The efficiency of strengthening of deteriorating concrete through the external bonding of prefabricated fiber reinforced composite strips to the concrete substrate depends on the durability of the adhesive. In this application, adhesives can be exposed to water and salt water, as well as alkali solution resulting from the permeation of water through concrete. This paper presents the results of a durability program designed to study the effects of ageing and environment on the durability of a typical adhesive used in external bonding, through characterization of moisture uptake and cure and investigation by dynamic thermal mechanical analysis, thermogravimetric analysis, differential scanning calorimetry, and tensile testing. It is seen that the adhesive shows two-stage diffusion with primary deterioration taking place in the first phase. Exposure to moisture is seen to cause plasticization and decreases in performance characteristics. The decrease in glass transition temperature due to hygrothermal ageing is such that the level is very close to the Federal Aviation Authority (FAA) recommended value of the wet glass transition temperature being at least 30 degrees F higher than the maximum operation temperature. Overall, the effect of salt solution exposure is seen to have the least effect, whereas exposure to concrete based alkali solution has the greatest deteriorative effect. (c) 2007 Wiley Periodicals, Inc.