ARCHIVED - A longer lifespan for bridges
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March 08, 2008— Ottawa, Ontario
Canadian winters may be hard on our cars, our clothing and our dispositions, but they're equally hard on the infrastructure that we use every day. The de-icing salts that help to keep us safe on the road can have a corrosive and damaging effect on reinforced concrete structures such as bridges.
"Concrete is a porous material, and moisture allows the salt to get into the pores," says Dr. Daniel Cusson of the NRC Institute for Research in Construction (NRC-IRC) in Ottawa. Over time, the salt penetrates the concrete, and when the concentration of salt around the steel reinforcement exceeds a critical threshold, the steel starts to corrode.
|The Vachon Bridge in Laval, Quebec, was the test site for a 10 year NRC study on the performance of corrosion inhibiting systems for concrete bridges.|
"Adding to the problem is the fact that many of Canada's highway bridges built in the 1960s and 1970s are approaching the end of their expected life spans," says Dr. Cusson. According to Statistics Canada, the average age of bridges is rising due to insufficient investments in bridge maintenance and renewal. As of 2007, Canada's bridges on average have passed 57 percent of their useful life.
Dr. Cusson and his research team recently completed a 10-year study of corrosion on the Vachon Bridge in Laval, Quebec. Their goal was to identify effective technologies that can delay corrosion and extend the lifespan of Canada's concrete bridges.
|There are many products on the market to delay the corrosion of reinforced concrete structures such as bridges. Testing their effectiveness under field conditions will help engineers to make informed choices.|
Working with the Ministère des transports du Quebec (MTQ) and several product manufacturers, Dr. Cusson's team took advantage of a major rehabilitation of the Vachon Bridge in 1996 to install nine different corrosion inhibiting systems. These systems have been used for decades to extend the life of bridges; however, there is limited scientific evidence and field data on how well they actually work in the field.
"We invited the manufacturers of each system to install their system their way," says Dr. Cusson. "We wanted it to be a real life scenario." The nine systems were installed in one of the barrier walls on the side of the bridge. They included various combinations of additives in the concrete mixture, protective coatings over the steel reinforcement, and sealers applied to the concrete surface.
The goal was to expose each system to the same rain, snow, ice and salt for 10 years and then compare steel corrosion and concrete damage, if any, among all the test sections of the barrier wall. Since a decade is not a long time in the life of a bridge, the team also embedded steel bars at shallow depths in the concrete barrier wall to shorten the time needed to assess corrosion. Once a year, they took non-destructive electrochemical readings on the surface of the concrete to find out whether corrosion might be happening deep within. The team also took core samples from the bridge for more testing, and measured the environmental conditions with sensors that were embedded in the bridge during reconstruction.
After 10 years, the test results confirmed that one of the most effective corrosion inhibiting systems was also one of the simplest. An inorganic admixture made with calcium nitrate, which was added to the concrete before casting, performed equally well or better than more complicated systems that included sealers applied to the concrete or coatings on the steel bars. "This is one of the least labour intensive systems," says Dr. Cusson.
The results of the study could be applied to other concrete portions of a bridge, such as the piers or concrete slabs. "This type of field data will help us to develop models that can accurately predict the behaviour of concrete structures using various corrosion inhibiting systems," says Dr. Cusson.
|The use of low permeability concrete combined with a corrosion inhibitor was found to offer good protection against reinforcement corrosion.|
The main recommendation from the study is to use good quality, low permeability concrete and a sufficiently thick concrete cover as the first line of defence against corrosion of the reinforcing bars (as was done during the rehabilitation of the Vachon Bridge). After that, a good corrosion inhibiting system such as the one identified in the study will help to ensure a long life for concrete bridges.
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National Research Council of Canada
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