Experimental research on the wet bonding properties between RFRP and concrete

In this work, an improved wet bonding method was developed for strengthening of fiber-reinforced polymer. A self-made roughened carbon fiber–reinforced polymer sheet (hereinafter referred to as RFRP sheet) was externally attached to the surface layer of a nano-kaolin-modified concrete test piece to form an RFRP–concrete wet-bonded test piece. Then, the pull-off bond test and the single shear test were performed on 32 and 30 test pieces, respectively. The performance of the wet bonding interface of RFRP–concrete in the normal and tangential directions was investigated by changing the length of glass fiber cellosilk in RFRP bonding resin, the diameter of RFRP porous pelelith rock, and the ratio of nano-kaolin. In addition, by comparing the scanning electron microscopy images of untreated fiber-reinforced polymer sheet and the concrete block without nano-kaolin, the mechanism of the adhesion enhancement of the RFRP–concrete interface was explained. The results show that the differentiation between fiber-reinforced polymer–concrete wet bonding failure and RFRP–concrete wet bonding failure was mainly based on the large-scale concrete with peeled off concrete surface. RFRP effectively enhanced the wet adhesion performance of the interface with concrete in both normal and tangential directions. The interface bonding ability increased by 900% and 42%, respectively, compared with the control test pieces. The diameter of pelelith rock was found to be the most important factor affecting the shear wet bonding performance of the RFRP–concrete interface. The second important factor was the ratio of nano-kaolin. The optimum conditions for the best tangential anti-peeling ability of the RFRP–concrete structure were found to be the addition of 5-mm-diameter pelelith stone, 3% nano-kaolin, and glass cellosilk of 89 mm length. When the RFRP and the concrete were wet-bonded, the uncured cement mortar effectively filled the holes of the original pelelith rock and acted as a mechanical lock, thereby increasing the bonding stress.