Novel FRP-UHPC-steel double-tube columns subjected to monotonic axial load: Compressive behavior and analytical model

Steel corrosion is a major cause of structural degradation, and utilizing advanced structural materials with high strength and corrosion resistance offers a promising solution. Novel FRP-UHPC-Steel Double-Tube Columns (i.e., Double-Tube UHPC columns, DTUCs for short), comprising an outer FRP tube, an inner steel tube, an UHPC (Ultra-High-Performance Concrete) interlayer between the two tubes, and an UHPC core within the inner steel tube, represent a new composite column design ideal for highly corrosive environments. DTUCs are anticipated to offer several advantages, including: the FRP outer tube's high corrosion resistance and the UHPC's impermeability effectively shield the steel tube from corrosive substances, thereby improving corrosion protection; UHPC enables DTUCs to achieve a high load-bearing capacity with a relatively small cross-section; the FRP enhances the overall compressive behavior of the component through effective hoop confinement. This study aims to study the compressive behavior of DTUCs, examining parameters like FRP tube thickness and steel tube type, and also included companion UHPC-filled FRP tubes (UCFFTs) for direct performance comparison. The experimental results indicate that: DTUCs exhibit exceptional axial compressive properties, significantly improving the ductility and strength of UHPC; no severe steel buckling is observed, highlighting the steel tube's effective role in enhancing compressive strength and stiffness within the DTUCs; the UHPC interlayer and core are well-confined, resulting in a favorable composite effect among the three constituent materials; the behavior of DTUCs is significantly affected by FRP thickness and specifications of steel tube. This paper presents two analytical models for DTUCs: the first model, built on a simple superposition principle, disregards the steel tube's biaxial stress state, while the second model accounts for the steel tube's biaxial stress state and the dual confining effect of both tubes to UHPC core. While both models demonstrate good accuracy, the simpler superposition-based model is more practical for engineering design applications due to its simplicity.