Reinforced UHPC beams: Investigating the synergistic effect of stirrups and steel fiber on cracking resistance

Abstract

Previous studies have mainly concentrated on the effect of steel fiber content on the cracking and bending behavior of Ultra-high-performance concrete (UHPC) beams. In contrast, research on the synergistic mechanism between stirrups and steel fibers remains limited. This study reports a four-point bending test on five UHPC beams and one C40 concrete beam, aiming to investigate the effects of parameters such as steel fiber content (1.0%, 2.0%, and 3.0%), stirrup configuration, and concrete type on the mechanical properties, failure modes, load-deflection responses, and strain distributions. The results indicate that under-reinforced and lightly-reinforced failures predominantly characterize the failure modes of UHPC beams. As the steel fiber content increases from 1.0% to 3.0%, the cracking load of the beams rises by 25.37%, while the maximum crack width decreases by 50.77%. The combined effect of stirrups and steel fibers enhances the overall mechanical performance, whereas stirrups alone reduce the cracking load by 7.46%. Moreover, stirrups contribute significantly to restraining crack development in the tension zone and improving ductility, thereby reducing the risk of brittle failure. Additionally, a formula for calculating the limit reinforcement ratio is derived, the optimal reinforcement ratio is determined, and based on the tensile strength contribution of the beam, formulas for bending strength and cracking moment relevant to this study are proposed. This study offers a theoretical basis and practical insights for improving the durability and safety of UHPC structures.