The plaque says that this is a pre-stressed concrete bridge. What that means is that it has steel bars going through it that are not just passive but have actually been tightened up so that they are squeezing the concrete. Concrete is incredibly strong under compression - it can been crushed with a huge force before it breaks. But it is not good when stretched under tension. The grains of sand come apart easily. By compressing it first, it makes sure that the concrete can carry a greater load than usual before reaching its limit for tension forces. This comes along the bottom side of the bridge.
