Spalling Resistance and Residual Strength of Hybrid Fibre-Reinforced Reactive Powder Concrete at Elevated Temperatures

Abstract

Reactive Powder Concrete (RPC) is an advanced construction material prized for its superior strength and durability. However, its dense, ultra-low porosity microstructure, while beneficial for mechanical properties, renders it highly susceptible to explosive spalling when exposed to temperatures between 200 °C and 400 °C. This dangerous phenomenon occurs as trapped moisture and air within the RPC’s pores rapidly expand upon heating, generating immense internal vapour pressure that causes sudden surface bursting. This study investigates a synergistic approach by combining steel fibres with low-melting-point polypropylene fibres within fibre-reinforced RPC (FRPC). The principle is that polypropylene fibres melt at approximately 170 °C, creating a network of micro-channels that provide pathways for the release of trapped vapour and air, thereby relieving the internal pressure that causes spalling. To evaluate this, cylindrical specimens (10 cm × 20 cm) were prepared, water-cured for 26 days, and then subjected to steam curing at 95 °C for 4 h. Subsequently, they were exposed to elevated temperatures of 200, 300, and 400 °C for 2 h to simulate fire exposure. The results conclusively show that the hybrid fibre combination effectively prevents explosive spalling. Furthermore, the hybrid FRPC maintained an impressive 80–90% of its original compressive strength post-heating. In stark contrast, FRPC specimens containing only steel fibres suffered severe damage and retained a mere 20–40% of their room-temperature strength. These findings demonstrate that hybrid fibre reinforcement is a highly effective strategy for enhancing the fire resistance of RPC, thereby enabling its safer application for structures prone to elevated temperatures.