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Numerous studies have been devoted to the investigation of the deterioration and behavior of fiber-reinforced polymer (FRP) sheets (made from a variety of materials such as carbon, glass, or aramid) bonded onto the concrete substrate under a variety of adverse environments. Results indicate that environmental conditions might exercise significant and undesirable effects on FRP-concrete bond performance. In many corrosive environments, there are the potential risks of premature debonding and failure of the bonding interface in externally bonded FRP-strengthened concrete structures. The effects of freeze–thaw cycles on the fiber-reinforced polymer (FRP)-to-concrete bond strength were investigated using the particle image velocimetry (PIV) technique. For this purpose, 18 specimens were prepared, including 12 specimens strengthened with carbon FRP (CFRP) strips as well as six control specimens subjected to 200 and 500 freeze–thaw cycles, each consisting of four steps according to ASTM C 666. In the first stage, the temperature was held constant at 5°C for 4.2 h. The next step involved rapid freezing to ‒18°C for 2.4 h. In the third step, the temperature was held constant at ‒18°C for 2.4 h. Finally, the temperature was raised and maintained at 5°C for 3 h in the fourth step. The freeze–thaw under wet conditions was selected in order to create harsher conditions than the dry freeze–thaw conditions would. According to ASTM C 666, the specimens were stored in saturated lime water from the time of their removal from the molds until the time of freezing and thawing tests started. In addition, the nominal freezing and thawing cycle consisted of alternately lowering the temperature of the specimens from +5 to −18°C and raising it from −18 to +5°C in not less than 2 nor more than 5 h. The freezing and thawing chamber was equipped with a user defined program. The temperature range of the chamber was −30°C to +65°C. The temperature was controlled by a sensor, which can be immersed either into the sample or into the water in which the sample was placed. The specimens were strengthened via externally bonded reinforcement (EBR) and externally bonded reinforcement on grooves (EBROG) methods. After the concrete prisms had been subjected to 200 and 500 freeze–thaw cycles, they were placed in the single shear test machine. During each test run, a tensile force was applied to the FRP composite while the concrete block was restrained from movement. The single shear test machine consisted of a hydraulic jack with a capacity of 400 kN that provided the required force for the single shear test. Moreover, a load-cell with a capacity of 50 kN was used to measure the force applied to the specimens. In the current investigation, the specimens were subjected to a quasi-static loading of 2 mm/min in accordance with ASTM D3039/D3039M. The results of PIV measurements revealed that, compared with the specimens strengthened via the EBR method, the EBROG-strengthened specimens exhibited considerably enhanced bond performance. When subjected to 200 and 500 freeze–thaw cycles, the EBR-strengthened specimens experienced a 3% and 9% decrease in their bond strength, respectively; the EBROG-strengthened specimens experienced no decrease in bond strength and increases in the range of 7%–19% when subjected to 200 and 500 cycles, respectively.

Keywords: EBROG, Debonding, Freeze–thaw cycles, Fiber-reinforced polymer (FRP), Durability, Bond behavior

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