May 28, 2010
Research suggests changing steel mix could extend lifecycle of bridges
Should we build our bridges in the future with stainless steel? Even given the high cost, the idea is not entirely a joke.
Research from the University of Windsor suggests that varying the amounts of chromium, molybdenum with nickel and copper might result in bridge components that are more corrosion resistant, require less maintenance and last longer, reducing the overall cost of ownership.
In fact, say researchers, it could lead to steel box girder bridges with a lifecycle of 75 years, double the current 30 to 40 years.
Sridhar Ramamurthy, senior researcher at Surface Science Western (SSW), a commercial and research materials analysis facility at the University of Western Ontario, says there’s some promising data coming out of research on rust around steel box bridges.
SSW has partnered with the Ministry of Transportation Ontario (MTO), Essar Steel Algoma Inc. and The Canadian Institute for Steel Construction (CISC) to scratch deeper past that patina and find what causes some steel to rust in some places more than others.
“The problem, of course, is corrosion,” says Ramamurthy, noting that oxidization in itself is not necessarily a bad thing. That layer of rust, in fact, coats the metal, preventing further penetration.
It’s when the patina falls off, he says, that things start to rot, since the metal surface is no longer “protected” and the cyclical process of deeper oxidization and disintegration continues.
“It’s also falling on cars below as they pass, which is one of the big problems for the MTO and why we are studying it,” says Ramamurthy.
Ramamurthy’s team has put samples of the patina to the test under electron microscopy, elemental X-ray analysis, laser Raman spectroscopy and Mossbauer spectroscopy.
“The findings were very clear,” says Ramamurthy. “In samples with more serious corrosion and de-bonding, we found a greater amount of a mineral called akaganeite.
In areas where corrosion was less prevalent, there was a greater amount of a mineral called goethite. This was our clue to the cause of the corrosion.”
The researchers didn’t have to look far for the cause of the corrosion. It was the usual villain — road salt.
The four common chloride salts used as de-icers in Canada are sodium chloride, calcium chloride, , magnesium chloride and potassium chloride, along with an anti-clumping agent, ferrocyanide salt. Sodium chloride is the most common, followed by calcium chloride and, to a lesser extent, magnesium chloride.
A combination of water and salts triggers akaganeite, which is more porous and this allows more salt penetration, which in time corrodes the steel girder, causing disintegration. Goethite, however, he says, is more compact and stable and offers more protection.
As it stands, Ontario’s steel box girder bridges typically need replacement after about 30 to 40 years — not a long life cycle.
“The bigger problem beyond whether they repaint the bridge or replace it, is managing the traffic, because it’s very disruptive,” says Ramamurthy.
The research is now looking at best practices to maintain and extend the life of existing bridges, with an eye eventually to designing steel box girder bridges with a 75-year lifecycle.
Among the best practices being explored, washing existing bridges to remove salt residues seems to help, says Ramamurthy, noting that this is producing good results in Japan.
Other coatings being explored include a zinc compound applied on-site, almost like galvanizing in place. There are a series of pilot projects that will track the effectiveness of various coating combinations.
Five different steel alloys have also been fabricated, adjusting the mix of copper, nickel, molybdenum and chromites to see which is better suited to weather the combination of road salt and the elements.
They are being tested in environmental chambers with salt added to test the metal’s resistance. When testing is complete, a bridge will be built with the best solution as identified by the data.
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