Push-off tests and comparative evaluation of direct shear transfer behavior in normal, steel fiber-reinforced, and ultra-high-performance concrete

This study investigates the direct shear behavior of normal concrete (NC), steel fiber–reinforced concrete (SFRC), and ultra–high–performance concrete (UHPC) through a systematic experimental program. Forty large–scale Z–shaped push–off specimens were tested under monotonic shear loading, with variations in fiber content, lateral confinement, reinforcement ratio, initial crack width, lap splice length, and reinforcement grade. Test results revealed that steel fibers significantly enhanced post–cracking resistance, residual strength, and energy dissipation, while lateral confinement and transverse reinforcement effectively limited slip and stabilized shear transfer. UHPC specimens exhibited superior shear capacity and stiffness, even with reduced lap splice lengths, establishing a clear performance hierarchy (UHPC > SFRC > NC) owing to their dense matrix and enhanced bond. Five performance indices—initial stiffness, peak shear strength, slip at peak, residual strength, and energy dissipation—were proposed for consistent evaluation, and regression–based sensitivity analysis highlighted the dominant roles of material type (especially UHPC), confinement, and reinforcement ratio. Furthermore, a unified regression–based shear strength expression was established, integrating cohesion, friction, reinforcement clamping, and fiber bridging, and achieved high predictive accuracy across all 40 specimens. Overall, the findings clarify the distinct contributions of NC, SFRC, and UHPC to shear transfer mechanisms and provide a unified expression and rational guidelines for the design of precast and composite joints in advanced concrete structures.