Advances in Building Technology, Volume 1 M. Anson, J.M. Ko and E.S.S. Lam (Eds.) 2002 Elsevier Science Ltd. All rights reserved

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ADVANCES IN CONCRETE TECHNOLOGY

M.F. Cyr and S.P. Shah

Center for Advanced Cement Based Materials, Department of Civil Engineering, Northwestern University, Evanston, IL

60208 USA

ABSTRACT

A survey of recent advances in concrete technology, with a focus on research performed at the Center for Advanced Cement Based Materials (ACBM Center) at Northwestern University, is presented. Ultra-high-strength concrete (UHSC), with compressive strength of 200MPa, has been developed. The properties and applications of reactive powder concrete, one type of UHSC, are discussed. Fiber reinforcement is used to overcome the inherent brittleness and increase the tensile strength of concrete, especially high- and ultra-high-strength concrete. Fiber-reinforced cementitious composites can be designed for specific applications with the use of special processing techniques, such as extrusion, and hybrid fiber reinforcement. Significant reductions in drying shrinkage are achieved with a newly developed shrinkage reducing admixture. Construction costs can be reduced with the use of self-compacting concrete (SCC), which does not require vibration at placement. The design of SCC is facilitated with a newly developed rheological model. A nondestructive evaluation technique has been developed to monitor the hardening process of fresh concrete.

KEYWORDS

High-performance concrete, ultra-high-strength concrete, fibers, extrusion, shrinkage, self-compacting concrete, nondestructive evaluation

INTRODUCTION

As concrete technology developed, an initial goal was to increase its strength. High-strength concrete (HSC) columns were first used in the construction of high-rise buildings in the 1970s. The same changes that increased the strength also improved the durability and other aspects of concrete performance. The term high-performance concrete (HPC) began to be used. Today, HPC refers to concrete with many different attributes. It is produced with specifically designed matrices, often containing special chemical and mineral admixtures and fiber reinforcement. HPC performance criteria include high strength and elastic modulus, improved toughness and impact resistance, high

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early-age strength, high durabilityincluding low permeability, resistance to chemical attack and free-thaw damageand ease of placement and compaction without segregation.

A selection of current research in HPC, with an emphasis on work performed at the Center for Advanced Cement Based Materials (ACBM) at Northwestern University, is presented. Ultra-high-strength concrete, with a compressive strength of 200 MPa, has been produced for specialized applications. The ductility of concrete, especially high- and ultra-high-strength concrete, has been enhanced with fiber reinforcement. Hybrid fiber reinforcement and special processing techniques, such as extrusion, have enabled the optimization of composite performance for specific applications. A shrinkage reducing admixture has been developed to improve shrinkage cracking resistance. A model to facilitate the design of self-compacting concrete has been developed. In addition to improving concrete performance, new nondestructive techniques have been employed to monitor the setting of fresh concrete and to assess early deterioration in hardened concrete.

ULTRA-HIGH-STRENGTH CONCRETE

The strength of brittle materials, such as concrete, is related to the porosity of the material. As the porosity decreases, the strength exponentially increases (Mindess and Young 1981). Powers and Brownyard (1948) showed that decreasing the water-to-cement (w/c) ratio reduced the porosity of the concrete, increasing the strength. This reduction in porosity also makes the concrete more durable. In addition to having a lower water-to-binder (w/b) ratio, HSC usually contains superplasticizer, and mineral admixtures, such as silica fume or fly ash. Its compressive strength is around 100 MPa compared to compressive strengths of 20-40 MPa for normal-strength concrete (Kosmatka et al. 2001).

Recently, special processing techniques have been used to produce concrete with even higher compressive strengths. Ultra-high-strength concrete (UHSC) can reach compressive strengths of 200 MPa. Two commonly produced UHSCs are macro-defect-free (MDF) cement and reactive powder concrete (RPC). Macro-defect free cement is a mixture of cement and a water-soluble polymer. High shear mixing causes a mechano-chemical reaction between the cement and the polymer resulting in tensile strengths of up to 200 MPa (Shah and Weiss 1998a).

Reactive powder concrete typically has a compressive strength of 200 MPa, although strengths as high as 810 MPa have been recorded (Semioli 2001). Its high strength and low porosity are obtained by optimizing particle packing and reducing water content. The mixture contains no coarse aggregates. Instead fine powders, such as sand, crushed quartz, and silica fume, with particle sizes ranging from 0.02 to 300 |nm are used. The grain size distribution is optimized to increase the matrix density. Superplasticizer is used to reduce the w/b to 0.2 as compared with w/b of 0.4-0.5 for typical normal-strength concrete. Steel and synthetic fibers are typically added to improve the ductility, and a post-set heat treatment is applied to improve the microstructure.

