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This research assessed the effects of elevated temperature and humidity exposures on the durability of fiber reinforced epoxy composite overlays on concrete substrates. Modified double cantilever beam samples were used to determine the Mode I strain energy release rate of the composite-to-concrete bond after exposure to temperatures of 23°, 60° or 100°C, humidity levels of 50 or 95% RH, and times up to 40 days. The only statistically significant degradation in toughness was observed after exposure to the 100°C, 95% RH environment. However, chemical changes in the matrix occurred during this extreme exposure condition that may not represent the in-service aging of composite-repaired concrete structures.