Autogenous Shrinkage of HPC and Ways to Mitigate it

摘要

Autogenous shrinkage strains may cause cracking of brittle cement-based systems. The lower the water/cement ratio of a mixture, the higher the autogenous shrinkage and the higher the probability of cracking. Cracking increases the probability of ingress of aggressive substances into the concrete, jeopardizing the durability of the material. Low water/cement ratios, however, result in a dense microstructure, which is one of the desired features of high performance concrete because of the assumed increase in durability. This results in a kind of dilemma: the high density of low water/cement ratio concretes is desired, but the increased risk of (micro)cracking of these mixtures, of course, not. This dilemma is discussed in this contribution. The search for ways to get out of this dilemma starts with understanding the mechanisms of autogenous shrinkage. It is shown that autogenous shrinkage is not only an issue in low water/cement ratio mixtures. In normal strength concretes, with w/c ratios between 0.40 - 0.50, autogenous shrinkage can make out a substantial part of the total strain. Ways to mitigate autogenous shrinkage of mixtures with water/binder ratios ranging from 0.18 to 0.50 are discussed. Finally brief comments are made on autogenous shrinkage values prescribed in current design codes.