Heat-damaged Fibre-Reinforced HPC: Physical and Mechanical Properties from Meso and Macro Investigations

Fire and concrete have still many issues open to investigation in terms of concrete spalling and mechanical decay at high temperature, the former mostly related to pore pressure and the latter to the addition of special constituents, like fibers and pozzolanic materials [1]. As these constituents have very different scales, the correlation between the properties measured at the micro- and macro-level stands out as a key factor to optimize cementitious mixes. Within this context, a comprehensive research project has been lately carried out in Milan and Bergamo on eleven concretes (fc = 40, 60, 90 MPa), with different aggregates (mixed, calcareous, basalt) and fiber amounts and types (vf = 0.05-0.8%; steel and pp fibers, either monofilament or fibrillated). In all mixes, the cementitious-mortar volumetric fraction was fixed (vcm = 0.40). For five reference temperatures (T = 20, 105, 250, 500 and 750°C) the properties at the micro-level were investigated by means of Thermogravimetric Analysis – TGA (to have information on free-water content, dehydration of hydrated compounds and decarbonation), X-Ray Diffraction – XRD (to determine the crystalline phases before and after heating), Mercury-Intrusion Porosimetry – MIP (to measure pore volume and size) and Scanning Electron-Microscopy - SEM (to investigate composition and possible microcracking). Also pore pressure was measured in a number of concrete slabs exposed to the standard fire on one face, with/without in-plane compressive stresses applied along the sides, to investigate concrete spalling.