Relationship between the effective strain of PBO FRCM-strengthened RC beams and the debonding strain of direct shear tests

Debonding of fiber-reinforced composites can limit the increase of strength that a reinforced concrete (RC) beam can achieve due to the addition of the external layer of reinforcement provided by the composite. Small-scale direct shear tests can help understand the debonding phenomenon. However, researchers have debated for decades whether the results of direct shear tests can be used to predict the strain level at which the composite would fail in strengthened beams. This paper is the first systematic attempt to address this issue. A database of RC beam flexural tests and of direct shear tests that feature externally bonded polypara-phenylene-benzo-bisoxazole (PBO) fiber-reinforced cementitious matrix (FRCM) composite is compiled. For the beam tests, two methods are employed to determine the strain in the composite at failure due to debonding, referred to herein as the cross-sectional analysis method and the ΔM method, and the results are compared with those of direct shear tests. The relationship between the effective strain in the FRCM when composite action is lost and the debonding strain in direct shear tests is critically discussed taking into account what is prescribed by current design guidelines. The limitations of the cross-sectional analysis method to determine the effective strain are also highlighted.