Prediction of cracking due to shrinkage restraint in concrete structures
Résumé
Cement-based materials undergo several types of deformations related to temperature, drying and hydration, namely, thermal, drying and autogeneous shrinkage. These strains may lead to tensile stresses, and sometimes to cracking, which can reduce the durability, limit mechanical material performance and reduce tightness (if required). The principal mechanisms of cracking by shrinkage restraint are presented, namely, self-restraint (gradients, strains incompatibilities due to the heterogeneity of cement-based materials) and external restraint (concrete lift, rendering mortar for instance). Then, 2 examples are given, including experimental results and numerical simulations. In the first one, shrinkage cracking in the case of a tunnel lining for a subway (40 cm thickness) is presented. Effects of autogeneous, drying and thermal (due to the heat release during hydration) are studied. After identification of material parameters, numerical simulations are performed. They show that thermal shrinkage at early-age is the main cause of snap-through cracks affected tightness. In the second example, the case of nuclear power plant construction is studied (1.2 m thickness). Cracking may occur because of thermal gradients and shrinkage restraint (by the previous lift or the raft foundation). Effect of creep is particularly unveiled. The numerical simulations show that differential creep in tension/compression leads to different results: no cracks or snap-through cracks. The evolutions of coefficient of thermal expansion and Poisson ratio are also addressed: their impact on the prediction of stresses is very limited, since they have a sharp evolution just before the setting of the material.