Experimental Investigation of Bond between Near-Surface-Mounted CFRP Strips and Concrete under Freeze-Thawing Cycling

Abstract

The technique of prestressing near-surface-mounted (NSM) carbon fiber–reinforced polymer (CFRP) has good potential and future prospects for structural strengthening due to its improvement of the bond between the CFRP and concrete. However, notably few investigations have been conducted to study the bond durability of the NSM CFRP–concrete interface. The present study was undertaken to increase the understanding of the durability of the bond between the NSM CFRP and concrete under freeze-thaw cycles. Pull-out tests were conducted to investigate the influence of freeze-thaw cycles on the interfacial bond with ordinary concrete and concrete with high strength or additional frost resistance. The mechanism of freeze-thaw cycles degrading the bond of the NSM CFRP is discussed. Additionally, the influence of the distance between concrete edge and groove (edge distance) and the bond length on the bond durability was studied. It was found that, first, freeze-thaw cycling results in a significant decrease in compressive strength of ordinary concrete but a unremarkable degradation of strength of high-strength concrete and concrete with water-reducing agents and admixtures of air-entraining agents. Second, freeze-thaw cycling also leads to a relatively significant decrease in the bond of the NSM CFRP systems with ordinary concrete; however, it has no substantial impact on the bond of specimens of concrete with high strength or frost resistance. This indicates that the deterioration of the bond of NSM CFRP under freeze-thaw cycles is caused by the degradation of concrete. Third, a decrease in the edge distance results in cone-shaped cracks at the loaded end. Finally, when the failure mode remains debonding at the NSM CFRP–concrete interface, the NSM CFRP system with a longer bond length exhibits more significant degradation in bond capacity compared with that with a shorter bond length. This is because the residual friction stress at the debonding interface also decreases due to freeze-thaw cycling; thus, a longer bond length corresponds to a great loss of residual friction force at the debonding zone.