Effect of micro-cracking and self-healing on long-term creep and strength development of concrete

When concrete is subjected to sustained load, it first deforms elastically and then continues to deform with time. The stress-induced time-dependent deformation is, by definition, creep. Creep plays an important role in view of the serviceability, durability and sometimes even the safety of concrete structures. Prediction of the long-term creep is still a challenge. Apart from the time-dependent deformation, the microstructure, strength and elasticity of concrete are also continuously changing under sustained load. This will, in turn, have an influence on the creep deformation. Micro-cracking has been detected experimentally by acoustic emission techniques during creep tests for concrete loaded at different stress levels. It could contribute to both an extra deformation and a reduction in the strength and elasticity. This is somehow contradictory to the experimental observation that there is an extra increase in strength (and elastic modulus) of concrete under sustained load, especially at low and medium stress levels, compared to load-free concrete. There must be another phenomenon during the creep process that "resolves" this contradiction. Despite a few theories which have been proposed in the paste to explain the extra increase in strength of concrete under sustained load, the mechanism behind this phenomenon has not been fully understood yet. Besides, how this extra increase in strength influences the long-term creep deformation has rarely been studied. In this research self-healing is considered as a promising mechanism to explain the extra increase in strength of concrete under sustained load. The main aim of this research is to study the effect of micro-cracking and self-healing on the long-term creep and strength development of concrete under sustained load and to gain a better understanding of the behaviour of concrete under sustained load.