A unified FRP-concrete bond model and new experimental insights on size effect

This study presents a novel discovery regarding the size behavior of fiber-reinforced polymer (FRP) bond to concrete, with three types of size behavior (Types 1, 2, and 3) discovered and their design charts developed. Type 1 has constant substrate width with variable FRP width, Type 2 features proportional increases in both substrate and FRP widths (the so called size effect) maintaining a constant width ratio, and Type 3 has constant FRP width with variable substrate width. With Type 3 being a previously unrecognized width effect—that existing bond models need to capture—the paper introduces its experimental investigation. Another contribution of this work is the development of a consistent FRP-concrete bond strength model using physics-driven semi-analytical approach, yielding multi-output, expressions for the capacity P_max, effective bond length L_e, maximum interfacial shear stress τ_max, fracture energy G_f, and ultimate slip s_f, all in one framework. Apart from the physics-driven approach used to develop P_max, the model is unified for the key bond parameters—L_e, τ_max, G_f, s_f, and surface preparation effect—to self-evolve without fitting their individually respective values, except that of P_max. Accuracy is validated using 710 experimental data showing remarkable performance of the unified model compared with top-ranked existing models and codes. In addition, the new size type tests are used to serve as further validation of the proposed model's consistency.