Only some bridge designs can use un-reinforced concrete. Many (most?) modern bridges need reinforced concrete to allow the concrete to bear tensile loads. Reinf

Only some bridge designs can use un-reinforced concrete. Many (most?) modern bridges need reinforced concrete to allow the concrete to bear tensile loads. Reinforcing concrete introduces new ways that concrete can degrade such as rusting of the reinforcing elements. I'm not really sure Roman concrete would be much help here.