This paper discusses the feasibility of using the impact-echo method (IE method) to evaluate the deterioration of near-surface mounted carbon fiber reinforced polymer (NSM CFRP) strengthening under outdoor exposure. In NSM CFRP strengthening materials, CFRP bars are embedded in pre-cut grooves on the surface of reinforced concrete members and the grooves are filled with epoxy. Prolonged outdoor exposure reduces the bonding strength of epoxy and generates debonding flaws at epoxy–concrete interfaces, preventing CFRP bars from fully exerting their material strength and inhibiting the reinforcement effectiveness of NSM CFRP. The IE method based on the principle of stress wave propagation was adopted in this study as a nondestructive technique for detecting debonding flaws at epoxy–concrete interfaces caused by the prolonged outdoor exposure of the NSM CFRP strengthening members. Firstly, the IE tests were conducted on the NSM CFRP beams without outdoor exposure. The IE spectra showed high-amplitude peaks at the dominant frequencies associated with the cross sectional mode of vibration of the NSM CFRP beams. Subsequently, the strengthening surfaces of the NSM CFRP beams were exposed to direct sunlight under outdoor environments. The IE and loading tests were conducted when the beams were exposed to the outdoor environment for 4, 8, 12, and 48 months. The experimental results indicated that prolonged outdoor exposure generated the debonding flaws at the epoxy–concrete interfaces and the IE method can be used to detect the interfacial debonding by identifying the decrease in the dominant frequencies. For a NSM CFRP beam with debonding interfaces, the result obtained from loading test revealed a decrease in the load-carrying capacity due to damages at the epoxy–concrete interfaces resulting in the inability of the CFRP bars to fully exert their material strength. Therefore, the IE method is applicable for assessing the deterioration of the NSM CFRP strengthening members under prolonged outdoor exposure
