Interfacial bond strength of SCFST composite columns after freeze-thaw cycles

This paper investigates the effect of freeze-thaw cycles (FTCs) on the interfacial bond strength of square concrete-filled steel tube (SCFST) composite columns through push-out tests on specimens subjected to FTCs. Key factors considered include the number of FTCs, the wall thickness of the square steel tube, and the interface length. By quantitatively analyzing components of interfacial bonding , the mechanisms underlying the reduction in bond stress after FTCs were elucidated. Results showed that interface concrete in specimens after FTCs exhibited more severe fracture failure under load. As the number of FTCs increased, there was a notable decline in bond failure load, interfacial bond strength, and slip values. Increasing the wall thickness of the steel tube mitigated the effects of FTCs on interfacial bond strength by stabilizing temperature fluctuations of the interfacial concrete and reducing the formation of frost cracks. Furthermore, extending the interface length exacerbated the impact of FTCs on chemical bonding force, yet it mitigated the effect of FTCs on overall bonding performance. Considering changes in bond stress after FTCs under various design parameters, three correlation coefficients were introduced, leading to precise formulas for calculating the interfacial bond stress of SCFST composite columns after FTCs. The calculation results showed that the maximum error did not exceed 6 %. Additionally, the critical shear transfer length of SCFST composite columns after FTCs was determined based on calculated results. These findings provide important recommendations for the safe application of SCFST composite columns in cold regions. • The bonding properties of SCFST after FTCs were studied. • The whole process was analyzed from the microscopic point of view. 3.The formula for calculating bond stress was presented.