Mineral Polymer’, was used as a supplementary cementing material in the production of precast concrete products.
Activating Medium
A combination of sodium or potassium silicate and sodium or potassium hydroxide has been widely used as the alkaline activator (Palomo et al, 1999; van Jaarsveld, van Deventer & Lukey 2002; Xu & van Deventer, 2000; Swanepoel & Strydom, 2002), with the activator liquid-to-source material ratio by mass in the range of 0.25-0.30 (Palomo, Grutzeck & Blanco 1999; Swanepoel & Strydom 2002).
Anurag Mishra (2008, 2009) conducted experiments on
FA based GPC by varying the concentration of NaOH and curing time. Total nine mixes were prepared with NaOH concentration as 8M, 12M, 16M and curing time as 24hrs, 48hrs, and 72hrs. The investigation indicated: an increase in compressive strength with increase in NaOH concentration and curing time, increase in compressive strength after 48hrs curing time not significant. Compressive strength up to 46 MPa was obtained with curing at 60ºC. Water absorption decreased with increase in NaOH concentration and curing time.
Swanepoel and Strydom (2002) conducted a study on geopolymers produced by mixing FA, kaolinite, sodium silica solution, NaOH and water. Both the curing time and the curing temperature affected the compressive strength, and the optimum strength occurred when specimens were cured at 600C for a period of 48 hours
Palomo, Grutzeck, and Blanco (1999) studied the influence of curing temperature, curing time and alkaline solution-to-FA ratio on the compressive strength. It was reported that both the curing temperature and the curing time influenced the compressive strength. The utilization of sodium hydroxide (NaOH) combined with sodium silicate (Na2SiO3) solution produced the highest strength. Compressive strength up to 60 MPa was obtained when cured at 85ºC for 5 hours. The type of alkaline liquid plays an important role in the polymerisation process. Reactions occur at a high rate when the alkaline liquid contains soluble silicate, either sodium or potassium silicate, compared to the use of only alkaline hydroxides confirmed that the addition of sodium silicate solution to the sodium hydroxide solution as the alkaline liquid enhanced the reaction between the source material and the solution. Furthermore, after a study of the geopolymerisation of sixteen natural Al-Si minerals, they found that generally the NaOH solution caused a higher extent of dissolution of minerals than the KOH solution.
Curing
Because heat is a reaction accelerator, curing of fresh geopolymer is carried out mostly at an elevated temperature (Palomo et al,1999). When curing at elevated temperatures, care must be taken to minimize the loss of water.
Swanepoel and Strydom,(2002), described the effects of curing at 40, 50, 60 and 70ºC for different durations (6, 24, 48 and 72 h) and the optimum condition was noted to be 60ºC for a period of 48 hours.
Curing at room temperature has successfully been carried out by using calcined source material of pure geological origin, such as metakaolin (Davidovits 1999; Barbosa, MacKenzie & Thaumaturgo 2000).
Cheng and Chiu (2003) found that the setting
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time of geopolymer paste made with GGBS as the source material along with metakaolinite, was affected by the curing temperature, type of alkaline activator, and the actual composition of the source material. The setting time of geopolymer paste was observed to range from 15 to 45 minutes at 60o C.
Van Jaarsveld et al. (2002) observed that curing at elevated temperature for long periods of time may weaken the structure of hardened material. The test data showed that curing temperature and its duration significantly influences the compressive strength. Longer curing time and higher curing temperature increased the compressive strength, although the increase in strength may not be significant for curing at more than 60oC and curing for periods longer than 48 hours. The compressive strength of fly ash-based geopolymer concrete cured at 60oC for 24 hours did not vary with the age and remained constant at approximately 60 MPa.
Chindaprasirt (2006) conducted the compression strength test on GPC specimens prepared by using the NaOH of 10M concentration. He concluded that the GPC specimens prepared by using above concentration can achieve high strength (70MPa) when cured in oven at a temperature of 75°C for two days.
GPC Mix Design
Djwantoro Hardjito, et al (2004), showed that the geopolymer paste binds the coarse aggregates, fine aggregates and other un-reacted materials together to form the GPC, and usual concrete technology methods to produce GPC mixes can be often employed. As in the Portland cement concrete, the aggregates occupy the largest volume, (about 75-80 % by mass) in GPCs. The silicon and the aluminium in the fly ash are activated by a combination of sodium hydroxide and sodium silicate.
Rangan and Hardjito (2005) have noted that unlike conventional cement concretes GPCs are a new class of construction materials and therefore no standard mix design approaches are yet available for GPCs. While GPC involves more constituents in its binder (viz., FA, GGBS, sodium silicate, sodium hydroxide and water), whose interactions and final structure and chemical composition are under intense research whereas the chemistry of Portland cement and its structure and chemical composition (before and after hydration) are well established due to extensive research carried out over more than century. While the strength of cement concrete is known to be well related to its water-cement ratio, such a simplistic formulation may not hold good for GPCs. Therefore, the formulation of the GPC has to be done by trial and error basis.
Rajmane (2006) studied the effect of geopolymeric binders such as GGBS and FA by activating silicon dioxide and aluminium oxide present in the binders, to form inorganic polymer binder system. This binder system can be used to produce concretes containing river sand as fine aggregate and coarse aggregate in the form of either sintered FA aggregates (SFFA) or crushed granite aggregates (CGA). It was concluded that the lightweight aggregate based geopolymer concrete have one day compressive strength of about 35 MPa and a 28 days strength of more than 50 MPa. CGA based geopolymer concretes produced marginally higher compressive strength of about 45 MPa at one day and 65 MPa at 28 days
František Škvára et al (2005) showed that the structure of the geopolymers prepared on the basis of fly ashes (cured at 60-80ºC 6-12 hours ) is predominantly of the AlQ4(4Si) type and SiQ4 (4Al), SiQ4 (2-3Al). The strength range was 15 to 70 MPa and is affected substantially by macro-pores (103 nm and more) formed in result of the air entrained into the geopolymers. There are fly ash particles that underwent only partial reaction. The presence of Ca-containing additives (slag, gypsum) reduces considerably the porosity. There was no transition phase of different composition between the geopolymers and the aggregate.
Geopolymers and Zeolites
Geopolymers are unique in comparison to any other aluminosilicate materials (e.g. aluminosilicate gels, glasses, and zeolites). The concentration of solids in geopolymerisation is higher than in aluminosilicate gel or zeolite synthesis. Geopolymers are believed to be an amorphous metastable phase of zeolites (i.e., zeolitic precursors) that can be converted to a more well-defined crystalline phase (zeolites) provided that the right conditions and reactant concentrations are used (Xu and Van Deventer, 2002b).
A recent review by Provis et al. (2005c) suggested that that geopolymer is constituted from agglomerates of zeolitic nanocrystals bound by an amorphous gel phase. The degree of Crystallinity is affected by reaction conditions and starting reactant concentration, particularly silicate and alkali concentrations.
Geopolymerisation Modelling
Sindhunata (2006) studied the conceptual model of geopolymerisation. He conducted studies under controlled conditions typically used for geopolymerisation, thus leading to findings, which improved the understanding of reaction steps. Various influencing parameters investigated, were the concentration of reactants (silicate concentration, alkalinity, and water content) and the curing conditions (temperature,
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time, humidity). A conceptual model of geopolymerisation was developed by incorporating the above mentioned factors. Three key aspects of GPC studied were: firstly, an investigation on the development of pore structure in geopolymers; secondly, an investigation on the competition between dissolution, polymerization, and crystallization of aluminosilicate gels during geopolymerisation and finally, an investigation on the ageing of geopolymers in alkali and carbonate solutions. The occurrence of different reaction mechanisms is influenced by the alkali-silicate concentration and the type of alkali metal cation used. The investigation of ageing provides insight into the reaction mechanisms of late geopolymerisation (i.e. post- set processes).
Reinforced GPCs
Past studies on reinforced fly ash-based geopolymer concrete members are extremely limited. Palomo et.al (2004) investigated the mechanical characteristics of fly ash based geopolymer concrete. It was found that the characteristics of the material were mostly determined by curing methods especially the curing time and curing temperature. Their study also reported some limited number of tests carried out on reinforced geopolymer concrete sleeper specimens. Another study related to the application of geopolymer concrete to structural members was conducted by Brookeet al. al (2005). It was reported that the behaviour of geopolymer concrete beam column joints was similar to that of members made of Portland cement concrete.
Shuguang Hu and Hongxi Wang (2008) investigated the mechanical properties of geopolymeric materials (steel slag based) and other conventional materials. The compressive strength, bond strength and abrasion resistances were experimentally studied. It was found that the bond strength of geopolymeric material with steel slag was 2.6% higher than those of other materials. It was also concluded that the steel slag was almost fully absorbed to take part in the alkali activated reaction and incorporated into the amorphous aluminosilicate geopolymer matrix.
Palomo et.al (2004) investigated the mechanical characteristics of FA based GPC concrete. It was found that the characteristics of the material were mostly determined by curing methods especially the curing time and curing temperature. Their study also reported some limited number of tests carried out on possible use of GPC concrete for the production of prestressed sleeper specimens.
Brooke et al. al (2005) studied the application of GPC concrete to structural members. It was reported that the behaviour of GPC concrete beam column joints was similar to that of members made of Portland cement concrete.
Sumajouw and Rangan (2006) conducted extensive studies
on low-calcium FA based reinforced GPC concrete beams and columns. The behavior and failure modes of reinforced GPC concrete columns and beams were similar to those observed in the case of reinforced Portland cement concrete columns. The results demonstrated that the methods of calculations used in the case of reinforced Portland cement concrete beams and columns are applicable for reinforced GPC concrete beams and columns. The results demonstrated that reinforced low-calcium (ASTM Class F) FA based GPC concrete structural members can be designed using the design provisions currently used in the case of reinforced Portland cement concrete members.Excellent correlation between experimental and analytical results is found.
Prabir Kumar Sarker (2008) reports study on analysis on GPC columns. It is found that the equation of Popovics (proposed for OPC concrete) can be used for geopolymer concrete with minor modification to the expression for the curve fitting factor, to better fit with the post peak parts of the experimental stress–strain curves. A good correlation is achieved between the predicted and measured ultimate loads, load–deflection curves and deflected shapes for 12 slender test columns
Durability
(i) Corrosion of Embedded Steel
Miranda et al (2005) gave details of corrosion potential and polarisation resistances for steel electrodes embedded in Portland cement mortar and two fly ash mortars (respectively activated with NaOH and waterglass+NaOH solutions). Chloride-free activated fly ash mortars were found to passivate steel reinforcement as speedily and effectively as Portland cement mortars. The polarization curves and the response to short-term anodic current pulses (galvanostatic pulse technique) corroborated the full and stable passivation of the steel. They concluded that the icorr value for both OPC and GPC mortar are similar (0.1 µA/cm2).
Yodmuneeand Yodsudjai (2006) studied the corrosion of steel bar located inside in fly ash-based geopolymer concrete in an accelerated corrosion test. All the GPC mixes had higher compressive strength than conventional concrete (10 to 16 MPa). The test results included the half-cell potential and cross sectional loss of steel bar and in both the respects GPCs performed better. He conclude that at 72 hrs, the GPC specimens gives the Half cell potential value of -175mV which is mostly equal to the OPC value (-200 mV).
Holloway and Sykes (2005) studied the Corrosion of mild steel reinforcement in an alkali-activated slag (AAS) cement
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mortar containing NaCl admixtures using an improved galvanostatic pulse technique. He concluded that, at initial stage highest corrosion rates are seen with the lowest chloride levels(0% and 2%), but over time(107days) the corrosion rate decrease to 1 µAcm-2 even when the NaCl level increase to 8%.
Shi 2003a reported that the Alkali activated slag showed much less corroded depth (<4mm) then OPC specimens (>14mm) which are immersed in nitric acid even after 90days continuous exposure.
Davidovits (1994) noted that unlike conventional Portland cement, geopolymeric cements do not rely on lime and are not dissolved by acidic solutions. Geopolymeric cements, Potassium-Poly (sialatesiloxo) type, remain stable with a loss in the 5-8 % range.
(ii) Acid Resistance
Bakharev et al (2003) investigated the durability of alkali-activated slag (AAS) concrete exposed to acetic acid solution of pH = 4. It was found that AAS concrete of Grade 40 had a high resistance in acid environment, superior to the durability of OPC concrete of similar grade.
Songa (2005) investigated the durability property of geopolymer concrete exposed to sulphuric acid corrosion. It was concluded that GPC is highly resistant to sulphuric acid; in terms of a very low mass loss, less than 3%. Moreover, Geopolymer cubes were structurally intact and still had substantial load capacity even though the entire section had been neutralized by sulphuric acid.
(iii) Fire Resistance
Van Jaarsveld, Van Deventer, and Schwartzman (1999) carried out experiments on geopolymers using FA and found them to be fire resistant with compressive strengths of 5 to 51 MPa. The factors affecting the compressive strength were the mixing process and the chemical composition of the FA. A higher CaO content decreased the microstructure porosity and, in turn, increased the compressive strength. Besides, the water-to-FA ratio also influenced the strength. It was found that as the water-to-FA ratio decreased, the compressive strength of the binder increased.
Lyon et al (1996) discussed the fire response of a potassium aluminosilicate (Geopolymer) matrix carbon fiber composite. At irradiance levels of 50 kW/m2(typical of the heat flux in a well developed fire), glass- or carbon-reinforced polyester, vinyl ester, epoxy, bismaleimde, cyanate ester, polyimide, phenolic, and engineering thermoplastic laminates ignited readily and released appreciable heat and smoke, while carbon-fiber reinforced Geopolymer composites did not ignite, burn, or release any smoke even after extended
heat flux exposure. The Geopolymer matrix carbon fiber composite retained 67% of its original flexural strength after a simulated large fire exposure
(iv) Sulphate Attack
Hardjito and Rangan (2005) studied the development and properties of low-calcium FA based geopolymer concrete. The research report described the development, the mixture proportions, and the short-term properties of low-calcium FA based GPC concrete. It was concluded that low-calcium FA-based geopolymer concrete had excellent compressive strength, suffer very little drying shrinkage and low creep, had excellent resistance to sulfate attack, and good acid resistance.
