Comprehensive FRP-concrete bond behavior: Impact of test methods and the innovative UBoT

The lack of a standardized bond test method for fiber-reinforced polymer (FRP)-concrete interfaces leads to variations in forms of the popular single-lap shear, double-lap shear, beam, mixed-mode, and pull-off tests. Recognizing the limitations of each bond test technique and the importance of analyzing all results as complementary, a detailed evaluation of these variations is essential for accurate bond behavior assessment prior to design and field deployment, to avoid subjective decisions and predictions. This study pioneers the evaluation of FRP-concrete bond test methods, offering the industry a consistent dataset for informed decision-making. Single- and double-lap shear tests yield trilinear bond-slip responses; however, the single-lap test's lack of symmetry reduces bond strength by 15% compared to the double-lap test. The beam test, unsuitable for bond-slip modeling, shows comparable average bond strength to the single- and double-lap tests. The mixed-mode test better simulates field applications, while the pull-off test provides the most consistent results. Given that each test method reflects only one field scenario, the Universal Bond Tester (UBoT) was developed. This device can convert to all FRP-concrete bond test types. It also addresses the modified double-lap shear test's limitations (e.g., inapplicability to pultruded laminate and FRP rupture in wet layup systems) and present a modified ASTM D7958 beam and mixed-mode tests cost-effectively. The UBoT captures full bond behavior and variabilities, overcoming the limitations of existing methods which fail to capture all failure modes in bond-critical applications. It reduces specimen sizes, weight, and data acquisition needs by half for most tests. This study allows test laboratories to use any preferred testing method with the UBoT. Link to video demonstrations for all tested specimens are provided in the supplementary material.