Autogenous Shrinkage, Residual Stress, and Cracking in Cementitious Composites: The Influence of Internal and External Restraint

The premise of this paper is that low water-to-cement ratio (w/c), higher strength concrete may undergo substantial volume changes, especially at early ages (<24 hours), and the prevention of these volume changes can lead to invisible micro-cracking as well as visible through-cracking. This paper compares the predicted response of concrete undergoing autogenous shrinkage using two different models. The first model considers the concrete as a homogeneous material with effective elastic properties. The second model considers the concrete as a heterogeneous two-phase composite. The paper begins with an overview of common effective elastic property computations. These effective properties are then used to compute residual stresses when external restraint is provided. Heterogeneous system simulations were performed to quantify the effect of both internal and external restraint on shrinkage. Internal restraint may be caused by aggregate particles while the structure surrounding the concrete composite may produce external restraint. These simulations considered a series of ‘model systems’ ranging from a single aggregate particle, to an assembly of hexagonal unit cells containing aggregate particles, to a more realistic system consisting of various sizes and shapes of aggregate particles. The finite element analysis (FEA) simulations illustrate how localized stresses develop, how effective equivalent properties can be determined for heterogeneous composites, and how localized stresses can lead to cracking.