Heated slabs under biaxial compressive loading: a test set-up for the assessment of concrete sensitivity to spalling

Explosive spalling of concrete members in fire is the violent expulsion of shards from the exposed face caused by the combined effect of pressure build-up in the pores due to water vaporization and of in-plane stress induced by both external loads and thermal gradients. Spalling progression leads to the reduction of bearing cross-section and often to the direct exposure of rebars to the flames. Since established predictive models are not available yet, experimental studies appear to be the most effective means of investigation on this phenomenon. To this purpose an experimental setup has been developed for the assessment of concrete sensitivity to spalling. The specimen is a concrete slab (800 × 800 mm) with a thickness comprised between 100 and 200 mm. The bottom face is heated via a horizontal furnace, in which a propane burner is actively controlled in order to follow the prescribed fire curve. During heating, a biaxial compressive load can be applied thanks to hydraulic jacks restrained by a steel frame. Load and slab thickness can be adjusted in order to represent the mechanical conditions achieved in the hottest region of thicker concrete members such as tunnel lining segments. The setup proved to be very effective in comparing spalling sensitivity among different concrete mixes, as is often required in initial material testing for strategic infrastructures such as tunnels.