Test results showed that heat-cured low-calcium fly ash-based geopolymer concrete has an excellent resistance to sulphate attack. Research data shows that geopolymeric materials performed significantly better in acid resistance compared to Portland cement (Davidovits, 1994; Gourley and Johnson, 2005).
Bakharev (2003) investigated the durability of alkali-activated slag (AAS) concrete exposed to sulphate attack. AAS concrete was immersed in 5% sodium, 5%magnesium and5% sodium + magnesium sulphate solution. The main parameters studied were the compressive strength, products of degradation, and micro structural changes. It was found that in AAS concrete the material prepared using sodium hydroxide had the best performance due to its stable cross-linked aluminosilicate polymer structure.
Douglas (1992) reported that the changes in dynamic modulus of elasticity, pulse velocity, weight and length of sodium silicate-activated slag cement concrete after 120 days of immersion in 5% sodium sulphate solutions. They noticed that the changes are even smaller than those in the controlled specimens immersed in lime-saturated water.
(v) Salt Environment
Nguyen Van Chanh et al [2008] found that compressive strength of heat-cured fly ash-based geopolymer concrete does not depend on age. Longer curing time (24 to 72 hours) produces higher strength, but, increase in strength beyond 48 hours is not significant. Geopolymer concrete has excellent properties within both acid and salt environments. Comparing to Portland cement, the geopolymers have a relative higher strength, excellent volume stability, better durability.
Gailius and Kazberuk (1998) monitored the long-term behaviour of concretes in a chloride exposure regime under influence of cyclic wetting and drying as well as freezing and thawing with chlorides. They concluded that the
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resistivity of concrete was closely connected with cement type and mineral addition content, cement mass as well as the time of storage. The resistance to chloride penetration was found to increase with time but the value of diffusion coefficient from migration test depended on cement type.
Yang and Cho (2001) Huang stated that the accelerated chloride migration test indicated a good correlation between the charge passed and the steady-state chloride flux.
2.4.3 Applications
(a) Hazardous Waste Encapsulation
Davidovits [2002] informs about zeolitic materials’ abilities to adsorb toxic chemical wastes. Geopolymers behave similarly to zeolites and feldspathoids. They immobilise hazardous elemental wastes within the geopolymeric matrix, as well as act as a binder to convert semi-solid waste into an adhesive solid. Hazardous elements of waste materials mixed with geopolymer get locked into the three dimensional framework of the geopolymeric/zeolitic matrix.
Davidovits (1999) suggested that the atomic ratio of Si-to-Al of should be about 2 for making geopolymeric binder based pastes, mortars and concretes. Geopolymer can also be used for waste encapsulation to immobilise toxic metals (van Jaarsveld, van Deventer & Lorenzen 1997).
Palomo and Palacios (2003), described the stabilisation/solidification capacity of a matrix made using alkali activation of fly ash, in the presence of toxic elements chromium and lead. Leaching tests proved that the matrix is able to stabilise and solidify lead efficiently (analysed lead concentrations from leaching were in parts per billion). However, geopolymer was not efficient for chromium fixation since this element strongly disturbed the alkali-activation mechanism of the ash
(b) Precast Products
Gourley and Johnson (2005) have reported commercial production of geopolymer precast concrete products. Reinforced GPC sewer pipes outperformed comparable Portland cement concrete pipes. Good performance of reinforced GPC railway sleepers on mainline tracks and excellent fire resistance of GP mortar wall panels were also reported.
Siddiqui (2007) demonstrated the successful commercial scale manufacture of reinforced geopolymer concrete culverts.
Davidovits and Sawyer (1985) used ground blast furnace slag to produce geopolymer binders. This type of binders was patented in the USA under the title Early High-Strength Mineral Polymer was used as a supplementary cementing
material in the production of precast concrete products.
(c) Structural Concretes
Zongjin Li et al (2004) terming the geopolymers as sustainable composites and found that they are a type of amorphous alumino-silicate product and can be synthesized by polycondensation reaction of geopolymeric precursor and alkali polysilicates. Geopolymers are energy efficient and environment friendly sustainable cementitious materials with superior properties compared to the Portland cement, such as high early strength, excellent volume stability, better durability, good fire resistance, and easy manufacturing process.
.Djwantoro Hardjito et al (2004) investigated geopolymer as the binder (in place of Portland cement) where binding action is achieved in fly ash by hydroxide-silicate based chemicals (as an initiators or catalysts for polymeric reaction) to produce concrete using the usual concrete technology methods.
Davidovits and Sawyer (1985) had used ground blast furnace slag to produce geopolymer binders. This type of binders patented in the USA under the title Early High-Strength Mineral Polymer for used as a supplementary cementing material in the production of precast concrete products. In addition, a ready-made mortar package that required only the addition of mixing water to produce a durable and very rapid strength gaining material was produced and utilised in restoration of concrete airport runways, aprons and taxiways, highway and bridge decks, and for several new constructions when high early strength was needed.
Concluding Remarks
The literature survey indicates that geopolymer word is one of the many names used for describing the binder formed with alumino-silicate gel structure which according to some researchers need not be in polymeric form. However, Davidovits has data to show that polymer is indeed formed. Commonly, Metakaolin (MK) is often used by some authors to produce so called pure ‘geopolymers’ since MK, mostly consist of alumina and silica. However, much literature exists on activation of MK in combination with FA, GGBS, etc. works on only FA based GPs were also reported, and notable among them is Prof. Rangan, of Curtin University. Davidovits advocates use of slag in combination with other GSMs such as MK and fly ash and he emphasizes on development with lower activation temperatures and lower alkali levels in AAS.
Comparatively, more papers are available in science of geopolymer where often paste is prepared for making test specimens. Concretes and mortars formulation are also
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reported, but, lesser in numbers. GP science has not yet came up with a unique way of describing the matrix of GP and the AAS has to be developed for each set GSMs used in any particular experiment.
About the mechanical strengths, only qualitative information is available which can be used to decide about any particular combination of GP mixes to achieve the desired level of strength.
Works on reinforced GPC are not many and however, the existing test results shows that structural behaviour of GPCs and CCs are essential and similar in nature, except that sometime at the same strength level, GPCs may tend to have lower modulus of elasticity.
Contrastingly, GP composites have performed better than P-C composites in durability related tests such as Sulphate, acid and corrosion resistance. This is mainly due to polymeric nature of GP matrix without presence of free lime.
Numerous studies on GPs indicated that though exact nature of GP microstructure is still to be decided, it is still possible to formulate the GP composites to achieve consistently the desired level of strengths for structural usages by suitable selection of GSMs, AAS, besides curing regimes.
Abbreviations/Notations
AAS = Alkaline Activator Solution Alumina = Al2O3 CCs = Conventional concretes CGA = Crushed granite aggregates C-S-H = Calcium-silicate-hydrate DW = Distilled WaterECO2 = Embodied carbon dioxideEE = Embodied energyFA = Fly ashFAA = Fly Ash Aggregates GGBS = Ground Granulated Blast Furnace Slag GP = Geopolymer GPC = Geopolymer concreteHVFA = High volume fly ash IR = Infrared MK = MetakaolinMR = Molar ratios NMR = Nuclear Magnetic ResonanceOPC = Ordinary Portland CementP-C = Portland CementSHf = Sodium Hydroxide flakes SHS = Sodium hydroxide solution
SiO2 = Silica SSD = Saturated surface drySSS = Sodium Silicate Solution W/C= Water-cement ratio
References
- Abrams, Duff A. [1913] Tests of Bond Between Concrete and Steel, University of Illinois in Urbana, Bulletin no. 71, Engineering experiment station, ISBN-13: 9781112226021 ISBN-10: 1112226028 2009, pages 238
- Alonso, S. and Palomo, A., Alkaline activation of metakaolin and calcium hydroxide mixtures: Influence of temperature, activator concentration and solids ratio, Materials Letters, 47, 55-62, (2001a).
- Alonso, S. and Palomo, A., Calorimetric study of alkaline activation of calcium hydroxide-metakaolin solid mixtures, Cement and Concrete Research, 31, 1, 25-30, (2001b).
- Anurag Mishra, Deepika Choudhary, Namrata Jain, Manish Kumar, Nidhi Sharda and Durga Dutt, (2008),” Effect of Concentration of Alkaline Liquid And Curing Time on Strength And Water Absorption of Geopolymer Concrete “ARPN Journal of Engineering and Applied Sciences Vol. 3, No. 1, February
- Bakharev T., J.G. Sanjayan and Y.-B. Chen, (2003), Resistance of alkali-activated slag concrete to acid attack. Cem. Concr. Res. 33 pp. 1607–1611
- Bakharev, T., (2005), Geopolymeric materials prepared using class F fly ash and elevated curing temperature, Cement and Concrete Research, 35, 6, 1224-1232
- Bakharev, T., Sanjayan, J. G. and Cheng, Y. B., 2003, Resistance of alkali-activated slag concrete to acid attack. Cement and Concrete Research, 33(1), 1607–1612
- Balaguru PS Balaguru P, Kurtz S, Rudolph J (1997). Geopolymer for Repair and Rehabilitation of Reinforced Concrete Beams. St Quentin, France, Geopolymer Institute: 5.
- Bao Y, Grutzeck MW, Jantzen CM (2005) J Am Ceram Soc 88:3287- Barbosa, V. F. F., Mackenzie, K. J. D. and Thaumaturgo, C., (2000),
Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers, International Journal of Inorganic Materials, 2, 309- 317
- Barbosa, V. F. F., Mackenzie, K. J. D. and Thaumaturgo, C., (2000), Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers, International Journal of Inorganic Materials, 2, 309- 317.
- Bjorn Lomborg, [2001], ‘Measuring the Real State of the World’, The Skeptical Environmentalist, Cambridge University Press, p 138
- Brooke N. J., L. M. Keyte, W. South, J. M. Ingham, L. M. Megget, (2006a), Seismic performance of inorganic polymer concrete joints, Source: Proceedings of the ICE - Construction Materials, Volume 159, Issue 4, pages171 –179 , ISSN:1747-650X, E-ISSN:1747-6518
- Brooke, N. J., L. M. Keyte, et al. (2005). “Seismic Performance of ‘Green Concrete’ Interior Beam-Column Joints.” Australian Structural Engineering Conference, Newcastle, Australia
- Brooke, N., Megget, L., Ingham, J. (2006b), Assessing the Seismic Performance of “Green Concrete” Interior Beam-column Joints, Fédération Internationale du Béton Proceedings of the 2nd International Congress June 5-8, 2006 – Naples, Italy ID 13-4 Session 13 – Concrete, p 12
- Chang, E. H., Sarker, P., Lloyd, N., & Rangan, B. V. (2007). Shear behaviour of reinforced fly ash-based geopolymer concrete beams. Paper presented at the The 23rd Biennial Conference of the Concrete Institute of Australia, Adelaide, Australia.
- Cheng, T. W. and J. P. Chiu, “Fire-Resistant Geopolymer Produced by Granulated Blast-Furnace Slag,” Miner. Eng., 15, 205-210 (2003)
- Chih-Hsing Wang, Cho-Liang Tsai, and Ching-Chang Lin, (2011), Penetration lag of chloride diffusion through concrete plate based on advancing model Journal of Marine Science and Technology, Vol. 19, No. 2, pp. 141-147
For a complete list of the references please visit: www.masterbuilder.co.inPublishers Note: Part - 2 to be features in May 2012 edition.
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Curing Methods for Precast Concrete Applications and Their Impact
As building development throughout the world continues, the desire to construct cheaper structures on sites that are more difficult to
build on, in shorter periods of time, all while providing improved performance will always be desirable in order to maximize both product economy and quality. As such, the construction industry is constantly searching for ways to improve their product. One means to this end is, rather than relying on improving construction implementation mechanisms such as scheduling, installation techniques, and quality control, is focusing on the industry’s improved knowledge and development of materials and
Sonjoy DebB.Tech.’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
their behaviour. One result of such motivation by the construction and engineering industry was the advent of prestressed concrete. This product was developed in order to take advantage of the desirable properties of concrete and steel, chiefly compressive and tensile strength, respectively, in order to achieve structural solutions that were not previously possible. For many projects, the best way to achieve this is through the use of precast concrete elements. Precasting prestressed concrete members eliminates many of the environmental and logistical problems associated with cast-in-place prestressed concrete, while taking advantage of the efficiency of factory-like operations and maintaining
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very strict quality control in order to produce a superior finished product. One of the most important characteristics of concrete that enables the use of precast members to be economically feasible is its ability, when under the right conditions, to gain strength extremely rapidly.
Need for Precast Concrete
Precast concrete is advantageous for several reasons: shrinkage and creep can be reduced, dead-load deflections can be controlled, quality control is improved, material availability can be improved and erection methods are similar to that for steel and thus total construction time is significantly reduced. One of the key properties of concrete that makes precasting economically feasible is its ability, under the proper conditions, to gain compressive strength extremely rapidly. The focus of this paper is a description of the various methods currently available for accelerating the curing of concrete, particularly for precast concrete applications.
Processes and Methods of Accelerated Curing
Two distinct methods for accelerating the curing process exist:
- the use of physical processes, and
- the use of admixtures to act as catalysts for the hydration process, resulting in the achievement of high compressive strengths in relatively short periods of time.
Typical physical processes used to accelerate the curing process are generally combinations of the following: increase in curing temperature, introduction of moisture to curing environment. Numerous methods exist, including conductive/convective heating, electrical resistance heating, and steam curing (low and high pressure). The use of admixtures in order to accelerate the curing process can be further subdivided into the use of mineral and chemical admixtures. Calcium Chloride has proven to be an extremely effective accelerator; however, due to corrosion concerns, its use in concrete with embedded metal is not recommended. The most common mineral admixture used as an accelerator is microsilica, or silica fume. While fly ash is frequently used in order to improve other properties of concrete, it has a retarding effect on the initial set and early strength gain of concrete, and should not be used for accelerated curing purposes. Some chemical admixtures, such as high-range water reducers (HRWR), or superplasticizers, have been used as indirect accelerators, primarily due to their ability to reduce the water demand for a given mix.
