Tests on the steel-concrete bond strength in steel reinforced concrete (SRC) columns after fire exposure

Bond behaviour between the steel and concrete is important in ensuring the interaction of different components in steel reinforced concrete (SRC) columns under various working conditions, and sufficient bond strength should be kept in these columns even after fire exposure. This paper presents the experimental results of bond strength between the embedded steel and concrete in SRC columns after fire exposure. A total of eight push-out specimens were prepared, including two unheated and six fire exposed specimens. The fire exposed specimens were heated and left cooling down in a fire furnace, and then were tested for residual bond strength. Concrete explosive spalling was found when the test specimens were heated. The elevated temperatures at the steel–concrete interface and at the location of stirrups were recorded by thermocouples. Effects of different parameters, such as fire scenario, lateral confinement, and encased steel type, on the bond behaviour were analysed. Time delay was found in the temperature increase across the concrete section, and the inner maximum temperature reached at a measurement location was 430°C during the cooling phase, which was much higher than the temperature of 180°C measured at the end of heating phase for the same location. The reduction in bond strength was as high as 54.2%, whereas the corresponding ultimate slip increased significantly from around 0.3 mm to over 4 mm due to fire exposure. An existing model was revised to predict bond stress-slip curves for SRC columns after fire exposure, and a reasonably good agreement was obtained between the measured and predicted results.