Police said the driver was not injured in the incident, which occurred near Yonge Street around 11 a.m. The off-ramp from the westbound lanes was closed, as were two lanes on Lake Shore Boulevard. All roads were reopened by mid-afternoon.
The debris hit a Mercedes sedan just east of Yonge Street. The driver of the vehicle, John Pandell, told CBC's Steven Bull that he wasn't sure at first what had happened after the debris fell on his car.
"It was like a bullet sound. So I just pulled over. And obviously something had hit my windshield and my car," he said.
"I was just kind of shaken to be honest with you about the whole thing."
Pandell then called police.
The incident comes as Toronto crews carry out a weeks-long inspection and repair blitz of the Gardiner between Jarvis and Bay streets.
Area identified as problem spot
Toronto staff had identified the area from which the piece fell on Monday as one that crews would do some "controlled chipping" on, meaning suspicious areas of the concrete were to be chipped off as a corrective measure, said John Bryson, a manager of the city's structures and expressways.
But staff had been unable to reach it in time, and other parts over sidewalks and roadways that were considered "critical areas" received attention first, said Bryson.
"This area was identified as an area to be done, but it wasn't put on the critical area [list]. But now a piece fell, so we are now working at it," Bryson said.
The piece that hit the car was between 10 and 13 centimetres long. It hit the hood, windshield and the top of the car, which suffered minor damage, Bryson said.
The incident comes after concrete fell from other parts of the busy expressway on three separate occasions in May.
Following the incidents, Toronto staff briefed a council committee on the state of the Gardiner, which is in need of significant rehabilitation over the next 10 years.
Coun. Denzil Minnan-Wong, chair of the public works committee, has called for more thorough and frequent inspections for the Gardiner, which staff say is inspected each year.