Steel corrosion is a significant problem of concrete‐based infrastructure, and it increases the repair costs of the damage to structures. Glass‐fiber‐reinforced polymer (GFRP) composite bars are corrosion‐proof and exhibit relatively higher strength than ordinary steel reinforcement. However, GFRP bars degrade when exposed to the highly alkaline environment of concrete over extended periods. This study focuses on the effect of the high alkalinity of concrete on the tensile strengths of several types of GFRP bars. Different accelerated aging techniques were employed to evaluate the effects of GFRP characteristics (fiber volume fraction, matrix composition, bar diameter, and presence of protective coating) on the alkali resistance. The test results indicated that the alkali attack significantly decreased the tensile strength of GFRP, increasing with age. The alkali resistance of the GFRP bars increased up to a fiber volume fraction of 50% but decreased when the fiber volume fraction increased to 60% and above. The effects of protective coating on alkali resistance were generally positive but not consistent, and the effects of the matrix type and bar diameter on the alkali resistance of GFRP bars were insignificant. Accelerated aging did not change the bond strength but altered the failure mechanism associated with the bond failure of the specimens.