RPC is produced commercially by two French construction companies (Semioli 2001). Beton Special Industriel (BSI) is produced by the Eiffage Group (EGI) in conjunction with Sika Corp., and Ductal is made by Bouygues Construction in partnership with Lafarge Corp. Ductal is reinforced with high-strength steel microfibers to improve ductility. It has a compressive strength of 200 MPa, a tensile strength of 8 MPa and a flexural strength of 40-50 MPa. Ductal is 100 times more resistant to water diffusion than normal-strength concrete and 10 times more resistant than typical French HPC. It has zero shrinkage after setting and 85-95% less creep than conventional concrete. The relatively low tensile strength requires the use of prestressing in more severe applications. With Ductal, very strong, lightweight and durable thin sections can be produced. Currently, there are plans to use Ductal in a 120-m long, slender arch pedestrian bridge near Seoul, Korea. The bridge will consist of six post-tensioned segments with a 30-mm thick walkway. In the United States, the Federal Highway Administration (FHWA) is currently testing Ductal in prestressed concrete girders (FHWA 2002).

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BSI has been used in French nuclear power plants. The producers of RPC also see potential for its use in pipes, tunnel and canal linings, paving, floors, liquid storage structures, nuclear waste containment, and long-spanning, slender, self-supporting structures such as stadium domes.

The high strength of UHSC is due to the homogeneity and low porosity of the matrix. These same characteristics also cause the material to be extremely brittle and increase the likelihood of shrinkage cracking. Microfiber reinforcement can increase the ductility of the concrete and improve shrinkage cracking resistance. The use of shrinkage reducing admixture also improves shrinkage performance.

FIBER-REINFORCED CONCRETE

Microfiber reinforcement

Microfiber reinforcement reduces the inherent brittleness of concrete, especially UHSC. Fibers spaced at the micron scale can interact with microcracks, delaying localization and increasing the tensile strength of the matrix (Shah 1991), in ways that steel reinforcing bars with spacing at the millimeter scale cannot. In addition, microfiber reinforcement delays the age of the first visible crack and reduces the crack width in restrained shrinkage tests (Grysbowski and Shah 1990).

The performance of fiber-reinforced concrete (FRC) is governed by the ratio of the elastic modulus of the fiber to the elastic modulus of the matrix, the strength of the fiber-matrix bond, the aspect ratio (fiber length/fiber diameter) of the fiber, and the material properties of the fiber (Bentur and Mindess 1991). Different fibers types and geometries yield different composite performance. Only relatively small amounts, usually less than 1% by volume, of fiber reinforcement can be added to conventional concrete because the fibers significantly reduce the workability of the fresh concrete. At this dosage, fibers can improve shrinkage cracking resistance and slightly enhance ductility. To maximize the benefits of fibers, including increasing the tensile strength and ductility of the composite and producing a strain-hardening response, larger doses must be added to the matrix. In cement-based materials, this requires special processing techniques. These composites are referred to as high-performance, fiber-reinforced cementitious composites (HPFRCC). The relative performance of plain, normal-strength concrete, conventional FRC, and HPFRCC is shown in Figure 1.

Tensile Stress

Strain

Figure 1: Tensile response of concrete, FRC, and HPFRCC.

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Extruded fiber-reinforced cementitious composites

Extrusion, a common processing technique for polymers and ceramics, was recently adapted for the production of HPFRCC at ACBM (Shao and Shah 1997 and Shao et al. 1995). A highly viscous, dough-like mixture of cement paste and fibers (2%-10% by volume) is forced through a die to produce an element of desired cross-section. Production is continuous, and a variety of shapes, such as thin sheets for siding and roofing tiles, pipe, and cellular sections can be extruded, as shown in Figure 2. The high compressive and shear forces required to extrude the composite result in a dense matrix, a strong fiber-matrix bond and alignment of fibers in the direction of extrusion. The extruded composites are stronger and tougher than a cast composite of the same material, as shown in Figure 3 (Shah et al. 1998b).

Figure 3: Flexural response of extruded and cast composites (Shah et al. 1998b).

