Based off the design description, I would guess that the pinned crown connection in the “canticrete” structure has failed in some form. This would cause the two ends of the arched cantilevers to act independently and thus create the visible offset seen in the photos. What actual forces are causing the problem is hard to determine without visiting the site and maybe seeing some bridge plans. It could be a settlement problem at the foundation or some other structural dilemma.
The spalled concrete with the exposed rebar while not desirable is quite a common occurrence on structures of this age. The settlement or offset problem will control whether this structure can be saved not necessarily the spalled concrete which can easily be repaired for the most part.
Just thought I would throw my two cents in on the matter.
MG
I am not an engineer. Clearly the temporary repairs have been in place for some time already and there appears to be no recent movement. I suspect that one pier shifted or "sunk" some years back. Of large concern to me are the two large areas of spalled and fallen concrete with visible and rusted structural steel visible.
Is the unusual design the reason for the failure mid-span?
If this bridge isn't rehabbed soon it will have to be replaced--that is a serious failure going on there!
This "canticrete" bridge is unusual. I don't know anything about it, but it sounds like the pin in the center is important, sort of like a three hinged steel arch? In Michigan, we have what is simply called a cantilevered concrete arch and it has no pin at the center and in fact there is a gap showing that each half is independent structurally (since cantilevers get their support from the pier) http://www.historicbridges.org/other/Belle/little_dscf7744.j...
http://www.historicbridges.org/other/Belle/