Durability of Glass Fiber-Reinforced Polymer Bars in Seawater-Contaminated Concrete

This research aims to investigate the durability performance and microstructure characteristics of two different types of glass fiber-reinforced polymer (GFRP) bars in severe environment. 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. Half of the specimens were conditioned under a sustained load of 25% of their ultimate strength whereas the other half was conditioned without load. 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 to 15% when conditioned without load. Their Type II counterparts exhibited higher moisture uptake, higher hydroxyl ions, lower matrix retention, and thus, substantial strength reductions in the range of 19 to 50% were recorded. 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 deboning after conditioning. Specimens conditioned under a sustained load exhibited higher moisture absorption than that of their counterparts conditioned without load. None of the loaded specimens conditioned at 20oC were creep-ruptured during conditioning. The presence of the sustained load during conditioning at 20oC for 15 months reduced the tensile strength retention by approximately 14 and 5% for Type I and Type II GFRP bars, respectively. In contrast, many bars were creep-ruptured and significant reductions in the tensile strength retention were recorded due to the presence of the sustained load during conditioning at the higher temperatures of 40 and 60oC. The accelerated aging test data along with the Arrhenius concept were employed to develop a durability design model that can predict the tensile strength retention of both types of GFRP bars in moist seawater-contaminated concrete