The head of a Quebec public inquiry probing the collapse of a Laval highway overpass last fall has warned the government that other road structures are deteriorating.

Pierre Marc Johnson, who is leading the provincial investigation into the Concorde overpass collapse, told the inquiry on Wednesday that for reasons of "public security," he alerted the Transport Department about his concerns about other roads.

"Researchers have demonstrated evidence of risk factors that could apply to other road works," Johnson said.

"For reasons of public safety, the commission has decided to alert the administrative authorities of the Quebec government."

Johnson would not elaborate on the risks he referred to, but said the government has acknowledged his concerns.

Transport officials would not specify which road structures were singled out by Johnson, but insisted there is no immediate danger.

"There is nothing to worry about in particular for the moment," said Jacques Gagnon, the department's assistant deputy minister. "We will check the plans."

Many factors caused Concorde collapse: expert

The warning came as expert witnesses told the inquiry that the deadly overpass collapse on Sept. 30 2006 was likely due to a combination of dodgy design, the use of inappropriate building materials and shoddy construction.

A section of the Concorde overpass over Highway 19 cracked and plunged to the ground below, crushing five people, including a pregnant woman.

Jacques Marchand, a Laval University civil engineering professor, told the inquiry he believes the southern section of the overpass broke off because a deep fissure in the structure was allowed to spread unchecked.

The type of concrete used to build the overpass was also not suitable for harsh Quebec winters because it is porous and couldn't adjust to annual freezing and thawing. 

The concrete reinforcement bars designed to reinforce the overpass were not installed properly, Marchand said in his testimony. And a repair job in 1992 to fix an expansion joint did nothing to reinforce it, he said.

Marchand was one of several engineers allowed to survey the overpass collapse scene last fall, before the structure was dismantled.

Other expert witnesses who examined the scene will testify at the inquiry this week.

The commission's final report is due in the fall.

With files from the Canadian Press