The coupling effect of mechanical and environmental loads is the main cause of deterioration in the performance of reinforced concrete (RC) structures. However, most studies on the durability of RC structures have focused only on mechanical or environmental loads. As a result, it is difficult to fully capture the effects of dry shrinkage and creep caused by temperature and humidity changes within the structure, as well as their impact on the overall deformation of the structure. To address this gap, this paper presents a numerical simulation of underground culvert projects using the durability concrete model-complex three-dimensional (DuCOM-COM3D) analysis software. The results of the simulation demonstrate that this approach offers a more precise characterization of the porosity, temperature, and humidity inside the concrete, resulting in improved accuracy in predicting the long-term deflection, crack width, and other macro-mechanical indices of the structure. Despite these advantages, some discrepancies were observed between the calculated and measured long-term deflection values. Additionally, a limit analysis was conducted to investigate the potential causes of the large deformation observed in the measurements. Overall, the results contribute to a better understanding of the complex mechanical and environmental loads affecting RC structures and provide a methodology for accurately simulating their long-term behavior.