Montreal’s Champlain Bridge slowly reopened to traffic Saturday after overnight emergency repair work was delayed by several hours.

The bridge, one of Canada’s busiest, was fully closed off to traffic at 2 a.m. Saturday, as crews installed a massive 75-tonne “superbeam.”

The bridge was expected to reopen at 7 a.m., but crews fell behind schedule when a median had to be removed so trucks carrying the giant concrete beam could load it onto the bridge.

The bridge authority was forced to speed up an emergency repair of the bridge after discovering in November a two-millimetre crack in one of the concrete beams. That crack has since doubled in size.

The affected beam was corroded from inside after years of exposure to road salt.

Some lanes had to be closed in order to reduce weight on the beam, causing major congestion in recent weeks for Montreal’s commuters.

The six-kilometre bridge crosses the St. Lawrence River and connects Montreal boroughs to the South Shore. With nearly 160,000 daily crossings, it is also where $20 billion worth of international trade travels every year.

The condition of the bridge has become a lightning rod for criticism on infrastructure needs in Canada.

NDP Leader Thomas Mulcair recently attacked the Conservative government's handling of the file, saying construction of a new bridge should have been in place a long time ago.

The federal government has said the bridge will be replaced by a toll bridge, which is expected to cost between $3 and $5 billion.

Construction is due to begin in 2021, but Ottawa said recently it was looking at speeding up the process.

Experts say emergency Band-Aids like the one applied to the Champlain Bridge this weekend will become commonplace as infrastructure continues to age and government investment declines.

“I would expect that we’ll see more disasters like this happening before we can get our infrastructure back up to snuff,” said Hugh Mackenzie, of the Canadian Centre for Policy Alternatives.

The repair work to the Champlain Bridge comes as Toronto prepares to demolish the 100-year-old Dufferin Bridge, which was deemed structurally unsafe a few months ago.

University of Toronto architect professor David Lieberman says the need to upkeep infrastructure like bridges, sewers and subways, can no longer be ignored.

“It is a cautionary tale for cities throughout the country and across North America,” he told CTV News.

With a report from CTV News’ Daniele Hamamdjian and with files from CTV Montreal