The New Democrats say documents show that Premier Kathleen Wynne came clean in July, 2013 when Ministry of Transportation staff suspected that The Windsor Star would break the story during her campaign stop in the city.
They say the documents also show that the ministry staff had discussed the problem internally since late 2012.
An expert panel later appointed by the government questioned the durability and safety of hundreds of support beams used in the construction of the $1.4-billion Windsor parkway.
The panel found that tack welding was used in assembling the girders, which violates federal code.
The company responsible for the parkway project has promised to replace 508 of the 562 concrete girders identified by the panel at no cost to the province.
The NDP say they find it hard to believe that Transportation Minister Glen Murray wasn't aware of the safety concerns before June 2013.
But Murray's spokesman maintains the minister was first briefed by officials in June and took action to ensure that no other girders were installed.
The expert panel's report said Infrastructure Ontario was aware that there was a problem with the girders in December 2012, months after the company in charge of the project — Windsor Essex Mobility Group — knew that they were made using tack welding.
But the consortium said last November that the decision to replace them is unrelated to the use of tack welding in the assembly of the girders, saying it's been used for years.