Since 1999, a chemical had been sprayed on the I-35W span to prevent the formation of black ice caused by mist from nearby St. Anthony Falls.
Federal investigators are looking at the Interstate 35W bridge's automatic de-icing system as one of the many facets of their probe, officials said Wednesday.
The National Transportation Safety Board, which is conducting an investigation of the bridge collapse on the Mississippi River as well as orchestrating debris removal at the site, announced that investigators are examining the chemical used in the de-icing system and "what type of corrosive properties it might have."
Safety Board spokesman Terry Williams said it is a standard part of investigation -- "one of the many things" that will be looked at.
The de-icing system examination comes after Safety Board officials said on Aug. 8 that they were looking at the design and makeup of steel connecting plates known as gusset plates, and the loads and stresses they bore.
On that day, Secretary of Transportation Mary Peters issued the first national alert to stem from the disaster, telling bridge engineers nationwide to "carefully consider the additional weight placed on bridges during construction or repair projects."
No such alert was planned on de-icing systems Wednesday, according to a Federal Highway Administration spokeswoman. One other bridge in the metro area also has a de-icing system: the I-35E Lexington Bridge south of downtown St. Paul.
The Safety Board said on Wednesday that it had determined that loads and construction vehicles on the bridge the day of the collapse weighed about 575,000 pounds. One expert observer said the weight didn't cause him concern.
Shortly after the bridge failed, Bob McFarlin, assistant to the commissioner at the Minnesota Department of Transportation, dismissed speculation that the collapse was somehow linked to corrosion from de-icing chemicals.
MnDOT was not concerned about the de-icing system on the I-35W bridge, and is planning to install a similar system on the bridge's replacement, Khani Sahebjam, the engineer for MnDOT's Metro District, said Wednesday.
Black ice was a problem
The de-icing system on the I-35W bridge was installed in 1999 to combat dangerous black ice that formed on the bridge because of the mist from the 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 MnDOT report, the agency chose a relatively expensive de-icing chemical called CF7, a liquid potassium acetate manufactured by Cryotech, a division of San Diego-based General Atomics. The agency said it selected the chemical, in part, "because it is safer for structural steel and reinforcing steel embedded in concrete" and is readily biodegradable.
"Also the fluid contains no nitrogen or chlorides," the report said. "Therefore, CF7 is considered much safer for the environment than glycol, urea, or chloride-based anti-icing chemicals."
MnDOT discovered that the chemical reacted with galvanized metals, however, when some was spilled on a grate. Cryotech issued a technical bulletin in 2005 saying a slow reaction can occur when potassium acetate and zinc come into prolonged contact. Zinc is used to galvanize steel.
In an update, the company said the reaction was not an issue during normal use. It cited a MnDOT project looking into whether the anti-icing chemical was creating advanced deterioration on the I-35W bridge. It concluded that the galvanization on the bridge components was thick enough.
Cryotech could not be reached for comment on Wednesday.
MnDOT records show that zinc-based paint was used on portions of the bridge in 1999 to try to stem corrosion. A review of MnDOT inspection reports from 1999 through 2006 found 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 percent of the paint was unsound, though it did not cite the de-icing chemical as the cause.
The $9 million bridge resurfacing contract with Progressive Contractors Inc., whose crews were working on the bridge at the time of collapse, included $397,000 to reconstruct the anti-icing system.