Crack instability of concrete in fire: a new small-scale screening test for spalling

The thermal energy accumulated by the concrete cover exposed to fire is several orders of magnitude larger than the elastic energy entailed by thermal stress. Water vaporization can quickly convert this huge energy source into the mechanical work required to drive the fracturing and acceleration of concrete shards, namely the explosive spalling phenomenon. In order to fully understand the role of moisture content in this mechanism, a novel small-scale screening test setup was developed, where a planar crack is triggered in a heated concrete disk by the high thermal dilation of an embedded polymer ring. The concurrent pore saturation level is changed by the heating rate, the addition of polypropylene fibre, and the preliminary drying of samples, with direct effects on crack instability. The ability of this setup to categorize the inherent spalling sensitivity of different concrete mixes was validated against standard fire tests on prestressed slabs.