Bond behavior of epoxy-coated reinforcement in FRP-confined seawater sea-sand geopolymer concrete

The incorporation of seawater and sea sand geopolymer concrete (SWSSGC) in reinforced concrete structures may accelerate corrosion of the internal reinforcement due to the prolonged exposure to seawater and sea sand. To address this issue, epoxy-coated rebar (ECR) has been proposed to enhance corrosion resistance. Furthermore, when fiber-reinforced polymer (FRP) is used to confine the concrete, it can further mitigate the degradation of bond performance associated with ECR, while also hindering the ingress of external corrosive ions into the concrete, thereby improving its mechanical properties and durability. This study designed and prepared 72 specimens to systematically investigate the effects of rebar diameter (16, 18, 20 mm), bond length (4d, 5d, 6d), concrete strength (C40, C60), and FRP wrapping on the bond behavior of ECR-SWSSGC. Throughout the testing process, digital image correlation (DIC) technology was used to monitor bond-slip behavior. The results indicate that specimens without FRP wrapping primarily exhibited a splitting failure mode, while specimens wrapped with FRP transitioned to a more ductile splitting-pullout failure mode, significantly enhancing bond stiffness and ultimate bond strength. The increase in concrete strength positively affected the bond performance, whereas larger rebar diameters and longer bond lengths resulted in a decline in performance metrics. Based on the experimental data, an empirical prediction model considering multiple factors for bond performance was established (average R² =0.83). Additionally, using a fictitious crack model, a theoretical strength model for bond-radial stress was developed (average R = 0.79). Furthermore, in conjunction with previous research, the mechanical constitutive model for the bond-slip process under both unconfined and FRP-confined conditions was fitted, with the model predictions aligning well with the experimental curves.