Bond behaviour of FRP NSM Systems in concrete elements

In the recent past, experimental results of bond tests performed by several researchers on the performances of Fiber Reinforced Polymer (FRP) applied according to Near Surface Mounted (NSM) strengthening technique showed that mechanical properties of materials and surface properties of FRP reinforcement as well as the grooves geometry and the test set up can affect the bond of this strengthening system. An experimental program aimed to compare the bond behavior of different types of FRP materials applied on concrete blocks according to Externally Bonded Reinforcement (EBR) and NSM strengthening techniques has been presented and discussed in a previous paper, showing that the NSM technique could represent a sound alternative to EBR systems since it allows the FRP tensile strength to be better exploited. Herein the results of bond tests on different types of NSM reinforcement are analyzed in terms of bond–slip relationships in order to show the appropriateness of using simplified bilinear relationships. Moreover, further experimental results of bond tests available in literature and characterized by different types of bond failures have been selected and joined to those widely analyzed in this paper in order to provide an experimentally calibrated relationship predicting the maximum debonding strain in the FRP NSM reinforcement.