Bond behavior between helically and tightly wound FRP bars and concrete

The bond behavior of fiber-reinforced polymer (FRP) bars is a critical factor in the mechanical perfor-mance of FRP rebar reinforced concrete structures, but until now, there has been limited research on the effect of surface treatment and geometrical features of ribbed FRP bars on the bond behavior, and no relevant provisions are incorporated into the current main design codes. In this paper, the helically and tightly wound FRP bar was taken into consideration, and large amount of existing test data were also collected to investigate the main test parameters on the bond failure modes and bond strength. Greater emphasis was placed on the effect of geometrical features of such FRP bars on bond behavior, and the bond mechanism was clarified. Results show that with the increase of concrete strength, the weak bond damage area in the bar-concrete interlocking layer transfers gradually from interface 1 (main damage in concrete) to the interface 2 (main damage in resin fiber), and the bond strength increases accordingly. Meanwhile, the larger the concrete lug ratio CLR, the greater the increase of bond strength due to the increase of concrete strength. With the increase of relative rib height hrd and CLR, the bond damage in the bar-concrete interlocking layer becomes severer, contributing to stronger mechanical interlocking and higher bond strength. Multivariate nonlinear regression was adopted to propose a formula to estimate the bond strength of the helically and tightly wound FRP bars. The calculated results by the proposed equation are in good agreement with the test results with greater accuracy than the current design codes. This is because the effect of surface geometrical features on bond strength is accurately accounted in the proposed equation. © 2022, Editorial Office of Acta Materiae Compositae Sinica. All right reserved.