Tensile behavior of C/GFRP-steel hybrid rebars: Effect of volume fraction and helical angle with a proposed analytical model

Abstract

This study examines the tensile properties of hybrid rebars consisting of a steel core wrapped with layers of Carbon Fiber-Reinforced Polymer (CFRP) or Glass Fiber-Reinforced Polymer (GFRP). The mechanical performance of the Steel-FRP composite bars (SFCBs) was evaluated through axial tensile tests considering two primary parameters: (i) the longitudinal FRP volume fractions (33 %, 40 %, and 47 %) and (ii) the helical wrapping angles (0 degrees, 30 degrees, and 60 degrees). Although SFCBs have gained increasing attention as an alternative to fully FRP or conventional steel reinforcement, the combined influence of fiber volume fraction and helical orientation on their tensile response has not been clearly established. Experimental findings demonstrated the influence of fiber volume fraction and helical angle on the tensile properties of SFCBs. Specimens coated with carbon exhibited greater strength than those coated with glass, especially at larger wrapping angles, while the glass-coated SFCB specimens demonstrated a wider range of deformation capability. The specimen with a 47 % volumetric fraction and a 60 degrees helical wrap exhibited a 21.8 % increase in yield stress compared with that of the steel bar, whereas its glass-fiber counterpart exhibited a 15.9 % increase. Maximum strength was significantly higher in carbon-SFCB rebars than in glass-SFCB ones. In contrast, glass-SFCB rebars showed 59 % higher ultimate strain. As the volume fraction increased, the influence of the helical angle on ultimate strength decreased. Increasing the helical angle from 0 degrees to 60 degrees enhanced the ultimate strength by up to 32 % in carbon-SFCBs and approximately 12 % in glass-SFCBs at a 33 % volume fraction. An analytical model was proposed to predict a five-zone stress-strain behavior, taking into account the effects of volumetric fraction and helical angle. The proposed model effectively replicated the stress-strain patterns of hybrid rebars in every zone with a satisfactory level of accuracy. Generally, the calculated mean absolute percentage errors for key mechanical parameters such as initial stiffness, yield stress, and fiber-contributed stiffness were below 15 %. This study offers a practical framework for designing SFCBs with customised mechanical properties for sophisticated RC applications.