Montreal's Champlain Bridge may even be closed to traffic if there are any doubts about its safety and security.
Glen Carlin, the general manager of the corporation which oversees Montreal's federally-run bridges, said the cracked beam is being inspected twice daily.
He insisted the corporation will take whatever measures are needed to ensure the safety of the structure.
"If we have to close three lanes, we will close three lanes," he said. "If we have to close the bridge, we will close the bridge."
In the short term, a steel "super" beam will be used to support a girder in the middle of the bridge after a two-millimetre crack widened.
The crack was discovered last week and other fissures have also been found in the beam since then.
The cracked girder is one of seven under the span which support the six lanes of the bridge.
Carlin said the other six girders are still in good condition, but he admitted to reporters Friday that the problem beam deteriorated faster than expected.
"We did not anticipate that this beam would continue to present structural cracks," he said.
"Last Friday, our repair method was completely different. We said, 'OK, it's under control and we have a minor crack which doesn't seem to be developing'.
"We did not expect to see the beam have signs of distress show up so quickly."
Carlin said the 75-tonne super beam, which was delivered in 2009, will be installed by mid-December. The three-metre-high beam, which cost $350,000, will be placed on top of the deteriorating girder.
Next spring, it is to be replaced with a modular truss which will be installed underneath the problem beam.
Carlin has previously estimated that 350 beams on the bridge are in different states of deterioration.
In the meantime, truck traffic will be restricted and speed will be reduced for all vehicles that use the bridge.
During an afternoon news conference, Quebec Transport Minister Sylvain Gaudreault said if the bridge has to be closed to traffic — for any reason — authorities are ready.
"We have scenarios in case of a complete closure," he said Friday. "But we hope that won't happen."
The Champlain Bridge was built in 1962 and plans are underway to replace it with a new structure in 2021. But federal officials are now looking at speeding up construction of the new bridge which will cost between $3 billion and $5 billion.
Quebec Premier Pauline Marois told reporters in Montreal she hopes the federal government will begin work on the new bridge as quickly as possible.
"What's urgent in the meantime is that we proceed with the construction of the new bridge and that the plans and preliminary estimates be adopted as quickly as possible," she said.
"We're ready to collaborate 100 per cent with the federal (government) so that it goes as quickly as possible."
Federal Transport Minister Denis Lebel said the government is working to bring forward — as much as possible — the construction of the new bridge on the St. Lawrence River.
"I'll be communicating a new agenda in the coming weeks," he said Friday during a visit to Saint-Felicien.
Lebel also tried to offer assurances that the bridge is safe, telling reporters: "the best engineering firms in the world in Canada are working on the dossier."
Every year, more than 50 million vehicles use the bridge, which links Montreal with its south shore.
Transport Canada also says on its website that 6.2 million trucks make up 10 per cent of the bridge's traffic and 200,000 buses also use it.
The federal department says the bridge was not designed to handle today's high volume of traffic and the use of de-icing salt has contributed to corrosion and the degradation of concrete.
(With files from Peter Rakobowchuk)