Effects of the fire decay phase on the bending capacity of a fire-exposed reinforced concrete slab

The decay phase can have an important role in the behaviour and failure of the structure during and after a fire, and therefore, it should be properly defined. The most commonly used method to define natural fire exposure, the Eurocode parametric fire curve (EN 1991-1-2:2002), adopts a simplified linear approximation of the fire decay phase with a constant cooling rate defined in function of compartment characteristics. In this study an analysis of the influence of the decay phase cooling rate assumption on structural behaviour is made, considering the case of a simply supported reinforced concrete slab exposed to fire from the bottom side. It is demonstrated that the cooling rate of the decay phase can have a significant effect on the reduction of the slab s bending capacity. It is also shown that the magnitude of this influence is affected by both the temperatures during the heating phase and the size of the concrete cover. The case study demonstrates that the difference between the cooling rates from the Eurocode parametric fire curve and available experimental test results are significant, and that this can have a significant effect on the assessment of the slab's structural performance.