Bond-slip response of novel half-hooked steel fibers in ultra-high-performance concrete

In this study, an attempt was made to investigate the effect of end-hook angle on the rate-dependent bond-slip behavior of novel half-hooked steel fibers embedded in ultra-high-performance concrete (UHPC). For evaluating the effects of the number of plastic hinges and length in the end-hook portion, commercially available hooked steel fiber and short half-hooked steel fiber were additionally used. Three different end-hook angles of 30°, 45°, and 60°, two different fiber inclination angles of 0° and 45° to take into account the random orientation of fibers in the composites, and various loading rates ranging from 0.018 mm/s (static) to 1186 mm/s (impact) were considered. Test results indicated that the most influential factor on the static pullout resistance in terms of the bond strengths and pullout energy was the length in the end-hook portion rather than the number of plastic hinges and end-hook angle if the fibers were pulled out without breakage. Increasing the end-hook angle was effective in enhancing the static and dynamic bond strengths and pullout energies of half-hooked fibers in UHPC given the pullout failure mode, and there was no effect of it if they were ruptured. In addition, the increase in length in the end-hook portion significantly improved the static bond strength and pullout energy in the aligned condition, whereas its effectiveness decreased under the impact loads. The use of half-hooked fiber or shorter length in the end-hook portion was effective in terms of the rate sensitivity to the pullout resistance compared with the commercial hooked fiber or the longer-length one, and the bond strength became more sensitive to the loading rate if the fiber was aligned rather than inclined.