Carbon fiber-reinforced polymers (CFRP) have been widely used as externally epoxy-bonded reinforcement for repair and retrofit of existing concrete structures. As the use of CFRP in construction increases, it is essential to consider the long term behavior. In this study, concrete specimens with externally bonded CFRP composite systems were exposed to various accelerated ageing environments: different elevated temperature water immersion, varying relative humidity exposure, real-time solar exposure, pressurized hygrothermal exposure, and fatigue loading. Three-point loading test and direct tension test were used to evaluate the flexural and tensile strength of CFRP specimens. This research recommends that the durability strength reduction factor be defined as the ratio of the flexural strength of a test specimen after exposure for 60 days at 60C to the strength of a control specimen at the initiation of exposure. Test specimens, exposure environments, testing procedures, and field application environments are selected to assure uniform reporting, testing and application of durability strength reduction factors. Two exposure environments are recommended for design, wet and air. Water can diffuse through concrete and accumulate on the epoxy/concrete interface in a wet environment and is the default condition for design. An air environment precludes water accumulation but requires engineering design and validation to guarantee compliance. The resulting durability strength reduction factor may be used to (a) calculate the reduced strength for environmental exposure, (b) qualify a specific CFRP system, (c) compare durability reduction factors to those provided by system suppliers, and (d) confirm compliance with project specifications.