Concrete structures retrofitted with externally bonded fiber reinforced polymer (EB-FRP) sheets have been widely introduced and applied for the last several decades. In order to study the coupling effect of concrete strength and bonding length on the bond properties of FRP–concrete system, a series of direct shear bond tests have been conducted. The test results show that not only the failure mode but also the bond capacity are affected significantly by the coupling effect between concrete strength and bonding length. Furthermore, a theoretical model is applied to demonstrate the two-way debonding failure for the specimens with bonding length less than the effective bonding length. Based on the existing bond strength model, an advanced model, which takes into account the coupling effect of concrete strength and bonding length, is then proposed. The advanced model can fairly satisfactorily predicate the bond strength of CFRP–concrete interface under different concrete strengths and bonding lengths.
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