Bond behaviour of CFRP/steel strap joints exposed to overloading fatigue and wetting/drying cycles

Carbon fibre reinforced polymer (CFRP) has been widely used in strengthening of deficient steel structures in subtropical regions following their application in concrete structure rehabilitation. However, after the strengthening, both hygrothermal conditions and overloading fatigue can further promote the mechanical degradation of the strengthened structures, and the tendency remains unclear. Therefore, to predict the service life of steel structures after CFRP strengthening, the bond behaviour of CFRP-strengthened steel plates that are affected by overloading fatigue damage and/or wetting/drying cycles (WDCs) with 3.5 wt% NaCl solution was investigated in this study. In total, 12 CFRP/steel strap joints were tested in six cases, where six specimens experienced overloading damage by axial fatigue tensile loading. Then, both specimens with and without pre damage were exposed to WDCs (0, 90 and 180). After WDC exposure, the tensile tests were conducted. The load strain curves, tensile-bearing capacities, failure modes and interfacial stress distributions were obtained. As observed, with increasing WDCs, the strength of the overloading fatigue-damaged specimens continuously decreased. However, the strength of the specimens without overloading damage decreased during the first 90 WDCs and then slightly increased during the next 90 WDCs. In addition, a simple expression of the maximum interfacial principle stress at the gap location of the CFRP/steel strap joints was developed. A model based on the maximum stress was proposed to evaluate the long-term bond strength degradation of CFRP-strengthened steel structures under overloading fatigue and/or WDC conditions, and good consistency was reached with the test results.