The use of fibre reinforced polymers (FRPs) as externally bonded reinforcements has gained widespread acceptance as an innovative technique for the maintenance, rehabilitation and upgrading of existing concrete structures. Significant enhancements in stiffness, strength and seismic performance of the structures can be achieved by this technique. Among extensive research efforts which have been paid worldwide, the author s research group at Ibaraki University in Japan has greatly contributed in this field. Although short-term behaviour of FRP-strengthened structures is intensively reported thus far, their long-term performance has not thoroughly elucidated yet. This paper presents the recent development in the long-term behaviour of concrete structures externally strengthened with FRP, particularly focusing on the FRP-concrete interface subjected to sustain and fatigue loads. Some suitable time-dependent models for creep and fatigue loading are also introduced. This review paper covers also a comparison between available guidelines and recommendations with the experimental results of time-dependent loading.

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.

This paper discusses experimental results of the short and long-term behavior of the anchorage zones of externally bonded prestressed fiber reinforced polymer (FRP) sheets. An experimental program was conducted to investigate seven beams bonded with prestressed FRP sheets including anchored or unanchored FRP sheet ends. Using different layers of FRP sheet, the prestress level of FRP sheets varied from 20% to 40% of the guaranteed tensile strength. The experimental observation was conducted in an outdoor environment and lasted about twenty months when temperatures were in the 7–30 ?C range. This study provides significant data on the development of the effective bonding lengths, as well as the initiation and propagation of debonding along the FRP-concrete interface due to creep effect of the adhesive layer. Although adhesive creep leads to debonding propagation at higher shear stress, this creep is favored at low shear stress because it increases the effective bond length which improves the bond capacity of FRP-concrete interface and prevents premature failure of the anchorages. The effective bonding length was found to increase to 50% due to creep of the adhesive layer. The anchored end of the FRP sheets using steel plates and anchor bolts is an effective solution to enhance the bond capacity of FRP-concrete interface for short and long-term loading.