Study on bond-slip between reinforcing bar and concrete during transient pull-out test under fire

The bond performance of reinforced concrete (RC) structures subjected to fire has received extensive attention due to the detrimental effects of high temperatures on the load-bearing capacity of RC structures. However, the existing understanding of bonding performance is flawed as it tends to overestimate the actual behavior. This overestimation arises from tests conducted under steady-state load excitation conditions, while the dynamic response of RC structures under fire is transient in nature. Therefore, this paper conducted eccentric pull-out tests between reinforcing bars and concrete under steady-state and transient conditions, comparing the differences in bond performance under different loading conditions. Test results indicate that the bond failure of the transient pull-out specimens is more abrupt than that of the steady-state test, and the maximum crack width observed in transient specimens reaches up to 2.93 mm. The failure time of transient specimens decreased with increasing heating rate, slip, and loading ratio. When the holding-load ratio is 40%, the bond failure temperature of transient specimen is 320 °C, and 180 °C for the specimen with 60% loading ratio. Subsequently, the failure time-slip constitutive relationship was established; the bond failure time prediction formula of transient specimens was proposed, and bonding correction coefficient was obtained based on force analysis of beam-type pull-out test specimen. Finally, the accuracy of bond failure time was verified by beam specimen's failure time prediction, and suggestions were made for the maximum holding-load ratio of RC structures during fire.