Microstructure Characteristics of GFRP Reinforcing Bars in Harsh Environment

Fiber reinforced polymer (FRP) composites have been suggested as corrosion-resistant alternatives to traditional steel reinforcement in concrete structures. Within this family of composites, glass fiber reinforced polymers (GFRPs) have been gaining momentum as the primary selection of FRP for construction applications. Despite being advantageous, its wide adoption by the industry has been hindered due to the degradation of its performance in severe environmental conditions. As such, significant studies have been carried out to assess the mechanical properties of GFRP bars subject to different conditioning schemes. However, the inconsistencies and wide variations of results called for more in-depth microstructure evaluation. Accordingly, this paper presents a critical review of existing research on the microstructure of GFRP reinforcing bars exposed to various conditioning regimes. The review analysis revealed that sustained load limits set by codes and standards were satisfactory for nonaggressive environment conditions but should be updated to include different conditioning regimes. It was also found that conditioning in alkaline solutions was more severe than concrete and mortars, where test specimens experienced irreversible chemical degradation, more hydroxyl group formation, and more intense degradation to the microstructure. The progression of hydrolysis was reported correlatively through an increase in hydroxyl groups and a decrease in the glass transition temperature. While moisture uptake was the primary instigator of hydrolysis, restricting it to 1.6% could limit the reduction in tensile strength to 15%. Further, the paper identifies research gaps in the existing knowledge and highlights directions for future research.