Bond Behavior of CFRP–Concrete Bonded Joints with Additional GFRP Layer: Effect of Bonding Sequence

Existing studies have shown that bonding a ±45° biaxial GFRP under CFRP laminate can significantly improve the load-carrying capacity and ultimate deformation of CFRP–concrete bonded joints. In such a bonding configuration, the GFRP interlayer is wider than the CFRP laminate so that the interfacial stress can be redistributed to achieve a higher fracture work; however, the effect of the bonding sequence,—specifically, the position of the GFRP layer—on the bond behavior is not yet clear. In this study, considering the same CFRP and GFRP usage, three types of CFRP–concrete bonded joints with CFRP bonded under, above, and between GFRP layers were prepared and tested under single-shear loading. Digital image correlation (DIC) was used to measure the deformation of the bonded joints during the test. Afterward, the failure mode, load–displacement behavior, and principal strain distribution were analyzed. The experimental results show that the dominant failure mode is the combined cohesion failure mode within the concrete and GFRP delamination, which is not affected by the bonding sequence. Compared to conventional CFRP–concrete bonded joints, bonding the CFRP laminate above, under, and between the GFRP layers achieved a 157.6%, 175.0%, and 177.2% increase in load-carrying capacity, respectively. Accordingly, the ultimate deformation also recorded an 83.0%, 103.6%, and 86.3% increase. However, the bonding sequence showed a slight influence on the initial stiffness of the load–displacement curve with a maximum difference of 16.1%, taking the minimum as a reference, which could be attributed to the differences in the strength and stiffness between the CFRP–concrete and CFRP–GFRP–concrete interfaces.

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