Punching shear of two-way RC slabs strengthened with NSM-BFRP and NSM-CFRP: experimental and analytical study

Punching shear failure in flat slabs is a sudden failure, brittle mechanism that can trigger progressive structural collapse, highlighting the urgent need for reliable strengthening solutions. This study experimentally and analytically investigates the punching shear behavior of two-way RC slabs strengthened using Near-Surface Mounted (NSM) Carbon Fiber-Reinforced Polymer (CFRP) strips and Basalt Fiber-Reinforced Polymer (BFRP) bars. Seven slabs measuring 1500 × 1500 × 150 mm, examining two key parameters: strengthening material type and layout configuration (radial, 2D, and 3D concentric arrangements). Results confirmed that both NSM systems significantly improved punching shear performance. The CFRP 2D and 3D configurations achieved the highest load capacity increases of 20.8% and 22.7% respectively, while the radial BFRP configuration yielded a 21.1% improvement. CFRP systems demonstrated superior stiffness, whereas BFRP systems offered greater ductility and energy absorption. Strengthened slabs also exhibited delayed crack propagation and more ductile failure modes. Analytically, the El-Gamal model provided the closest agreement with experimental results at an average ratio of 1.11, while ACI and EN standards offered conservative predictions. The JSCE-97 model underestimated capacity by neglecting bond-slip and confinement effects. Overall, NSM-CFRP and NSM-BFRP techniques prove effective and practical for enhancing the strength, stiffness, and ductility of RC slabs in structural rehabilitation applications.