Post-fire load-reversed push-out performance of normal and lightweight concrete-filled steel tube columns: Experiments and predictions

In this study, the push-out experiment with load reversal and up to four load half-cycles was conducted on concrete-filled steel tube (CFST) members to investigate parameters affecting the natural bond capacity (chemical adhesion, micro-locking, and macro-locking) and bond-slip curves before and after heat-induced damage. Variables under study included the exposure temperature, length-to-diameter ratio of steel tube (L/ D), diameter to thickness ratio of steel tube (D/t), grade of cementitious materials, and type of concrete (pumice lightweight concrete and normal concrete). The results showed that exposure to heat often had a negative effect on chemical cohesion and friction between the steel tube and concrete core. In this regard, after exposure to 200, 400, and 600 degrees C, the bond capacity of the lightweight CFST specimens respectively remained relatively un-changed (negligible increase or decrease) and declined by 84 and 94% compared with that at the ambient temperature. An increase in parameter L/D due to an increase in length and a decrease in diameter led to a drop and a rise in the bond capacity, respectively. As the D/t ratio increased (due to a rise in D or a drop in t) in the lightweight CFST specimens, the bond capacity declined after exposure to all the target temperatures. In addi-tion, lightweight concrete specimens with a smaller cement grade showed greater bond capacities compared with the corresponding columns with a higher cement grade for most exposure temperatures. Moreover, the bond capacity of the specimen with normal concrete was significantly higher than the corresponding specimen with lightweight concrete (around 8 times) for the exposure temperature of 600 degrees C. By comparing half-cycles 1 and 3 of CFST specimens, the share of macro-locking was obtained as 12 and 34% for 600 degrees C. The initial stiffness and energy absorption of the specimens with the lightweight concrete experience a significant drop after exposure to 600 degrees C compared with the ambient temperature; however, an inverse trend was seen for the specimens with normal concrete. Finally, two simple models with proper accuracy were proposed to predict the normalized bond capacity of the CFTS specimens with lightweight and normal concrete in terms of temperature.

Automated summary

In this study, a push-out experiment was conducted on CFST members to investigate the parameters affecting the natural bond capacity and bond-slip curves. The results showed that exposure to heat had a negative effect on chemical cohesion and friction between the steel tube and concrete core. The bond capacity was influenced by variables such as exposure temperature, length-to-diameter ratio of steel tube, diameter to thickness ratio of steel tube, grade of cementitious materials, and type of concrete.