This paper aims to investigate the durability performance and microstructure characteristics of two different types of commercially-produced glass fiber-reinforced polymer (GFRP) bars conditioned in moist seawater-contaminated concrete. GFRP bars encased in seawater-contaminated concrete were immersed in tap water for 5, 10, and 15 months at temperatures of 20, 40, and 60 °C. Following conditioning, the GFRP bars were retrieved then tested to failure under uniaxial tension. Microstructure analysis was performed by employing differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and matrix digestion using nitric acid. Type I GFRP bars, with the lower moisture uptake, exhibited insignificant strength reductions in the range of 2–15% whereas Type II, with the higher moisture uptake, exhibited substantial strength reductions in the range of 19–50%. Type II GFRP bars featured lower matrix retention and developed higher hydroxyl ions during conditioning than those of their Type I counterparts. The extent of degradation was more sensitive to the conditioning temperature rather than conditioning duration. A decrease in the glass transition temperature (Tg) of both types of GFRP bars was recorded, indicating matrix plasticization. Results of SEM highlighted matrix disintegration and fiber debonding after conditioning.
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