Aging bridge infrastructure worldwide faces challenges due to increasing load demands and material degradation. Many bridges, designed decades ago, now have insufficient load capacities for modern traffic. In the U.S., more than half of the bridges are over 40 years old. A significant portion of the population classified as structurally deficient raises concerns for public safety and calls for cost-effective solutions to extend bridge service lives. One such method is Proof Load Testing (PLT), a non-destructive technique that applies controlled loads to assess bridge performance under stress. When combined with Finite Element Analysis (FEA), PLT allows engineers to validate computational models with real-world data, enhancing bridge evaluations and strengthening methods. This study focuses on the I-39 Kishwaukee River Bridge, a 5-span post-tensioned concrete box girder bridge built in 1979. The bridge developed significant cracks and shear key slippage due to construction deficiencies. External post-tensioning tendons were added to address this issue and improve its load-bearing capacity. The study integrates PLT and FEA to evaluate the bridge behavior before and after rehabilitation. The findings indicate that the post-tensioning system significantly minimized deflection by 88.72% and reduced tensile strain at the female shear key by 80 μɛ. These findings demonstrate the effectiveness of the post-tensioning intervention and suggest that the bridge can accommodate future traffic load increases. Additionally, this paper outlines the methodology for combining field testing with computational modeling, offering an efficient approach to assessing aging infrastructure.