Effect of GM patterns on ductility and debonding control of FRP sheets in RC strengthened beams

Strengthening and retrofitting of old structures under current use, are important issues that have been considerably studied in the last few decades. There are many aims for structural strengthening; among them, resisting higher design loads, compensating strength loss due to deterioration, correcting design or construction deficiencies and increasing ductility are the most important. Fiber reinforced polymers (FRPs) have been recognized as one of the best materials used for strengthening of RC structures; especially in the form of externally bonded reinforcement (EBR). However, a major problem in its application is debonding of FRP from concrete surface. Surface preparation is a conventional method to overwhelm debonding recommended by many researches and by ACI 440-2R code. Nevertheless, this method is only partially beneficial and is unable to prevent debonding and yet, debonding causes premature failure in FRP strengthened RC members. Grooving method (GM) is a recently proposed alternative of surface preparation that can improve the behavior of strengthened members and control debonding phenomenon; however, the method must be optimized. In this paper the effect of different longitudinal grooving patterns is investigated and three criteria of ultimate load capacity, ductility and failure mechanism are studied for the strengthened beams. Furthermore, the effect of different thicknesses of CFRP sheet, i.e., thicknesses of 0.17, 0.30 and 0.43 mm are also investigated. Grooves are applied on critical areas called high-stressed zones and are omitted from zones with low state of stresses. The results showed that the specimens strengthened by grooves at the end of the plate performed better regarding the strength and ductility compared to other specimens; where the fibers ruptured in all selected thicknesses.