Compression Behavior of Concrete Columns Reinforced with Hybrid Steel-FRP Bars with Enhanced Buckling Resistance

The steel fiber-reinforced polymer (FRP) composite bar (SFCB) is a novel rebar that consists of an inner steel bar and an outer continuous FRP layer. Its configuration enhances durability and provides a stable positive postyield stiffness, which can improve the seismic performance of SFCB-reinforced concrete structures. The buckling resistance of rebars inside concrete columns serves as a crucial safeguard against the collapse of structures under the ultimate limit state. Inspired by the connection in precast concrete structures, a corrugated pipe-confined (CPC) component was designed in this study to enhance the buckling resistance of rebars, which consisted of bundled reinforcement, high-strength grout, and corrugated pipes. Eighteen concrete columns were exploratively tested under axial compression to investigate the responses of rebars inside concrete [SFCBs, basalt FRP (BFRP) bars, and stainless-steel bars], as well as the effect of bundled reinforcement and the CPC component on the buckling resistance. It was found that the maximum lateral deformation of the SFCBs and stainless-steel bars occurred in the middle position between stirrups, whereas the BFRP bars fractured near the stirrups. The external rebars of three- and four-bar bundled reinforcement could prevent the internal rebars from buckling. The CPC specimens displayed double peaks in load-displacement curves due to the function of the CPC components after the weakening of the surrounding concrete. The use of the CPC components could increase the load-carrying capacity of SFCB- and BFRP-reinforced columns by up to 13.9% and 41.6%, and the deformation ability from the peak load to failure could be improved by 73% and 209% at most. Furthermore, the reduction in rebar strain was decelerated, indicating that CPC components can significantly enhance the buckling resistance of rebars and improve the overall performance of reinforced concrete columns. Among the rebars utilized in the test, BFRP bars contributed the least to the peak load of the specimen, accounting for 12%, while SFCBs with similar ultimate tensile load contributed the most by 34%, suggesting a higher compressive efficiency of SFCBs as reinforcement in normal service conditions.