Effect of fiber on residual strength and explosive spalling behavior of ultra-high-performance concrete exposed to high temperature

Plain concrete, steel fiber and hybrid fiber reinforced ultra-high-performance concretes, with 140-160 MPa compressive strength at 56 d, were prepared using common raw materials. The residual compressive strength and tensile splitting strength of these concretes after exposure to high temperature were determined experimentally. Explosive spalling test was carried out on the specimens with 100% moisture content and two heating rates were employed to study the effect of heating rate on spalling behavior of ultra-high-performance concrete. The results indicate that residual compressive strength of each type of concrete increases firstly, and then decreases with increasing temperature. After exposure to 800℃, residual compressive strength is approximately 30% of that at normal temperature. Residual tensile splitting strength of both steel fiber reinforced concrete and hybrid fiber reinforced concrete also increase firstly and then decrease, which are 15.1% and 35.4% of the original strength, respectively. Residual tensile splitting strength of plain concrete decreases with increasing temperature, which is 20.3% of its original strength after exposure to 800℃. At 7.5℃/min heating rate, explosive spalling of each type of concrete specimens with 100% moisture content is quite severe. Nevertheless, steel fiber can alleviate explosive spalling but does not avoid the occurrence of explosive spalling, and the effect of hybrid fiber on improving spalling resistance behavior of ultra-high-performance concrete is not significantly superior to that of steel fiber. At 2.5℃/min heating rate, incorporating hybrid fiber inhibits the occurrence of explosive spalling in some specimens.