Modeling of mechanical bonding between concrete and reinforcing steel bars under elevated temperatures

Abstract

A mechanical bond model capable of characterizing the bonding behavior of reinforced concrete structures under elevated temperatures is proposed in the current study. The proposed model is developed based on the smear crack theory and the thick-wall cylinder theory by considering the concrete cover in its partially cracked elastic stage. The relationship between the splitting resistance and the inner crack radius of the concrete cover at the elevated temperature is established by taking into account the variation of the material properties with temperature and the differential thermal expansion of the steel rebar and the concrete cover. The proposed model is verified by using previous experimental results on the splitting bond strength of reinforced concrete pull-out specimens at normal and elevated temperatures. Furthermore, to investigate the bonding effect on the behavior of reinforced concrete structures, the relationship between the bond-slip curve and the proposed model is also established based on previous experimental results. A series of load-bearing tests is conducted for reinforced concrete beams with 25-mm and 40-mm concrete covers subjected to elevated temperatures. The test data of the reinforced concrete beams in terms of the load-deflection relationship and the crack pattern are compared with the results obtained from the finite-element analyses assuming two types of bonding: perfect bonding and slip bonding (the proposed model). It is found that the FE models with slip bond are capable of predicting the tensile splitting cracks in the beam specimens which is consistent with the experimental results whereas the FE models with perfect bond tend to overestimate the bond stresses. Based on the results of the current study, the proposed model can be used to assess the bonding degradation and the tensile splitting cracks as well as their influences upon the behavior of reinforced concrete beams at elevated temperatures.