Fatigue bond behavior of FRP-to-concrete joints with various bonding adhesives

Bond degradation due to cyclic or fatigue loading has significant negative impacts on the service life of fiber-reinforced polymer (FRP) strengthened concrete structures. Existing studies have shown that the debonding failure of FRP-to-concrete joints under fatigue loading mainly occurs in the adhesive layer or the adhesive-concrete interfacial layer, and the fatigue bond behavior varies significantly among specimens with different bonding adhesives. This study evaluates the fatigue behavior of FRP-to-concrete joints with various bonding adhesives through experimental tests and numerical simulations. First, fatigue testing was conducted on the FRP-to-concrete double-lap shear specimens with various bonding adhesives (including soft, normal, stiff, and liquid rubber-modified epoxy adhesives). According to the test results, specimens with soft adhesive exhibited higher interfacial fatigue lives and superior fatigue bond behaviors than their normal adhesive counterparts. In comparison, stiff adhesives resulted in much lower interfacial fatigue lives and unsatisfactory failure modes. The use of liquid rubber-modified epoxy contributed to enhancing the bond performance under fatigue loading. Then, a fatigue bond-slip model considering the degradation of bond-slip stiffness was proposed, and the corresponding interfacial fatigue damage was quantified. On this basis, a simplified finite element (FE) model was developed and verified by the test results. The influence of adhesive properties on the fatigue bond behavior of FRP-to-concrete joints was investigated further using FE modeling.