Bond behavior of early-age concrete and deformed steel bars under unidirectional cyclic loading: experimental study and bond stress–slip model

The bond behavior between early-age concrete and deformed steel bars under unidirectional cyclic loading is of great significance for evaluating the bearing capacity of reinforced concrete structures during construction. In this study, pull-out tests were conducted on specimens of four concrete ages and three strength grades under unidirectional cyclic loading to study the bond failure mechanism. The results showed that, regardless of whether the load level was high or low, unidirectional cyclic loading affected the bond performance between the steel bar and early-age concrete, resulting in irreversible residual deformation. The bond energy increased with the strength grade and concrete age. When the concrete strength grades were the same, the peak and residual strengths increased with increasing age, and the slip at the slip point and peak strength decreased with increasing age. Taking C50 concrete as an example, when the age was increased from 1 to 28 days, the peak and residual strengths increased by 167.87% and 199.76%, respectively, whereas the slip at the slip point and peak strength decreased by 36.84% and 17.48%, respectively. For concrete with the same curing age, as the concrete strength grade increased, the peak bond strength and residual strength gradually increased, and the slip at the slip point and peak strength decreased progressively. For example, when the concrete age was 28 d, if the strength grade increased from C30 to C50, the peak bond strength and residual strength increased by 64.79% and 151.75%, respectively, and the slip at the slip point and the peak strength decreased by 14.29% and 13.24%, respectively. Based on the characteristics of the curve under unidirectional cyclic loading, a bond stress–slip model was established and compared with the test results. The results showed that the proposed models are acceptable.