Structural behavior of ultra-high performance concrete beams with different rebar and fiber reinforcement ratios

This experimental study comprehensively analyzes the flexural performance of ultra-high performance concrete (UHPC) beams with combined rebar and fiber reinforcement. The study examines the synergistic effects of longitudinal reinforcement ratio (varying from 0.31 to 5.13%), steel fiber content (0−2% by volume), and fiber geometry (comparing straight and wavy shapes) through testing of seventeen beam specimens under four-point bending conditions. Results demonstrate that steel fiber incorporation significantly enhances structural performance by improving crack control, with maximum crack widths remaining below 0.25 mm at service load levels (65−70% of ultimate capacity). The benefits of fiber reinforcement show strong dependence on longitudinal rebar reinforcement ratio, with maximum strength improvements reaching 47% for beams containing 2% fibers at the lowest reinforcement ratio (0.31%). Comparative analysis reveals the superior performance of wavy fibers, which provide up to 25% greater strength enhancement compared to straight fibers in lightly reinforced specimens. The study identifies two distinct failure modes: abrupt failure after crack localization in lightly reinforced beams ( ≤ ≤ 0.87%) versus gradual strength gain at post-localization stage in highly reinforced specimens ( ≥ ≥ 2.56%). Fiber effectiveness diminishes significantly in highly reinforced beams due to rebar dominance and fiber distribution challenges in congested tensile zones. These findings provide quantitative evidence for optimizing fiber-rebar combinations in UHPC design, particularly highlighting the importance of fiber geometry selection and dispersion quality. The research establishes clear relationships between reinforcement parameters and structural performance, offering practical guidance for engineers while identifying key areas for future investigation, including advanced fiber dispersion techniques and hybrid reinforcement strategies for improved structural efficiency.