Hello, I've been lurking for a couple months but this was worth making an account for. The arches on this bridge are actually being strengthened with carbon fiber sheets. The white pasty stuff on the arch is the epoxy being used to adhere the carbon to the concrete, and the black sheet that the worker is holding up is the fiber that's about to go up. Then, the fiber will be sprayed with sand and painted over to match the concrete. I had a co-op the first half of this year working as a field engineer for the subcontractor doing this work. Here's the finished product on the span over the basketball courts, taken sometime late May: