Thermal Debonding Of Concrete-filled Steel Tube Members

Concrete filled steel tube (CFST) composite members have been widely used in engineering structures. This is because of the favorable structural interaction between the steel tube and the concrete core. However, normal debonding, especially thermal debonding, it is recently found that this debonding is a common phenomenon in CFST arch bridges, between the steel tube and the concrete core degrades this interaction. In order to fully understand the debonding and its influence, it is necessary to know the normal bonding strength between the steel-concrete interface. However, literature review shows that there is no consensus on the bonding strength and its influencing factors. Therefore, this research aims to obtain the normal interfacial bonding strength and the temperature distribution in the cross section of the CFST members exposed to natural and fire environments.The bonding strengths were measured by direct tension tests and the third point flexural tests.Test results show that the bonding strength is affected by both the composition and curing condition of concrete. Hence,a regressive formulation for the bonding strength is obtained statistically.With the bonding strength obtained, together with the local temperature data and solar radiation intensity in the location of the Ganhaizi Bridge, the radial tensile stresses along the interface CFST rib are analyzed using the commercial software ANSYS. It is shown that in summer, under intensive solar radiation at noon, the magnitude of interfacial tensile stresses exceeds the bonding strength,indicating the happening of thermal debonding. Further study focuses on the interfacial tensile stresses of CFST members in fire. FE analysis show that thermal debonding of CFST members will definitely happen in fire.An FE model based on ANSYS is developed to model the debonding and the temperature distribution after debonding in CFST members. The accuracy of this model is validated by test data.Parametric study shows that the temperature difference in the cross section of CFST members increases with magnitude of the debonding gap. In fire conditions, the influence of debonding is most significant at around 30-60min of fire exposure.In bridge engineering, most of the CFST arch bridges are closed at a relatively lower temperature at night. This increases the debonding tendency. Therefore, it is suggested that low absorptive color of coating be used to minimize the thermal debonding. Also, a small diameter-thickness ratio helps to lower the interfacial stresses so as to mitigate thermal debonding.