Contribution to the explanation of the spalling of small specimen without any mechanical restraint exposed to high temperature

The behaviour of concrete subjected to high temperature is studied. The aim of the study is to explain the spalling or bursting phenomenon observed during experimental studies in the laboratory. Mechanical computations are carried out with the finite element code CAST3M developed at the French Atomic Energy Agency (CEA). Heat gradient and water vapour pressure inside the concrete element are determined by using a thermo-hydrous model. Then, the mechanical stresses generated in the studied concrete element are calculated according to two behaviour assumptions: the linear isotropic elastic law and an elastoplastic model. Numerical simulations show that, during the heating cycles, tension stresses are developed in the central part and compression stresses at the surface of the cylindrical concrete element. The highest stresses appear when the surface temperature of the concrete element is about 300 oC. The tension stresses in the specimens then exceed the concrete tensile strength.