This paper presents an analytical solution for the evolution and distribution of shear stresses along the entire bond length of FRP-concrete interfaces due to mode-II fatigue loading. The creep-fatigue interaction and fatigue crack growth after debonding initiation are incorporated into a nonlinear interfacial constitutive law. While the creep-fatigue interaction is represented by the degradation of the interfacial stiffness, the debond growth is governed by a form of the Paris equation and the fracture energy ratio, Gmax/Gc. Furthermore, a new form of energy ratio is adopted to be debond-dependent. Through a series of experimental double-lap shear specimens, the results showed that the debond growth rate (da/dN) along the FRP-concrete interfaces diminishes with fatigue cycles and that 30% of the static bond capacity of the FRP-concrete interface can be considered as the endurance limit of fatigue loading for FRP-strengthened beams. The agreement between the theoretical predictions and experimental results is valid, with a good degree of accuracy.

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This paper presents recent developments in long-term behavior of fiber reinforced polymer (FRP) composites and FRP-strengthened concrete beams with special emphasis on the FRP-concrete interface subjected to creep and fatigue loads. Although short-term behavior of FRP-strengthened structures is intensively reported thus far, their long-term performance has not been thoroughly elucidated yet. This state-of-the-art paper provides a synthesis of recent findings on the long-term performance of FRP composites and their time-dependent bond behavior for strengthening concrete structures, including externally-bonded FRP sheets and near-surface-mounted FRP strips. The review examines the bond-stress slip response, damage accumulation associated with progressive crack propagation, and failure modes, including descriptions of the predictive models. The identified research needs to further advance FRP-strengthening technologies are addressed.

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