Focus of This Paper
The focus of this paper is the analysis of the various methods employed in the precast industry for the purpose of accelerating the curing process of concrete, and their effects on the short and long term compressive strength of concrete. These various methods of accelerated curing can be divided into three main categories: physical processes, mineral admixtures, and chemical admixtures. First, research and development of various accelerated curing methods will be presented, followed by a brief discussion of current methods predominantly employed by commercial precast manufacturers.
Reported Research Works in This Area
The relationship between the rate of compressive strength gain in concrete and curing temperature has been long established. To an extent, an increased curing temperature will result in an increased rate of strength gain. Beyond a certain point, increases in temperature not only prove to be less efficient, but can actually be detrimental to the properties of the concrete. A typical maximum curing temperature used in commercial precast plants is 160oF (Corcoran, 2004). Various methods of increasing the curing temperature of concrete have been employed in order to achieve high early strength. These methods include simple convection through the circulation of hot water or oil through formwork, or even through pipes inside the concrete members in the case of hollow elements, electric resistance heating, and both low and high pressure steam curing. One of the drawbacks to an increased curing temperature is the increased rate of humidity loss
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to the surrounding environment, which can result in severe shrinkage and cracking. Another problem is the rapid change of temperature within concrete members, resulting in potentially large thermal stresses. In order to alleviate these problems, any method of increasing the curing temperature must also involve the provision of adequate humidity in order to prevent excessive moisture loss, as well as careful cyclic implementation of temperature increase and decrease, in order to prevent the development of thermal stresses (Heritage, 2000). Three main factors are to be considered when using elevated temperatures in order to increase the curing rate of concrete: rate of temperature rise, maximum curing temperature, and heating time. Traditionally, it has been thought that early strength gains are offset by lower 28-day strength. As such, specifications often restrict maximum curing temperatures to between 140 and 160oF. However, a study by Pfeifer and Landgren (1982) showed that the use of a maximum curing temperature of 180oF resulted in no significant decrease in 28-day strength when compared to concrete cured at maximum temperatures of 110 or 145oF. While this does not dispute the general relation of increased early strength gain to decreased long-term strength, it may indicate that current restrictions on maximum curing temperatures are too low. A more recent study has reinforced the relation between increased early strength and decreased long-term strength. This report showed that increased curing temperatures resulting from direct electrical curing techniques, “increase the 1-day compressive strength, but reduce 28-day strength,” (Heritage, 2000). Regardless of the actual technique used to elevate concrete curing temperatures, and thus increase the rate of strength gain, two precautionary steps to prevent negative impacts of the process should be taken. First, “before any induced temperature increase is started, the time to the commencement of the initial set is required to allow the hardening phase to sufficiently resist thermally induced stresses,” (Heritage et. Al, 2000). This delayed increase in temperature allows a minimum development of strength necessary to prevent cracking resulting from the formation of thermal stresses. In addition, the supplementation of heat prior to the initial set has been shown to be relatively ineffective in increasing the rate of strength gain (Pfeifer and Landgren, 1982). At this point, the rate of hydration is extremely slow, and is affected little by increased temperature. An increased curing temperature also results in an increased rate of humidity loss to the environment. As such, “all efforts must be made to stop the evaporation of water from the surface of the sample by the use of a suitable covering,” (Heritage et. Al, 2000). If the effect of humidity lost to the environment is not controlled during the accelerated curing process, the impact on long-term compressive strength can be detrimental (Mehta and Monteiro, 2001).
Conduction/Convection Used for Accelerated Curing
One of the most fundamental methods for rapidly increasing the curing temperature of concrete is through the employment of simple conduction/convection techniques. The temperature of the forms may be increased either electrically or by pumping hot oil or hot water through them (Gerwick, 1993). The direct contact between the concrete and the forms with an elevated temperature results in conductive heat transfer. By utilizing convection as well, in the form of flowing hot oil or water, the rate of thermal energy transfer is increased, thereby increasing the rate of curing temperature increase. As with all accelerated curing methods involving elevated temperatures, precautions should be taken to provide sufficient humidity to prevent drying of the concrete, and proper insulation of the formwork will result in a more energy efficient increase in curing temperature.
Types of Accelerated Curing Used for Precast Concrete Members
A. Electrical Resistance Curing
Two primary types of accelerated curing processes involving elevated temperatures resulting from the dissipation of heat through electrical resistance have been attempted. One type of process involves the use of additional elements, such as special coils of wire, or even the reinforcement itself, as a means to generate heat through electric resistance (Heritage, 2000). By imposing an electrical current through reinforcing steel, or through additional wires, heat is generated inside the concrete as a result of the provided electrical resistance, resulting in an increased curing temperature. When steel forms are used, this method may also be used by applying electrical currents directly to the formwork, or by attaching electrical resistance elements to the forms.
More recently, an additional method of electrical resistance curing has been employed. Direct electrical curing, “is based on the fact that fresh concrete has an electrical resistivity of approximately 100 ohms-meter and, as such, can be heated ohmically when an alternating electric current is passed through it,” (Heritage, 2000). With direct electrical curing, the electrical resistance of concrete itself is taken advantage of and additional wires for the purpose of electric resistance curing are unnecessary. In addition, a more even distribution of heat generation occurs when compared to the use of either reinforcement or additional wires as resistors.
B. Low-Pressure Steam Curing
Steam curing is a process in which elevated curing
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temperatures and the addition of moisture during the curing process are both used in order to accelerate the rate of strength gain. These methods can be applied simultaneously, with an increase in temperature as a direct result of steam injection, or individually, in which case an initial temperature elevation is achieved through some alternate means and is followed by an increase in humidity through steam injection. Low-pressure steam curing is frequently used in very dry climates and in applications when the controlling the loss of moisture is imperative (Gerwick, 1993). The basic method of steam curing at atmospheric pressure, for the most part, follows the same stages present in any accelerated curing process involving elevated curing temperatures. First, an initial delay period, usually of three to four hours, is necessary for the concrete to attain its initial set. Next, a heating period, with a
temperature rise of 40 to 60oF per hour, is employed in order to reach a maximum curing temperature, generally between 140 and 160oF. This is followed by a steaming period, typically lasting six hours while maintaining the maximum curing temperature. Next, a cooling period is employed, during which time it is sometimes necessary release the boundary constraints of the forms, prestressing tendons, etc., in order to prevent damage from the development of thermal stresses. In general, the concrete elements are still covered with the steam hoods, or with tarps, during this process. Finally, a stage unique to steam curing, an exposure period, is necessary. At this time, the steam hoods or tarps are removed, and the concrete surface is exposed to the natural environmental conditions (Gerwick, 1993). The combined use of high curing temperatures and moist curing conditions results in the attainment of very high early strength.
Pressure vessels used for High-pressure steam curing
C. High-Pressure Steam Curing (Autoclaving)
Although generally reserved for the production of concrete masonry units in the United States, high-pressure steam
curing, also known as autoclaving, has been successfully employed in the production of prestressed precast concrete elements in Japan and Germany (Gerwick, 1993). During this process, the increase of curing temperature and humidity are combined with an increase in pressure; as such, elements in this manner must be cured in some type of enclosed vessel. This restriction limits the use of the technique to relatively small elements for typical applications. One of the benefits of this technique is that extremely low water-cement ratios can be utilized in the initial mix design. In the case of concrete blocks, the elements are produced through extrusion machines, without the use of formwork, using no-slump concrete. By utilizing high-pressure steam curing, sufficient moisture necessary to complete the hydration process is introduced to the concrete elements (Polisner and Snell, 1985).
Figure 1 (French et al., 1998)
Mineral Admixtures for Accelerating Curing
A. Microsilica
Microsilica, or silica fume, is an extremely reactive, pozzolanic material. In one study it was used as a cement replacement for the primary purpose of increasing overall concrete compressive strength (French et. Al, 1998). Not only did the results show an increase in long term strength, but they indicated an increase in concrete strength at all ages. Figure 1 demonstrates the relationship between compressive strength and time for the concrete with the addiction of microsilica compared to concrete without microsilica.
B. Fly Ash
Like microsilica, fly ash can be used as a cement replacement material. Fly ash is one of the byproducts formed by modern power plants; it is a coal-combustion byproduct, and is collected by electrostatic precipitators used to filter combustion gases. Unlike mircrosilica, however, fly ash does not result in improved early strength of concrete. In fact, the results of the same study mentioned previously in which microsilica was shown to increase concrete strength show that the replacement of cement by fly ash resulted in decreased early strengths (French et al., 1998).
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Fly ash particles (which look like bubbles) bind with other components in concrete to create a stronger, more durable concrete
Chemical Admixtures for Accelerating Curing
A. Calcium Chloride
The use of 1% of calcium chloride relative to the weight of cement in a mix has resulted in an increase of strength after 24 hours of 300% (Levitt, 1982). But inclusion of calcium chloride in reinforced and prestressed concrete can be extremely detrimental, as the chloride can contribute greatly to corrosion of the reinforcing steel.
Void system produced by early types of high range water reducers
B. High-Range Water Reducers (Super Plasticizers)
Although not technically characterized as accelerators, high-range water reducing (HRWR) admixtures contribute to, “large increases in early concrete strengths under both normal and accelerated curing conditions,” (Hester,
1978). When compared with standard concrete mixes, the inclusion of HRWR admixtures has showed a marked increase in early strength gain when exposed to a variety of curing temperatures.
Conclusion
The implementation of elevated curing temperatures is a relatively straight forward process, and can be achieved without the need for a great deal of research and development. As a result, this is the primary method currently employed by commercial precast manufacturers. With recent advances in material technology, a number of admixtures (mineral and chemical) can be used, both directly and indirectly, as accelerating agents. However, compared with increased curing temperatures, the use of admixtures as accelerators can introduce numerous potential problems and difficulties. Until some significant incentive or motivation is provided, such as significant increases in energy costs and decreases in admixture costs, currently employed curing methods involving elevated curing temperatures will likely continue to prevail.
Reference
- Corcoran, James, (2004). Concrete Technology Corporation, 1123 Port of Tacoma Road, Tacoma, WA 98421. Telephone interview on 3/15/04.
- French, Catherine, Alireza Mokhtarzadeh, Tess Ahlborn, Roberto Leon, (1998). “High-Strength Concrete Applications to Prestressed Bridge Girders,” Construction and Building Materials, Vol. 12, Elsevier Science Ltd., Great Britain, pp. 105-113.
- Gerwick, Ben C. Jr., (1993). Construction of Prestressed Concrete Structures, Second Edition, John Wiley & Sons, Inc., New York, NY, pp. 19-23, 91-94.
- Heritage, Ian, Fouad M. Khalaf, and John G. Wilson, (2000). “Thermal Acceleration of Portland Cement Concretes Using Direct Electronic Curing,” ACI Materials Journal, January-February, 2000, pp. 37-40.
- Hester, Weston T., (1978). “High-Range Water-Reducing Admixtures in Precast Concrete Operations,” PCI Journal, July-August, 1978, pp. 68-85.
- Levitt, M., (1982). Precast Concrete, Materials, Manufacture, Properties and Usage, Applied Science Publishers, INC., Englewood, NJ, pp. 33-38, 53-73.
- Mehta, P. Kumar and Paulo J.M. Monteiro, (2001). Concrete, Microstructure, Properties and Materials.
- Pfeifer, Donald W., (1982). “Development of the Concrete Technology for a Precast Prestressed Concrete Segmental Bridge,” PCI Journal, September-October 1982, pp. 78-99.
- Pfeifer, Donald W., and Robert Landren, (1982). “Energy-Efficient Accelerated Curing of Concrete for Plant-Produced Prestressed Concrete,” PCI Journal, March-April, 1982, pp. 94-107.
www.fhwa.dot.gov, www.4.uwm.edu, www.reflect.ifl.ac.uk
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Precast Concrete for Building Systems
The concept of precast (also known as “prefabricated”) construction includes those buildings where the majority of structural components are standardized
and produced in plants in a location away from the building, and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at low cost. The main features of this construction process are as follows:
- The division and specialization of the human workforce
- The use of tools, machinery, and other equipment, usually automated, in the production of standard, interchangeable parts and products
Sonjoy DebB.Tech.’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
This type of construction requires a restructuring of the entire conventional construction process to enable interaction between the design phase and production planning in order to improve and speed up the construction. One of the key premises for achieving that objective is to design buildings with a regular configuration in plan and elevation.
Urban residential buildings of this type are usually five to ten stories high (see Figure 1). Many countries used various precast building systems during the second half of the 20th century to provide low-income housing for the growing urban population. They were very popular after the Second World War, especially in Eastern European countries and former Soviet Union republics. In the former Soviet Union,
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different precast buildings systems are denoted as “Seria,” whereas in Romania they are called “Sectiunea.”
In general, precast building systems are more economical when compared to conventional multifamily residential construction (apartment buildings) in many countries. The reader is referred to the UNIDO report for detailed coverage on precast systems and their earthquake resistance.
Figure 1: A typical precast slab-column building (WHE Report 68, Serbia and Montenegro)
Categories of Precast Building Systems
Precast buildings constitute a significant fraction of the building stock in the republics of the former Soviet Union and Eastern European countries. These systems have been described in the following eight WHE reports: 32 (Kazakhstan); 33, 38, and 39 (Kyrgyzstan); 55 (Russian Federation); 66 (Uzbekistan); 68 (Serbia and Montenegro); and 83 (Romania). Depending on the load-bearing structure, precast systems described in the WHE can be divided into the following categories:
- Large-panel systems- Frame systems- Slab-column systems with walls- Mixed systems
Large-Panel Systems
The designation “large-panel system” refers to multistory structures composed of large wall and floor concrete panels connected in the vertical and horizontal directions so that the wall panels enclose appropriate spaces for the rooms within a building. These panels form a box-like structure (see Figure 2a, 2b). Both vertical and horizontal panels resist gravity load. Wall panels are usually one story high. Horizontal floor and roof panels span either as one-way or two-way slabs. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls.
Depending on the wall layout, there are three basic configurations of large-panel buildings
- Cross-wall system. The main walls that resist gravity and lateral loads are placed in the short direction of the building.
- Longitudinal-wall system. The walls resisting gravity and lateral loads are placed in the longitudinal direction; usually, there is only one longitudinal wall, except for the system with two longitudinal walls developed in Kazakhstan (WHE Report 32).
