Performance and Durability of In-Plant Partially Cured GFRP Bent Bars in Steam-Cured Precast Concrete Elements

Abstract

Glass fiber-reinforced polymer (GFRP) bent bars were used as shear reinforcement for precast/prestressed concrete box beams in Ontario, Canada. Differential scanning calorimetry (DSC) tests performed on the bent GFRP bars revealed a curing ratio varying from 88% to 94% and a glass transition temperature ranging from 63°C to 71°C (lower than the limits of 95% and 100°C, respectively, specified in the CSA S807 FRP-material specifications). This study investigated the consequence of implementing partially cured GFRP bars in constructing steam-cured precast-prestressed concrete bridge box beams. The physical and mechanical properties of the partially cured GFRP bars were evaluated and compared with the properties of GFRP bars postheated at 80°C for 4 h. The durability performance was also assessed by immersing the GFRP bars in alkaline solution at 70°C for 2,200 and 4,300 h. Moreover, scanning electron microscopy (SEM) observations were performed to assess the microstructure of the GFRP bars. The results show that the glass transition temperature, Tg, and interlaminar shear strength were the criteria most affected by the degree of curing. Exposing the bars to steam treatment (as is the case for precast concrete box beams) increased the cure ratio and, consequently, the Tg and interlaminar shear strength of the bars. This was verified by simulating the concrete mix design used in the precast concrete process [including the same water-to-cement ratio (w/c) and aggregate type] and exposing the concrete to the same steam-treatment conditions. The steam treatment promoted the postpolymerization of the GFRP bars, leading to a cure ratio of 98%, which exceeds the limit specified in CSA S807 and enhanced bar properties. The results of this study have been instrumental in developing the new edition of CSA S807 standard incorporating evaluation of the physical, durability, and mechanical properties of bent FRP bars. Moreover, the reported study constitutes a valuable contribution to the state-of-the-practice.