Experimental analysis of concrete spalling due to fire exposure

Concrete thermal instability is observed when a concrete structure is totally, or partially, exposed to an accidental fire. This sudden phenomenon, which can appear in the first twenty minutes of a fire, is called concrete spalling. Spalling can seriously jeopardise the stability of a concrete structure during a fire (loss of load-bearing cross section, exposure of the reinforcement to fire) as well as after a fire (decrease in the residual mechanical properties and weaker durability). Moreover, the costs of repairing the damages induced by spalling can be very high. Since the early 1980s, many studies (mainly theoretical) have tried to find the most probable causes for concrete spalling. However, we still do not exactly know the mechanisms of spalling, and this phenomenon still cannot be evaluated by calculations. We present in this paper the results from an experimental research dealing with the behaviour of concrete at high temperatures. The comparison of experimental results, from material characterisation at high temperature to fire tests on medium-sized concrete slabs, allows us to discuss the influence of different parameters of spalling risk. Some important conclusions are drawn on the most probable causes for spalling of concrete when exposed to fire. In particular, we show that pore gas pressure cannot be the only physical origin for concrete spalling.