Quebec has released long-awaited inspection reports on Montreal's Mercier Bridge that confirm rapidly accelerating decay in the aging structure forced its closure earlier this summer.

Corrosion noted in a 2011 inspection was so advanced that some bridge parts were perforated and deformed, the reports say. 

In particular, the report said, 10 gusset plates that hold beams in an interlocking pattern are severely eroded.

Of 346 bridge parts inspected, 86 were given a "1" rating, meaning that they were deemed "incapable of perfoming required task."

The 2011 report was dated June 11, and the Transport Ministry banned most traffic from the bridge three days later, citing the need for critical repairs to remedy safety-threatening corrosion and rust.

But Transport Minister Pierre Moreau was quick to point out that long-term repairs to remedy decay were underway when the span was shut down.

"The deterioration was going at a faster rate than what we expected," but the  bridge was in no danger of collapsing, Moreau said at a news conference Monday.

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Rust was so advanced on some bridge sections that perforations were visible to inspectors. (CBC)

All emergency repairs have been completed, but other work is in progress.

The Mercier Bridge partially reopened Sept. 6, with remaining lanes scheduled to open in December if all repairs are completed.

The summer closure angered South Shore residents, officials and business owners who rely on the Mercier Bridge for daily commutes into the city.

The Transport Ministry has released inspection reports from 2007, 2008, 2009 and 2011 on its website.

The Mercier Bridge comprises two structures, one built in the 1930s and another inbound arm built in the 1960s.

With files from the Canadian Press, Alison Northcott