Corrosion-resistant glass fiber reinforced polymer (GFRP) composite bars are emerging as an alternative for traditional steel reinforcement in concrete structures exposed to aggressive environments such as bridges and box culverts. Although GFRP eliminates the problems related to corrosion of steel reinforcement, its long-term behavior in commercial applications needs to be confirmed. A box culvert bridge consisting of precast concrete units entirely reinforced with GFRP bars (constructed in 1999, on Walker Avenue in the city of Rolla, Missouri) was chosen as a case study. It replaced the original bridge that was built in the early 1980s and had been diagnosed as unsafe to operate due to excessive corrosion of encased steel pipes. Material samples were extracted from different locations of the box culvert and analyzed to monitor possible changes in GFRP and concrete after more than 16 years of service. Initially, carbonation depth, pH, and chloride diffusion measurements were performed on concrete cores surrounding the GFRP bars. Subsequently, scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS) were conducted on GFRP samples to monitor any microstructural degradation or change in chemical compositions. In addition, glass transition temperature (Tg) of the resin and fiber content were determined and results were compared with pristine samples produced in 2015. Results from the concrete tests were consistent with expected values corresponding to the type and age of the structure. The SEM images and EDS test did not show any signs of GFRP microstructural deterioration. Moreover, Tg and fiber content of GFRP coupons were comparable to values from samples tested in 2015. The results of this study validate the notation that GFPR material properties are maintained during two decades of service. Hence, using GFRP internal reinforcement in box culverts eliminates corrosion problems, reduces long-term maintenance costs, and increases the service life of a structure.