...consisted three to five thin coats of alkyd paint containing toxic lead and chromate. Over the course of several days, bridge workers would apply the paint directly onto the mill scale (black corrosion analogous to rust) that adheres to the formed steel when it is heated. Subsequent painting for preventive maintenance and corrosion protection was rare and generally reserved for larger spans.

Because the majority of the steel bridges in the interstate system were built between 1950 and 1980, many have little protection from corrosion because their coating systems have outlasted their useful lives. Often, harsh environments and exposure to roadway deicing chemicals (salts) intensify the effects of the natural aging process. Further, the presence of potentially hazardous substances in the existing paint complicates maintenance processes and dramatically increases related costs.

Nearly 20 years ago, research led to the current standard, which is three-coat system of zinc-rich primer/epoxy/polyurethane paint. Many States use the three-coat paint system as the preferred method of protection. In humid environments, for example, maintenance personnel use three coats of zinc-rich moisture-cured urethane (MCU)/MCU/polyurethane paint system.

A new class of coating systems consisting of a zinc-rich primer topcoated with fast-dry, high-build (thick film) polyaspartics, polyurethane, or polysiloxane promises anti-corrosive results that are comparable in some situations with the three-coat systems. These two-coat systems eliminate the intermediate epoxy layer, so painting a steel overpass can be completed overnight. When application specifications are followed, two-coat systems can reduce labor as well, increasing worker productivity and decreasing the overall cost of coating applications.

The Basics of Bridge Paints

Zinc Primers. There are two types of zinc-rich primers: organic and inorganic.

Organic primers are epoxy-containing zinc-rich primers. Work crews can apply them using brush, roller, or spray. If not topcoated, zinc primers do not protect steel as well as inorganic primers. However, both organic and inorganic primers offer the same degrees of protection if they are topcoated.

Moisture-cured urethane (MCU) zinc primer is a new type of organic. MCU coatings are more tolerant to humid environments than epoxy-based primers and are one-component products.

Inorganic zinc-rich primers are silicate-containing primers. They may be used as a stand-alone coating but typically are topcoated with a compatible paint or epoxy. Generally, inorganic primers must be spray applied.

Polyurea is a polymer technology used in coatings to protect steel from corrosion and abrasion. Conventional polyurea is known to cure very rapidly, but it needs special equipment to apply.

Polyurethane is a polymer coating that is formed by reacting polyisocyanate with polyol or base resin. It is a high-performance topcoat.

Polysiloxane, better known as silicone, is an inorganic polymer that is resistant to water, chemicals, and oxidation, and has good color and gloss retention.

Polyaspartics are a new coating technology that builds on conventional polyurethanes and provides even faster dry times. Polyaspartics dry quickly and can be applied with high thickness.

To assess the performance of these new two-coat systems, researchers at the Federal Highway Administration (FHWA) recently conducted a series of...

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



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