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Lowering water-to-cement (w/c) ratio is a conventional method for reducing the diffusion rate of chlorides in concrete and hence reducing the corrosion risk of internal steel reinforcements. However, the potential effect of w/c ratio on the mechanical performance of glass fiber reinforcement polymer (GFRP) is unknown. In this study, concrete mix design was first carried out with different w/c ratios of 0.3, 0.4 and 0.5. Experimental results showed that a lower w/c ratio effectively reduced the concrete porosity and increased the concrete compressive strength and GFRP-concrete interfacial bonding strength. Then mortar-wrapped GFRP reinforcements were exposed to 40 degrees C water for up to 12 months. Tensile test results demonstrated that the tensile strength of GFRP decreased with exposure durations, but the varying w/c ratio (0.3 - 0.5) had no significant effect on the tensile strength retention (63.3 % - 69.3 %) of GFRP reinforcements from the life-span perspective. Furthermore, flexural strength and inter-laminar shear strength of GFRP reinforcements basically remained intact after water exposure and were hardly affected by the w/c ratio. To explain the various observations, the glass transition temperature, surface morphology and hydrolysis reaction of GFRP reinforcements were examined to reveal the degradation process at the microscopic level. The findings of this study on the effect of w/c ratio provide engineers with a more comprehensive database to incorporate engineering and durability properties of concrete and GFRP reinforcement into project considerations.