Just hours after the Minneapolis I-35W bridge collapse on August 1, 2007, we pointed out that a de-icing system was installed on the ill-fated bridge in 1999.
Now the NTSB is looking at that de-icing system, according to today's St. Paul Pioneer Press. The article quotes several recent research papers that conclude the active agent in the liquid de-icer, potassium acetate, can be as caustic as lye when in contact with certain kinds of concrete aggregate (the rocks and sand that are mixed with cement to form concrete) and cause severe corrosion, as seen at the Colorado Springs Airport in Colorado, which took 18 months to repair. Most intriguingly, the article quotes two Finnish researchers at the Helsinki University of Technology, Ville Alatyppo and Jarkko Valtonen, who told the Pioneer Press, "in addition to degrading the concrete they [the deicing chemicals] could have 'eaten' the reinforcing rebar within the roadway."
Most speculation as to the cause of the collapse has centered around the steel gusset plates that held the box girders together. But the line of inquiry into potassium acetate will no doubt focus not only on whether the chemical weakened the concrete deck, but whether it might have dripped down and worked away at the steel superstructure that supported the bridge. Chillingly, the company that makes the de-icing system used on the I-35W bridge, Cryotech, is looking into alternative chemicals should fears about potassium acetate pan out. Such systems have become increasingly common on American and European bridges over the last two decades.
Another question that will no doubt be asked by investigators: what effect does the combination of potassium acetate and salty water have on steel and concrete? Afterall, the de-icing system was on the bridge, but the Interstate highway was liberally spread with salt during snowstorms.