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Effective Use of Duplex Coating Systems to Improve Steel Bridge Structure Durability


Steel bridge coatings greatly contribute to the longevity of a bridge structure and reduce the total life-cycle cost by protecting steel bridge members from corrosion. For most steel bridges across the nation, the most popular practice is to provide a 3-coat, zinc-rich primer system. The 3-coat system generally requires regular maintenance (touchup) and may have to be completely overcoated or repainted after as little as 15 years. This approach has proven to be a maintenance challenge for bridge owners. While efforts are on-going to improve the service life of traditional paint systems another approach would be the use of duplex systems (hot dip galvanized or metallized with a traditional finish coat) to provide long-term protection against corrosion. The combination of a metallic coating and a traditional finish coat is often referred to as a “duplex coating.” Van Eijnsbergen1 suggests that the synergetic life of duplex coatings is 1.5 to 2.3 times the sum of the individual lives – this means that may result in service lives in excess of 75 years.

Unfortunately, bridge owners have had mixed experience with duplex coatings (including the galvanizing and metalizing process and/or finish coat application). Some DOTs have had great success with the approach while others have had issues with galvanizing and metalizing quality, experienced adhesion problems with the finish coating, or have had to perform maintenance painting of the duplex system sooner than anticipated. The processes are currently reliant on the experience of the galvanizers, metalizers, and coating applicators and as such, may be considered more of an art than science. The proposed project will close the gap between art and science by developing design guidelines, guide specifications, and a state of the art report.


The objective of this research is to collect technical data and develop practical guidelines that allow bridge engineers to position duplex coating systems among alternative corrosion control options for steel bridge structures.


SCOBS global focus:

Foster innovation

SCOBS prioritized objective:

1 Extend Bridge Service Life Duplex coating systems could be an option for improved durability for a large percentage of the nation’s bridge population. While economic considerations vary, duplex systems are available for shop and/or field application for new bridges, maintenance coating for existing bridges, and to protect steel components replaced during major rehabilitation. Effective use of duplex coating systems should make steel bridge designs more competitive by reducing service life cost through increased performance. The use of duplex systems should more than offset any increased application costs through the elimination costs associated with additional recoating cycles, such as: mobilization, maintenance and protection of traffic, user costs, surface preparation containment and disposal, as well as additional application costs. This project directly addresses the 2013 SCOBS strategic vision objective to “Extend Bridge Service Life.” SCOBS T-9, T-14, T-18; SCOM; the AASHTO National Bridge Preservation Partnership’s (NBPP) Coatings Group; TRB AHD 30, Standing Committee on Structures Maintenance, and TRB AHD30(2), Steel Bridge Coating Subcommittee have expressed support for this project.

Related Research:

NCHRP Synthesis 20-05, Topic 48-03, Corrosion Protection for Extending Steel Bridge Service Life


The researcher shall use a combination of laboratory testing and field studies to accomplish the project objectives. The main tasks would be the development of three documents:

  • Design guideline for the use of duplex coatings. This document will provide the designer guidance on technical and economic issues relating to duplex coating systems and alternative systems. Issues to be addressed will include considerations for new construction and maintenance painting and considerations when selecting among galvanizing and metallizing.

  • Guide specifications for duplex coatings. These guide specifications will be developed in AASHTO format and will address the proper procedures for quality application of both the metallic and the finish coatings. They will also address testing procedures for qualification of duplex coating systems. State of the art report. This report will document laboratory testing and field performance of duplex coating versus other systems on bridges. The report will identify state-of-the-art training and qualifications for applicators and inspectors. The report will also identify gaps in technology, training and other resources which, if filled would improve the state-of-the-art.


Implementation would be through a combination of approaches:

  • Publication of AASHTO guide specification (described above)

  • Development of training and qualifications for applicators and inspectors (described above)


This research would be relevant and useful to the bridge design, maintenance, and construction community throughout the US.

Sponsoring Committee:AFF20, Steel Bridges
Research Period:24 - 36 months
Research Priority:High
RNS Developer:J. Peter Ault, P.E and Jeffrey A. Pouliotte, P.E. (FL DOT)
Source Info:The TRID database was searched using terms such as “galvanizing,” ”metallizing,” and “duplex coatings.” About half of the database hits related to concrete bridges (either using galvanized rebar, or thermal zinc treatments). The other half predominately addressed comparison studies on the relative performance of traditional coatings and metallic coatings, the benefits of metallic coatings, or adhesion of paints to galvanizing. Much of the directly relevant work was anecdotal. No comprehensive study was identified that covered the design considerations, proper specifications, or performance of duplex coating systems.
Date Posted:03/19/2018
Date Modified:04/20/2018
Index Terms:Coatings, Steel bridges, Durability, Galvanizing, Metal working,
Cosponsoring Committees:AHD30, Structures Maintenance
Maintenance and Preservation
Bridges and other structures

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