Successful extrusion requires the mixture to be soft enough to flow through the extruder but stiff enough to maintain its shape after exiting the die. The highly specialized cementitious matrix has a low w/b (w/b ~ 0.25) with admixtures, and mineral additives. The rheology of the extrudate has been improved with the replacement of a portion of the cement with Class F fly ash (Peled et al. 2000a). The spherical fly ash particles make the paste easier to extrude. Fly ash is also less expensive and more

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environmentally friendly than cement. It improves the flexural performance of fiber-reinforced extruded composites by increasing the likelihood of fiber pullout instead of fiber fracture at failure. Fly ash also improves the durability of glass-fiber-reinforced extruded composites by reducing the alkalinity of the cement matrix (Cyr et al 2001)

Hybrid fiber reinforcement

Material performance can be optimized for given applications by combining different types of fiber reinforcement in hybrid fiber composites. For example, glass fibers tend to be strong but relatively brittle and form a strong fiber-matrix bond. Polyvinyl alcohol (PVA) fibers are weaker, but more ductile than glass fibers. These fibers were combined in extruded composites to produce composites that were both strong and tough with strain hardening behavior (Peled et al. 2000b). In addition, it was shown that a portion of the PVA fibers can be replaced with less expensive polypropylene (PP) fibers without any significant reduction in performance. The performance of extruded single-fiber composites and extruded hybrid-fiber composites is shown in Figure 4.

0 1 2 3 4 0 1 2 3 4 Deflection (mm) Deflection (mm)

Figure 4: Flexural response of single-fiber and hybrid-fiber reinforced extruded composites. Total Vf = 5% for hybrids (Peled et al. 2000b).

Different size fibers can also be combined to enhance performance. Lawler (2001) found that combining 0.5% steel macrofibers (500 urn diameter, 30 mm length) with 0.5% PVA microfibers (14 l^ m diameter, 12 mm length) significantly improved both the pre- and post-peak performance of mortar. The microfibers bridge microcracks as they form, preventing them from coalescing and increasing the tensile strength of the composites. As the cracks grow, the steel macrofibers bridge the larger cracks, increasing the ductility of the composite. This hybrid combination also significantly reduces the permeability of cracked concrete.

SHRINKAGE REDUCING ADMIXTURES

It is well known that concrete shrinks as it dries. If the concrete is restrained, tensile stresses will develop and cracking can occur. This is of particular concern in pavements, bridge decks, and industrial floors where the volume-to-surface ratio is low. The likelihood of shrinkage cracking depends on the free shrinkage, the creep relaxation, age-dependent material properties, such as tensile strength, and the degree of restraint of the concrete. Shrinkage cracking is of greater concern in HSC where increased early-age free shrinkage, reduced creep and increased brittleness result in earlier cracking (Wiegrink et al 1996). Attempts to reduce shrinkage cracking have included the use of secondary reinforcement to keep cracks from widening, the use of fiber reinforcement to prevent microcracks from coalescing, the use of expansive cementa cement that expands during hydration creating a compressive prestress that counteracts the tensile stresses that develop under restrained shrinkageand reducing the w/c. Research at the ACBM Center has included the development of

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experimental techniques and theoretical models to assess the cracking potential due to shrinkage (Grysbowski and Shah 1990, Weiss et al. 1998, Shah et al. 1998c). In addition, a shrinkage reducing admixture (SRA) has been developed to improve the shrinkage cracking resistance of concrete (Shah etal 1992).

The SRA is a propylene glycol derivative sold by Grace Construction Products as Eclipse that reduces free shrinkage by reducing the surface tension of water. One cause of drying shrinkage in concrete is the surface tension that develops in small pores as water evaporates (Balogh 1996). As cement reacts with water, calcium silicate hydrate (CSH) forms in water-filled spaces. These spaces are not completely filled by the CSH so a capillary pore network develops. As the water evaporates, a meniscus forms in the pores. The surface tension of the water pulls the pore walls inward causing the concrete to shrink. This phenomenon occurs pores with radii from 2.5 nm to 50 nm. The reduction in the surface tension of the water by the SRA reduces the capillary pore forces that cause shrinkage, thus reducing the drying shrinkage of the concrete.

