A Discussion of Differences Between Single-Lap Tests and Full-Scale Beam Tests in Terms of FRCM-Concrete Debonding

Fiber-reinforced cementitious matrix (FRCM) composites are a recent addition to the family of fiber-reinforced composites employed as externally-bonded reinforcement to strengthen reinforced concrete members. FRCM composites use a cementitious matrix rather than epoxy, which is typically used for the well-known fiber-reinforced polymer (FRP) composites. For both FRCM and FRP, the weakness of the technology is the premature debonding of the composites, which in most cases is a brittle phenomenon. The study of the debonding is often carried out at the small-scale level by employing small blocks of concrete with a strip of composite applied to one face. Different set-ups exist to study the phenomenon at the small scale. The most common set-up is the pull-push single-lap direct-shear test, in which the composite strip is pulled while the concrete block is restrained in such a way that the interfaces between the composite and the block is subjected mainly to shear stresses. The set-up is designed to represent the shear stress-transfer that occurs at the composite-concrete interface in full-scale strengthened beams. Single-lap direct-shear tests are commonly used to study the debonding phenomenon because the specimens are easy to construct and handle, especially compared to full-scale beams. Nevertheless, the open question in the scientific community is whether the results of single-lap direct-shear tests can provide useful information on the debonding phenomenon in strengthened beams. This paper aims at providing an insight into this open question. The paper focuses on single-lap shear tests and full scale beam tests that employ a polyparaphenylene benzo-bisoxazole (PBO) FRCM composite.