Experimental tests on intermediate crack debonding failure in FRP-strengthened RC beams

In the present paper, the results of an experimental campaign concerning the failure due to intermediate crack-induced (IC) debonding of RC beams strengthened with FRP composites are presented. Both CFRP plates and sheets have been adopted for strengthening. The simply-supported beam scheme with a multiple-load distribution has been adopted in order to have a loading condition the more close possible to the real cases. During the tests, mid-span deflection, compressive strain, mean tensile elongation, crack opening and FRP strains along the beams have been measured. The effect of the FRP reinforcement on flexural strength and ductility has been studied. Moreover, the IC debonding type of failure has been identified from the FRP strain distribution profiles. The IC debonding occurs where the FRP strain is high and a gradient of it (associated with shear stresses along the interface according to classical beam theory) is also present. Finally, the comparison with the theoretical prediction of the FRP strain distribution at IC debonding failure is presented. It is shown that the strengthening with FRP sheets is more effective than adopting pultruded plates, because: i) the anchorage surface is much greater, being equal to the beam width; ii) the debonding surface is more irregular and, consequently, the mode II fracture energy is greater. The role of the amount of the longitudinal steel reinforcement on the FRP strain at IC debonding is also discussed