An experimental study on pullout and tensile behavior of ultra-high-performance concrete reinforced with various steel fibers

This study investigates the effects of steel fiber type on the pullout and tensile performance of ultra-high-performance fiber-reinforced concrete (UHPFRC). For this, four different types of steel fibers, i.e., straight, twisted, hooked, and half-hooked, were used. In order to consider random fiber orientation in UHPFRC, various inclination angles, ranging from 0° to 60° were considered for the pullout tests. Test results indicated that better pullout resistance was obtained in the deformed (twisted, hooked, and half-hooked) steel fibers embedded in ultra-high-performance concrete (UHPC) than that of the straight steel fiber as fiber breakage was prevented. The highest bond strengths of all steel fiber types were found when they were inclined 30° or 45° while their slip capacities increased with increasing inclination angle. The hooked steel fiber exhibited the highest bond strengths at all inclination angles, while the twisted and half-hooked steel fibers exhibited the highest pullout energies at aligned and highly inclined (45° and 60°) conditions, respectively. In contrast to the pullout test results, the best tensile performance of UHPFRC was achieved by incorporating straight steel fibers, followed by the twisted, half-hooked, and hooked steel fibers. The poorer tensile performance of UHPCs reinforced with the deformed steel fibers was because of the severe matrix damage from excessive mechanical anchorage and fiber congestion, leading to insufficient matrix volume. Similarly, a relatively weak correlation between the fiber pullout and tensile behaviors of UHPFRC was observed. Therefore, a new pullout test method, which is able to consider the fiber random orientation and spacing, is required in order to improve the correlation.