Bond behavior in concrete-filled steel tubes

In this paper, the bond behavior between the steel tube and the concrete in concrete-filled steel tubes (CFST) is investigated. A series of push-out tests on circular and square CFST specimens were conducted, and the main parameters considered in the test program were: (a) cross-sectional dimension (120–600mm); (b) steel type (carbon and stainless steels); (c) concrete type (normal, recycled aggregate and expansive concretes); (d) concrete age (31–1176days); and (e) interface type (normal interface, interface with shear studs and interface with an internal ring). The experimental results indicate that stainless steel CFST columns have lower bond strength compared with carbon steel counterparts, and the bond strength decreases remarkably with increasing cross-sectional dimension and concrete age. To enhance the bond strength, welding internal ring(s) onto the inner surface of the steel tube is the most effective method, followed by the methods of welding shear studs and using expansive concrete. •CFST columns have lower bond strength when using stainless steel.•Bond strength decreases with increasing section size and concrete age.•Specified design bond strengths in current design codes may not be safe.•Welding internal rings and shear studs is reliable in improving bond strength.