Abstract: Steel/polypropylene (PP) hybrid fibers can enhance the high-temperature resistance and corrosion durability of ultra-high performance concrete (UHPC) structures, but simultaneously affect their structural mechanical performance. To investigate the influence of reinforcement ratio on the flexural behavior of steel/PP hybrid fiber-reinforced UHPC beams and quantify the improvement in mechanical properties, the authors design a series of such beams with different reinforcement ratios (0.28% to 1.27%) for flexural tests, comparing them with normal C50 reinforced concrete beams of identical cross-section. Key performance indicators including load-displacement curves, crack development patterns, load-steel strain curves, strain profile at mid-span, and ultimate load-carrying capacity are observed. The results indicate that: (1) Under the same cross-sectional conditions, compared to C50 concrete beams, the steel/PP hybrid fiber-reinforced UHPC beams exhibit increases of 6.67%, 16.00%, 25.00%, and 36.30% in cracking load, yield load, peak load, and ultimate load, respectively. Flexural stiffness increases by 56.4%, the ductility coefficient improves by 26.2%, and the average crack spacing reduces by 28.5%. (2) Reinforcement ratio is identified as a primary influencing factor. As the reinforcement ratio increases from 0.28% to 1.27%, the yield load, peak load, and ultimate load increase dramatically by 128.3%, 128.8%, and 177.1%, respectively, flexural stiffness improves by 67.6%, and average crack spacing decreases by 28.5%. The results confirm that the contribution of fibers to the tensile resistance cannot be neglected. Based on back-calculation from experimental data, a recommended equivalent coefficient of 0.40 for the tensile zone is proposed. Calculations using this value show good agreement with test results.

Key words: tunnel segment, hybrid fiber, ultra-high performance concrete beams, bending performance, structural test