It is some kind of policy they use in Illinois. It has to do with how they construct new highways. They tend to build the overpass first then they let it stand for a while for some reason. I dont claim to be some kind of bridge construction expert. Maybe it makes the overpasses last longer or something.
Of course this could be construction slowdown technique that Unions use. I know often that construction funding comes in phases. So maybe they do the overpasses first because it costs so much.
I think a lot depends on if the support structures were damaged or not. I dont know what that kind of heat can do to reinforced concrete. They had a barge hit a footing on the bridge once in St Louis, MO and everyone was freaking out.