Federal Highway Administration 100-Year Coating Study

CHAPTER 1. INTRODUCTION

The coating industry switched from lead-based to zinc-based three-coat systems in the 1970s due to health hazards associated with lead in the coatings to protect steel bridges from corrosion.⁽¹⁾ The current state of practice in steel bridge coatings usually involves multi-layer coating typically consisting of a zinc-rich primer over an abrasive blast-cleaned surface and one or two additional coating layers on top of the primer. Both inorganic and organic zinc-rich primers provide galvanic corrosion protection by sacrificing itself to the less electrochemically active steel substrate in the presence of corrosive conditions. The intermediate coat provides a physical barrier to the passage of moisture, oxygen, and other aggressive ions, such as chloride ions, while the top coat protects underlying coating layers against deterioration caused by ultraviolet (UV) radiation and physical damage and enhances the aesthetics of the coating system. It was reported that some three-coat systems with zinc-rich primers can have a service life up to 30 years of protecting steel from corrosion before a major touch-up is required.⁽²⁾

Typical costs for conventional coating systems include costs associated with removing mill scale and creating adequate surface condition, coating application, and logistics of moving coated steel bridge members to the field. Such shop operations also require downtime and space during the application of multiple coating layers. The high cost of mill scale removal and surface preparation can be attributed primarily to expensive open-blast cleaning equipment and labor.⁽²⁾ The cost of typical three-coat systems ranges from under 4 percent to more than 24 percent of the cost of fabricating the steel.⁽³⁾ After the first shop application, repainting the same bridge after approximately 30 years is very expensive. A practical alternative in reducing these repetitive coating costs is to extend the service life of the shop-applied original steel bridge coatings.⁽²⁾

The Federal Highway Administration (FHWA) Coatings and Corrosion Laboratory at the Turner-Fairbank Highway Research Center (TFHRC) initiated an in-house study in August 2009 to identify coating systems that can provide 100 years of virtually maintenance-free service life at comparable costs to the existing coating systems, even in adverse environments. Coating systems in this research study were selected based on past experience and results from previous FHWA studies that evaluated coating systems such as moisture-cured urethanes, waterborne acrylic and epoxy systems, two-coat systems, and one-coat systems. (See references 4–8.) Particularly, previous FHWA in-house studies indicate that the two-coat systems may perform well relative to the well-performing legacy zinc-rich three-coat systems. While critical factors in the field include being cost effective due to a lower number of coats and having reduced application time, two-coat systems have the potential to replace the conventional three-coat systems without sacrificing corrosion protection properties.⁽⁹⁾ This report presents research findings from a performance evaluation of eight coating systems based on experimental data from accelerated laboratory testing (ALT) and outdoor exposure testing.