Refined simulation of bond-slip behavior between ribbed steel and concrete: Effects of initial stress and temperature

The bond performance between steel and concrete exposed to fire is influenced by many factors, including the presence of an initial load before the temperature increase. To investigate such effect, a 3-D refined simulation model is presented accounting for the heterogeneity of concrete at the mesoscopic level and the external geometry of ribbed steel bars. The rationality and validity of the refined simulation model and of the multi-step analysis were verified by comparison with existing experimental data. Subsequently, the effects of aggregate gradation and distribution, initial loading level and temperature on the bond behavior were analyzed. The load transfer mechanism at the steel–concrete interface as a function of temperature under different loading levels is captured and discussed. It is found that the influence of aggregate grading and distribution on the bond-slip curve can be ignored under the aggregate volume fraction set in this work. Additionally, results show that the damage initially concentrates at the rib tip of the deformed steel bars, with an increase across the concrete section as the slip increases, as it could be expected. However, the bond stress and both stress and strain of steel at the same position decrease as the temperature increases. Finally and most importantly, it is found that the heating and loading sequence induce a different damage evolution inside the specimen, with a significant effect on the bond stiffness, the plateau portion of the bond stress-slip curve and the bond strength associated to a given critical temperature.