Combined effects of wetting–drying cycles and sustained load on the behaviour of FRP-strengthened RC beams

This paper presents the results of an experimental study designed to investigate the performance of reinforced concrete (RC) beams strengthened with fibre-reinforced polymer (FRP) sheets subjected to wetting–drying cycles and a sustained load. Twenty-one RC beams were manufactured and externally bonded with carbon-FRP (CFRP) or glass-FRP (GFRP) sheets. Two U-shaped FRP anchorages were also applied on the ends of the beams to prevent end peeling failure. The FRP-strengthened beams were subjected to combined wetting–drying cycles in 5% NaCl solution and sustained loads for 0, 90, 180, and 360 days at load levels of 0%, 30%, and 60% of the ultimate loading capacity of the unstrengthened beam. During this period, the mid-span deflections and cracks of the beams were monitored, following which the beams were monotonically loaded to failure under a four-point bending. The failure patterns, mid-span deflections, and debonding loads were also examined. The test results showed that during sustained loading, the time-dependent and residual deflections of the FRP-strengthened RC beams increased with the increase in their periods of sustained loading and load levels. Intermediate crack-induced (IC) debonding of the CFRP-strengthened RC beams was induced by concrete separation and CFRP–concrete interface debonding, whereas that of the GFRP-strengthened RC beams was mainly caused by GFRP–concrete interface debonding. Combined wetting–drying cycles and a sustained load had an adverse effect on the yielding and debonding load of the FRP-strengthened RC beams, and the ductility of the GFRP-strengthened RC beams was higher than that of the CFRP-strengthened beams. In addition, a model considering the effects of wetting–drying cycles and sustained loads was proposed to predict the debonding load of FRP-strengthened RC beams, and it showed a reliable prediction.