As cranes and other heavy equipment machinery began the arduous task of removing metal and cement materials from the I-35W bridge site Aug. 22, the National Transportation Safety Board began looking into the bridge’s de-icing system as a possible factor in the collapse.
Since 1999, a chemical had been sprayed on the I-35W bridge to prevent the formation of black ice caused by mist from nearby St. Anthony Falls. It used temperature- and precipitation-activated nozzles embedded in the bridge deck to spray a chemical on the roadway.
According to a 2001 Minnesota Department of Transportation report, the chemical used, CF7, is a liquid potassium acetate manufactured by Cryotech, a division of San Diego-based General Atomics. The relatively expensive chemical was selected in part “because it is safer for structural steel and reinforcing steel embedded in concrete.”
When CF7 was spilled on a grate, it was found to react with galvanized metals, however. Cryotech issued a technical bulletin in 2005 stating that a slow reaction can occur when potassium acetate and zinc come into prolonged contact. Zinc is used to galvanize steel.
An update from the company said the reaction was not an issue during normal use, and citing an MnDOT project investigating whether the chemical was causing advanced deterioration on the bridge, concluded that the galvanization on the bridge components was thick enough.
A review of MnDOT inspection reports from 1999 through 2006 revealed just one problem specifically linked to the de-icing system—in 2006, inspectors cited “paint failure from leaking de-icing system” on one girder. The report said that overall 15% of the paint was unsound, though it did not cite the chemical as the cause.
Safety Board spokesman Terry Williams said the scrutiny of the system is a standard part of investigation, “one of many things” that will be studied.
Shortly after the Aug. 1 collapse, assistant to the commissioner at MnDOT Bob McFarlin dismissed the notion that it was related to corrosion from de-icing chemicals.
MnDOT was not concerned about the system, and is planning to install a similar one on the bridge’s replacement, engineer for MnDOT’s Metro District Khani Sahebjam said Aug. 22.
The $9 million bridge resurfacing contract with Progressive Contractors Inc., whose crews were working on the bridge when it collapsed, included $397,000 to reconstruct the de-icing system.