- Two-way system. The walls are placed in both directions (Romania, WHE Report 83).
Thickness of wall panels ranges from 120 mm for interior walls (Kyrgyzstan, WHE report 38) to 300 mm for exterior walls (Kazakhstan, WHE Report 32). Floor panel thickness is 60 mm (Kyrgyzstan). Wall panel length is equal to the room length, typically on the order of 2.7 m to 3.6 m. In some cases, there are no exterior wall panels and the façade walls are made of lightweight concrete (Romania, WHE Report 83). A typical interior wall panel is shown in Figure 3.
Figure 2a: A large-panel concrete building under construction (WHE Report 55, Russian Federation)
Lateral stability of a large-panel building system typical for Romania is provided by the columns tied to the wall panels (WHE Report 83). Boundary elements (called “bulbs” in Romania) are used instead of the columns as “stiffening” elements at the exterior. The unity of wall panels is
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Figure 2b: Cross Wall construction
Figure 3: Precast interior wall panel with steel dowels and grooves (WHE Report 38, Kyrgyzstan)
achieved by means of splice bars welded to the transverse reinforcement of adjacent panels in the vertical joints. Longitudinal dowel bars placed in vertical and horizontal joints provide an increase in bearing area for the transfer of tension across the connections. Wall-to-floor connection is similar to that shown in Figure 4.
Frame Systems
Precast frames can be constructed using either linear elements or spatial beam-column subassemblages. Precast beam-column subassemblages have the advantage that the connecting faces between the subassemblages can be placed away from the critical frame regions; however, linear elements are generally preferred because of the difficulties associated with forming, handling, and erecting spatial elements. The use of linear elements generally means placing the connecting faces at the beam-column junctions. The beams can be seated on corbels at the columns, for ease of construction and to aid the shear transfer from the beam to the column. The beam-column
joints accomplished in this way are hinged. However, rigid beam-column connections are used in some cases, when the continuity of longitudinal reinforcement through the beam-column joint needs to be ensured. The components of a precast reinforced concrete frame are shown in Figure 5.
Figure 4: Plan of a large-panel building showing vertical connection details (WHE Report 32, Kazakhstan)
Figure 5: Components of a precast reinforced concrete frame system of Seria IIS-04 (WHE Report 66, Uzbekistan)
Slab-Column Systems with Shear Walls
These systems rely on shear walls to sustain lateral load effects, whereas the slab-column
structure resists mainly gravity loads. There are two main systems in this category:
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- Lift-slab system with walls- Prestressed slab-column system
Lift-slab systems were introduced in the last decade of the Soviet Union (period 1980-1989) in some of the Soviet Republics, including Kyrgyzstan, Tadjikistan, and theCaucasian region of Russia, etc. This type of precast construction is known as “Seria KUB.”
The load-bearing structure consists of precast reinforced concrete columns and slabs, as shown in Figure 6. Precast columns are usually two stories high. All precast structural elements are assembled by means of special joints. Reinforced concrete slabs are poured on the ground in forms, one on top of the other, as shown in Figure 7. Precast concrete floor slabs are lifted from the ground up to the final height by lifting cranes. The slab panels are lifted to the top of the column and then moved downwards to the final position. Temporary supports are used to keep the slabs in the position until the connection with the columns has been achieved.
Figure 6: A lift-slab building of “Seria KUB” under construction (WHE Report 39, Kyrgyzstan)
Earthquake Performance
There is a general concern among the earthquake engineering community regarding the seismic performance of precast construction. Based on experience in past earthquakes in Eastern European and in Central Asian
countries where these systems have been widely used, it can be concluded that their seismic performance has been fairly satisfactory. However, when it comes to earthquake performance, the fact is that “bad news” is more widely publicized than “good news.” For example, the poor performance of precast frame systems of Seria 111 in the 1988 Spitak (Armenia) (M7.5) earthquake is well known (see Figure 8). However, few engineers are aware of the good seismic performance (no damage) of several large-panel buildings under construction at the same site, remained undamaged in the 1988 Spitak (Armenia) earthquake (far back), whereas the precast frame buildings suffered extensive damage (foreground)3 (WHE Report 32, Kazakhstan); these large-panel buildings were of a similar seria as the large-panel buildings described in the WHE Report 55 from the Russian Federation (Seria 464). The buildings of Seria 111 were similar to the precast concrete frame system of Seria IIS, described in the WHE report 66 (Uzbekistan). The precast prestressed slab-column system (IMS Building System) described in WHE Report 68 (Serbia and Montenegro) has undergone extensive laboratory testing that predicted excellent resistance under simulated seismic loading. These building have been subjected to several moderate earthquakes without experiencing significant damage.
Figure 7: Plan of a typical lift-slab building (WHE Report 39, “Seria KUB,” Kyrgyzstan)
Due to their large wall density and box-like structure, large-panel buildings are very stiff and are characterized with a rather small fundamental period. For example, a 9-story building in Kazakhstan has a fundamental period of 0.35 to 0.4 sec (WHE Report 32). In general, large-panel buildings performed very well in the past earthquakes in the former
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Soviet Union, including the 1988 Armenia earthquake and the1976 Gazly earthquakes (Uzbekistan). It should be noted, however, that large-panel buildings in the area affected by the 1976 Gazly earthquakes were not designed with seismic provisions. Most such buildings performed well in the first earthquake (M 7.0), but more damage was observed in the second earthquake that occurred the same year (M 7.3), as some buildings had been already weakened by the first earthquake (Russian Federation, WHE Report 55). Large-panel buildings performed well in the 1977 Vrancea (Romania) earthquake (M 7.2) and in subsequent earthquakes in 1986 and 1990 (Romania, WHE Report 83).
Figure 8: Building collapse in the 1988 Spitak (Armenia) earthquake (WHE Report 66, Uzbekistan)
Seismic-Strengthening Technologies
According to WHE reports, no major efforts have been reported regarding seismic strengthening of precast concrete buildings. However, seismic strengthening of precast frame buildings was done in Uzbekistan (WHE Report 66). The techniques used include the installation of steel straps at the column locations (see Figure 9) and reinforcing the joints with steel plates to provide additional lateral confinement of the columns.
Figure 9: Seismic strengthening of precast columns with steel straps (WHE Report 66, Uzbekistan)
Benefits of Using Precast elements in Building Construction
(A) Hotels
Precast Structures uses a system of precast elements which link together to form a cross-wall format.
Panels can be formed in solid or twin wall styles to suit the design requirements of the structure. Whichever solution is selected the selections are produced in high quality finish which is suitable for direct decoration, with minimal preparatory work, obviating the need for plaster finishes, leading to cost and programme savings.
The philosophy of PCS is to produce a design which will provide the most cost effective solution, utilising the most appropriate materials for the project. This can include such items as hot rolled steel sections and cold rolled steel infill panels as appropriate.
Benefits of Using Precast Concrete Structures Include
- High quality concrete designed for direct decoration or exposure.
- Architectural and structural quality components.
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- Large volume supply capacity.- Dedicated experience project management.- In house Erection by trained and qualified erection
personnel.- Solid room size slabs- Prefinished for direct ceiling decoration.- Suitable for direct carpet application.- Reduced structural zones free from downstands.- Erection of stair and lift cores as erection progresses
allowing safe access for subsequent trades.- Pre-fitted windows option.
Figure 10: Use of precast concrete in hotel construction
Precast Concrete Structures Ltd specialise in the fast efficient delivery of the building structure, where minimal wet trades and high quality finish are essential to follow on trades. PCS strive to be market leaders in quality of finished product and offer an innovative and non-contractual approach to building structures. Some typical hotels made of precast concrete shown in Figure 10.
(B) Student Accommodation
Cross-Wall Construction
The use of cross-wall construction in student accommodation (See Figure 11) gives significant benefits for short-term build projects where a deadline for opening is critical.
Precast concrete construction offers extremely durable accommodation, capable of sustaining even the toughest conditions of student living.
By the use of direct finishing techniques to the walls and ceiling, together with solid room-sized slabs, and the pre-installation of bathroom pods, cross-wall construction offers speed of construction together with economy.
Key requirements for economical construction in student accommodation include:
- Repetition of room layout.- Consistency of vertical alignment to division walls.- Repetition of elevational treament.
By adhering to these basic principles, Precast Concrete Structures Ltd will provide advice and innovative solutions
on the most economical means of manufacturing the components and sequencing the erection to the maximum benefit of the client. These benefits include:
- Fast-build programme within term-time constraints.- Direct decoration to walls and ceilings, with only minor
pre-decoration treatment.- Pre-installation of windows.- Early “dry-box” working for subsequent trades.- A variety of elevational treatments using non-load-
bearing cladding systems (loads are transferred via the cross-walls and do not rely upon external walls for support).
- Reduced structural zone without downstands.
Figure 11: Use of precast concrete in hostel construction
(C) Apartments
Apartment construction has become increasingly popular as a modular build (Refer Figure 12) alternative to traditional steel and insitu concrete frame methods. The system adopted uses cross-wall construction in a similar method to the hotel construction system, but differs in that the variability of room layouts and external elevations require differing techniques and innovative thinking to produce fast-build economical solutions.
The options for apartments are both extensive and flexible providing key criteria in design are met. Precast Structures Ltd has broad experience in developing solutions for alternative construction, particularly suited to the Design & Build market.
Benefits include
- Direct decorative finish to walls with only minor pre-decoration treatment, negating the requirements for wet plaster.
- Optional methods of floor construction, allowing flexibility for individual client requirements, including:
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- Traditional hollow-core.
- Wide slab composite flooring.
- Pre-finished solid slabs.
- Direct soffit finishing in replacement of suspended ceilings, significantly reducing construction build costs
- Reduced structural zone without downstands.
- Construction of common stairs and lift cores as the erection progresses, permitting early access for subsequent trades.
- Pre-fitted windows.
- External pre-finished cladding panels, grey concrete inner leaf only, or curtain-walling / metal stud permitting total flexibility in elevational treatment.
Figure 12: Use of Precast concrete technology in apartment construction
Apartment construction is usually designed with traditional building solutions which are subsequently modified during the design process to obtain a competitive edge in Design & Build solutions. The benefits of early consultation with Precast Structures will result in significant savings in both cost and time, resulting from economical manufacture solutions and reduced erection periods.
(D)Architectural Concrete
Precast Concrete Structures has extensive use in manufacture and erection of architectural and structural building components (Refer Figure 13).
Sections are bespoke and can be manufactured within the programme for our standard materials with a wide range of finishes and colours including:
- Brick.- Wet cast reconstituted stone cladding and dressings.- Composite Architectural / Structural insulated
columns.- Exposed structural elements.
Buildings are considered on an individual basis and assessed for integration of structural components to reduce programme and to ultimately drive down costs.
Figure 13: Precast concrete in making special architectural look
PP5 shown to the right is a flagship example of the benefits of incorporating Architectural Cladding into the structure. Originally concepted as an insitu frame, with independent cladding, the revised PCS proposal produced cost saving and significant overall programme reduction.
Progressive Collapse
Concrete building structures whether, insitu or precast, are required to perform in the event of accidental damage or explosion by meeting the design criteria set out in BS8110 for progressive collapse.
Within the building structure, ties are incorporated to resist calculated forces determined by a variety of factors, including:
- number of stories- centres of walls / size of spans- total loads carried
These are achieved by the use of the following ties incorporated into the precast cross-wall design:
- vertical ties- horizontal ties- peripheral ties- internal ties
Joints between panels are tied together using pre-shuttered insitu-fill to create a robust joint with minimal finishing required. The joints use wire ties designed to meet the specific tie-force criteria, but also to allow flexibility in assembly tolerances during erection.
Peripheral and internal ties use high strength steel strand within the nominal insitu joints at cross-wall locations and around the perimeter of the building to create a continuous tie arrangement.
Building design is analysed for structural stability by Precast Structures consultants who have extensive knowledge in the design stability of cross-wall building structures.
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Examples of Projects with Precast Concrete Technology
- MOD Barrack Blocks
As part of the MOD’s wider Project MoDEL works at RAF Northolt, Precast Concrete Structures Ltd have successfully completed the erection of 2 x junior ranks, ‘barrack blocks’ for Norwest Holst Ltd. The project, completed to programme and budget, provides single living accommodation for some 182 airmen and airwomen at this prestigious West London unit.
- Federation Headquarters in Precast
The Police Federation of England and Wales which represents 140,000 police officers, Sergeants, Inspectors and Chief Inspectors commissioned a new Headquarters building in Leatherhead, Surrey.
- Carver’s Warehouse
The listed Carvers Warehouse is the oldest surviving and only stone built warehouse remaining in Manchester City centre having been completed in 1804 and in recent times had been the home of a bathroom showroom.
- Project Hal, Broxbourne
News International, publishers of The Times, The Sunday Times, The Sun and News of the World, undertook a redevelopment of its printing facilities on 3 sites, Broxbourne, Liverpool and Glasgow. The largest of these sites was a new 27,250 square metre development at Broxbourne near the M25.
- 1400 Student Rooms, Brunel University
With growing student numbers Brunel University in Uxbridge decided to expand its en-suite student accommodation on its West London Central campus by 1,400 units.
- 290 Apartments, Budenberg
Pioneering developer Urban Splash teamed up with one of the UK’s leading Architects, Foster and Partners to develop an environmentally sustainable, residential scheme on the banks of Bridgewater Canal in Altrincham, incorporating a number of innovative features.
- The Hub, 5 Piccadilly Place
The Hub is a 10 storey ‘U’ shaped development, wrapped around a hard landscaped plaza, of 167 new apartments in the heart of a Manchester mixed use development area, by Argent Group in Manchester City centre.
- Ramada Encore Hotel
The new £8m Ramada Encore Warrington, located on Birchwood Business Park, is a new hotel with a simple concept of being fresh, stylish vibrant and upbeat whilst offering comfortable contemporary accommodation.
This hotel located near Warrington town centre provides 103 ensuite bedrooms, and 2 meeting rooms over 4 stories, with all public areas and bedrooms being fully air-conditioned.
