Agreed. The deck is almost certainly composite with the girder, meaning the piece that crumbled and fell was an integral part of the structural system. It would likely cost more to restore the structural integrity of the deck/girder top flange, than to replace the entire superstructure, or at least that entire span.
That looks like a classic case of re bar in the base for the main structural slab then being used as a cantilever the bending changes and the re bar is in the wrong location.
This looks like composite concrete with the girder being a bulb T.
If this was a job handed to me I would (as the previous responders indicate)
Close access to the public as the walkway cant be safe for pedestrians.
Start a hunt for a structural engineer specializing in concrete structure restoration.
I agree with Bridge Smith that attempting to repair this is likely to be substantially more costly than replacement of the structure. I am also concerned that whatever caused this failure could be present in the remaining (unfailed) spans.