Flexural behavior of concrete beams reinforced by partially unbonded steel-FRP composite bars

Abstract

Fiber-reinforced polymers (FRPs) are linear elastic materials that are widely used to reinforce newly built bridges and strengthen existing structures due to its high strength and good durability. To avoid brittle failure, the strength utilization of FRPs in practical applications is relatively low. A steel-FRP composite bar (SFCB) is a composite bar compound of an inner steel bar and outer longitudinal FRPs; its advantages include a good initial modulus, high durability and controllable postyield stiffness. When a concrete structure is reinforced by SFCBs, due to the limited rupture strain of the SFCB’s outer FRP, the deformability of the corresponding member is often controlled by the rupture of the FRP. To improve the deformability of an SFCB beam, an experimental study on concrete beams with partially unbonded SFCBs was conducted, and the failure modes, load–deflection curves and strain distribution were investigated. The test results showed that with partial unbonding treatment of the SFCB, the failure mode involving rupture of the SFCB’s outer FRP during SFCB reinforced concrete failure can be delayed or transferred into failure due to concrete crushing after the SFCB’s inner steel bar yielded, and the corresponding deformability and energy ductility can be improved. Then, a numerical 3D model was built and validated with the test results, and a parametric study of partially unbonded SFCB beams with different parameters was carried out. The simulation results show that the partial unbonding treatment can increase the deformability of the SFCB beam (with rupture of FRP) by two times while maintaining the stability of the load carrying capacity. When the unbonded length lub is between 0.5D and 1.0D (D is the height of the concrete beam), balanced failure of the SFCB beam is more likely to occur (the postyield stiffness is between 0.20 and 0.25). An excessive unbonded length lub will prematurely lower the depth of the compression zone, and the corresponding bearing capacity will decrease. As a result, when designing an SFCB reinforced concrete beam, the postyield stiffness ratio, reinforcement ratio and unbonded length should be comprehensively considered.