Effect of wet-dry cycles on the bond behaviour of concrete elements strengthened with NSM CFRP laminate strips

The near-surface mounted (NSM) strengthening technique is capable of effectively increase the bearing capacity of structural concrete elements. This technique which basically consists of placing FRP reinforcements inside small grooves cut in the concrete cover, has been widely investigated in terms of structural performance and ability to improve the flexural and shear behaviour of reinforced concrete beams and columns. However, little research has been carried out concerning to the NSM long-term performance and durability. Motivated by the need of increasing the knowledge on the expected durability of the NSM technique using CFRP laminates, this paper presents an experimental program in which direct pull-out tests are carried out for evaluating the bond behaviour of specimens aged through wet-dry cycles. A total of 30 specimens are tested, analysing the effect of the bond length, the groove width, the groove depth and the aging effect on the bond behaviour. Digital image correlation method is also used to identify the bond resistant mechanism developed in an element strengthened using NSM technique. Finally, using the experimental results, an analytical–numerical strategy is applied to establish the local bond stress–slip relationship.