Evaluation of zero-stress temperature and cracking temperature of high performance concrete at early ages

Assessing the risk of cracking of high performance concrete induced by restrained volume changes from early ages is of considerable significance. To estimate and control such cracking risk of high performance concrete, two characteristic temperatures, namely zero-stress temperature (T_z) and cracking temperature (T_x) are crucial. In this study, the two temperatures are investigated in-depth by both theoretical analysis and experimental studies. For predicting the evolutions of T_z and T_x from early ages, rigorous yet practical models are proposed, which crucially take the visco-elastic behaviour of concrete into account. The reliability and predictive capability of the proposed models are demonstrated through a series of comparisons between the predicted and the measured results. Based on the predicted T_z and T_x profiles, practical thermal control criteria for preventing concrete from cracking caused by restrained strain are put forward. In principle, the actual temperature (T) of concrete should be kept higher than both T_z and T_x to properly maintain the stress induced by restrained strain in compression at early ages. If T becomes lower than T_z and reduces continuously, the lower the value of T, the higher the risk of cracking of concrete induced by restrained strain. As a consequence, once the value of T reaches or becomes lower than T_x, cracking is highly likely to occur. For a given actual temperature condition, lowering T_z and T_x can mitigate the risk of the cracking of concrete. Finally, effective measures for such lowering of T_z and T_x are also proposed.