Interfacial Performance of Concrete-Filled Double-Skin Steel Tubes in Low Temperatures and Freeze–Thaw Conditions

A concrete-filled double skin steel tube (CFDST) boasts performance advantages in cold regions by virtue of its composite effects and reduced cast-in-place concrete. Its interfacial performance is the foundation for the composite effect, yet effective methods to clarify and quantify low-temperature influence remain lacking. In this paper, a decoupling method for interfacial force components of the CFDST in cold regions has been proposed. Cyclic pushout tests under low-temperature environments were designed and conducted on its outer and inner interfaces, covering 20°C to –60°C different sustained temperature and 0–300 freeze–thaw (FT) cycles. Their influences on interfacial forces were systematically analyzed via interfacial failure modes, bond stress–relative slip curves, cyclic loading curves, and interfacial normal pressure. Inner interfacial forces are generally higher than outer interfacial forces, while the low-temperature influence on the two are opposite. Then, the evolution mechanism of each component throughout the entire freezing and thawing processes was revealed. The nonuniform thermal deformation, frost heaving microcracks, and enhanced bonding effect due to ice, respectively, affected each component at different stages of the interfacial slip. The chemical adhesion and freezing force were the first to deteriorate, then the micro-interlocking was gradually damaged, and finally, the bond stress was dominated by the friction. Low-temperature-induced changes in interfacial normal pressure had a greater impact than the generation of freezing force. Finally, a practical design method for the outer and inner interfacial forces under low temperatures and after FT cycles was proposed based on interfacial normal pressures, which could effectively ensure the composite action of the CFDST in cold regions.