GFRP as Interface Shear Reinforcement in Concrete Cold Joints

Interface shear reinforcement plays a critical role in ensuring load transfer across concrete cold joints. Although the increasing use of glass fiber–reinforced polymer (GFRP) bars in structural applications, their application as interface shear reinforcement remains not yet comprehensively addressed in current design provisions. This study investigates the performance of GFRP reinforcement in cold-jointed reinforced concrete (RC) through an experimental program comprising 18 push-off specimens, varying the interface condition (intentionally roughened, as-cast, and smooth), reinforcement ratio (0.48%–2.87%), reinforcement shape (closed stirrups, U-shaped bars, and straight bars), and embedment length from the interface plane. Results show that at low reinforcement ratios (≤0.53%), concrete strength minimally affected the cracking load and the peak load but enhanced postpeak resistance. At higher reinforcement ratios, shear capacity gains were evident; for instance, increasing compressive strength from 35 to 59 MPa enhanced capacity by 27% at a 1.05% reinforcement ratio. Anchorage details significantly influenced postpeak behavior; closed stirrups and U-shaped bars maintained residual resistance, while straight bars with embedment lengths of 6–9 times the bar diameter failed abruptly at approximately 85% of the capacity of stirrup-RC specimens. Extending embedment to 12 times the bar diameter achieved a similar peak capacity but with reduced postpeak deformability. Interface roughening markedly increased load capacity and reduced slip and crack opening compared to smooth joints, whereas roughening from the as-cast condition primarily reduced slip and crack opening without substantially enhancing load capacity. A regression analysis further produced an improved predictive model for the interface shear resistance of GFRP-RC cold joints.