Moisture-Cure Coatings Help With Wisconsin Bridge Renovation

By: Carl Angeloff P.E.

The 66-year-old Cass Street Bridge is an exceptionally busy thruway for commuters and a critical artery between LaCrosse, Wis., and La Crescent, Minn. The open-grid deck bridge features 18 spans over the Mississippi River with 17 piers that support more than 2,500 ft of steel.

After years of harsh Wisconsin winters, the heavily traveled bridge was showing its age and in need of extensive renovation. Dan Kleinertz, project manager, Wisconsin Department of Transportation, noted there were several problems with the deck. “While rain was able to drain through, cold weather brought hard-packed snow and ice that sunk into grid spaces and sat on the underlying structures.” This, combined with roadway deicing chemicals, resulted in significant corrosion to these areas.

To help solve the problem, the grid deck was replaced with a new concrete roadway and the underlying support system was fortified to support the new load.

The next step in the renovation process was painting. Rainbow Inc., the painting contractor, prepared the metal surfaces to meet SSPC standard SP-10, “Near White” Blast. Since bridges in the state are repainted every 20 to 25 years, a coating was needed that would offer long-lasting protection. Given Wisconsin’s inclement fall, winter and spring weather, and the project’s timetable (May 2005 to June 2006), a coating system that could also withstand wet weather during application and extend the application window was important to keeping the project on time and on budget.

Chosen was a three-part moisture curing urethane coating system from Sherwin-Williams. The system includes Corothane I Galvapac Zinc Primer , intermediate coat and Corothane I Ironox A topcoat. All three coatings contain polyurethane resins manufactured by Bayer MaterialScience . With these coatings, ambient humidity aids the curing process rather than threatens it. The three coatings in the system are VOC-compliant. They were designed for low-temperature application with conventional spray equipment at temperatures as low as 20°F.

“These coatings are great for extending the painting season, especially in colder climates,” said Mike Debuhr, industrial division manager, Rainbow Inc. One of the more versatile types of coatings for use on steel substrates, moisture-curing urethanes are surface-tolerant, quick drying and applicator-friendly. They offer excellent impact resistance and wear and abrasion resistance, providing long-term durability. In abrasion resistance tests, moisture-curing polyurethanes average only about 10 to 30 milligrams film loss.

Moisture-cured polyurethanes penetrate into pores and tight crevices, where moisture is usually present, to form strong chemical bonds, reducing the risk of blisters caused by water trapped under the coating film.

Department officials were pleased that the project was completed on time--June 30, 2006--and the bridge will provide yet another generation with convenient access to downtown La Crosse.