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Performance Evaluation of One-Coat Systems for New Steel Bridges

CHAPTER 1. INTRODUCTION

Identification of health hazards associated with lead-based paints in the 1970s led to their replacement with three-coat systems to protect steel bridges from corrosion.⁽¹⁾ Bridge coating technology has been vastly redefined in the past 30 years by changes in surface preparation methodologies, coating processes, and coating material science. Technological advancement in these areas has aided in creating high-quality bridge coating systems with enhanced corrosion protection and minimal environmental impact.

The current state of practice in bridge coatings usually involves multilayer coating typically consisting of a zinc-rich primer over an abrasive blast-cleaned surface and two additional coating layers on top of the primer. The inorganic or organic zinc-rich primer provides cathodic 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 electrolytes, while the top coat protects against deterioration caused by ultraviolet (UV) radiation while enhancing the aesthetics of the coating. Conventional three-coat systems have demonstrated a long-term service life. Studies have shown that these three-coat systems with a zinc-rich primer can have a service life of 30 years before a major touch-up is required.⁽²⁾

Although current coating technology provides a comprehensive solution to improve corrosion protection of steel bridges, the overall cost involved is relatively higher than its predecessors. Data obtained from 20 fabrication shops in the United States for a recent Federal Highway Administration (FHWA)-sponsored study to investigate and analyze the cost of shop painting indicated that the painting cost of steel bridges ranged from under 4 percent to more than 24 percent of the cost of fabricating the steel.⁽³⁾ The median cost of application of a one-coat system is 8 percent of the cost of the girder, while the cost of a three-coat system is 12 percent for the same application.⁽³⁾ These increased costs can be attributed to enhanced preprocessing steps, such as higher levels of cleaning and surface preparation, and the direct influence of the number of protective coats on the overall cost of coating fabrication. In addition to the cost involved, the time and space required for proper shop application of a three-coat system are a burden to fabricators and bridge owners. Optimizing cost and productivity is a major challenge for the bridge-coating industry.

In an effort to minimize fabrication costs, novel fast deployment two-coat systems were studied in an FHWA project in 2002. Test results from surface failure and rust creepage at the scribe in both the laboratory test and outdoor exposure revealed that the two-coat systems performed on par with the widely established well-performing zinc-rich three-coat systems. While they are cost-effective due to fewer coats, these two-coat systems have the potential to replace the conventional three-coat systems without sacrificing much corrosion resistance.⁽⁴⁾

Since the performance evaluation of two-coat systems demonstrated promising potential to replace three-coat systems, FHWA sponsored a small research project to investigate the viability of one-coat systems. Three one-coat materials, including a polyaspartic (ASP), a polysiloxane (SLX), and a waterborne epoxy (WBEP), were evaluated in this study.⁽⁵⁾ All three one-coat systems developed severe blistering along and away from the scribe area after 5,000 h of salt fog exposure according to American Society for Testing Materials (ASTM) B117-09, "Practice for Operating Salt Spray (Fog) Apparatus." ⁽⁶⁾ Two of the three one-coat systems did not blister (away from the scribe) after about 5,000 h of cyclic weathering exposure according to ASTM D5894-05, "Standard Practice for Cyclic Salt/Fog/UV Exposure of Painted Metal (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet)."⁽⁷⁾ This may be due to continuous salt fog exposure in ASMT B117-09 compared to cyclic salt fog in combination with UV exposure conditions in ASTM D5894-05.^(6,7) Although none of the one-coat systems performed as well as a standard three-coat system, two one-coat systems showed encouraging performance characteristics such as strong adhesion, edge retention, and minimal to no surface blistering in the cyclic weathering test.

In light of the encouraging results obtained from the 2002 study, FHWA performed extensive one-coat research at FHWA’s Turner-Fairbank Highway Research Center (TFHRC) Coatings and Corrosion Laboratory (CCL) in McLean, VA. The purpose of this study was to evaluate the performance characteristics of various commercially available high-performance coating materials that can be applied as one-coat systems to steel bridges in shop application. Eight one-coat systems were selected based on their performance in previous FHWA research projects and also after researching the North East Protective Coat Qualified Products List and many commercially available coating products.⁽⁸⁾ A three-coat and a two-coat system, both consisting of zinc-rich based primers, were included in this study as controls. The 10 selected coating systems were tested using the cyclic testing method ASTM D5894-05 in addition to a freeze cycle, an accelerated laboratory test (ALT) for 6,840 h, and three outdoor exposure conditions including a marine exposure (ME) in Sea Isle City, NJ, for 24 months, mild natural weathering (NW) for 18 months at TFHRC, and mild natural weathering plus salt solution spray (NWS) tests for 18 months at TFHRC.^(7,9)

This report presents performance evaluation results and major findings for the 10 coating systems based on experimental data and subsequent data analyses.