Glass fiber-reinforced polymer (GFRP) bars have been used in concrete structures as an alternative to steel bars due to their noncorrosive behavior. However, due to the lack of full understanding of long-term performance, their use as internal reinforcement is still limited. To evaluate the durability of in-service GFRP bars under natural exposure, a collaborative project including four organizations investigated the conditions of GFRP bars and their surrounding concrete from bridges with 15-20 years of service. The aim of Part I of a two-paper series is to describe the bridge structures, methods of extraction, and the results of concrete testing, wheras Part II focuses on GFRP bar performance. The extracted bars were tested for physical, mechanical, and chemical properties, and the surrounding concrete was evaluated for chloride penetration, pH, and carbonation depth at the level of reinforcement. Results showed that carbonation and chloride may have reached the depth of the GFRP bars. This paper discusses the process of extraction of the bars, including the location and type of the selected bridge, and the concrete tests performed in terms of procedure, results, and observations.