Influence of different types of steel fiber on explosive spalling behavior of ultra-high-performance concrete exposed to high temperature

To explore the steel fiber which can improve the fire resistance of ultra-high-performance concrete (UHPC), explosive spalling behavior of different types of steel fiber (three types of normal steel fiber and two types of recycled steel fiber from waste tire) reinforced UHPC and plain UHPC exposed to high temperature up to 800℃ were experimentally investigated in this paper, and fracture energies of these concretes were also tested. All of plain UHPC specimens encounter severe explosive spalling. Steel fiber significantly alleviates the explosive spalling of UHPC but cannot avoid the occurrence of explosive spalling. UHPC incorporating normally available commercial steel fiber with hooked-end (35 mm long and 0.55 mm in diameter) exhibits the best resistance to explosive spalling, and the second is the UHPC incorporating recycled steel fiber without attached rubber. This suggests that the characteristics of steel fiber affect the explosive spalling of steel fiber reinforced UHPC significantly. The steel fiber which has a large distribution density or that without a large distribution density but can make concrete have high toughness (fracture energy), is more appropriate to be utilized in UHPC structures with high fire resistance requirements.