MONTREAL - Canada's second-biggest city is being called a No. 1 example of the potential consequences of rushing into infrastructure projects and neglecting their upkeep.

Longtime structural engineers say Montreal's woes are an illustration for other Canadian cities of what could happen when a municipality doesn't have a solid maintenance plan in place — from the very beginning.

They made those remarks Monday, a day after part of a Montreal tunnel collapsed under the city's downtown core and slammed onto a major expressway.

The city's roadway problems are perhaps the most extreme example of what the Federation of Canadian Municipalities calls a $123-billion shortfall in municipal infrastructure spending across the country.

"It's like a cancer in a human being: if you catch it in time, it will be OK, but if you don't it will spread," said Saeed Mirza, a professor emeritus of civil engineering at McGill University.

"What I would like to say to the government is: if you cannot maintain it, do not build it."

The aging road network in the Montreal area has forced lane closures on several overpasses and bridges this summer, as engineers scramble to deal with the city's crumbling infrastructure.

The closed routes in Montreal have resulted in traffic-congestion nightmares across the city — even at the oddest hours — and there are fears that it will be prohibitively expensive simply to replace what already exists.

Amid public pressure Monday, the Quebec government released a pair of alarming engineering inspection reports conducted on the tunnel in 2008 and 2010.

The 2008 study warned that the tunnel was in a "critical" general state concerning "user safety," and it recommended "safety work to be undertaken." It also warned that parts of a duct bank could collapse onto passing cars.

Transport officials shut down the Ville-Marie expressway Sunday morning after a 15-metre-long concrete slab crumpled and smacked the road below.

There were no injuries as the morning collapse came during a quiet period along the normally busy stretch of highway — where an average of 100,000 vehicles roll by every weekday.

Following the cave-in, transport inspectors noticed another unsteady concrete beam in the same tunnel and used a crane to rip it down Monday. A spokeswoman for the department said workers are now trying to stabilize a third beam.

The falling concrete has brought back fears triggered by the 2006 collapse of a highway overpass in the neighbouring city of Laval — an incident that killed five people and injured several more.

Mirza said Montreal's problems stand out among Canadian cities because the city experienced a building boom during the 1960s and '70s, just ahead of Expo '67 and the 1976 Olympics.

Today, the island city's spaghetti-like network of overpasses and bridges prominently features an unsettling mix of exposed rusted steel, giant cracks, water leaks, and huge patches made of metal mesh.

"We built extensively and in a hurry and the result was that we did not have the same quality control on them as should have been exercised," said Mirza, adding that much of the infrastructure now eroding at 40 years old should have lasted 75 to 100 years.

"Because of lack of quality control, the quality was poor and these are the results we're seeing now."

Mirza said the problem was compounded by budget cuts in the provincial transport ministry that targeted maintenance programs in the early 1980s.

A change in philosophy is also necessary in the field of civil engineering, he added.

He said engineers across North America are trained to build, but they must also learn to focus on implementing detailed maintenance plans for their projects — such as when parts must be replaced and when repairs will be needed.

Mirza said he's disheartened by the state of the city's roadways and he doesn't buy attempts by provincial Transport Minister Sam Hamad — who is also a civil engineer — to reassure the public.

"(Hamad) says if anything is open it is safe — hell, it is not," he said.

"We are very lucky that nobody was hurt or killed."

Another civil engineering expert blamed Montreal's woes on its old infrastructure and the increasingly heavy traffic — a lethal combination other Canadian cities don't have to contend with.

Concordia University's Adel Hanna agreed Montreal's inspection procedures as very poor, and he urged other municipalities to pay attention, even when Quebec hasn't.

"We didn't learn our lessons from the Laval bridge," said Hanna, who has 32 years' experience in the field. "It's a lesson to everybody — it's a lesson to the entire country."

His colleague at Concordia, Bala Ashtakala, said the constant freezing and thawing during Montreal's harsh winters and the city's liberal use of road salt in winter, which penetrates the concrete, has shortened the lifespans of its infrastructure.