Windsor Essex Mobility Group, which is responsible for the $1.4-billion Windsor parkway, said 508 of the 562 concrete girders identified by the panel will be replaced, which will delay the project.
The panel found that tack welding was used in assembling the girders, which violates federal code. It also makes steel brittle and susceptible to early corrosion, it said.
The consortium said it will replace 500 girders made in Ontario by Freyssinet Canada Ltd. at no cost to the province. They include 288 girders that were installed and 212 that were made but not installed.
Eight girders made by Prestressed Systems Inc. and installed on the North Talbot Road bridge will also be replaced, it said.
The overpass was shut down Aug. 29 when concerns arose over the safety of the support beams.
But the consortium said the decision is "unrelated" to the use of tack welding in the assembly of the girders, saying it's been used for years.
It's replacing the Freyssinet girders because new information came to light during recent inspections and testing of some of the girders, WEMG said.
"I don't agree with a lot of the stuff that's in the report itself," said Michael Hatchell, WEMG's technical director.
"We've made the decision we've made, we think it's the best for the province to move forward."
Transportation Minister Glen Murray said he's "pleased" that WEMG "has decided to do the right thing and take responsibility for the actions of its subcontractor."
"From the beginning, I have been clear that the girders in question would be removed, unless their safety and durability could be assured," he said in a statement.
Last month, Murray said the government wouldn't replace the Freyssinet girders, even though the panel couldn't confirm that they were safe. Instead, he said the government would monitor them to see if they show signs of cracking or corrosion and fix them if necessary.
The panel revealed Friday in its second report that 62 of Prestressed's support beams don't meet federal code either.
They include the eight girders that are to be replaced and another 54 that were used in constructing a tunnel.
Not only was tack welding used in the 62 Prestressed girders, but the concrete strength was reduced, which "made matters worse," the report said.
Since the tunnel isn't used for heavy traffic, the 54 girders' "structural deficiency is therefore relatively insignificant," the report said. But durability is still an issue because tack welding makes them vulnerable to accelerated corrosion.
"Corrosion of steel, if not arrested in time, can cause spalling, and falling chunks of concrete on highway traffic," the report said.
"In a worst case scenario, corrosion that may not be visible to a naked eye could weaken the structure to a point that failure could take place without any warning."
Hatchell said the company isn't concerned about corrosion and is working with the government and conducting inspections.
All girders have some small cracks, but it's not a structural concern because most of them are covered in concrete, he said.
Infrastructure Ontario was aware that there was a problem with the girders last December, months after the company in charge of the project knew that they were made using tack welding, the report said.
WEMG, whose engineers warned against tack welding, approved its use by both Freyssinet and Prestressed, it said. WEMG didn't tell the government or the expert panel about it until the end of August.
"We did allow it to happen, but again, I have no concerns about tack welds themselves and what's there," Hatchell said.
"And the testing and the things that we've done have actually shown that it doesn't, it would not impact it from a structural standpoint."
The panel said some of the Freyssinet girders were manufactured before the Ontario plant obtained certification from the Canadian Standards Association, which was responsible for quality control. Most were installed in two tunnels.
Infrastructure Ontario and the Ministry of Transportation discovered that tack welding was used during a meeting with WEMG and Freyssinet in December 2012. Murray says he wasn't aware of the problem until May.
The Liberals stopped the installation in July, but critics say they should have acted months earlier.
In the end, the buck stops with the governing Liberals, said New Democrat Taras Natyshak, who represents the riding of Essex.
People in his community want to know how the girders were allowed to be produced and installed without anyone noticing that they didn't meet federal requirements, he said.
"How did we get this far without the proper oversight?" he said. "Those questions have yet to be answered by the minister."
The Herb Gray Parkway is an 11-kilometre corridor that includes 11 tunnel sections covering 1.8 kilometres of freeway, a six-lane below-grade freeway from Highway 401 and a four-lane at-grade road extension from Highway 3.