The accident happened around 11:30 a.m. ET.
One car's hood and windshield were heavily damaged after the concrete fell onto it, while several other cars experienced minor damage, mostly owing to debris on the ground that caused punctured tires, said Gino Paré of the SûretéduQuébec.
The right-hand lane of the eastbound Hwy 40/Trans-Canada was also closed and traffic was diverted to Highway 13.
Transports Québec officials closed the road so that workers could inspect the overpass structure.
They have not yet confirmed whether the concrete came from the overpass itself or from another source, such as a truck travelling on the overpass.
Transports Québec officials said they do not know when the roads will be reopened and are advising drivers to avoid the area.
No injuries have been reported.