Effect of Steel Fiber Hybridization on the Shear Behavior of UHPC I-Beams

This study aims to investigate the synergistic effect of hybridizing steel fibers on the shear behavior of I-shaped reinforced concrete beams (I-beams) produced with Ultra-High-Performance Concrete (UHPC) without shear reinforcement. For this purpose, five I-beams were prepared using UHPC mixtures with three fiber volume fractions (0%, 1% and 2%), incorporating either straight micro steel fibers alone or an equal combination of straight micro and hooked-end macro steel fibers, and tested under three-point loading. In addition, the experimental program evaluated the effects of hybridization on the compressive strength, splitting tensile strength and fracture behavior of UHPC. The test results showed that beams with 1% microfibers and hybrid fibers demonstrated substantial improvements in shear resistance, achieving 2.7 and 2.0 times higher shear strength than the reference beam without fibers, respectively. Moreover, the beam reinforced with only microfibers exhibited 37% greater shear strength than the beam with hybrid fibers, indicating that the synergistic effect was limited at this dosage. At a 2% fiber volume, the failure mode shifted from shear to flexure. These findings highlight the critical influence of fiber type and dosage on the shear behavior of UHPC I-beams.