...Department Transportation (Caltrans) estimates that full removal of paint can cost as much as $35 per square foot. Because old paint typically includes lead, which is hazardous to human health and the environment, full paint removal requires abrasive blasting, dust containment, environmental monitoring, and waste removal and disposal. Worker health and safety must be protected as well.

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To keep costs down while maintaining the service conditions of the Nation's steel bridges, Caltrans and many other State departments of transportation are turning to overcoats--applying a new coating on top of the existing one(s)--as an alternative to removing old paint.

"We're overcoating the majority of our steel bridges," says Senior Chemical Testing Engineer Andy Rogerson with Caltrans. The department maintains nearly 800 steel bridges statewide. Most have a red, lead-based primer coat, which for the most part is performing well, Rogerson says. When the topcoats start to fail, Caltrans applies waterborne primers and acrylic latex topcoats or, for harsher coastal climates, three-coat, moisture-cured urethane (MCU) overcoat systems.

Cost is the main advantage. Overcoat applications cost the agency $6 to $10 per square foot--nearly two-thirds less than the cost of full removal. "If rust covers less than 20 percent of a bridge, then we'll keep the lead primer and do an overcoating," Rogerson says.

But how well do these overcoat materials work in the long term? And under what circumstances do they perform most effectively? As new coatings emerge, such as new products with low levels of volatile organic compounds (VOCs), those will need to be evaluated for performance.

Between 2004 and 2006, to fill the need for data on the latest overcoat products, researchers at the FHWA Turner-Fairbank Highway Research Center (TFHRC) conducted an inhouse study to evaluate how various overcoat materials perform when they are applied to different types of aged steel substrates.

The FHWA researchers and their contractor staff selected six lead-free and low-VOC materials to apply over coated, aged, and rusted surfaces. Using a cyclic, accelerated testing method, they studied the overcoat systems in the laboratory and through field exposures, evaluating performance by assessing surface failures and rust creepage developed at scribes (scratches made through the overcoat surface down to the steel substrate). Comparing the results yielded a number of insights into overcoat performance when applied to the three types of substrates.

Experimental Procedures

Most aging steel bridges are covered with one of two types of coating systems: (1) a two-coat system with an alkyd primer and topcoat (both coats could contain lead or just the prime coat) and (2) a two-coat system with an inorganic zinc (IOZ)-rich primer and a vinyl topcoat. Therefore, the researchers chose to study the performance of overcoats when applied to both types of coating systems and to a rusted steel base, cleaned using power tools according to the Society for Protective Coatings' (SSPC) specification SSPC-SP3.

To set up the study, the researchers created rectangular steel test panels measuring 10.0 by 15.0 by 0.48 centimeters (4.0 by 6.0 by 0.19 inches). To prepare the substrates, they applied two coats of lead-based alkyd paint to some of the test panels, which had been cleaned using solvent according to specification SSPC-SP1.

For the samples that would have the IOZ/vinyl coating system as...

NOTE: All illustrations and photos have been removed from this article.



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