Bond behaviour between steel fibre and cement mortar exposed to elevated temperature

In order to accurately evaluate the deterioration of the mechanical properties of concrete reinforced with steel fibre (SF) after exposure to elevated temperature, it is particularly important to study the bonding properties between the SF and the concrete matrix. In this work, four types of SFs and five cement mortars with different strengths were used to investigate the bond behaviour of the SF–mortar interface after exposure to elevated temperature. The results showed that the compressive and flexural strengths of each mortar decreased significantly after elevated temperature treatment, and the attenuation degree of flexural strength was greater than that of the compressive strength. In addition, the ultimate pull-out load of specimens with straight SFs gradually decreased with an increase in exposure temperature. However, the ultimate pull-out load of specimens with hooked-end SFs showed no significant change between exposures to 20°C and 400°C. When the exposure temperature exceeded 400°C, the ultimate pull-out load gradually decreased with an increase in temperature. The damage mechanism of the SF–mortar interface under elevated temperature was investigated by analysing the interfacial microstructure and phase change of cement pastes. Finally, a formula to related the ultimate bond strength with temperature was established.