Experimental Study on the Axial Compressive Behavior of Concrete-Filled Steel Tube Short Columns Confined by Carbon Fiber-Reinforced Polymer Composites

This research presents an experimental investigation on the compressive behavior of fiber reinforced polymer-confined concrete-filled steel tubes (FRP-CFSTs). The study evaluated 72 specimens, including CFST and FRP-CFST columns, with varying numbers of FRP layer (0–3), steel tube thickness (1.8 to 3.8 mm), and nominal concrete strength (20, 30, 40 MPa). Concrete mixes enhanced with polypropylene fibers and silica fume were used. Material properties for the infill concrete, steel tube yield strength (307 MPa), and CFRP tensile parameters (ultimate strain 2.1%, tensile strength 4900 MPa) were determined. The test specimens were wrapped with CFRP sheets using a wet lay-up process and subjected to axial compression through a 4000-kN capacity machine. The load–deformation behavior until failure, which typically occurred due to FRP rupture from lateral concrete expansion, was recorded. Results revealed that FRP confinement increased the ultimate axial load capacity of CFST columns and enhanced ductility with improvements correlating positively with the number of CFRP layers. Steel tube thickness contributed to an increase in stiffness and load capacity by roughly 15–25%. Concrete mixes incorporating polypropylene fibers and silica fume demonstrated superior performance compared to conventional mixes by reducing brittleness and improving tensile and flexural strengths. These quantitative findings demonstrate the significant influence of FRP confinement and concrete mix design on the enhancing strength, stiffness, and ductility of CFST columns, supported by rigorous experimental characterization and systematic analysis of their composite behavior.