Bond of steel-FRP composite bar embedded in FRP-confined concrete: Behavior, mechanism, and strength model

Abstract

In the steel-FRP composite bar (SFCB) reinforced concrete filled in an outer FRP tube, the bond behavior of SFCB in such FRP-confined concrete should differ from that in unconfined concrete. This paper aimed to understand the local bond behavior of SFCB in FRP-confined concrete. Sixty pullout specimens with variables (including the concrete strength, concrete cover-to-bar diameter ratio, and FRP confinement stiffness) were tested. The test results showed that the bond failure mode of SFCB was changed from concrete splitting in unconfined-concrete to pull-out in FRP-confined concrete. The study revealed that the development of FRP confinement promoted significant bond growth after concrete splitting and meaningful post-failure ductility. Greater FRP confinement stiffness resulted in the more pronounced the bond strength enhancement. In addition, a bond-confinement relationship at the bond interface was developed. Accordingly, the enhancement mechanism of SFCB in FRP-confined concrete is clarified: 1) FRP confinement substituted the cracked concrete in resisting to the wedge-action of SFCB; 2) FRP confinement improved the strength and deformability of interface concrete in resisting the shear-action of SFCB; 3) FRP confinement improved the friction. Finally, a bond strength model with a simple form was proposed, which can provide an accurate prediction of the bond strength of SFCB in FRP-confined concrete based on a confinement factor and the existing model in unconfined concrete.