Unified analytical approaches for determining shear bond characteristics of FRP-concrete interfaces through pullout tests

The pullout test is a conventional test method for calibrating interfacial shear bond characteristics of Fiber Reinforced Polymer (FRP)-concrete interfaces. However, due to the small bending stiffness of FRP sheets/strips and the highly non-linear interface fracturing mechanism, a well-recognized analytical approach to the accurate interpretation of the pullout test results remains to be achieved despite extensive studies particularly when the aim is to calibrate a local bond stress-slip model, which is necessary for developing bond strength and anchorage length models avoiding the use of empirical formulations. This paper introduces a newly developed non-linear bond stress-slip model for analyzing full-range strain distributions in FRP and shear bond stress distributions in the interface bond layer during pullout tests, along with a new anchorage length model and bond strength model that were developed accordingly. Compared with other existing bond models, the bond model described here has two advantages besides its simplicity: (1) it incorporates the most important interface parameter, the so-called interfacial fracture energy, in all analytical processes and links it successfully with all other important bond parameters; (2) it is a general and unified approach that allows for the first time consideration of the effects of the adhesive bond layer in non-linear analysis of FRP-concrete interfaces. Further, a unified bond stress versus slip expression is formulated to show the differences in local bond stress-slip relationships at the loaded and free ends in pullout tests, so that the effects of the bond length used in a pullout test on the calibration of the interfacial bond stress-slip model can be clarified. The reliability of all proposed models is verified through a comprehensive comparison of the experimental and analytical results.