[Arrowhead1952]

It may be old pavement overlay.
They were working on the inside lanes.

I haven't ever seen overlay that dark with stripes. I saw another picture that the dark pavement seemed to be thicker than the active traffic surface.

[concretebob]

Depends on the material. If it was temporary, they may have used asphalt, which accounts for the stark contrast.
It looks like they were re-surfacing the bridge with some very dark material.
Without knowing exactly what type of repairs, or how they executed the work procedures, anything I say is speculation based on experience.
They had to have access to the center lanes to perform the work.
Logistically, a bridge is the worst structure to execute repairs, because there are usually only two ingress/egress points for equipment and materials. (Each end of the bridge.)
That darker lane could be the completed overlay, which would have had no traffic on it, until all inside lanes were complete.