FRP Strips Externally Bonded to Quasi-Brittle Substrates: Discussions and Advances of Open Research Topics

In the first part of the paper, a critical review of the fundamental knowledge of the debonding mechanism of externally bonded
fiber-reinforced polymer (FRP) composites from a quasi-brittle substrate (such as concrete and masonry) is presented. Then, the paper focuses
on five selected research topics. Some of these topics have reached a quite mature state. However, the authors build upon their research carried
out in the last 20 years and the work published in the literature to present new perspectives or clarify aspects that have not yet reached a shared
consensus. Among these five topics, the following two topics are discussed more extensively and advances in their knowledge are proposed:
(1) the specific features of the interfacial law (interfacial shear stress versus slip relationship) and its shape required to obtain a finite length of
the stress-transfer zone and (2) the relationship between the bond capacity in direct shear tests and the force in the FRP strip corresponding to
the full debonding of FRP-strengthened beams.