Decoupling Method for Steel Tube–Concrete Interface Force in Low-Temperature and Freeze–Thaw Conditions: Normal Bond Performance

Concrete-filled steel tubes have broad applications in cold regions, and their interfacial performance is the foundation for the composite effect. However, due to the complex interactions among multiple factors in cold regions, there is currently no effective method for decoupling interfacial forces in low-temperature environments, making their quantitative evaluation difficult. In this paper, a decoupling method for steel tube–concrete interface force components in cold regions is proposed, and the first part, normal bond performance, is highlighted in detail to decouple the chemical adhesion and adfreeze stress. An interfacial normal bond test method was developed to address the previous problems of alignment deviation and accidental damage. Using this test method, 15 interfacial bond specimens were tested, covering 300 freeze–thaw (FT) cycles and different sustained temperature conditions ranging from 20°C to –60°C. The influence of low temperatures and FT cycles on the normal bond strength was analyzed. The results show that, as the temperature decreases from 20°C to –60°C, the bond strength increases by 2.45 times, and the failure mode changes from interface separation to concrete failure. The bond strength after 300 FT cycles decreases by 73% and reaches an extremely low level (<0.05 MPa). The mechanism of variation in each component of the interfacial force in low-temperature environments is revealed. Low temperature generates adfreeze stress and damages the chemical adhesion, yet this damage only manifests at room temperature due to ice filling. Finally, a calculation method for the interfacial normal bond strength in low-temperature environments is proposed to decouple the chemical adhesion and adfreeze strength with good accuracy.