Research on the High Temperature Behaviour of Ultra-High-Performance Concrete (UHPC) with Polypropylene (PP) and Steel Fibres

The research features a series of tests on two types of Ultra-High Performance Concrete (UHPC, 150 and 180 N/mm2) with polypropylene (PP) fibres (0.27% of volume) and variable content of steel (S) fibres (0% to 1.92%), aimed at investigating the residual mechanical properties of the material after high temperature exposure. The results are compared to available research on small UHPC specimens exposed to high temperatures, with PP fibres from 0.03% to 2%, and S fibres from 0 to 3% of volume. The results demonstrate that UHPCs need hybrid fibre reinforcement (PP + S) to withstand high temperatures, and that the residual strength increases after 200 ℃ exposure, at all steel fibre dosages; this is in line with literature. Available research also shows that strength loss is possible in hot conditions, as found in the present research, while PP fibres alone do not always prevent the occurrence of spalling in small UHPC samples.