Fire spalling sensitivity of high-performance concrete in heated slabs under biaxial compressive loading

Explosive spalling of concrete exposed to fire consists in the violent expulsion of shards from the hot surface due to the interaction between cracking and pore pressure build-up. Fire spalling relevantly increases the overall thermal damage of a structure exposed to fire, thus leading to much higher costs in the repair intervention, and in some cases it can even jeopardize the structural stability due to loss of reinforcement protection and reduction of the bearing cross-sections. High-performance concrete is particularly sensitive to spalling phenomenon due to inherent material features, such as the unstable fracture behaviour and the low permeability (favouring high values of pore pressure). In this context, an experimental campaign has been carried out on high-performance concrete (fc ≈ 60 MPa with silico-calcareous aggregate), without or with one of three different fibre types (steel fibre, monofilament or fibrillated polypropylene fibres). Tests were performed by means of a special test setup developed at Politecnico di Milano, based on slabs (800 × 800 × 100 mm) subjected to Standard Fire at the bottom and to biaxial compressive loading in the mid-plane, while monitoring pore pressure, temperature and deflection. Explosive spalling was observed in both plain concrete slabs and in one of the two slabs with steel fibre, this casting some doubts on the use of steel fibre alone against spalling. No detachment was observed when polypropylene fibre was added to the mix.