Conclusion
By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete. The production process for Precast Concrete is performed on ground level, which helps with safety throughout a project. There is a greater control of the quality of materials and workmanship in a precast plant rather than on a construction site. Financially, the forms used in a precast plant may be reused hundreds to thousands of times before they have to be replaced, which allows cost of formwork per unit to be lower than for site-cast production. The use of precast concrete in Indian construction industry will definitely enhance the efficiency of the contractor interms of quality, safety and time of project completion. In developing country like India, adoption of this technology for building construction will boost the Government’s development plans, as this gives really faster way of construction and also quality of work far more better than onsite casting concrete which is really value for money.
Reference
[1] Definition of “Mass Production” in “Industrial Engineering and Production Management” Britannica Macropaedia, The New Encyclopaedia Britannica, 15th Edition, Vol. 21, p. 204, 1989.
[2] UNIDO, 1983. Design and Construction of Prefabricated Reinforced Concrete Frame and Shear-Wall Buildings. Building Construction Under Seismic Conditions in the Balkan Region. Volume 2. UNDP/UNIDO Project RER/79/015, Vienna, Austria.
[3] EERI (1989). Armenia Earthquake Reconnaissance Report. Special Supplement to Earthquake Spectra, El Cerrito, California.
[4] http://www.precaststructures.com
[5] Precast Concrete Construction, Svetlana Brzev, British Columbia Institute of Technology, Canada, Teresa Guevara-Perez, Architect, Venezuela
Photo Courtesy
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Practical and Economical Design Aspects of Precast Concrete Building Structures
Precast concrete shear walled structures, also called large panel systems, are a good solution for low rise and medium rise residential and commercial buildings. This paper describes the practical and economical aspects of designing and detailing these kinds of structures.
Precast concrete structures with load bearing wall panels have several advantages compared to rcc frame structures with brickwork infill walls.
- No brickwork infill walls required- Superior quality of finishing (smooth surface)- At mould face no plastering required- Saves time and manpower- Thin wall structure increases the carpet area- Precast concrete is more durable than clay brick- Better health and safety standards during construction
The external precast wall panels shall be a finished product and no plastering shall be required. The precast wall panels should be load bearing members and shall be capable of carrying the vertical and lateral loads. The wall panels can be connected to each other in various ways and together with the floor diaphragm they will form box type structures (figure 1 and figure 2).
Precast Floor Slabs
For residential and commercial building projects two different kind of precast floor systems can be considered.
Hollow core slabs (figure 3.)
These are prestressed floor slabs with longitudinal voids. The presence of the voids results in material savings and weight savings. With hollow core slabs large spans can be achieved and no temporary propping is required. Hollow
core slabs only have longitudinal prestressing reinforcement and no other reinforcement. Due to manufacturing methods it is not possible to make slabs with anchored tie bars, protruding stirrups or embedded welded plates. Diaphragm action is achieved through special joint design. Especially in seismic zones an rcc topping has to be added to join the slabs properly.
Bob Van GilsManaging DirectorWBK Engineering Services Pvt. Ltd.and Van Boxsel Engineering Pvt. Ltd.
Figure 1. Large precast panel construction
Figure 2. Large precast panel construction
Precast planks with lattice girders (figure 4.)
These are composite slabs made of precast concrete planks of 50mm thick with an rcc topping. The bottom reinforcement is placed within the precast planks and the top reinforcement is placed within the rcc topping. Basically the composite slab behaves the same as an rcc one way slab. The precast planks serve as the shuttering and have to be supported during casting and curing of the concrete. It is a very flexible system where size of planks can be easily adjusted and MEP (Mechanial Electrical Plumbing) services can be placed in the rcc topping.
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In every precast concrete project the following aspects are important for the design team:
- Architecture- MEP Services- Structure- Manufacturing- Erection
One by one these points shall be discussed in this paper.
Architectural aspects
Full advantage of precast concrete construction is achieved when the building has been designed for high construction speed and maximum repetition. The architect should be considering the following points:
- Simple and symmetrical layout and elevations- Achieve standardization and repetition- Modular grid: multiple of 1200mm- Modular design has big impact on the costing- Design with larger floor spans- Minimize joints- In façade minimize horizontal or low sloped elements
that collect dirt- Keep precast elements as large as possible, but max.
around 10,000 kg- Wall panels are usually one storey high- Design shouldn’t be a conversion of cast in-situ
structure- Don’t try to make everything in precast- Integrate architecture, services and structure- Avoid last minute changes
Modular design
Modular design principles can be strictly followed but give less freedom to the architect. A balance should be found between modular design and customized prefabrication.
Architectural features
Sunshades are a common feature in Indian building projects. The precast walls are made on flat steel moulds and it is not possible to make the sunshade as one part with the wall. Sunshades can be prefabricated and connected
in a later stage to the precast walls.
Cantilevered balconies can be made by providing cantilevered brackets on the adjacent precast walls and resting the balconies on these brackets. In case there are no precast walls nearby, the precast balcony can be made cantilevered with protruding top reinforcement connecting to the rcc topping of the floor slab.
Shafts should be strategically placed where they don’t complicate the layout of the load bearing walls and precast floor units.
Precast wall panels can be provided with false joints to achieve a better architectural design pattern (see figure 5).
Flexibility
Flexibility in precast concrete building projects can be achieved by creating larger floor spans with larger open spaces. Especially in office buildings this concept will provide a lot of advantages to the end user (see figure 6).
Figure 3. Hollow core slabs Figure 4. Precast planks with lattice girders
Figure 5. False joints in precast panel Figure 6. Flexibility with large open spaces
MEP Services
In precast concrete building projects it is important that the MEP services consultants and the MEP vendors are part of the design team. Services like air-conditioning, electrical and plumbing have to be an integrated part of the precast design. Wall panels can be provided with electricity conduits, electricity boxes and openings for ducts (see figures 7 and 8).
Figure 7. Electricity in precast wall panel
Figure 8. Several openings in precast wall panel
Hollow core slabs can be provided with electricity boxes and block outs. Placing MEP services within the hollow core slabs is not possible. Services have to be placed above or below the floor, special hangers can be used (see
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figure 9). Precast planks with rcc topping can be provided with electricity boxes and block outs. Furthermore small conduits, ducts and plumbing pipes can be embedded in the rcc topping (see figures 10 and 11)
Design aspects of hollow core slabs (see figures 14 and 15):
- Standard width is 1200mm- Slabs can be cut in longitudinal direction- Minimum thickness can be 100mm but most
manufacturers offer minimum thickness of 150mm- No propping required- Camber and deflection should be checked- Minimum 60mm rcc topping is recommended in seismic
zones- In case of rcc topping the top surface of the hollow core
has to be roughened- It is difficult to place MEP services within the hollow
core slabs, services have to be placed below or above the slab.
- Maximum shear stress in longitudinal joints can be 0.10 N/mm2
- Connection of hollow core slab to shear wall has to be properly detailed
- Pay attention to fixation of hollow core units in between load bearing walls
Design aspects of precast planks with lattice girders (see figures 16 and 17)
- Standard width is generally 2.4m or 3.0m- Flexible system, any type of slab size can be made- Bottom reinforcement is placed in the precast plank- Top reinforcement is placed in the rcc topping- Minimum thickness is generally 50mm precast plank
with 100mm rcc topping- Propping of the slabs during casting and curing of
concrete is required- MEP services can be placed in the rcc topping
Figure 9. Typical load hangers for hollow core slabs
Structural aspects
India being an earthquake prone country the seismic resistant requirements are the most important criteria of the structural design. Looking at the requirements we can draw the conclusion that the basic earthquake resistant design rules are favoring precast concrete. Generally the following design rules should be followed:
- Simple and symmetrical layout- Uniform distribution of mass and structural stiffness
over the height- Avoid torsion- Ductile behavior of the structure
Simple, symmetrical and uniform buildings are normally easy to optimize for precast concrete construction.
The structural behavior of precast concrete shear wall structures is different than rcc frame structures. The walls are to be considered as cantilevering from the foundation (see figures 12 and 13).
Vertical Load Path
Hollow core slabs and precast plank floors are considered to be one way slabs. Enough seating on the load bearing walls should be provided. Transfer of vertical loads between wall panels should be achieved by direct structural connection.
Figure 10. MEP services in topping of plank floor
Figure 11. Plumbing pipes in topping of plank floor
Figure 12. Forces acting on shear wall Figure 13. Forces acting within shear wall
Figure 14. Complicated layout with hollow core slabs
Figure 15. Simple layout
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Lateral Load Path
The precast floor units have to be properly joined together to act as a floor diaphragm that transfers the lateral loads to the shear walls. The connections between the floor diaphragm and the shear walls have to be properly detailed. The shear walls will transfer the lateral loads to the foundation by acting as cantilevered walls.
Connections
The wall panel connections can be classified into horizontal joints and vertical joints. The horizontal joints have to transfer vertical loads as well as lateral loads. The vertical joints can be open and not transferring any loads or they can be connected to transfer shear loads.
In many countries the horizontal joints between precast wall panels are made with grouted corrugated ducts. The precast wall panels are lowered into position over the vertical reinforcement bars which are protruding from the below element (see figure 18.). The ducts and the horizontal joint are fully filled with non shrink high strength grout with at least 10MPa higher strength as the precast concrete. In the plastic hinge regions the ducts can be provided over the full height of the precast wall and the reinforcement bars can be lapped inside the duct. Another option is to use the splice sleeve type 2 connection according to ACI 318 (see figure 19.). It can also be decided to design the bottom stories, where yielding will occur, in cast in-situ concrete.
Filling of horizontal joints with non shrink high strength cement based mortar or grout can be done in several ways:
- Placing the precast wall in thixotropic mortar bed (see figure 20)
- Fill the joint with mortar by hand placement- Pump thixotropic grout in the joint (see figure 21)- Fill the joint with flowable grout- Injection of flowable grout
Because of high temperatures in India and because clean filling has to be achieved it is advised to follow the third method and fill the joints by pumping thixotropic grout in the joints.
Filling of the corrugated ducts is generally done by pouring flowable grout from the top or by injection/pumping from the bottom of the duct.
Figure 16. Layout of precast plank floors
Figure 17. Precast plank floors withvarious shapes
Figure 18. Connection through grouted corrugated ducts
Figure 19. Splice sleeve connections
Figure 20. Placing wall in mortar bed
Vertical joints can either be structural joints which have to transfer shear forces or non-structural joints which don’t have to transfer any forces. In case fully monolithic behavior has to be achieved the best option is to use a protruding
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reinforcement connection in combination with drop-in stirrups. To ease the manufacturing process the protruding reinforcement can be replaced by coupler bars (see figures 22 and 23). It is advised to use these connections only for internal shear walls as the vertical joint has to be finished with plastering at both sides and this requires a lot of extra work.
Manufacturing Aspects
Difference has to be known between Industrialized building systems and Prefabrication systems. Industrialized building systems are consisting of standard prefab elements made in standard moulds with minimum customization (see figure 24).
Prefabrication means precast elements according to a standard concept but with flexibility to customize according to the requirements of the project.
Figure 22. Vertical connection detail Figure 23. Vertical connections between internal walls
Figure 24. Industrialized building system components
Figure 25. Circulating pallet system (flat moulds)
Figure 26. Battery mould (vertical moulds)
The design team has to understand the capabilities of the manufacturing unit.
Design aspects
- Conventional, semi automated or fully automated precast plant?
- Horizontal tables or vertical battery moulds (see figures 25 and 26)
- Wooden side shuttering or steel side shuttering?- Custom made wooden moulds for special elements
(see figure 27)- Minimum and maximum size and weight of the precast
elements
Figure 27. Custom made wooden mould
Figure 28. Standard coupler bars
Figure 21. Pumping grout in horizontal joint
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- Standard embedded parts Ò anchors, lifting eyes, reinforcement etc.
- Minimize the variation in embedded parts (see figure 28)
- Avoid penetrations through the mould- Pay attention to stripping of the elements
Figure 29. Space for crawler crane
Figure 30. Casting rcc topping
Figure 31. Typical detail precast wall with hollow core slab
Figure 32. Typical detail of sandwich panel with hollow core
- Block outs and rebates should be made tapered- Chamfer the edges of wall panels to reduce edge
damage and to mask differences in alignment between panels at the joints.
- Prefab reinforcement cages- Different types of finishing
Execution / Erection Aspects
Erection of the precast elements will be done by cranes which can be placed at a fixed location like tower cranes or by mobile cranes which can move around the building (see figure 29). In case of mobile cranes or crawler cranes there should be enough space to maneuver comfortably around the building. The speed of the crane often determines the speed of erection, especially in high rise structures where it takes more time to lift the elements. Another important aspect of the erection sequence is the casting of the rcc topping on hollow core or plank floors.
Design aspects
- Crane position and lifting capacities- Lifting speed and speed of erection- Space for mobile cranes or crawler cranes (see figure 29)- Easy access to connections- Clean construction process- Tolerances- Easy and fast erection- Erection sequence- Filling of joints with grout / mortar- Casting rcc topping (see figure 30)- Position of props and supporting structures
Typical Details - Commercial Buildings
Commercial buildings are generally constructed with false ceilings and therefore the usage of precast wall panels with corbels is preferred. The advantage of this solution is the direct load transfer from wall to wall and there is more space for seating of the floor units and connections. For commercial buildings large spans are required because of flexibility and hollow core slabs are best suited in this case (see figures 31 and 32).
Typical Details - Residential Buildings
In residential buildings the precast walls with corbels are not preferred by the client. Also the floor spans are smaller and the advantages of hollow core are diminished (see figure 33). Precast planks with rcc topping become a more feasible option with advantages like better structural behavior, more flexibility, weight reduction and easier to install MEP services (see figure 34).
Figure 33. Typical detail of precast wall with hollow core
Figure 34. Typical detail of precast wall with plank floor
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Hollowcore Manufacturing and Factory Design
This document is designed to provide guidance and outline the main considerations in the initial planning of a Hollowcore plant. The information presented is based on 40 years of Spiroll experience. To summarise the key points: -
- The Plant should be planned with both the short term and long term capacity targets for hollowcore manufacture based on beds numbers and length. The normal bed length can vary from 60 to 150 metres - 120 metres is the most common as it provides good flexibility and fits well in to the daily production cycle.
