Experimental and analytical investigation of PBO FRCM-concrete bond behavior using direct and indirect shear test set-ups

Externally bonded reinforcement (EBR) represents an effective solution to strengthen existing reinforced concrete (RC) members. EBR comprising fiber-reinforced cementitious matrix (FRCM) has been increasingly adopted due to some advantages with respect to the more diffused fiber-reinforced polymer (FRP). Externally bonded FRCM often reported debonding failure at the matrix-fiber interface. In the literature, the matrix-fiber bond behavior was studied using mostly single- and double-lap direct shear (DS) tests. An alternative is represented by modified beam (MB) test set-ups. In this paper, a MB test set-up is adopted to study the effect of flexural deflection on the bond behavior of a polyparaphenylene benzobisoxazole (PBO) FRCM composite bonded to a concrete substrate. Two different MB test layouts are investigated and the results obtained are analyzed and compared with those of corresponding DS tests with the same PBO FRCM. Then, an analytical model able to describe the results of both DS and MB tests accounting for the presence of stresses normal to the matrix-fiber interface is proposed. The experimental results and analysis presented in this paper help to clarify the influence of the test set-up on the bond behavior of FRCM composites, which represents a key parameter for the design of FRCM EBR.