For the past several decades, bridge owners have relied on a conventional three-layer coating system to protect their steel bridges from corrosion. This time-proven system consists of a zinc-rich primer, an epoxy intermediate and a polyurethane finish coat. The time to first significant maintenance of this three-layer system is 20+ years depending on service environment. The excellent balance of cost versus performance has made this three-layer system the standard for protecting steel bridges from corrosion in highly corrosive environments.
In the early 2000s, first-generation polyaspartic (PAS) coating systems were commercialized and introduced to the bridge market. Now, almost two decades later, a dozen bridge owning authorities across the United States and Canada have used PAS coatings on steel bridges. In comparison to conventional three-coat systems, two-coat polyaspartic urethane coating systems (zinc primer / polyaspartic urethane finish) offer a number of advantages to bridge owners. PAS coatings reduce maintenance painting costs up to 20 percent while increasing painting efficiency up to 30 percent without sacrificing long-term asset protection.
Lowering maintenance painting costs is achieved through reduction in labor to complete painting and inspection operations, reduction in mobilization and maintenance of traffic (MOT). Being able to complete maintenance painting operations 30 percent faster significantly reduces MOT costs, which contributes to half of the overall cost reduction. Reduced MOT also has a direct impact on the number of hours workers spend in that construction zone with moving traffic. The increased throughput contractors can achieve with PAS coatings makes these coatings an ideal choice for maintenance painting of overpass structures crossing heavily traveled roadways.
In recent years, Covestro has developed the second generation of PAS coating resins (DESMOPHEN NH 2850 XP) and cross-linkers (DESMODUR XP-2763)* to address application issues encountered in warm climates with high humidity. While first-generation products offered excellent performance, they were difficult to apply due to their relatively short pot lives and recoat windows in these specific conditions.
When touch-up of the topcoat was necessary after 24 hours from the initial application, abrading between coats was required. If the surface of the initial PAS coating was not abraded, delamination of the second layer could occur. Recent advances in PAS resin and cross-linker technology addressed the aforementioned issues while also improving wet adhesion and corrosion resistance.
The Virginia Department of Transportation (DOT) was one of the early adopters of PAS coatings for maintenance painting of steel bridges. The first bridge structures in the state painted with PAS date back to 2005. Now, with over a decade of service history in the field, two-coat PAS coatings are proving to perform on par with the convention three-coat system.
Virginia DOT: I-64 over Simpson Creek in Clifton Forge, VA, repainted in 2005
This performance is gaining the attention of a number of bridge owners. Over the last several years, several bridge-owning authorities have completed projects with second-generation PAS coating systems, including the Maine DOT, Michigan DOT and Maryland State Highway Administration (MDSHA).
Maine DOT: Deering Rd over I-295 in Portland, ME, repainted in 2016
Michigan DOT: West Rd over I-95 in Woodhaven, MI, repainted in 2017
MDSHA: Baltimore Annapolis Blvd over Arundel Expressway in Glen Burnie, MD, repainted in 2017
PAS two-coat systems provide significant value to bridge owners in the form of cost reduction in maintenance painting, accelerated painting schedules and fewer traffic congestion headaches. Advancements in PAS resin technology have alleviated application complexities around short pot life and recoat windows that had occurred in past in higher temperature and humidity conditions. PAS two-coat systems continue to have a bright future in the bridge market as a high-performance and cost-savings solution for long-term asset protection.