A detour is now in place in Pointe-Claire while crews work to secure the structure.
Officials say the structure is safe, however, and the damage is only superficial.
The bits of concrete started falling from the St.John's overpass, which travels over Donegani Avenue, yesterday afternoon.
Safety officials closed down the road underneath to vehicle and pedestrian traffic a short time later. Vehicles are still able to drive over the top of the overpass.
Bill McMurchie, mayor of Pointe Claire, said inspectors were brought in right away.
"There are other pieces of concrete in the same area that appear to be loose," he said.
"Repair crews at the present time are removing anything that appears to be less than firmly attached."
He says inspection reports show there is no structural danger.
Crews are almost finished the work on the structure. Donegani Avenue is expected to reopen this afternoon.