April 18, 2007
Bridges to undergo more inspections
Quebec government not demolishing 15 bridges
The media were scrambling last Friday when La Presse reported that Transport Quebec was planning to demolish and rebuild 15 highway overpasses over the next five years.
The overpasses have a similar design to the one that partially collapsed in Laval and claimed five lives.
However, Mario St. Pierre, a Transport Quebec spokesperson, said the bridges, which were inspected last October following the Laval incident, will be thoroughly inspected on a yearly basis and the results of those inspections will determine whether they will be repaired or rebuilt.
“We were talking about four bridges — the de la Concorde bridge that collapsed and the de Blois bridge which was demolished as a safety precaution,” he said. “There was a bridge near Joliette, over Highway 31, that was reinforced last fall and is very safe. The other is a little bridge in the Eastern Townships, an overpass over Highway 10 and will be demolished and replaced by the end of the year.”
The 15 bridges at the centre of the La Presse report, St. Pierre said, are “not exactly the same design. It is a modified type of cantilever bridge.
“We are going to monitor them more closely.”
The Angrignon overpass in Montreal, which spans Highway 20, is one of the structures that will be inspected. Close to 140,000 vehicles pass under it every day.
Bala Ashtakala, a professor emeritus with Concordia University, who specializes in transportation engineering, unofficially examined the Angrignon overpass last Friday.
“As it stands now, there is no danger, but you never know what is happening inside,” he said. “Damage is there, but it is not very extensive. I saw reinforcing rods on the pillars and on the beams. That means the concrete is gone. It had chipped off. There is also a wire mesh underneath the overpass. It shows there is probably more chipping concrete.”
Ashtakala explained that concrete becomes brittle over time and the bonding becomes weak.
“It has nothing to do with the design. It is the materials, concrete and steel, where the real problem is. Concrete is chipping off and the steel is exposed to the natural elements like the rain, snow and sleet and with elements such as salt, it deteriorates fast.
“Cement is meant to be an integral part of reinforced concrete,” he added. “Once you just have a bunch of steel rods, they don’t maintain the strength. The rebar needs protection.”
The overpass was built in 1966.
“Its life is almost over,” said Ashtakala. “Over the next few years, until they replace it, they have to be very careful.
“In this case, it is better to replace and put up a new bridge. Regardless of how it was maintained, the age element is there.”
Ashtakala endorses the annual inspections and urges modern equipment be used to determine the state of the internal workings of the structures to detect faults and cavities.
This should be bolstered by taking core samples to examine the density and strength of the materials.
“A bridge can collapse any time,” he said. “There are things we should be guarding against. At the de la Concorde bridge, the (inspector) saw all this white stuff coming from the crack. That was cement that had disintegrated — it had combined with water. We call that ‘frying’ and that happens inside.
“When you mix cement with water and all that, after a period of time, it changes chemically. You see this on many sidewalks.”
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