Influence of Time on Crack Formation and Failure of Concrete

Abstract

First of all experimental results are summarized and discussed all of which document the influence of time on crack formation and failure of concrete. The observed behaviour can be explained qualitatively on the basis of a phenomenological approach. In this case the time-dependent degradation of the composite structure of concrete is considered. Refined methods are necessary for a quantitative prediction of the influence of time on crack formation and failure of concrete. It is shown that crack theory provides us with a powerful tool to study crack propagation under sustained load. If the elastic modulus is replaced by a time-dependent operator crack growth can be linked with creep of the material. In this way life-time of concrete under sustained load can be predicted realistically. In addition a stochastic model based on rate theory is outlined. This model enables us in principle to predict failure under arbitrary loading conditions including high rate and impact. Variability of strength can be determined on the basis of structural parameters. Mechanisms of crack growth are finally discussed. It is concluded that crack growth is a rate process similar to creep. So far there i s no direct evidence of stress corrosion. Degradation mechanisms of porous materials such as crystallisation pressure, however, can be fortified by an external load.