Analytical and statistical study of the bond of FRP bars with different surface characteristics

Bond performance of Fibre Reinforced Polymer (FRP) bars to concrete can considerably differ from those of traditional steel because of the different physical and mechanical properties. The surface of FRP bars is most often greatly different from the ribbed shape of steel bars. This study focuses on the influence of surface type on the bond characteristics of FRP bars. Analytical expressions of the constitutive bond stress-slip laws, describing the bond behaviour, can be applied to define characteristics such as the development length. Available models (e.g., mBPE and CMR) involve parameters to be defined by calibrating to the experimental results. In this paper data from 184 pull-out tests are used to calibrate the parameters of mBPE and CMR models. Next, a comprehensive statistical analysis methodology has been developed to study the statistical significance of the influence of included factors on the experimental bond characteristics (bond strength and corresponding slips) and analytical parameters (alpha, beta, sr). The studied factors involve bar surface (sand coated (SC), helically wrapped (HW), helically wrapped and sand coated (HWSC), indented (In) and ribbed (Rb)) and diameter (6 to 12 mm) as well as concrete compressive strength (35 and 66 MPa). Due to similar mechanical properties, both Glass FRP (GFRP) and Basalt FRP (BFRP) bars were used. The statistical methodology was applied to the bond strength results and to the parameters of the ascending branch of the mBPE model corresponding to free end slip (i.e., alpha fe). Statistically significant difference was observed in the bond strength not just in case of bars having different surface profiles but within the same surface category as well. Furthermore, the interaction between the influence of bar surface and diameter as well as between the bar surface and concrete strength was found to be statistically significant. Based on the statistical model, coefficients are provided - to account for the influence of involved factors. Analytical parameters were defined dependant on the surface category.