The shrinkage reducing admixture was tested in both normal- and high-strength concrete at 1% and 2% by weight of cement (Weiss et al. 1998). In general, 2% SRA showed much greater improvements over mixtures without SRA than 1% SRA did. With 2% SRA, the free shrinkage at 49 days was reduced by 42% in both normal- and high-strength concrete. Because the SRA greatly reduced the free shrinkage, the age of cracking of restrained ring specimens containing SRA was increased. Rings of normal-strength concrete cracked 10 days after casting, on average. With 2% SRA, only one of the rings cracked before the end of the tests, 50 days after casting. For high-strength concrete, the mixture without SRA cracked at 3.2 days, while 2% SRA delayed cracking until 11.6 days. The rings that cracked at later ages also had much smaller cracks.

SELF-COMPACTING CONCRETE

Self-compacting concrete (SCC) is concrete that is designed to flow under its own weight. This eliminates the need for vibration, making it easy to place in dense reinforcement and complicated formwork and reducing construction time and costs. SCC must be fluid enough to fill a mold without vibration but not segregate. Viscosity agents, such as superplasticizer, and fine mineral admixtures are commonly used.

SCC is characterized by deformability, segregation resistance, and passing ability. Deformability, represented by the flow or fluidity of the SCC, is a measure of yield stress (Ozawa et al. 1992). It is quantified as the slump flow diameter, which is obtained from a modified slump test (Takada 2000). The segregation resistance is sufficient if the aggregates are uniformly distributed throughout the cement paste. It is evaluated using a penetration apparatus (Bui 2000). The segregation of aggregates has been modeled using Stoke's Law at the ACBM Center (Saak et al 2000). The passing ability indicates how well the fresh SCC can flow through the spaces between rebar. It is measured using an L-box apparatus (Tangtermsirikul and Khayat 2000). These properties of SCC are affected by the rheology of the cement paste and the average diameter and spacing (Dss) of aggregates.

To facilitate the design of SCC, a paste rheology model was developed at the ACBM Center (Bui et al 2001). The goal was to determine the rheology of cement paste required to obtain SCC with sufficient deformability and segregation resistance for given aggregate properties. The paste rheology is characterized by yield stress, measured as the paste flow diameter, and the viscosity, measured with a standard rheometer. The model establishes, for given aggregate properties and doses, the minimum apparent paste viscosity, the minimum paste flow and the optimum flow-viscosity ratio required to achieve SCC with acceptable segregation resistance and deformability.

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In a recent study at the ACBM Center, the parameters for the paste rheology were examined by varying the total aggregate ratio, the paste volume, the w/b, and the cement, fly ash and superplasticizer contents (Shah et al. 2002). These mixes had different degrees of deformability and segregation resistance. Plots of Dss vs. flow diameter and Dss vs. apparent viscosity were obtained for a constant average aggregate diameter (Shah et al. 2002). For a constant aggregate diameter and a given Dss, there exists a minimum paste flow diameter below which the SCC exhibits poor deformability and a minimum viscosity below which the SCC segregates. Fresh SCC with a larger Dss requires a smaller paste flow diameter and higher viscosity to achieve acceptable performance. To increase the aggregate spacing (Dss), the paste volume of the mixture is increased, or the aggregate volume is decreased. This results in reduced friction between aggregates (Bui and Montgomery 1999). A higher viscosity is required to hold the aggregates together, and a smaller paste flow diameter is necessary to achieve good deformability.

NONDESTRUCTIVE EVALUATION

Nondestructive evaluation (NDE) uses stress waves to determine mechanical properties, the presence, location, and extent of damage, or the degree of hydration of concrete structures. Stress pulses are applied to the structure, and the transmission or reflection of the resulting waves or the vibration response of the structure is measured. Early NDE techniques relied on the transmission of a wave through a structure, which required access to both sides of the structure, making them inappropriate for concrete pavements or slabs. They were also unable to detect early stages of deterioration, i.e., the presence of microcracks, resulting from damage due to freeze-thaw cycling, sulfate attack, or rebar corrosion.

Recently, several new NDE techniques have been developed at the ACBM Center (Shah et al. 2000). These techniques are sensitive to the early stages of damage. A self-calibrating, one-sided technique, suitable for use on concrete pavements, was developed and shown to be sensitive to the presence of cracks (Popovics et al. 1998). The structural vibration frequency response can be tracked during test loading (Subramaniam et al. 1998) and was used to predict the remaining life of a specimen subjected to fatigue loading. Another newly developed technique, discussed in detail here, uses the reflection of wave energy at a steel-concrete interface to monitor the setting of fresh concrete.