- The aim of any plant is to achieve maximum efficiency by filling every bed every day. To achieve this, adequate time must be allowed for curing the concrete, cutting the slabs to length, lifting the slabs and cleaning and preparing the beds for casting again. Of particular importance is the curing time as the strength of the concrete must be adequate to hold the bond when the wires are cut. Before releasing the tension in the free strand to transfer the strain energy into the concrete, the concrete must have enough strength.
- The factory layout must also take account of finished product handling and storage, concrete batching and distribution, and maintenance and service requirements.
- An economic approach to the investment is to have staged investment. The factory layout should then include provision for immediate and future production levels so that the production can grow to meet market demand without disrupting the ongoing production.
- A low cost start-up can also be considered with minimal plant. This can be achieved with mobile plant, initially handling of the concrete and the product can be with a five tonne forklift. This system can be replaced by a gantry cranes, overhead cranes or travel lifts in the future.
- This system using extruder casting machines with mobile plant can be extended to a mobile hollowcore production plant. In this way the plant is sited at the construction project site and moved when the project is complete.
Hollowcore Slabs
In terms of selling hollowcore slabs into your local market, the slab is a versatile precast element that can be utilised in a wide range of applications and thus expand the available markets.
Few building materials available today offer the economy, flexibility and reliability of precast prestressed concrete. The advantages of hollowcore slabs are significant for the following reasons: -
Durability: Hollowcore slabs provide long-term performance in extremely harsh conditions that could destroy lesser materials. It is extremely resilient to deterioration from the
weather and the dense concrete and high cover to the steel allows design for high fire ratings.
Speed: Factory production of hollowcore allows the producer to have full control over all the variables, which affect the durability, strength and appearance of the slab. The high quality and excellent finishes of the slabs reduce site work to an absolute minimum.
Flexibility: Hollowcore slabs used for floors have good soffits which allow for direct application of ceiling finishes. They can also be used for wall panels. Speed and economy make them a good solution to which can add decorative finishes by using a thin layer of different aggregates and colours on the soffits and tops of the slabs.
Stephen Carr C Eng MI Mech EManaging DirectorSpiroll Precast Services Ltd.,
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Economical: Hollowcore slabs themselves are up to 30% lighter than the equivalent in-situ floor. With the prestress and the low self weight, longer spans can be achieved for the same loads or greater loads for the same depths. The building foundations can be lighter as they are required to support less weight. Alternatively the number of supporting columns and beams can be significantly reduced. Hollowcore therefore gives the opportunity for longer spans, greater loads and less foundation costs.
Features of High Freq. Vibration
- Fire resistance (2-4 hours fire rating) depending on design
- High density product- No strand slippage- Low cement content- Greater span/depth characteristics- Consistent camber- Greater span load characteristic- Eliminates costly propping during installation- All weather construction- Immediate working surface- Custom made to length and detail- Excellent sound barrier (due to hollows)- Carpet direct top surface- Speedy erection, reducing interim financing- Maintenance free- Economical long line Production- Unlimited design possibilities, compatible with almost
all building materials- Flexibility in design and application
In summary there are a variety of uses for hollowcore with applications for floors, roofs and wall panels being the most common. Also some of the more innovative producers
have found use for hollowcore in such projects as parking decks, bridge deck (permanent forms), basement walls, retaining walls, pedestrian bridges and parapet walls (air displacement).
Tests and Approvals
Hollowcore slabs have world-wide recognition and acceptance as a building element. Many tests have been done for different purposes and in different countries. The design of hollowcore is covered in the British Standards, the EC by Euro Codes and in the USA by the Precast Concrete Institute (PCI).
Many tests on hollowcore have been carried out initiated by some of the early tests, for example
- Report on Structural Test on Spiroll Extruded Hollow Core Slabs, Report K68-05 Stockholm, Sweden, August 1968.
- Report on Test to Demonstrate the Adequacy of Floor or Roof Assemblies using Spiroll Panels (By: S.B. Barnes and Associates).
- In addition to the published design codes mentioned above some more recent publications included.
The Extrusion Process
The most common casting system for casting hollowcore is the extruder. It was the World’s first machine for producing hollowcore slabs that did not require a separate driving force to move the machine along the production bed. The same effort that feeds the concrete mix through the machine and forms it into the final precast slab also provides the motivation to drive the extruder along the bed. This natural process propels the extruder along the production bed and allows the compacted concrete to reach the required density.
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With high frequency (HF) vibration in the Spiroll machines, the intense vibration and pressure within the machines, means the concrete mix is effectively ‘plasticised’ during the short time that it is passed through the extruder. This results in dense concrete with little air retention and moulds the concrete to form the required section.
The formed slab then reverts to its ‘dry’ state and reaches a density high enough to stand on the slab immediately after the extrusion process. After a period of natural or accelerated curing, the slabs are then cut to length, stripped from the casting bed and transported to the storage area. Concrete strength of a minimum of 35N/mm2 is required to hold the bond between the concrete and the strands.
The casting beds are prepared by cleaning and the application of a release agent. The high tensile steel strands are pulled down the length of the bed and stressed. The extruder hopper is filled with concrete and the machine moves along the bed, pushed by the pressure generated by the compacted concrete. The casting takes one and half to two hours depending on the length of the bed. The daily routine is established depending on the number of beds to be cast and the shift operated.
Extruders are by far the simplest hollowcore machine on the market in terms of their design and ease of use. Once the machine has been commissioned and set-up to suit the local material, it simply requires the required mix to be put in the hopper and the machine to be started. Some adjustment of the mix may be necessary to achieve the desired quality and curing times and once set, one man is required to operate the machine. Maintenance is extremely easy. Wear components are designed for extended life. The simplicity, reliability, low maintenance, low labour costs and high strength of the finished product make the machine extremely popular and have stood the test of time with many reputable customers.
Extruders are capable of producing hollow core slabs from depths of 150mm-470mm with widths from 600mm-1800mm.
Factory Design
Scope of Plant Layout
The layout of a new plant should be considered with a view to the future requirement for increased numbers of beds. This leads to reviewing the product handling and the distribution of the concrete. Consideration is required of the maintenance facility, the drainage, access, wiring of the beds, stressing of the beds and storage of the finished product.
For a low cost start up, the plant would be designed with a
production facility with two (2) 120 meters long Production Beds and 1.2 metres wide. This will provide an approximate output of 65,000m2 of slabs per annum based on an average of two hundred fifty (250) working days per year.
Provision would be made for future expansion by the addition of two (2) to four (4) identical beds in the future. The basic system would include one (1) extruder; one (1) saw, stressing equipment and lifting equipment.
By locating the mixer in the middle of the factory the distribution of the concrete and the lifting and handling of the finished product can be completed with two overhead cranes. This minimizes the travel time for the concrete and allows the second crane to continue with other activities. If concrete is to be distributed to more than one bay then a batching plant at the end of the factory is usually necessary. Concrete distribution can then be aided by using an overhead travelling bucket and transfer crane.
With both systems, overhead cranes are used to strip the product. Also the opportunity exists to extend the crane longitudinal travel beyond the production buildings. This enables it to be used for transfer of product to the yard and some for yard functions in the future.
The batching plant should have the provision for handling of two (2) or three (3) aggregates and silo storage of cement. Batch size should match the machine usage of concrete to ensure continuous operation during casting.
Transport of the concrete delivery skip/buckets to the extruder is accomplished by forklift truck(s), overhead cranes or other suitable methods.
Stressing Abutments and Production Bed foundations are to be designed as per details provided by your consultant and Soil Investigation Report provided by the customers.
Civil Work
- Foundations for Batch Plant, electrical and mechanical
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distribution centre and cement silos.- Roofed, insulated structure to cover production area
( Although this depends on local climate).- Fully enclosed areas for parts storage and
maintenance.
Production Beds
The bed length is dictated by a number of factors. These factors are plant capacity, available space, concrete distribution time, batch size, bed production time, flexibility of product depth and strand patterns, bed utilisation and bed end wastage.
Shorter Production Beds give quicker production cycle time. They are more flexible for scheduling of multiple machine sizes, but are not so productive. We would normally recommend a bed length of 120 metres if space is available. In practise bed lengths vary between 60m to 150m.
Bed construction techniques vary, but generally heavier construction gives more dimensional stability and longer life.
To reduce heat loss insulation should be installed under the beds. Hot water pipe for heating is installed above the insulation. The bed can be filled with concrete, before turning onto insulation to improve bed stability and reduce transmission of vibration.
Abutments
The capacity of the abutment design should take account of future requirements for deeper hollowcore slabs as a small additional cost at the installation stage will save significant costs later (The stressing load for a 500mm deep unit could be as high as 400 tonnes). When preparing the ground works for the abutments provision should be made for future beds to minimise costs.
Fixed Steel Posts
The simplest and cheapest method is to have fixed steel posts at both ends of the production bed. With this method the strands are tensioned individually using a hydraulic pump unit and stressing jack.
These posts can be in line with the stressing load or a deflected strand system can be used with the post below the bed level. This later system allows strand patterns to be readily changed, facilitates drainage and is a safer system.
If ‘shock-detensioning’is employed there is a potential for cracking and damaging to the slabs. Extra care needs to be taken when cutting the strand. Preferable is the slow release of tension using a hydraulic detensioning system.
Hydraulic Detensioning System
To avoid the problems associated with shock detensioning, Spiroll has developed a simple and cost effective hydraulic detensioning system.
The design of the abutment is based on two posts, which are cast on site into the concrete at an angle; this allows the highest point of the assembly to be below the level of the beds. The Stress is transferred to the posts by a yoke, which fits over the posts and is locked off, to allow the hydraulic detensioning assembly to be fitted and removed.
Multi-Stressing System
The most effective system for stressing and release of tension is hydraulic multi-stressing. This method allows all the strands to be both stressed and detensioning at the same time. Multi-stressing significantly reduces the time it takes to stress and eliminates the possibility of bond slip or damage to the slabs caused by shock detensioning. However this is the more expensive option and not recommended for start ups.
Drainage
Control of the water used during sawing, and maintaining a uniform level of water on the bed ahead of the casting machine can be achieved more easily if the beds or the channels between beds are installed with a fall of approximately 3 to 4mm per metre of bed run, over the length of the beds.
Production Processes
Batching/Mixing
Concrete usage is approximately 1m3 every 6minutes per machine. For concrete distribution to match a batch size of 1m3 is preferable. To run two machines together the
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minimum batching capacity would therefore be 20 to 30m3/h (note: if the pan size is reduced then the capacity needs to be increased).
Needs only 20-70 litres of water but must be accurate to 1litre. Admix needs to be able to mix with water before going into mix.
Concrete Mix Design
The Spiroll system uses an extremely dry concrete mix, typically a water/cement ratio of approximately of 0.30 The mix design will depend on the availability of local cement and aggregates and can be easily fined tuned to suit local conditions. To reduce curing times and to allow ‘double casting’ within a 24-hour period the cement proportions can be increased. A survey of customers suggests that the proportion of course to fine aggregates does vary to suit local conditions. Admix is normally not required but can be added to improve flow and workability with angular aggregates or assist to reduce curing times.
Material Recommendations
Course Aggregates: 10mm/14mm Aggregates (Max. Size 16mm for mechanical clearance). Irregular shape is recommended. Extremes of very rounded or extremely angular respective are prone to sagging and lower speeds or are difficult to compact.
Sand: Clean Zone 2 or equivalent.
Cement: Cement can be normal Portland cement or high early strength cement as they contribute to workability and benefit to rapid curing.
Water: This could range from 23 to 70 litres per cubic meter of mix depending on the moisture content and/or degree of absorption of the aggregates.
Admix: Admixtures may be useful for workability or set control, but are not normally required.
Concrete Distribution
While the Extruder is the heart of the Hollowcore Plant, additional equipment is essential to perform other tasks. Most important is the transportation of the concrete mix from the batching plant to the Extruder.
Delivery of the concrete must match the requirements of the Extruder so that it does not run out of mix and slow down production. Several methods of concrete delivery can be used such as overhead cranes, fork lifts or automated Concrete Distribution Systems (CDS).
Method Benefits Disadvantages
Forklift Low initial cost Readily available
Floor space required
Portal Crane Low cost, reliable, flexible, no building
required
Reduced floor spaceDanger of Legs
Overhead Gantry crane Low cost, reliable, flexible, clear of floor
space, faster than PortalCrane
Part of building Cost ofstructural supports
CDS System Automation, more than one bay, speed, low
labour content
High cost of investment,maintenance. Poor
reliability
Method Benefits Disadvantages
900 Cross Cut Low initial cost Faster cutting times
Need a secondary cutting station for long
and angle cuts
Long Rip Cuts Cut slabs longitudinally when still wet, which is
faster
Poorer Finish
Multi Angle Cuts any angle and longcuts on the bed
Heavy and more expenSive Saw
Secondary Cutting Station
Frees up production bedfaster. Cheap method of
cutting angles
Two stage cutting
To maintain continuity of supply of concrete to a Spiroll Extruder producing (as an example) a 200mm deep slab would require 1m3 every 6/7 minutes.
The Extruder can be stopped between loads but it is preferable to maintain the continuity of the casting once the line has been started. The permissible standing time before the machine has to be lifted clear of the curing concrete would be established by trials but would normally be between 5 and 10 minutes.
When delivering concrete the transfer between skips should be kept to a minimum to avoid segregation. The skip should be bottom opening with a wide mouth (1m2) to avoid trapping and segregation of the stone from the fines.
Using an average extrusion speed of 1.2m/min and a Bed length of 120 metres, the casting time per bed would be around 100 minutes. Transfer for lifting of the Extruder, setting-up, cleaning time etc. would add approximately 15 minutes.
Consideration should be given to the systems available to distribute the concrete as follows.
Curing
The curing process is the longest part of the production cycle. As such it is critical to the overall production cycle time.
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This means that all efforts to reduce this process will most affect the whole length of the production cycle. By having a concrete mix with a low water content the curing time is greatly reduced. The application of heat into the cast slabs through pipes under the bed initiates and accelerates the curing of the concrete. The Production Beds can be heated by either hot oil steam or hot water. Of these hot water is the cheapest to install and maintain and is by far the most popular as it is reliable, cost effective and manageable. Inlet temperatures of 60 – 80°c should be maintained with enough flow to maintain outlet temperatures at around 25 – 35°c. To ensure good early strengths, the beds should be hot when casting and the heat applied during the casting to maintain a concrete temperature of 60 degree centigrade. To trap the moisture and for efficient use of heat the product should be covered at the earliest opportunity after casting. Plastic sheet can be used but for efficiency particularly in colder climates the concrete should be covered with a good quality insulated sheet.
