Study on the bond performance of glass fiber-reinforced polymer bars considering the relative position between longitudinal bars and stirrups

Fiber-reinforced polymer (FRP) reinforcements are increasingly recognized as an effective solu-tion to the corrosion problems faced by reinforced concrete structures, and the bond between FRP bars and concrete is an essential basis for the application. Previous research has shown that stirrup restraints on the concrete can significantly enhance the bond performance of middle -arranged longitudinal FRP bars. However, in practical applications, longitudinal bars are typi-cally placed at the corners or sides of stirrups to form reinforcement cages, and there has been limited research on the effects of stirrups on the bond performance of FRP bars under these conditions. To address this gap, this study attempts to further reveal the stirrup effects by varying the relative positions between the longitudinal bars and stirrups. Based on forty-four axial pull-out tests on glass fiber-reinforced polymer (GFRP) bars equipped with Distributed Optical Fiber Sensors (DOFS), the overall and local bond performances of three GFRP bars (sand-coated, helical wound, single helical wrapped) considering four relative positions between the GFRP bars and stirrups (no stirrups, side, middle, corner) were investigated. The test results indicated that the bond strength of FRP bars increased significantly, up to 71.5%, for the relative position of the middle case, but could be reduced by up to 17.5% for the side and corner positions compared with the no-stirrup situation. DOFS proved to be an effective method to obtain the continuous strain distribution and reveal the interfacial bond mechanism. Based on DOFS results, a bond stress-slip model considering the effect of relative positions was developed. Using this model, the devel-opment lengths of GFRP bars were proposed and compared with the ACI, CSA, and JSCE codes.