Shear performance and steel fiber reinforcement mechanism of prestressed ultra-high performance concrete rectangular beams

Ultra-High Performance Concrete (UHPC) is widely used in bridges and other infrastructure due to its superior mechanical properties and durability. The shear performance is a critical design factor for UHPC beams, and the incorporation of steel fibers significantly enhances their shear capacity. However, the underlying reinforcement mechanism and quantitative models remain incomplete. This study aims to systematically investigate the effects of steel fiber content, shape, and dimensions on the shear performance of prestressed UHPC rectangular beams, elucidate the shear reinforcement mechanism, and provide a scientific basis for the design of UHPC beams through experimental research, numerical simulation, and theoretical analysis.Methodology：Eight prestressed UHPC rectangular beams, each measuring 3.1 m in length and 0.35 m in height, were subjected to four-point loading tests. Key variables included steel fiber volume content (1%, 1.5%, 2%, 2.5%), shape (straight, twisted, hooked), and dimensions (14 mm/0.22 mm, 6 mm/0.12 mm, 13 mm/0.32 mm). The study analyzed load-deflection relationships, crack propagation, and failure modes. Numerical simulations were conducted using the Concrete Damage Plasticity (CDP) model to explore the sensitivity of shear capacity to different design parameters. Furthermore, a shear capacity prediction model was developed using the limit equilibrium method, and its results were compared with design codes, including France's NF P18-710, Germany's DAfStb, and the U.S. FHWA-HRT-23.

The experimental results demonstrated that the shear process of UHPC beams can be divided into three stages: elastic, cracked, and ultimate. Steel fibers improved the stress distribution in shear compression zones by bridging cracks, delaying crack propagation and structural instability, thereby significantly enhancing shear capacity and ductility. Increasing steel fiber content from 1% to 2.5% improved shear capacity by an average of 10.7%. Hooked steel fibers exhibited the best performance, enhancing ductility by 41.7% and effectively reducing crack width and number. Fiber content and shape strongly influenced failure modes: beams with low fiber content (1%) showed brittle diagonal tension failure, whereas higher fiber content and more complex fiber shapes led to ductile shear-compression or flexural-shear failures. Beams with 2.5% fiber content exhibited flexural failure.Numerical simulations using the CDP model accurately replicated the load-deflection curves and failure patterns observed in experiments, with an average error of 3% for shear capacity. However, the predicted deformations at ultimate load were slightly lower than experimental values (-7.7%). Parameter analysis indicated that the contribution of steel fibers decreased with a reduction in shear span-to-depth ratio (a/d). At a/d = 1, increasing fiber content from 1% to 2.5% resulted in only a 2.6% improvement in shear capacity. A synergistic effect was observed between steel fibers and longitudinal reinforcement or stirrups, but the effectiveness of steel fibers diminished with higher stirrup ratios. Changes in longitudinal reinforcement ratio and prestress level had minimal impact on shear capacity.Theoretical analysis revealed that the shear capacity of UHPC beams comprises contributions from uncracked concrete shear resistance, steel fiber bridging stress, and stirrup action. Existing design codes account for these contributions to varying degrees. France's NF P18-710 and Germany's DAfStb independently considered fiber bridging stress but relied on simplified variable-angle truss models, resulting in conservative predictions (average errors of -8.9% and -6.7%, respectively). The U.S. FHWA-HRT-23, based on a modified compression field theory, provided predictions closer to experimental results (average error of -4.6%) but underperformed for low fiber content. The limit equilibrium method achieved the highest accuracy, with an average error of only 1.3%.Conclusions：This study systematically revealed the influence of steel fiber content, shape, and dimensions on the shear performance of prestressed UHPC rectangular beams and clarified the mechanism by which steel fibers enhance shear capacity and ductility through crack bridging and improved stress distribution. High fiber content (≥2%) and complex shapes (hooked or high aspect ratio fibers) yielded the most significant improvements in shear capacity and ductility. The benefits of steel fibers were particularly pronounced for beams with large shear span-to-depth ratios or low stirrup ratios. The proposed limit equilibrium method outperformed existing design codes in predicting shear capacity, providing a reliable basis for the precise design of UHPC beams.