When engineers from Short Elliot and Hendrickson Inc., St. Paul, Minn., were asked to specify a coating for a bridge in International Falls, Minn., one of their primary concerns was selecting a coating that could be applied in cold temperatures and withstand harsh weather over time. They also needed a coating that could speed job turnaround. After all, as one of the coldest states in the continental U.S., their painting season is short.
Owned by Minnesota, Dakota & Western Railway Co. and the International Bridge & Terminal Co., the bridge serves as an important crossing on the U.S.-Canada border. More than 1 million trucks and cars use the bridge every year. The original through-truss steel bridge was constructed in 1908. A concrete bridge to accommodate increased traffic was added next to the bridge in 1979.
Spanning approximately 940 ft over the Rainy River, the bridge’s primary purpose is to convey trains and process piping that runs from one section of a paper mill to another located across the river in Fort Francis, Ont.
After completion of a bridge inspection and coatings evaluation process, the engineering firm recommended a coating system consisting of Corothane I Galvapac Zinc Primer and Fast Clad Urethane from Sherwin-Williams. The system, which incorporates new resin technology, was selected for its ability to provide long-lasting protection against corrosion and abrasion. An additional benefit is that it is a two-coat, not a three-coat system, so it has faster turnaround.
The two-coat system has a proven track record in other parts of the country. The Northeast Protective Coatings Committee has tested and recently approved it for new and 100% bare existing steel for bridges. The coatings are the only two-coat system approved for such projects.
Corothane I Galvapac Zinc Primer is a moisture-curing urethane, zinc-rich primer. It offers excellent bonding over blast- or tool-cleaned steel surfaces. The primer can be applied at 3 to 4 mils dry film thickness (DTF).
Fast Clad Urethane, the topcoat, features new polyaspartic resin technology that provides excellent application features like high-volume solids, a tightly cross-linked film with no outgassing and sharp edge retention, and excellent color and gloss retention. The raw material resins for the paint were supplied by Bayer MaterialScience LLC. For the International Falls project, the coating was applied at 6 to 9 mils DTF.
The primer and topcoat are VOC-compliant, can be applied at temperatures as low as 20°F and dry fast, so painters can work quickly. Painting crews also save time because the system requires only two coats. Doni Riddle, vice president of industrial and marine marketing at Sherwin-Williams, noted that the system provides the same long-term performance and protection as conventional, three-coat systems. The coatings are applied with conventional spray equipment.
“The two-coat system allows a bridge to be painted in two-thirds the time it would normally take, with a cost savings of approximately 22%,” he said.