Bond behaviour between steel bar and concrete under sustained load and dry–wet cycles

Abstract

Reinforced-concrete (RC) structures are inevitably subjected to sustained loads during their service life. In the natural environment, the exposed surfaces of RC members often experience dry–wet cycles due to rainfall and tides. An important parameter in structural design is the the bond behaviour between steel bars and concrete, and this is affected by the coupled effect of sustained loads and dry–wet cycles. In this study, 32 RC beam specimens were subjected to different levels of sustained load (25%, 50% and 75% of ultimate load) and dry–wet cycles for a period of 120 d. The immediate slip and time-dependent slip (TDS) between the steel bar and concrete during testing were recorded and analysed. It was found that the TDS increased with an increase in the duration of sustained loading and greater sustained load levels resulted in larger TDSs. After long-term testing, the bond strength and bond stress–strain were investigated using a flexural-bond test. The results showed that bond strength increased due to the coupled effects of sustained load and dry–wet cycles. Based on the test results, an empirical model for steel bar–concrete TDS considering the effect of sustained load and dry–wet cycles was developed.