Over the last three decades, industrial maintenance coating systems used to protect steel bridges from corrosion have improved dramatically as alkyd paints containing lead and chromate have been replaced by advanced technology zinc-rich urethane primers and fluoropolymer topcoats. Fueling this evolution toward more durable, higher value-added coating systems has been a fundamental understanding of the life-cycle cost (LCC) impacts of bridge painting operations.

Extensive research studies of bridge coating materials and processes by the Federal Highway Administration (FHWA) have confirmed that:

• The relative cost of paint material is almost always insignificant when viewed in terms of the overall cost of the bridge maintenance job, and

• The advantage in the relative durability of the superior coating systems often far outweighs the nominally increased cost of these materials at the time of application.

Labor typically represents 80 to 90 percent of the cost of field-coating a bridge, according to Winn Darden, business manager for AGC Chemicals Americas and author of dozens of papers and presentations for the Society for Protective Coatings (SSPC) and the National Association of Corrosion Engineers (NACE). “The cost of the coating on bridge projects is irrelevant compared to the cost of getting on that structure, and having to deal with blasting, cleaning and doing all of the application work,” Darden noted.

Among the coatings that provide long-term corrosion protection for steel are advanced technology fluoropolymers, which have been used as topcoats on bridges in Japan since the mid-1980s, and more recently in the U.S. “The Japanese Ministry of Land, Infrastructure and Transport has determined that the life-cycle cost of fluoropolymer coatings is much lower than that of comparative coatings,” Darden acknowledged. “In Japan, a minimum service life of 30 years is expected and fluoropolymer topcoats are required to be used on all bridges.”

“The chemical bonds found in fluoropolymers are strong enough where they’re not broken by ultraviolet (UV) light,” Darden explained. “When a coating chalks, its chemical bonds are being broken down by UV radiation. But because the chemical bonds are so strong in fluoropolymer coatings, that degradation doesn’t occur, resulting in longer color and gloss retention. For the bridge designer, fluoropolymer topcoats open the possibility of doing more aesthetically pleasing things with color than have traditionally been done.”

This was true in Topeka, Kan., where the outside beams of the 3,375-foot Topeka Boulevard Bridge were coated red using Tnemec’s high-solids fluoropolymer, Fluoronar. “The project’s engineer knew enough about the color red to realize that he didn’t want to see the bridge turn pink over time,” recalled Tnemec coating consultant Rick Penner. “After searching for the best performing coating in terms of gloss and color retention, he determined that fluoropolymer technology was best suited for the application and used it as the standard for the specification.”

The exterior beams for the Topeka Boulevard Bridge were shop-primed with a two-component organic zinc, Tneme-Zinc, which was chosen for its ease of application and ability to be touched up on the job site. The beams also received an intermediate coat of an aliphatic acrylic polyurethane, Endura-Shield II, before the Fluoronar topcoat was applied.

In western Pennsylvania, the galvanic protection capabilities of another highly durable Tnemec coating system are still evident on three steel bridges after more than 23 years. Professional, third-party inspection of the Windgap Bridge in Allegheny County indicate that additional years of service life can be expected from the three-coat system consisting of a shop-applied prime coat of Tneme-Zinc; an epoxy tie-coat of F.C. Typoxy; and a topcoat of an aliphatic polyester polyurethane.

Numerous FHWA and private studies have concluded that from a LCC standpoint, quality industrial coatings with long-term performance are the optimum choices for new bridge construction and maintenance applications. According to the FHWA, “We must now realize that tradeoffs in quality long-term corrosion protection will be paid for several-fold in the future.” 

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