Spalling Test on Concrete Slabs Under Biaxial Membrane Loading

Concrete spalling is a rather complex phenomenon ensuing from the interaction of different aspects – often hard to be monitored – namely, temperature, pressure in the pores and stress. It is, in fact, commonly agreed that spalling is triggered by the mutual influence of hygro-thermal and thermo-mechanical processes. Aimed at investigating these two critical issues, an ad hoc test setup was developed at the Politecnico di Milano, based on in plane-loaded slabs. Concrete specimens of dimensions 800x800x100 mm were subjected to the Standard Fire at the intrados, while a constant biaxial compressive load was applied. Pore pressure and temperature at 6 different depths, as well as the flexural behaviour, were continuously monitored during the test. Taking advantage of this facility, an experimental campaign was carried out on one High-Performance Concrete (fc ≥ 60 MPa with silico-calcareous aggregate), without or with one among 3 different fibre types (steel fibre, monofilament or fibrillated polypropylene fibre). So far, tests on concrete without and with polypropylene fibre were carried out. Explosive spalling was observed in plain concrete only, with a remarkably homogeneous spalled layer. In all cases, the mechanical response was characterized by sagging deflection due to thermal strain followed by hogging deformation due to creep and plastic strain.