Flexural and Shear Response of RC Beams Reinforced with 4D Steel Fibers Exposed to High Temperatures

This study investigates the effects of 4D steel fiber reinforcement on the shear and flexural performance of slender reinforced concrete (RC) beams under ambient conditions and after exposure to elevated temperatures (600 °C). A total of 48 beams were tested, including control specimens and beams with fiber volume fractions ranging from 0.25% to 1.25%. The results indicate that incorporating steel fibers significantly enhances both the ultimate load capacity and ductility of RC beams, with the degree of improvement strongly dependent on fiber dosage and structural behavior. In shear-dominant beams, ultimate load capacities increased by 148–192% under ambient conditions and retained 118–128% of their capacity after heating, while ductility improvements ranged from 197% to 437%. However, workability at high fiber dosages is critical for shear members, as volumes above 1.25% may cause placement and compaction difficulties. Flexure-dominant beams achieved optimum performance at 0.5% fiber content, providing 21–28% enhancement in ultimate load, 15–22% residual strength post-heating, and 12–58% improvement in ductility, without compromising workability. Higher fiber dosages in flexural members offered minimal additional benefits. The findings demonstrate that steel fiber reinforcement is a highly effective strategy for improving the structural resilience of RC beams, enhancing post-fire performance, and mitigating brittle failure, with the optimum fiber dosage dependent on the desired structural response and practical casting considerations.