External restraint factors in early-age massive foundation slabs

Massive foundation slabs are one of the structures in which early age effects play a significant role. These effects are mainly related to the exothermic nature of cement hydration and a consequent temperature rise in the structure. The inhomogeneous volume changes have consequences in arising stresses in a concrete foundation slab. Two types of early-age stresses can be distinguished: self-induced stresses caused by the internal restraints, and restrained stresses resulting from the restraint existing along the contact surface of foundation slab and subsoil. This paper is focused on the distribution and the magnitude of the restrained stresses caused by the restraint between the foundation slab and soil. The results of the study have been expressed by the restraint factor R, defined as a ratio between the real stress generated in the analyzed slabs to the stress generated at full restraint of the slabs. The dependence of the external restraint factor on length/thickness ratio and thickness of the slab, type of subsoil, age of concrete, and presence of slip layers is investigated in the study. The general equations for the R factor, dependent both on length/height ratio (L/H) and thickness of the slabs, are proposed.