Hybrid effect of macro and micro steel fibers on the pullout and tensile behaviors of ultra-high-performance concrete

This study aims to investigate the effect of hybrid use of macro and micro steel fibers on the pullout and tensile behaviors of ultra-high-performance concrete (UHPC). To this end, three different macro steel fibers (i.e., straight, hooked, and twisted fibers) and a single micro straight steel fiber were used, whereby a portion of the macro fibers was replaced with the micro fibers at the constant total volume fraction of 2%. In order to fabricate ultra-high-performance hybrid fiber-reinforced concrete (UHP–HFRC), five different replacement ratios of the macro fibers with the micro fiber were employed, including 0, 0.5, 1.0, 1.5, and 2.0%. The pullout behaviors of multiple aligned fibers embedded in UHPC were also compared with the tensile behaviors of UHP–HFRC to analyze their correlations. The test results indicated that the average bond strength and normalized pullout energy of the macro straight fibers in UHPC were improved after their replacement with micro fibers, whereas those of the hooked and twisted macro fibers were reduced according to the replacement ratio. Lower fiber efficiency ratios were obtained after the replacement of the macro fibers with the micro fibers in the hooked and twisted fiber cases, while similar ratios of approximately 0.3–0.4 were obtained in the cases of the macro straight fibers. In comparison, the post cracking tensile strength and energy absorption capacity of UHPC reinforced with macro straight fibers decreased, whereas those of UHPCs with hooked and twisted fibers increased when they were replaced by the micro fibers. The negative correlation between fiber pullout and tensile behaviors of all UHP–HFRCs occurred because the actual fiber orientation and bonding area in the composites could not be considered in the fiber pullout tests. Thus, it is concluded that the effects of the use of hybrid steel fibers on the tensile performance of the UHPC composites could not be predicted based on the pullout test results of the aligned fibers.