Bond-slip behaviour of steel fibres in concrete after exposure to elevated temperatures

The bond-slip mechanisms, associated with the pull-out behaviour of steel fibres embedded in concrete after exposure to elevated temperatures, are experimentally investigated. A series of pull-out tests have been performed on straight and hooked-end steel fibres embedded in four different types of concrete, namely, normal strength concrete (NSC), medium strength concrete (MSC), high strength concrete (HSC) and ultra-high performance mortar (UHPM). Ninety days after casting, the specimens were heated to a target temperature of either 100, 200, 300, 400, 500, 600, 700 or 800 °C. The effect of temperature on the mechanical and thermal properties of the steel fibres and concrete was also studied. The results showed that the bond behaviour of straight fibres is significantly influenced by heating. The influence of elevated temperatures on the bond characteristic of hooked-end fibre was twofold: the bond strength does not vary significantly for all matrixes in 20-400 °C, while the bond dramatically degraded in 400-800 °C, especially at temperatures greater than 600 °C. The reduction in bond strength at elevated temperatures is found to be strongly related to the degradation in properties of the constituent materials, i.e. the fibres and concrete.