A one-sided, ultrasonic technique has been developed to monitor the hardening and setting of fresh concrete (Ozttirk et al. 1999, Rapoport et al. 2000, and Akkaya et al. 2001). The change in ultrasonic shear wave reflections over time between a steel plate and hardening concrete are measured to monitor the setting process. High-frequency, ultrasonic shear waves are transmitted through the steel plate into the fresh concrete. As the wave reaches the steel-concrete interface, a portion of it is transmitted through the concrete and the rest is reflected back to the transducer. The wave energy reflected at the steel-concrete interface is called the wave reflection factor (WRF). It depends on the differences in the acoustic impedance, the product of material density and wave velocity, of the steel and the hardening concrete. Initially, the value of the WRF is unity because the concrete is in a liquid state, which cannot transmit shear waves, so all of the wave energy is reflected at the interface. As the concrete stiffens, more of the wave energy is transmitted into the concrete and the WRF decreases. A typical WRF vs. time plot is shown in Figure 5. In the initial stage, the WRF equals one. At the end of the induction period, there is a sharp drop in the WRF as the concrete begins to harden. The WRF eventually approaches a final asymptote. The significance of this asymptote is not yet known. The WRF test setup is shown in Figure 6.

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1.00

0.90 u. | 0.80 CO

0.70

0.60 0 4 8 12 16 20 24 28 32 36

time (hours)

Figure 5: A typical WRF curve (Rapoport et al. 2000).

^ H j ^ ^ ^ ^ l l i l ^ ^ e r s ':' computer

Figure 6: The WRF test apparatus.

Initial tests demonstrated the sensitivity of the WRF method to the presence of different admixtures, including accelerators, retarders, superplasticizer, and silica fume (Rapoport et al. 2000). WRF, pin-penetration measurements and dynamic modulus tests were performed simultaneously. The results were correlated and critical points on the WRF curve were shown to correspond to critical points in set time, temperature and dynamic modulus curves (Rapoport et al. 2000).

Additional work has further demonstrated the sensitivity of the technique to changes in mixture design and curing conditions and correlated the wave energy attenuation, or the inverse of the WRF, with early-age strength gain. Akkaya et al. (2001) measured wave energy attenuation for two different mixtures at three different curing temperatures. It is well understood that temperature affects hydration (Mindess and Young 1981). This trend is evidenced in WRF attenuation measurements. Akkaya et al. (2001) also compared the wave energy attenuation in mortar and concrete. Three batches of each mixture were tested. The mortar specimens showed good repeatability, while the concrete did not. The attenuation curves for mortar had the same shape and approached the same final asymptote. The shape of the attenuation curves for concrete was consistentdifferent stages of hydration, as indicated by distinct points on the curve, were reproducedbut each sample approached a different asymptote. This might be due to local differences in the concrete, and suggests that the homogeneity of the mixture and the final attenuation value are related.

Compressive strength tests and ultrasonic measurements were performed simultaneously to correlate wave energy attenuation with early-age strength gain (Akkaya et al. 2001). The results were

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correlated to predict strength evolution from the change in wave energy attenuation. The relationship between strength and attenuation is linear up to three days. The linearity is not affected by changes in curing temperature or mixture design. Several tests performed outdoors demonstrated that fluctuating ambient temperatures also did not influence the linearity of the relationship between strength evolution and change in attenuation. This procedure requires the determination of one or two compressive strength values at the beginning of strength evolutionwithin the first dayto calibrate the strength-change-in-attenuation relationship.

CONCLUSIONS

Some of the most recent developments in concrete technology were discussed. Reactive powder concrete, one form of UHSC, has a compressive strength of 200 MPa. It is currently used in long, slender pedestrian bridges and nuclear power plants and has potential for use in pipes, tunnel linings, and nuclear waste containment. Micro fiber reinforcement is used to overcome the inherent brittleness of concrete. Fiber-reinforced cementitious composites for specialized applications are produced with special processing techniques, such as extrusion, and hybrid-fiber reinforcement. Significant reductions in drying shrinkage, and thus the potential for shrinkage cracking, are achieved with the use of a newly designed shrinkage reducing admixture. The design of SCC is facilitated with a recently developed rheological model. Finally, a new nondestructive technique is used to monitor the setting and predict the strength gain of fresh concrete.

ACKNOWLEDGEMENTS

Much of the work presented here was funded by the Center for Advanced Cement Based Materials at Northwestern University. In addition, the research on extrusion is currently being funded by a grant from the National Science Foundation in support of the Partnership for Advancing Technology in Housing. The assistance of these organizations is gratefully acknowledged.

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