Effects of high temperature on the residual compressive strength of high-strength siliceous concretes

The of high-strength concretes (f(c) > 60 MPa) in special structures designed to work in a high-temperature environment or to withstand severe thermal accidents requires the mechanical properties of the material to be assessed with regard to high-temperature effects. In this context, the "residual" mechanical properties of two high-strength concretes (f(c) = 72 and 95 MPa), with siliceous aggregates (mostly flint) are studied here under uniaxial compression after a single thermal cycle at 105, 250, 400, and 500 C. The results show that while concrete toughness increases after a cycle at high temperature, strength and stiffness decrease dramatically, and the recovery of strength in time is either nil or negligible. Furthermore, the stress-strain curves exhibit a rather pronounced softening branch, which has never so far been measured in a high-temperature context, and is instrumental iii assessing concrete toughness in compression. The knowledge of the the residual mechanical proper-ties of a concrete is necessary whenever the thermally-damaged structure is required to bear a significant share of the loads even after a severe accident, as may occur for instance, in the case of a core heatup in the secondary-containment shell of a nuclear power plant.