John J. Myers, The University of Missouri-Rolla
The modulus of elasticity of concrete is an important mechanical property since it affects the camber of prestressed concrete beams at release of prestressing strands and deflections under superimposed dead and live loads. The modulus is closely related to the properties of the cement paste, stiffness of the selected aggregates, and the method of determining the modulus. The standard test method is ASTM C 469—Static Modulus of Elasticity and Poisson’s Ratio of Concrete in Compression.
One approach to increase the modulus of elasticity of concrete for a given mix design is to increase the coarse aggregate content of the mix. In doing so, the concrete producer might be required to adjust other mix constituents to satisfy placement and workability requirements. Figure 1 illustrates the effect of three different coarse aggregate contents on the modulus of elasticity for the same concrete constituent materials.
For ASTM standard-cured specimens, the modulus at 56 days was enhanced by approximately 3.5 percent for every 2 percent increase in the coarse aggregate content. The gain in modulus occurred more gradually at early ages with higher coarse aggregate contents for a given cementitious material content. Increasing the coarse aggregate content beyond 40 percent by weight increased the modulus but did not increase the compressive strength. The modulus appeared to be independent of aggregate size although a smaller-size aggregate for the same aggregate content resulted in a small increase in compressive strength.
For concrete subjected to high heat of hydration, as was the case with the high strength beams investigated in Texas [1], over 90 percent of the 56-day modulus was achieved within 24 hours after casting. This may be attributed to the high early-strength development and the improved paste matrix and bond characteristics. When compared to ASTM standard-cured specimens, the high strength precast concrete had a lower later-age modulus due to the reduced later-age compressive strength development and increased microcracking [1,2].
A second approach to enhance the modulus of elasticity for given mix proportions is to use a hard, dense aggregate which is compatible with the paste matrix characteristics. Figure 2 illustrates the modulus of elasticity for comparable mix designs with various types of aggregates. While stiffer, denser aggregates improve the modulus of the concrete, they can act as stress risers resulting in stress concentrations at the transition zone and subsequent microcracking at the bond interfaces. This reduces the compressive strength of the concrete. Thus, the compatibility of materials in producing high strength concrete is important for development of mechanical properties. To develop optimum strength and modulus of elasticity, it is desirable to match the stiffness characteristics of the aggregates and paste matrix. Crushed or angular aggregates are preferable because of their enhanced aggregate-matrix bond characteristics. The net result is a more homogeneous material with optimal performance characteristics, and is also cost effective.
This article describes the results obtained in Texas. Optimized mix designs should always be determined from trial batches using locally available materials.
References
[1] Myers, J. J. and Carrasquillo, R. L., “Production and Quality Control of High Performance Concrete in Texas Bridge Structures,” Center for Transportation Research, The University of Texas at Austin, Preliminary Research Report 580/589-1, to be published.
[2] Myers, J. J. and Carrasquillo, R. L., “Effect of Curing Temperature on Compressive Strength Development,” HPC Bridge Views, Issue No. 2, March/April 1999.