Temperature, pore pressure and mass variation of concrete subjected to high temperature — Experimental and numerical discussion on spalling risk

Spalling at high temperature is a phenomenon that can be observed in different materials such as ceramics, rocks and bricks. For concrete, this phenomenon, considered as a thermal instability of the material, can seriously jeopardize the integrity of a whole structure during fire and can even constitute a risk for people. Many explanations to the spalling risk exist but still no model can accurately predict it. Among them, models based on thermo-hydral behaviour of concrete have been proposed and developed by several authors. In particular, an important role is given to the pore vapour pressure, considered by many authors as the main mechanism for the trigger of such a thermal instability. However, pore vapour pressure is not easy to measure and numerical works still need more experimental results to validate their assumptions regarding the spalling risk. This paper presents the results of an experimental study carried out on five different concrete mixtures. We used a device intended for measuring temperature, pore vapour pressure and mass loss of concrete specimens. The aim of the study was to better understand the thermo-hydral behaviour of concrete exposed to high temperature and the possible link to spalling risk. In particular, we focused on the influence of matrix compactness on the transfer properties of concrete and we discussed about the importance of pore vapour pressure on spalling risk. Moreover, based on our experimental observations, a numerical analysis of the influence of water content on the thermomechanical behaviour of concrete during heating is done.