Experimental Study on the Compressive Performance and Enhancement of Buckling Resistance for Composite Bars

A steel-fiber reinforced polymer (FRP) composite bar (SFCB) consists of an inner steel bar and an outer continuous FRP layer, which exhibits excellent corrosion resistance, a high elastic modulus, and customizable postyield stiffness in tension. However, its response and buckling characteristics under compression remain unclear. This paper presents an experimental study on the axial compressive properties of bare SFCB with varying equivalent slenderness ratios (λeq) and compares them with those of BFRP bars and stainless steel bars. To enhance the buckling resistance of the rebars and to solve the rebar congestion in precast members, corrugated-pipe confined (CPC) specimens with bundled reinforcement were designed and tested under compression with two pipe diameters (40 mm and 60 mm). The compressive damage of SFCB included FRP fracture, debonding between fibers, or debonding between the FRP and the inner steel bar. SFCBs with λeq less than 8 demonstrated a slight postyield stiffness, and the load capacity exhibited a slight decrease with an increase in λeq. The compressive load capacity of SFCBs was significantly lower than that under tension and could hardly exceed the yield load, with a maximum of only 34% of the tensile load. The CPC specimens could result in a higher overall load capacity than the sum of the compressive loads of the individual components as well as better axial deformation ability of the inside rebars. The maximum load capacity of SFCB inside the CPC specimens increased by approximately 100%, exceeding the yield load and demonstrating postyield stiffness characteristics, which is effective in enhancing the seismic performance of the structure. BFRP bars are sensitive to the pipe diameter of CPC specimens, and the compressive load capacity and deformation capacity in D60-CPC specimens can reach more than twice that of bare BFRP bars, while the performance of D40-CPC was significantly degraded.