Evaluation of fire spalling behavior of ultra-high-strength-PVA fiber-reinforced concrete using the ring-restrained heating test

Ultra-high-strength-fiber-reinforced concrete (UFC) can help in reducing the labor required for construction work and can extend the service life of structures owing to its high compressive strength and durability. Furthermore, the reinforcing fiber suppresses the crack width and provides high bending toughness. However, explosive fire spalling tends to occur when UFC is exposed to high temperature, and the load bearing capacity of the structure drastically deteriorates. The fundamental mechanism of fire spalling remains unclear, but it is known that compounding synthetic fiber can reduce the risk of fire spalling. This study analyzes the fire spalling behavior of UFC compounded with polyvinyl alcohol fiber (PVA-UFC) using the ring-restrained heating test. This test represents spalling behavior under the actual constraint condition of structures by heating the bottom surface of the specimen which comprised concrete placed in a steel constraining ring. To analyze the fire spalling suppression effect of the PVA fiber, the tests were conducted using specimens with fibers and those with no fiber, whose compressive strength is 100 N/mm2 . Consequently, specimen with the PVA fibers exhibits less fire spalling depth and spalling volume ratio than the one without fiber. Furthermore, we analyze the effect of the PVA fibers on the occurrence factor of fire spalling based on the correlation between the occurrence time of spalling, restraint stress, and vapor pressure in the specimens under high temperature.