Influence of high level of cyclic loading on the bond strength of the FRP-concrete interface

It is well known that unpredicted failure modes (FRP delamination, shear failure, etc.) may significantly reduce the theoretical bearing capacities of FRP-strengthened R/C beams when the retrofit intervention is designed with respect to ultimate bending moment only. Furthermore, many strengthening interventions on concrete structures by means of FRP plates/sheets, are designed to increase the strength and the ductility of elements against the earthquake actions. In these cases, the FRP-concrete interface is subject to cyclic shear forces inducing very high stress regime which lead to debonding. A clear and comprehensive understanding of this type of debonding mechanism has not been reached at the moment. The number of cycles is usually small but with very high force level and a behavior different from fatigue cases has to be expected. In the present paper, the first results of an experimental campaign on the cyclic behavior of the FRP-concrete interface are presented. A number of CFRP plates/sheets have been bonded to concrete prisms and tested according to a previous established procedure, where the concrete back side is fixed to an external restraining system. The adopted set-up allows to obtain a stable delamination process. Both strain gages along the FRP plate and LVDT transducers have been used. Each specimen has been subject to different sets of force cycles at increasing levels of applied traction, up to the onset of delamination. The effect of the cyclic loads on the debonding process has been observed; FRP-concrete slips evolution corresponding to the application of repeated cycles at different levels of prescribed displacement have been also investigated and measured.