Cutting the Slabs
The estimated time for a cut is 1½ to 2 minutes. With moving and positioning this gives a cycle time of 4 to 5 minutes per cut. The blades are diamond tipped and require water during the cutting process. Water can supplied to the saw using a hose Cable Reeler or directly with a trailing hose. The later is not an efficient system.
Lifting (Stripping) Clamps
Special Lifting Clamps are utilised to lift the product off the beds either by crane or forklift. The product can be transferred from the bed to the Stock Yard by crane, boggie trailers, forklift, purpose made lifters or directly onto trailer.
Care is required to match the logistics of handling the finished product with the production cycle to ensure the beds are stripped at the optimum rate.
Transporting Slabs
Options for transfer of product are:-
- Forklift Truck Front Loader
- Forklift Truck Side Loader
- Stacker Lifters
- Overhead Crane
- Direct onto road trailers
- Low trailer system
- Bogie Trolleys
The production rate will call for movement of: approximately 80 square metres per hour (or approximately 8 to 10 pieces per hour assuming average lengths 6 to 8 metres).
Preparing the Beds
Once the hollowcore slabs have been cut to length and lifted away from the production beds, the beds then need to be cleaned and oiled. The prestressing strands are then pulled the full length of the bed from the strand dispensers, threaded through the abutments and the anchors fitted prior to stressing.
The stages of preparation are: -
- Clean the Casting Bed- Clean the Bed Rails- Push Debris off the Bed- Spray the Bed Oil/Release Agent- Pull the Prestressing Wires/Strands- Stress the Wires/Strand
These activities can be done by hand. Equipment is also available to speed up the processes and reduce the labour costs.
Quality Control Equipment
Efficient Hollowcore Production requires good quality control systems to ensure the consistent quality of the aggregates, the concrete, curing conditions, good bond and dimensional accuracy of the finished product.
To achieve this, the normal aggregate testing and cube testing equipment is required. Consistent concrete is achieved with batching calibration procedures. Preparation of the cubes with heavy vibration to match the extruder is necessary and extra cubes should be made to check the “transfer strength of the concrete is required in addition to the 28 day strengths.” Stressing and detensioning procedures require to be established with correct calibration.
Conclusion
The manufacture of hollowcore is not difficult. Low cost start up units can be designed with the potential to increase the capacity to match future demand.
The degree of automation depends upon the capacity required and the local cost of labour to ensure good pay back periods.
Start up factories can be run with a low level of automation; this will reduce the capital expenditure and increase the reliability of the plant.
A high standard of product can be guaranteed by using the correct procedures and equipment.
High Frequency Vibration Extruders as made by SPIROLL produce the strongest and most consistent product.
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An Overview of Construction of Buildings in Precast Concrete
India is developing at a very rapid rate with a result that demand is far more than the current pace of construction. With growing standards of living and the presence of multinationals in India, the expectations on quality have also increased. This has led to adaption of innovative methodologies in construction which reduce the requirement of manpower and material and increase efficiencies, quality and pace of construction. Precast concrete construction fulfills most of the requirements. And therefore, though not new to India, precast concrete construction is only now gaining acceptance in an unprecedented manner. The principles for structural design for precast concrete mostly remain the same as that for conventional in situ construction. However there are a few nuances arising due to the fact that concrete is cast away from the location where the element is supposed to be for its service life. Beautiful shapes and architectural finishes can be achieved which otherwise are very difficult or even impossible to achieve in a conventional in situ construction.
Like the game of Lego, Precast concrete buildings are structures made up of numerous small individual elements of concrete cast at an off-site location. These
precast elements such as beams, columns, slabs and walls are transported to the site for assemblage and erection. Wind and earthquake loads are resisted by moment carrying connections of beams to columns for moment frame resistance, and coupling of wall elements together for composite shear wall resistance. Thus, generally in precast concrete buildings, the individual element on its own plays no role in gravity and lateral resistance. It is the assembly of all these elements by proper connections which gives the building its stability against vertical and lateral resistance.
Precast concrete usually is either ordinary reinforced concrete or prestressed reinforced concrete. Prestressing gives advantages of reduced cross-sections and steel requirements (reduced weights). However, prestressing needs additional equipments, abutments etc.
Precast Concrete is either a factory-cast (off-site) precast or site-cast (on-site) depending on the volume of work and logistics. Factory cast is typically more popular. Factory cast precast gives more control to the producer and the designer with better options for prestressing, architectural finishes and grade of concrete. A better quality can be
obtained as workers and supervisors are well trained and experienced. Work does not hamper due to bad weather.
Site-cast precast is adopted when the project volume is so large that setting up a plant at site is economical. It is also adopted when the transport of precast products becomes very expensive or difficult due to large distances and adverse road conditions. Setting up of long line prestressed beds is difficult on site and may not be economical, hence most of the site-cast precast is non-prestressed.
Wet concrete is poured in forms (moulds) and stripped out when it attains certain minimum strength. It is stored in a storage area and later transported to the site for erection. Forms are basically either stationary steel plate forms or Tilting tables or battery moulds or moving carrousel systems with production pallets. Selection of a system depends upon the volume of production of a particular element and flexibility desired in production. There are various patented systems for forms systems available in India.
When compared to conventional construction, precast formwork can be vibrated in place using vibrating tables thereby giving excellent quality. In case of precast formwork it is very important to maintain shapes and dimensional accuracies (tolerances) or else the product may not fit at
Mangesh Kumar HardasDirector, Precision Precast Solutions Pvt. Ltd.
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its place at the time of erection. Steam curing and heating of the bed is possible in precast formwork which increases the rate of strength gain of concrete.
Precast formwork presents unlimited possibilities of architectural finishes such as brick, stone, ribbed finish sand blasted or acid itched exposed aggregates.
Cost of precast concrete structures comes down with repetitions. Sound Architectural and Structural Planning is a must. With precast just starting up in India, a regular precast concrete building may seem to be expensive at the estimation stage when compared to conventional one. However when completed, the cost of structure in Precast would come out to be lesser as the construction period is significantly less, thus substantially reducing the effect of spiraling costs of building materials. Time saved would bring in additional benefits.
Procedure of Precast Concrete Construction
Construction procedure of precast concrete buildings starts with architectural planning. Architect should be fully aware that the building would be made using precast components. Converting an existing plan designed for conventional reinforced concrete into precast would result in expensive and difficult propositions. There are some inherent advantages of precast concrete and some drawbacks which must be considered at the planning stage only. The architect should be aware of precast options available to him. Early involvement of precast specialty structural engineer and precast manufacturers is always advantageous. 3D models made using software result in accurate and speedy construction.
Selection of a particular element in the building depends on the availability of the technology with the local precast manufacturer or in case setting up of the plant by the contractor – the economics of the formwork and cost of setting up of plant. For example whether to use Hollowcore or half slab or solid slab for slabs will depend on the spans intended, loads, volume of work, speed of work desired and availability of any nearby manufacturer. A 2.4m wide hollowcore slab spanning say 10m or more will be very fast to erect, thus reducing the erection time and maybe costs. However if there are a lot of non-structural partition walls, the cost of these should be also considered. There are many light weight low cost options available now.
Once the production is done, the elements are usually stacked in a yard preferably close to the construction site. In project planning phase, the availability and cost hiring of such land for limited period should be considered. The maximum weight and size is governed by local transportation limitations. It makes sense to consider
transporting of maximum number of elements in a single trip. The sizes of elements considered while planning should not overlook this aspect.
Erection is done with the help of cranes. These cranes are huge – with capacities depending upon the weight of precast element to be erected at the farthest distance from the crane. There are various cranes namely crawler or mobile cranes, tower cranes and tower cranes on the rails. Erection sequence is a specialized job and one has to take care of stresses generated during erection. Temporary shoring may be needed to handle such stresses.
Type of Elements and Production Methods
Precast buildings are made up of structural and non-structural elements, which may be prestressed or non prestressed based on the use.
Prestressed systems are usually long line systems wherein large numbers of elements are produced in a single bed. Typical elements produced in such a system are Hollowcore planks, Double Tee floor elements, Spandrels and Inverted Tee girders. A prestressed bed needs stressing abutments at the ends and a long form in-between. Generally, the forms for prestressing elements are either self stressing forms which take the hydrostatic forces of concrete and compressive forces from prestressing, or non-self-stressing or free forms which take only hydrostatic forces leaving the compressive forces coming from prestressing to the abutments. End abutments for stressing is a good solution but sometimes one needs setup for small quantity of elements where self stressing beds can be used. Sometimes post tensioning is also done within the factory for small number of elements.
The forms must be designed properly so that they do not deform during any of the operations of production - pouring concrete, vibrating, stressing, distressing and stripping the element out of form. The end product must comply with the specified tolerances as specified in the BIS codes.
Maximum reuse of formwork is the key to economy. The Architect must keep the number of different shapes to a minimum and design shapes which can be stripped easily, preferably cast in single pour. Even so, it should achieve the desired edges, surfaces and textures.
Typically forms should be made for standard cross sections of columns, beams etc. The Architect should try to use these standard sizes as much as possible so that new forms are not required to be made.
The form side(s) of the precast are usually on exterior of the building. When a panel is cast horizontal, the bottom side may be exposed aggregate, rubber form lined (to give
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desired texture) or just plain surface. The upper surface of the concrete in the mould which is not as smooth is on the interior of the building.
The interior edges of the form should be radiused or chamfered at least 10mm to avoid edge damage during stripping. This can be done using chamfer strips made up of wood or steel.
In long line - prestressed method of casting during detensioning of strands concrete shortens, and so the inside forms need to be removed before detensioning. The design should be such that these inside forms can be removed without disturbing the strands.
The form surface against which concrete is cast should be smooth. These are cleaned by wirebrush, scrapping, scrubbing and even chipping. The form sheet should be thick and strong enough to maintain its smooth surface. The plywood used is raisin coated.
If steel bed is chosen, which normally is the case, magnetic systems can be used to fix side forms. Side forms are needed for not only defining the boundaries of the panel but also for door and window openings.
Heavier construction of formwork as a rule, gives more dimensional stability and helps reduce transmission of vibration and results in longer life. Fabrication tolerances are typically half the product tolerances. The steel forms have thickness of plates of about 5mm to 8mm and have gussets at every 200mm to 500mm depending upon the forces. The smoother the steel surface, the better is the finish.
Sometimes accelerated curing is achieved by heating. To do so, elobrate piping is done under the form bed and hot water or steam is passed through it. To reduce heat loss, insulation is installed under the beds. The pipe for heating is placed above the insulation.
Vertical Elements
In a long line method, there is a long form of about 50m to 100m with side fixed rail on one side which makes the common side for all the panels. The second rail is usually movable and is kept such that it is on the largest width in the pack. Others in-between are wooden. Sometimes the bed is capable of vibrating and heating.
Tilting tables are used to cast wall panels. These tables are equipped with heating and vibrating bed as well. Tilting tables are hydraulically operated and are horizontal at the time of casting. At the time of striping, tilting tables tilt to almost vertical – thus need lifting inserts only on the edges. They also reduce the steel required or can be stripped quickly.
Figure 1 - A long moving Prestressed bed showing blockouts
Battery moulds are designed for the vertical fabrication wall panels. Each layer can have a variable area and reinforcement. They consist of bulkheads between which 5 to 10 panels can be simultaneously formed. Vibrators facilitate the effective compacting of concrete. Battery moulds offer to produce architectural wall panels with both inside and outside surfaces as smooth.
Another system is based on production pallets (a steel table) which pass through various workstations manually over a set off protruding wheels before concrete products are complete. Various transport systems (such as central
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shifter, side shifters, and rollers) transport the pallets from workstation to workstation. Each workstation has a role – preparing, concreting, curing and stripping. This system offers the flexibility of horizontal casting and economizes
on tilting table.
Strong magnetic systems are available which help in fixing the side forms. The force is more than 500kgs for a magnetic clamp of 5kgs.
A fully automated system called carrousel system is also available. It is computer controlled and gives a very high rate of production. Lattice Girder Slabs can be made with such a system.
Double wall formwork is essentially the same but it additionally needs a system to rotate one half of the already cast and set slab all around and keep it on the one which is recently concreted.
Columns and Beams
Usually precasters keep standard width and height forms. Column forms are usually non prestressed and can be made up of steel or wood. The sides of these forms can be detached. Long line prestressed forms have arrangements for prestressing steel. They need permanent abutments and hence are fixed to a particular place.
2.4m. These are typically 150mm, 200mm, 250mm or thicker. These have hollows in them saving concrete and reducing the weight. The spans of hollowcore slabs vary from few meters to usually a maximum of 15m though it could be more based on use and thickness. Manufacture of hollowcore is a propriety system and a hollowcore machine manufacturer normally provides the equipment and beds. No side forms are required as hollowcore production needs a very dry mix concrete and remains there on its own.
For larger spans such as 20m, Double Tees are used. These are prestressed elements with very high strength to weight ratio. Parking garages, podiums and IT buildings can be made using these. Triple Tees and Single Tees have also being used.
Other types of slabs used in precast construction are Solid Slabs, Half Slabs, Lattice Girder Slabs, etc. These can be prestressed or non prestressed with or without voids. Prestressed slabs can span longer and be more economical with reinforcement however one time setup cost is more.
Figure 7 – Lattice Girder Slab
Conclusions
Precast project needs a lot of thinking to go into the process in the planning phase of the project. All related activities such as casting, curing, stacking, transport and erection are required to be planned and finalized on paper at the start as per project timelines. There are various systems available for manufacture of precast elements. There are various ways to configure a building. The most economical method depends on the building type. Regular buildings can be most economical with precast concrete.
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Figure 3 - Battery Mould
Slabs
Worldwide, approximately half of the floors used in commercial and domestic buildings are of precast concrete. It offers both design and cost advantages over conventional methods.
