The aging of reinforced concrete structures is one of the biggest concerns in civil engineering today since billions of dollars are spent annually on deck repairs and replacements. This study focuses on the rehabilitation of reinforced concrete box girders used in bridge construction. Bridge rehabilitation with a new concrete overlay possesses many challenges that involve cracking and debonding of the overlay caused by the restraining effect of the substrate. This effect leads to the development of tension stresses in the overlay, compression stresses in the substrate, and shear stresses at the interface. In order to mitigate this type of cracking and to ensure a desirable monolithic structural behavior of the rehabilitated bridge, a long-term assessment of the free shrinkage strains acting in the overlay is necessary. This study conducts a two-dimension finite element analysis of a reinforced concrete box girder bridge to evaluate humidity and free shrinkage strain profiles at different times. The humidity gradient between the overlay and the substrate generates differential volume changes between substrate and overlay. The substrate deformations are negligible, while the overlay is subjected to high shrinkage; 78% of the ultimate shrinkage strain is reached after 3 years, indicating a high susceptibility to cracking.