Glass fiber reinforced polymer (GFRP) rebars can be utilized as a substitute for steel reinforcements in concrete structures to eliminate possible steel corrosion. However, embedded GFRP rebars may also degrade with aging time due to the high concrete alkalinity (pH ∼ 13). Previous studies only focused on the GFRP degradation in extreme conditions (pH = 7 and 12.5 to 13.5), whereas little research covered pH down to 11, which could be achieved by modifying the cementitious binder. The purpose of this paper is to evaluate the GFRP durability under exposure to various pH levels (i.e., pH = 7, 11, 12 and 13). Experimental results showed that lowering pH from 13 to 11 could considerably reduce fiber/matrix defects induced by alkali attack, leading to a reduction in diffusion rate from 6.2 × 10-7 to 4.2 × 10-7 mm2/s at 60 °C. Further, a significant increase from 27.6% to 45.9% in GFRP rebar tensile strength retention after 12-month exposure to 60 °C was found when the exposure pH dropped from 13 to 11, but the flexural strength and inter-laminar shear strength were not affected. This change in macroscopic mechanical properties was attributed to the continuous ingress of external solution and the microstructural corrosion of fibers and matrix. The results of this study provide in-depth explanations of the alkali-induced degradation mechanism as well as comprehensive information on the durability performance of GFRP rebars for engineering applications.

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