Effect of groove spacing on bond strength of near-surface mounted (NSM) bonded joints with multiple FRP strips

In the strengthening of existing deficient structures using the near-surface mounted (NSM) FRP method, a group of parallel NSM FRP strips are usually needed to meet the capacity enhancement requirement. When the groove spacing (i.e., the net distance between grooves) is relatively small, the bond behaviour of each NSM FRP strip is detrimentally influenced by the adjacent grooves/FRP strips, and such detrimental effect should be taken into account for a safe design of the NSM FRP strengthening system. All the existing models, however, have been proposed for NSM bonded joints with a single FRP strip and thus cannot consider the effect of groove spacing on the bond behaviour, due to the insufficiency of data from tests or numerical simulations. Against this background, a numerical parametric study, was conducted to clarify the effect of groove spacing on the bond strength of such bonded joints; the numerical parametric study involved the use of a three-dimensional meso-scale finite element model developed in the present study for NSM bonded joints with two FPP strips separately embedded in two parallel grooves. Based on the results from the parametric study, a reduction factor to account for the detrimental effect of insufficient groove spacing on the bond strength is proposed and extended to NSM bonded joints with three or more evenly-spaced FRP strips. By combining the proposed reduction factor and the bond strength model previously developed by the authors for NSM bonded joints with a single FRP strip, a bond strength model for NSM bonded joints with multiple FRP strips is proposed and the accuracy of the proposed model is verified with test results.