Toronto police said the concrete appears to have fallen near the area where the eastbound Gardiner splits into the northbound Don Valley Parkway and eastbound Lake Shore Blvd.
Police tweeted the concrete fell from the eastbound Gardiner onto the westbound Gardiner.
A CBC manager driving on the westbound Gardiner around 9 a.m. reported seeing “very large chunks of concrete” on the roadway.
The left lane of the eastbound Gardiner will be closed as city crews clean up.
Concrete falling from the expressway has been a recurring issue for the city. At the end of February, CBC Toronto spoke with one woman whose windshield was smashed by falling concrete.
At the time, city officials gave assurances that the elevated highway is structurally sound.