Effect of Lightweight Aggregate on Early-Age Cracking of Mass Concrete

Abstract

Early-age cracking in mass concrete structures is a severe problem which could lead to long-term serviceability related problems in the structure. In this dissertation, the effect of using lightweight aggregates (LWAs) on the early-age cracking tendency of mass concrete was evaluated. Concretes were made with 30% Class F fly ash to be representative of mass concrete and the following concrete types were made at water-to-cementitious materials (w/cm) ratios of 0.45 and 0.38: 1) normalweight concrete, 2) internally cured concrete, 3) inverse sand-lightweight (ISLW) concrete, 4) sand-lightweight (SLW) concrete, and 5) all-lightweight (ALW) concrete. Rigid cracking frames were used to measure from the time of setting until the onset of cracking the development of concrete stresses caused by autogenous and thermal shrinkage effects. Rigid cracking frame specimens were tested under isothermal and match-cured temperature conditions. The match-cured temperature condition simulated the edge of an 8 × 8 ft mass concrete column. In addition, three concrete cross-section sizes, (4×4, 8×8, and 12×12 ft) were modeled using ConcreteWorks for four different concrete mixtures to determine the maximum concrete temperatures, maximum concrete temperature differences, stresses, and cracking risk. The results indicate that the maximum in-place concrete temperatures increase as more lightweight aggregates were used in each mixture; therefore, care should be taken when using LWA concrete in mass concrete to make sure that the delayed Ettringite formation (DEF) temperature threshold is not exceeded. The use of LWAs in concrete with low w/cm is beneficial to control early-age cracking, because it helps to mitigate autogenous shrinkage and lower the modulus of elasticity of the higher strength concrete. The presence of LWAs in concrete delayed the time to cracking, with SLW concrete providing the best overall resistance to early-age cracking. Although an increasing amount of LWA in the concrete will increase the maximum concrete temperature in mass concrete applications, the increasing use of LWA will reduce the modulus of elasticity, reduce the coefficient of thermal expansion, and eliminate autogenous shrinkage effects, which all contribute to improve the resistance to early-age cracking. The commonly used maximum temperature difference limit of 35°F in mass concrete applications is inappropriate for concretes containing lightweight aggregates. A simplified version for computing the maximum concrete temperature difference limit for all concretes is proposed.