Experimental investigations of FRP-interlayer-steel confined concrete columns under axial compression

A fibre-reinforced polymer (FRP)-interlayer-steel confined concrete (FISC) column is proposed in this study to improve the corrosion resistance and bearing capacity of concrete-filled steel tube (CFST) structures. In FISC columns, grouting material is used to bond the FRP tube and steel tube to produce a dual-confined composite skin for the core concrete. To establish the performance levels of FISC columns, 35 specimens in two batches of four types of columns, including FISC columns, FRP-confined concrete-filled steel tubular columns, concrete-filled FRP tubular columns, and CFST structures were used. The specimens were tested under axial compression, and the FRP types (glass- and carbon-fibre-reinforced polymer), FRP thickness, and steel thickness were the main parameters examined in the tests. Failure patterns, load–axial shortening curves, and strain development were observed, and the dual confining effect of FRP tube and steel tube on core concrete was discussed. The test results showed that the FRP tube and grouting material suppressed the outward buckling of a steel tube, and the load-bearing capacity of the core CFST was substantially enhanced. In addition, owing to the existence of an interlayer, a strain lag was observed on the strain transfer from the steel tube to the FRP tube, effectively delaying the cracking of the resin on the FRP tube. Furthermore, the bearing capacity and ultimate axial deformation of FISC columns increased linearly with the addition of FRP thickness but had little changes when the thickness of the steel tube differed.