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1 Corrosion is one of the most often-cited problems with steel bridges. To varying degrees, corrosion affects both the appearance and structural integrity of a bridge. According to the 2015 National Bridge Inventory (NBI), there are over 180,000 steel bridge structures in the United States, which is nearly 30% of the bridge inventory. A 2001 FHWA study identified the annual direct cost of corrosion for highway bridges in the United States to be $6.43 bil- lion to $10.15 billion. This synthesis presents information on practices to extend steel bridge service life by protecting bridges from corrosion. The information was gathered through a literature review and a survey of transportation agencies. Forty-six states and three agencies responded to the survey. To optimize steel bridge structure design, industry offers various options for corrosion con- trol of steel bridges, with varying combinations of durability, cost, aesthetics, service life, and other characteristics. The technical literature generally addresses âuncoatedâ and âcoatedâ steels separately, and coated steel bridges can be further divided into the categories of liquid coatings and metallic coatings (e.g., galvanizing and metallizing). Although the literature tends to address these approaches to corrosion control separately, most bridge owners have a com- bination of structures with each of these approaches, and may see multiple corrosion-control strategies employed on the same structure. The survey results indicate that uncoated steels are predominately weathering-grade steels, which are designed to form a protective rust layer (patina). Estimates of the current proportion of planned and newly constructed steel bridges designed with weathering steel outnumber the proportion of in-service weathering steel bridges by a factor of 4, suggest- ing that many states are becoming comfortable with the material. Structural stainless steel alloys are the most recent innovation in uncoated steels. Bridges have been fabricated with structural stainless alloy in at least four states since 2004. An industry guideline for design- ing bridges with structural stainless steels is being developed. Stainless steel alloys are also beginning to be used on corrosion-prone elements of bridge structures, such as bearings, to extend their service life. The survey results indicate that the current state of the art for liquid coatings on new con- struction and major rehabilitation involves a three-coat system consisting of a zinc primer, epoxy or urethane intermediate, and an aliphatic urethane finish coat. Metallic coatings, such as galvanizing and metallizing, are well-established technologies that are being used increasingly on steel bridges in the United States. Both galvanizing and metallizing can be painted with an organic paint system in what is often referred to as a âduplexâ coating system. The added organic coating provides aesthetic benefits and extends the life of the metallic coating. Properly galvanized or metallized parts can provide service longer than do liquid coatings, especially when installed as a duplex system. S U M M A R Y Corrosion Prevention for Extending the Service Life of Steel Bridges
2 Corrosion Prevention for Extending the Service Life of Steel Bridges Strategies for the maintenance painting of existing structures are more varied than for new steel. A wide range of surface preparations and coating materials are used for spot painting, zone painting, and/or overcoating to extend the life of existing coating systems. The strategies share the goal of reducing cost and public inconvenience with an acceptable sacrifice in performance, versus removing and replacing the entire coating system. Although these strategies are well established, the maintenance decision is often driven by various externalities (e.g., structure access, environmental rules, funding availability, and public or political influences) as well as technical issues [e.g., existing coating condition, material compatibility, and life-cycle costs (LCCs)]. The survey showed that bridge washing or cleaning is becoming more common as a preser- vation practice. This practice benefits steel bridges by reducing water collection and poultice buildup on steel. Ideally, a steel structure should be designed to drain so that water doesnât collect on steel surfaces. Both uncoated and coated steels will last longer if they are not subject to the immersion condition that exists under ponding water. Furthermore, when dirt and debris collect on steel surfaces, they tend to retain moisture and can contribute to a locally corrosive condition, depending on the chemistry of the dirt and debris. Several areas of research needs have been identified that could help reduce the cost of steel bridge corrosion. They can be grouped in the following broad categories: ⢠Optimize current corrosion prevention schemes (e.g., improve weathering steels or improve coating processes). This may include developing design details to reduce corrosion, per- forming field studies to document the real-world performance of current technologies, and making incremental improvements to current practices. ⢠Develop and demonstrate state-of-the-art approaches to corrosion prevention. These approaches include metallic coatings, duplex coatings, high-durability finish coats, improved corrosion-resistant steels, and stainless steels. Research projects would generate specifications and standards, training programs, case studies, and so forth. ⢠Improve methodologies for service life design and prediction based on environmental conditions at the location of the structure. This would include projects to move corro- sion prevention design from a âdeemed to satisfyâ approach toward partial probabilistic approaches. ⢠Improve methodologies for life-cycle planning, risk management, and financial planning for preservation of corrosion-prevention systems. These methodologies would help owners justify preservation needs, allow them to demonstrate the value of executing preservation activities, and help them optimize their preservation practices.
