INFLUENCE OF EARLY HEAT CURING ON PROPERTIES OF 100-MPA AIR-ENTRAINED CONCRETE

An air-entrained, high-strength concrete was prepared in a precasting plant to evaluate the effect of initial heat treatment and subsequent outdoor storage on concrete characteristics. One set of standard specimens was cured in lime-saturated water at 23 deg C until testing. Another set of specimens and a large panel were heat cured on a steel bed heated by 50 deg C water circulating underneath it. Heating began 6 hr following concrete casting and lasted for 8 hr. Heated concrete remained on the bed for an additional 6 hr before demolding; it was then cured indoor at 15 deg C until the age of 7 days before outdoor storage. Investigated properties included temperature rise, compressive, splitting tensile, and flexural strengths, modulus of elasticity, chloride ion permeability, and frost durability. Mechanical properties were evaluated at 1, 4, 28, 91, and 365 days. Test results indicate that the investigated concrete can secure high early strength without early heat curing. Initial heating increased compressive strength; however, such heat treatment coupled with subsequent outdoor storage had a deleterious effect on ultimate strength, especially for concrete from the precast panel where initial peak temperature was 56 deg C, compared with 42 deg C for accompanying heated specimens. Maximum reductions in compressive, splitting tensile, and flexural strengths and modulus of elasticity of concrete subjected to early heat curing and subsequent outdoor storage compared with standard-cured specimens were approximately 15, 35, 40, and 15 percent, respectively. Regardless of curing history, the concrete exhibited excellent frost durability. Mercury intrusion porosimetry of 1-yr old samples demonstrated that heated concrete had greater porosity than standard-cured concrete and higher concentrations of pores with 1-5 micrometer diameter. That was confirmed by microscopical examination; however, no particular increase in microcracking in the paste or transition zone of heated concrete, as compared with moist-cured concrete, was observed.