Shear Behavior of Reinforced Concrete Diaphragms Strengthened with FRP

This study investigates the shear behavior of reinforced concrete diaphragms strengthened with externally bonded fiber-reinforced polymer (FRP) composites. Conventional diaphragm retrofits, such as concrete overlays, steel trusses, reinforced collectors, or additional shear walls, are often costly, time-consuming, and increase seismic mass. FRP retrofits offer a more efficient alternative with a high strength-to-weight ratio, chemical inertness, cost-effectiveness, and minimized downtime. This research aims to develop experimentally validated design guidance for FRP retrofits of reinforced concrete diaphragms to increase shear resistance. Six cantilever diaphragm specimens, approximately 2.6×3.0×0.1 m, were subjected to reversed cyclic loading. Five specimens were retrofitted to have additional shear strength using various FRP configurations, while one served as a baseline. The test results were compared to understand the effect of varying FRP parameters, e.g., composite modulus, coverage, sheet orientation, and FRP anchor layout, on response parameters such as stiffness, FRP strains, strength and ductility. All retrofitted specimens exhibited similar behavior up to a peak load that was on average 27% greater than the control specimen, with peak strength governed by FRP debonding initiated at diagonal shear cracks. Variations in composite modulus, sheet width, spacing, and anchorage did not affect response up to peak load, provided the retrofits maintained constant ply stiffness. End anchorage promoted load path redundancy after debonding, allowing sustained load levels after the peak. However, intermediate FRP anchors, while not significantly impacting peak shear strength, led to localized FRP rupture and reduced deformation capacity, suggesting that they should be designed to fail before the FRP sheet. Measured FRP strains at peak load were generally consistent with backcalculated values based on peak shear strength and averaged 20% higher than the design shear debonding strain from ACI PRC-440.2R.