Bond Durability of GFRP and BFRP Bars embedded in PVA Fiber–Reinforced Seawater and Sea-Sand Concrete under Seawater Freeze–Thaw Cycles

Structures made of fiber-reinforced polymer (FRP) bars encased in seawater and sea-sand concrete (SSC) have received significant attention in recent years owing to the high corrosion resistance of FRP, and the environmental friendliness and economic advantage of SSC. However, studies on their long-term bonding performance, particularly in cold coastal environments, are limited. This study investigates the feasibility of using polyvinyl alcohol (PVA) fibers to improve the resistance of normal-strength SSCs to seawater freeze–thaw cycles (FTCs). The interlaminar shear, flexural, and tensile properties of bare glass-FRP (GFRP) and basalt-FRP (BFRP) bars and their bond behaviors in the designed SSC with good seawater freeze‒thaw durability were studied. The failure modes and microstructural tests revealed a bonding-deterioration mechanism. The test results revealed that the chloride ions in seawater, SSC-encased alkalinity, and FTCs adversely affected the mechanical properties of the saturated GFRP and BFRP bars. Additionally, the effect of FTCs was greater than that of SSC alkalinity. The deterioration of the BFRP was marginally more severe than that of the GFRP because the corrosion at the basalt fiber–resin interface caused interface and resin cracking, and thereby increased ice expansion during freezing. Seawater FTCs reduced the bond strength and residual bond strength, and increased the corresponding slippages of the SSCs with GFRP and BFRP bars. This is attributed to the degraded resin-rich surface layer of the bar and radial cracking of the cement matrix at the bonding interface. The radial cracking was caused by the hoop and radial stresses generated by the radial expansion of the saturated bar during freezing. These exceeded the tensile strength of the deteriorated concrete after seawater FTCs. The bond degradation of the BFRP–SSC was marginally more severe than that of the GFRP–SSC owing to the larger extent of cracking in the bar and cement matrix.