Hollowcore slabs are available in the widths of 1.2m or
Figure 6 – Double Tee as a roof under construction and its typical section
Formwork for IT beams Steel Formwork for Rectangular Column
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Effect of Yarn Size on the Performance of Hybrid Braided Composite Rebar
Replacement of the steel reinforcement in concrete structures with more corrosion resistant substitutes such as composites is rapidly becoming a more
economical option for construction facilities worldwide- Mufti et al., 1991, Iyer and Sen, 1991, Nanni and Dolan, 1993, Basham 1994, Saadatmanesh and Ehsani, 1996, 1998. Composites can be used in new or repaired reinforced concrete structures. In general, composites have high strength, a range of moduli and low ultimate tensile strains compared to steel. The stress-strain behavior of all of these fiber systems is linear up to failure, which makes it impossible to have significant hysteretic behavior. In spite of their superior light weight, corrosion resistance and non-magnetic properties, the lack of material ductility and energy absorbing capabilities is a severe limitation of all these fiber systems if they are to be considered for earthquake resistant applications.
Design Concept
In order to achieve ductility in reinforced concrete structures without using conventional steel rebar, a new design methodology was introduced to identify suitable composite materials that mimic the stress-strain characteristics of steel [Somboonsong et al., 1998]. The technology of braiding, as detailed by Ko and Pastore [1989], is a well established technology which intertwines three or more strands of yarns to form a tubular structure with various combinations of linear or twisted core materials. By judicious selection of fiber materials and fiber architecture for the braid sleeve and the core structure, the load-deformation behavior of the braided fibrous assembly can be tailored. The rebar sleeve is fabricated from tough aramid tows (Kevlar 49) and the core structure composed of high modulus carbon (Thornel P75S-2K) to provide the initial resistance to deformation. The rib effect is built into the sleeve structure during the
Dr. Christopher M. Pastore1, Eileen Armstrong-Carroll2, Frank K. Ko3
1,2School of Textiles and Materials Technology Philadelphia University 3Department of Materials Engineering, Drexel University
Composite Technology Rebar
braiding process.
A 24 carrier braiding machine was employed to form the structure. The core consisted of 12 plies of 2,900 denier P75S-2K. The braiding yarns were 3 ply 1,240 denier Kevlar 49, except two of the bobbins was loaded with a 15 ply 1,140 denier Kevlar 49. These large bundles were used to create two spiraling ribs in the braid for mechanical bonding between the composite rebar and the concrete. The design of this rib is similar in concept to the current steel rebars.
A process, called “Braidtrusion” was used to form the rebar (Figure 1). This process takes the braided fabric through a forming ring, and runs the braid through an infusion zone wherein Epon 828 epoxy resin is dripped onto the fell of the cloth. The wet fabric is then run through a heated chamber to cure the resin. The fabric has a 30 minute dwell time before being collected. A resin system from Shell Chemical, consisting of EPONTM Resin 9310/EPI- CURETM Curing Agent 9360/EPI-CURETM Curing Agent Accelerator 537 is used for consolidation.
A typical rebar made with this process is shown in Figure 2. The rib yarns running in opposite helices can be clearly seen. The core carbon yarns are completely encased by the aramid sheath yarns.
Figure 1. Schematic Illustration of Braidtrusion process
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The effect of the large braiding yarns on creating a rib structure is shown in Figure 3. These very large yarns create substantial distortions in the braided structure which provide the mechanical connection to the concrete system, but provide additional difficulties in modelling.
Figure 2. Micrograph of typical hybrid rebar showing rib yarns on surface
Figure 3. Effect of rib yarns on braided fabric geometry
Considering the effect of core yarns on the rebar diameter a geometric model was constructed accounting for the core yarn size, number of aramid braid carriers, and rib effect. The cross-sectional area of the area changed with movement of the two rib yarns. The local increase in diameter was most marked when the two rib yarns were along the same diameter line. At these instances (when the ribs are on top of each other and when they are 180° from each other) the diameter was 30% greater than a diameter measurement across a line with out rib yarns. The two rib yarns crossed each other in 2mm intervals.
The graphite core had an ellipticity of 1.3 (b/a) and accounted for 19% of the cross sectional area. The nominal braided tube cross-section was calculated by subtracting the core area from the tube area as calculated using the minimal rebar diameter. The braided tube area calculation was then refined to account for the ribs. The average width, height, and length of the ellipsoid protrusions formed when two rib yarns were on top of each other were measured. The average area contribution from the rib yarns was determined by dividing the ellipsoid by its length. The area of overlap with the nominal tube and average rib yarn areas was calculated and subtracted out. Thus
At = At* + Ar - Ao (1)
where At = area of the tube, At* = nominal area of the tube, Ar=area of the rib and Ao = area of overlap between rib and braid.
Fabric Geometry Model
The fabric geometry model (FGM) REF was used predict the elastic properties of the rebar. In this model the fibers are considered rods that are classified according to their orientation. Since the rebar was tested in tension the orientation of the fibers relative to the x-axis was determined. The core was straightforward, the fibers were considered aligned with this axis. The braid tube contained 4 distinct fiber orientations as shown in Figure 4. The angles and represent the braid and crimp angles.
Due to symmetry with the x-axis and the tensile loading condition being characterized, each braid yarn orientation could be considered equivalent. The transformation tensor for rotating the braider yarns so they were aligned with the x-axis was developed by rotating the yarn an angle (made by projecting the rod into the yz plane) so the rod lies in the xy plane, and rotating by the braid angle so the rod is aligned with the x axis.
Using the geometrically defined unit cell, fiber mechanical properties and matrix mechanical properties, a 6 x 6 stiffness matrix can be formed for each system of yarns using the stiffness matrix of a comparable unidirectional composite and transforming it appropriately for the system’s fiber orientation. This stiffness matrix will form a link between applied strains and the corresponding stress responses. For each system of yarns, this stiffness matrix is expressed as:
(2)
where [Ci] = stiffness for the ith system of yarns
[T ,i] = Hamiltonian strain transformation for the ith system of yarns and [C] = stiffness matrix for a comparable unidirectional composite.
The transformation matrix [T ,i] is a Hamiltonian tensor transformation matrix which can be defined in the following form:
where l = cos , m = 0, n=-sin ,
l’=sin cos , m’ = sin , n’ = cos cos ,
l’’ = sin sin , m’’ = - cos , n’’ = cos sin
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and , are the two angles which define the orientation of any fiber, i.e.
= the orientation of the fiber with respect to the longitudinal axis of the structure and = the azimuthal angle of the fiber.
Figure 4 Geometric Relationships Used in the Mechanical Analysis of Braided Composite Materials.
Because a braided structure can have longitudinal and braiding components, and the 3-D braided structure can have longitudinal, transverse and braiding components, the orientation of all possible components ( , )are:
(0,0) for the longitudinal components
(90,0) for the transverse components
( * , *)for the braiding components.
Thus, the Hamiltonian transformation is based singly upon the orientation of the yarn. As seen in the previous equation, the other term required for the calculation of the braided composite stiffness matrix is [C], which can be given as the symmetric matrix
where c11 = (1- 232)E11 / K*c22 = c33 = (1- 12 21) E22 / K*c12 = c13 = (1+ 23) 21 E11 / K* c23 = ( 23 + 12 21)E22 / K*c44 = G23
c55 = G13
c66 = G12
and K* = 1 - 2 12 21 (1 + 23) - 232
The terms which fill the [C] matrix are derived from fiber mechanical properties, matrix mechanical properties and fiber volume fraction. It should be noted that the fiber volume fraction is the particular fiber volume fraction associated with the system of yarns under investigation. The particular derivation of the elements of the stiffness matrix are achieved through the well established micromechanical analysis using the following representations.
(5)
where m = 3 - 4 m and fL = 3 - 4 f
LT
(6)
(7)
where fT = 3 - 4 f
TT
(9)
where m = 3 –4 m.
(10)
A stiffness matrix is determined for each system of yarns, and the stiffness matrices for each of these systems are superimposed proportionately according to contributing volume to determine the fabric reinforced composite system stiffness:
(11)
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where [Cs] = total stiffness matrix and ki = fractional volume of the ith system of yarns.
In order to account for the potentially nonlinear behavior of the materials, the system stiffness matrix should be calculated anew at each strain level. Thus, the incremental stress-strain behavior of the composite can be determined as
(12)
where {∆ } = incremental stress vector (6x1) and {∆ } = incremental strain vector (6x1) From this, the stress vector can be determined as
(13)
where { } = stress vector (6x1)
Tensile Response
Tensile stress-strain characteristics have been obtained for the rebar and compared to the FGM. The stress-strain behavior of the 5 mm D-H-FRP bar is compared with theoretical predictions in Figure 5. Note that the 5 mm hybrid bar achieved high initial modulus as well as a ductile failure mode characterized by the trapezoidal area under the stress-strain curve. The model correctly predicted the initial failure point, demonstrating that the rib yarns could a modelled as interchangeable with the other braider yarns. The definite yield strength is achieved by the hybridization process and is a manifestation of the fracture of the fibers with the lowest failure strain, the graphite core fibers. The model contains a damage parameter to describe the initial drop in stress.
In this type of damage modelling, the carbon core yarns begin to fail at a determined strain level, but the core yarn contribution decreases linearly over a range of strains to account for gradual failure of individual filaments in the core due to inherent fiber waviness. This approach enables the load bearing capability of the core to degrade over a strain range in which the unidirectional core is evolving into a chopped-fiber filled core. Typically in core-filled braids that fail initially in the core there is an elastic recovery period in which the braided sleeve continues to take load.
A second damage mechanism is proposed and modelled wherein the rib yarns fail somewhat earlier than the other braiding yarns due to the increased bending strain on these yarns. Again they initiate failure and gradually lose full contribution to the system. The whole system fails when the remaining braiding yarns fail.
With the actual rebar, this tendency is interrupted and damage progresses in a series of increments. The reasons
for this post-damage mechanical behavior is not yet known. Perhaps the spiraling orientation of the ribs yarns well outside the overall geometry of the part induces a bending moment onto the rebar sufficient to cause flexural failure at the point of double rib overlap, and thus creates this unique damage progression.
The theoretical model of stress-strain behavior illustrated in Figure 5 is based on the process modelling of the braid structure and use of a stiffness averaging predictive model. Failure of the core yarn is predicted by strain to failure criteria. The braided yarns reorient themselves during loading, resulting in the slight non-linearity of the curve. The next step of modelling is to include the resin cracking at the interlacing points and the residual contribution of the fractured carbon yarns in the core. Additionally the possibility of flexural failure due to bending forces from the ribs need to be added.
As can be seen in the experimental data, there is a characteristic step phenomenon which recurs throughout loading. This is not predicted by the damage model presented. Although the damage model does fall within the range of stresses throughout much of the experimental regime, it does not fully capture the behavior of the rebar.
Further investigation is required to identify the actual cause of these stress drops and rises. The next step of research will be to examine specimens at different damage levels and identify causes.
Conclusions
Predictive models of the hybrid braided composite show promise. Processing conditions and initial failure can be predicted well. Post-yield ductile response requires additional modelling. The effects of yarn size and process parameters on the tensile response have been
Figure 5. Comparison of Experimental and Theoretical stress-strain response for 5 mm hybrid braided rebars
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demonstrated. Remaining to be done is post-yield behavior and cyclic response.
Results have been presented of the cyclic flexural behavior of a new ductile hybrid braidtrusion reinforcing bar for earthquake resistant concrete structures. Load-deflection and moment-curvature relations from small beams show that the D-H-FRP rebar can achieve a ductile behavior with ductility indexes similar to those of mild steel reinforcement.
References
- K. D. Basham, Proc., Third Materials Engineering Conf., American Society of Civil Engineering, New York, NY, 1994.
- M. M. El-Badry, Advanced composite materials in bridges and structures, The Canadian Society of Civil Engineering, Montreal, Canada, 1996.
- H. G,. Harris, W. Somboonsong and F. K. Ko, Proceedings of the 1997 International Conference on Engineering Materials, Ottawa, Canada, Vol. I, 1997, pp 593-604.
- H. G. Harris and G. M. Sabnis, Structural Modeling and Experimental Techniques, Second Edition, CRC Press, Boca Raton, FL. (1999).
- R. Huesgen, Flexural behavior of ductile hybrid FRP rebars in singly reinforced concrete beams, MSc Thesis, Drexel University, Philadelphia, PA. (1997)
- S. L. Iyer and R. Sen, “Advanced composite materials in civil engineering structures”, Proc. Specialty Conf., American Society of Civil Engineering, New York, NY (1991).
- F. K. Ko, W. Somboonsong and H. G. Harris, ed. M. L. Scott, Proceedings of the International Conference on Composite Materials, Vol. VI Composite Structures, pp VI-723-VI-730. (1997),
- Ko, F.K., ed. Chou T. W. and Ko, F.K., Textile Structural Composites: Series 3, Elsevier, New York, (1989)
- Mufti, A., Erki, M. A. and Jaeger, L. (1991). Editors, Advanced composite materials with application to bridges, Canadian Society of Civil Engineers, Montreal.
- Naaman, A. E. and Jeong, S. M. (1995).”Structural ductility of concrete beams prestressed with FRP tendons”, Non-metallic (FRP) Reinforcement for Concrete Structures,” Edited by L. Taewere, RILEM, Published by E & F N Spon, 2-6 Boundry Row, London.
- Nanni, A. and Dolan C. W. (1993). Editors, “Fiber-reinforced-plastic reinforcement for concrete structures”, Proceedings of International Symposium, Vancouver, ACI SP-138.
- Saadatmanesh, H. and Ehsani, M. R. (1996). Editors, “Fiber composites in infrastructure”, Proc. First International Conf. on Composites in Infrastructure, Tucson, Arizona, Jan. 15-17, Dept. of Civil Eng. and Eng. Mech., University of Arizona.
- Somboonsong, W. (1997). “Development of ductile hybrid fiber reinforced polymer (D-H-FRP) for concrete structures”, Ph.D.Thesis, Department of Civil and Architectural Engineering, Drexel University, Philadelphia, PA, December 1997.
- Somboonsong, W., Ko, F. K., and Harris, H. G. (1998). “Ductile hybrid fiber reinforced plastic (FRP) rebar for concrete structures: design methodology,” ACI Materials Journal, V. 95, No. 6, Nov.-Dec., pp. 655-666.
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