Bond behavior of concrete-filled steel tubes at elevated temperatures

Extensive studies have been conducted into the bond behavior in concrete-filled steel tubes (CFST) at ambient temperature. However, the bond behavior of CFST columns subjected to fire is still unclear. A total of 24 push-out tests were conducted to investigate the bond strength of CFST columns at elevated temperatures, 12 reference specimens at ambient temperature, and 16 postfire specimens were also tested for comparison. The main test parameters explored in this test program include: steel type (carbon and stainless steels), concrete type (normal and expansive concretes), cross-section type (circular and square sections), interface type (normal interface, interface with shear studs, and interface with an internal ring), temperature level (20, 200, 400, 600, and 800°C), hold time period of heating (45, 90, 135, and 180 min), and applied axial load during heating. The experimental results indicated that the bond of specimens with normal interface could be completely broken in fire. However, welding internal rings or shear studs onto the inner surface of the steel tube can effectively retain a portion of the bond strength in fire