Early-Age Behavior of Lightweight Aggregate Concrete
Type of Degreedissertation
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Early-age cracking can be a severe problem in concrete as it can reduce the service life of the structure. In this project the early-age behavior of concrete was measured, evaluated, and modeled. The early-age stress development of concrete was measured with rigid cracking frames. The free strain was measured with a free-shrinkage frame and corrugated tubes. Rigid cracking frames measure the stress development of concrete due to thermal and autogenous effects from setting until the onset of cracking. The stress and strain development of match-cured concretes with various water-cement ratios, densities, and curing temperatures were measured. Mixtures with various proportions of expanded clay, shale, and slate lightweight coarse and fine aggregates were examined, and their behavior was compared to that of normalweight control mixtures. The effects that mechanical properties and internal curing have on cracking tendency, stress development due to autogenous stress, and stress relaxation were examined. The B3 compliance model was modified to accurately model the early-age stress development, and the effect of lightweight aggregates on relaxation was examined. Increasing the amount of pre-wetted lightweight aggregate in the concrete systematically decreases the density, modulus of elasticity, coefficient of thermal expansion, and thermal diffusivity of the concrete. When compared to a normalweight control concrete, the use of lightweight aggregates in concrete effectively delays the occurrence of cracking at early ages in bridge deck applications. The use of pre-wetted lightweight aggregates in concrete can reduce or eliminate the stress development caused by autogenous shrinkage. The decrease in autogenous stresses is due to internal curing, because water desorbed from the lightweight aggregates fills capillary voids formed by chemical shrinkage. The B3 Model is a compliance model that was designed and calibrated to estimate the instantaneous and time-dependent strain behavior of mature concrete with an age of one day or more. Modifications to the B3 Model were made that provide a better fit to early-age stress data. Using the Modified B3 Model, it was found that the reduced modulus of elasticity of the lightweight aggregate increases the amount of relaxation compared to normalweight aggregates; and decreasing the water-cement ratio, with constant paste volume, increased the paste quality and decreased relaxation. A simplified version of the Modified B3 Model is proposed that has minimal loss of accuracy compared to the Modified B3 Model.