Size Effect on Direct Shear Resistance of Monolithic Cast Ultrahigh-Performance Fiber-Reinforced Concrete Components

Effective shear stress transmission through weak shear planes is essential for realizing the full load-bearing potential of ultrahigh-performance fiber-reinforced concrete (UHPFRC) structures. This study investigates the underexplored size effects that govern the direct shear behavior of monolithic cast UHPFRC components through both experimental and theoretical analyses. A systematic experimental program involving 34 Z-shaped UHPFRC specimens was designed to evaluate the effects of critical parameters, including shear plane height and thickness, steel fiber content, and interfacial groove configurations. The results demonstrated that the shear stiffness increases proportionally with the decrease of the shear plane height, regardless of the presence of shear plane grooves. For every 1.0% increase in steel fiber content, the shear resistance of UHPFRC specimens showed an increase of 3.1–6.9 MPa. As the shear plane height increases from 50 to 450 mm, the strength of the intact specimen without grooves on the shear plane decreases by 24.5%, and that of slotted specimen with grooves on the shear plane decreases by 28.4%. Thickened shear planes (300 mm) exacerbate stress heterogeneity, reducing direct shear strength by 24.1% compared with thinner configurations (200 mm). The experimental insights are synthesized into a prediction equation for direct shear capacity that incorporates size effects, groove geometry, and the mesomechanical constitutive of UHPFRC, achieving prediction errors below 10% compared with experimental data. This work provides a mechanistic framework for optimizing the design of shear-critical UHPFRC components.