Novel FRP-steel-ultra-high-performance concrete (UHPC) hybrid bars: Concept and compressive behavior

Fiber-reinforced polymer (FRP) bar has attracted extensive attention as an alternative to steel bar to eliminate the issue of steel corrosion over the past decade. However, FRP bar reinforced concrete (FRP-RC) structures may exhibit lower ductility and stiffness compared to steel reinforced concrete (steel-RC) structures due to the lower elastic modulus and linear tensile behavior of FRP bars. Furthermore, FRP bars have a much less favorable performance in compression because of the micro-buckling damage of fibers, which makes them rarely used as longitudinal reinforcement in compression. To tackle these challenges, a novel FRP-steel-ultra-high performance concrete (UHPC) hybrid bar was proposed. This novel hybrid bar consists of an outer FRP layer (with or without ribs), an inner steel tube and in-filled UHPC. In this paper, an experimental program was conducted to investigate the compressive behavior of the proposed novel hybrid bars, focusing on variables such as the thickness of FRP layer, the type of FRP layer, the thickness of steel tube and the steel fiber content in UHPC. The test results demonstrated that FRP-steel-UHPC hybrid bars generally exhibited a bilinear stress-strain behavior with good ductility and high strength under concentric compression. Existing strength models for FRP-confined UHPC-filled steel tubes were evaluated using the test results of FRP-steel-UHPC hybrid bars.