Effects of diameter on the durability of glass-fiber-reinforced-polymer (GFRP) bars conditioned in alkaline solution

Current standards do not consider the diameter of glass-fiber-reinforced-polymer (GFRP) bars used as internal reinforcement in concrete structures to be a factor influencing bar durability. This paper investigates the effects of bar diameter on the physical and mechanical properties as well as the durability of GFRP reinforcing bars conditioned for three months at 60°C in an alkaline solution simulating a concrete environment. Five diameters (nominal diameters of 9.5 mm, 12.7 mm, 15.9 mm, 19.1 mm, and 25.4 mm) were considered; bar properties were assessed before and after conditioning. Microstructural analyses and measurement of physicochemical properties were also carried out. The results show that bar size had no significant effect on bar physical properties, except for water absorption. The smaller diameter bars had higher water absorption than the larger ones due to their higher surface-area-to-volume ratios. In the case of the unconditioned bars, the tensile strength and modulus were not significantly affected by bar diameter, but there was a size effect for interlaminar shear strength and flexural strength. On the other hand, the conditioning in the alkaline solution had a greater negative effect on the tensile strength of the larger bars than on the smaller ones. Scanning-electron-microscope (SEM) observations and Fourier-transform-infrared-spectroscopy (FTIR) analysis revealed that the degradation remained at the surface of all the conditioned specimens. Nevertheless, there were only small variations between the physical and mechanical properties of the GFRB bars of different diameters. This indicates that the current provisions in standards that do not relate strength-retention limit to bar size are acceptable.