Bond strength in full-scale concrete-filled steel tubular columns

This paper studies the bond strength between the steel tube and core concrete in full-scale concrete-filled steel tubes (CFST). A series of push-out tests were carried out to measure the bond stress versus slip curves of circular CFST specimens with a diameter of 400 mm and square CFST specimens with a width of 600 mm. The primary test parameters include steel type (stainless and carbon steels), concrete type (normal and expansive concretes), concrete age (28 days and about 3 years) and interface type (normal interface, interface with shear studs and interface with an internal ring). The experimental results indicate that CFST specimens with stainless steel tubes have lower bond strength compared with the CFST specimens with carbon steel tubes, and the bond strength decreases remarkably with increasing concrete age. It is also found that welding internal rings onto the inner surface of the steel tube is the most effective method in enhancing the bond strength in CFST specimens, followed by the method of welding shear studs onto the inner surface of the steel tube. Although using expansive concrete can also enhance the bond strength, this method is less reliable to be used in practice.