Push-out test on the interface bond-slip behavior and calculation on bond strength between steel tube and recycled aggregate concrete in RACFST structures

To reveal the interaction mechanism of bond-slip performance at the interface of recycled aggregate concrete (RAC)-filled steel tubes (RACFST), the concrete with accomplished service of full 50 years was used as a source of recycled coarse aggregate (RCA), and a total of 25 RACFST short column specimens were designed for the push-out test. Five parameters, including the shape of cross-section, replacement rate of RCA, grade of concrete strength, ratio of length to diameter and interface embedment length, were considered for their influences on the bond-slip behaviors of RACFST. The load-slip curves, distribution laws of bond strain and bond load for these specimens were obtained, and the calculation formula of bond shear transfer length was derived. For the purpose of comparison, practical formulas of bond strength in concrete filled steel tube (CFST) were used to calculate the bond strength of RACFST. The results show that there is a similarity in the whole process curves of load-slip between the loading end and free end, at which the initial slip develops more slowly. The longitudinal strain of steel tube in the early and late loading exhibits a negative exponential distribution and a linear distribution, respectively. The measured results are larger than the values of nominal bond strength. By comparison, the bond performances of circular specimens are superior to those square specimens. With the replacement rate, the bond strength of circular specimens may fluctuate to a certain degree, while the bond strength of square specimens may increase slightly. When the strength grade of concrete is high, its ultimate strength can be improved. Along with the increase of the ratio of length to diameter, the bond strength of RACFST may be reduced to some extent.