Bond-slip law for the analysis of ribbed steel-FRP composite bars embedded into FRP-confined concrete

Bond behaviour of composite bars whose steel core is enveloped by a ribbed FRP layer is investigated when the bars are placed inside a concrete member uniformly confined by outer FRP wrappings. The test results show that passive confinement – by resisting concrete transverse dilation induced by bar slip – brings in a change in bond failure mode, from splitting to pull-out (both failure modes being controlled by concrete), and even to bar damage (failure mode controlled by the reinforcement). Furthermore, moderate levels of FRP confinement stiffness (0.56 − 0.85 GPa) are advantageous for both bond ductility and strength, while high confinement levels may lead to a brittle bond behaviour. Existing bond-slip laws are examined with the focus on their limitations when they are used to model the effect of FRP confinement on bond. A refined 3D rib-scale FE model is developed to accurately describe bond static and kinematic behaviour at the local level in the case of ribbed composite bars embedded into a concrete member uniformly confined via FRP wrappings. To optimize bond performance, a regular bond-slip law is formulated for ribbed composite bars, taking into account such parameters as concrete strength, cover-to-bar diameter ratio, bar-rib spacing, and the level of confinement. • Bond failure mechanism of ribbed steel-FRP composite bars in FRP-confined concrete is elucidated. • A rib-scale FE model characterizes ribbed steel-FRP composite bar bonding. • A continuous bond-slip law is formulated for ribbed steel-FRP composite bars in FRP confined concrete.