Corrosion Prevention for Extending the Service Life of Steel Bridges (2018)

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68 Available Technologiess In an effort to optimize steel bridge structure design, industry offers various options for cor- rosion control of steel bridges which offer varying combinations of durability, cost, aesthetics, service life, and other characteristics. The technical literature generally addresses “coated steels” and “uncoated steels” separately, and coated steel bridges can be further divided between liquid coatings and metallic coatings (e.g., galvanizing and metallizing). Although the literature tends to separately address these approaches to corrosion control, most owners have a combination of structures with each of these approaches and may see multiple corrosion-control strategies employed on the same structure. Uncoated steels are predominately weathering grade steels, which are designed to form a protec- tive rust layer (patina). Since the introduction of uncoated steel to highway bridges in the 1960s, industry has improved the product performance through alloy improvements, better design detail- ing, and improvements in maintenance and inspection procedures. Estimates of the current pro- portion of new steel bridges designed with weathering steel outnumber the proportion of existing weathering steel bridges by a factor of 4, suggesting that many states are becoming comfortable with the material. More recent innovations in uncoated steels include new corrosion-resistant steel alloys: ASTM A1010 and duplex stainless steels. Bridges have been fabricated with these alloys in at least four states since 2004. An industry guideline for designing bridges with corrosion-resistant steel alloys is being developed. Finally, these alloys are beginning to be used on corrosion-prone elements of bridge structures, such as bearings, to extend their service life. Traditional liquid coatings for steel bridges have come a long way since the legacy lead-based alkyd coatings fell out of favor because of environmental and health concerns. The current state of the art for liquid coatings on new construction and major rehabilitation involves a three-coat system consisting of a zinc primer, epoxy or urethane intermediate, and an aliphatic urethane finish coat. This coating system includes three coats of paint, each with a specific functional purpose applied over an abrasive-blasted steel substrate. The zinc-containing primer is designed to protect steel from corrosion and resist undercutting (delamination) at coating damage. The intermediate coat functions as a durable barrier to environmental effects. The finish coat provides the desired appearance and is designed to maintain color and gloss. Strategies for maintenance painting of existing structures are more varied than for new steel. A wide range of surface preparation 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 same goal of reducing cost and public inconvenience with an acceptable sacrifice in performance versus removing and replacing the entire coating system. Although these various strategies have been well established for years, the maintenance decision is often driven by various externalities (e.g., structure access, environmental rules, funding availability, public or political influences) as well as technical issues (e.g., existing coating condition, material compatibility, LCCs). C H A P T E R 6 Conclusions

Conclusions 69 Metallic coatings such as galvanizing and metallizing are well-established technologies that are seeing increasing use on steel bridges in the United States. They are essentially coating techniques but involve different design and technical considerations than liquid coatings. Galvanizing is a well- established process performed by immersing a fabricated part in molten zinc. However, structural steel must be properly designed for galvanizing. This is not particularly challenging but may require changes to established design details. Galvanizing is also becoming more common as a treatment for steel used during structural repairs. Metallizing is the process of spraying molten metal onto a properly cleaned steel surface. The procedure is well established, relying on elements of both welding and painting processes. Metallizing has a different set of design constraints than does galvanizing. For example, galvanizing bath capacity limits the size of members that can be galvanized; virtually any size part can be metallized. On the other hand, metallizing is a “line-of-sight” process. Parts with complex geometries that cannot be metallized can be coated during galvanizing if the design allows molten zinc to flow into and drain out of the complex geometries. Projects have been constructed with both galvanized and metallized components on the same structure. Both galvanizing and metallizing can be painted with an organic paint system in what is often referred to as a “duplex” coating system. The added organic coating provides aesthetic benefits and extends the life of the metallic coating. Properly galvanized or metallized parts can provide a longer service than liquid coatings, especially when installed as a duplex system. Bridge washing or cleaning is becoming more common as a preservation practice. This prac- tice benefits steel bridges by reducing water collection and poultice buildup on steel. Ideally, a steel structure should be designed to drain so that water doesn’t collect on steel surfaces. Both uncoated and coated steels 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 tends to retain moisture and can contribute to a locally corrosive condition depending on the chemistry of the dirt and debris. Although a clean, well-drained steel surface lasts longer, the cost–benefit of cleaning is highly dependent on the ability and cost of cleaning. Cleaning activi- ties may include flooding drainage paths, pressure washing, and/or physically removing debris. In addition to the usual access constraints, cleaning activities may be further limited by local environmental regulations, such as water discharge rules. A few recent studies have looked at cleaning in detail (Berman et al. 2013; Alland et al. 2013; Palle et al. 2003). For the Bridge Designer The current state of the practice for steel bridge design is to use either weathering steel or steel with a three-coat, zinc-based coating system. Weathering steel has some environmental limita- tions; there are federal and state guidelines to help designers identify unsuitable locations. For either material, the structure should be designed to minimize locations where corrosion is more likely to initiate. For example, geometries should not be designed that will trap water or debris, joints should be minimized, and water should drain away from the steel. The designer should consider more robust protection on beam ends and other areas subject to a more-severe environment. For weathering steel structures, beam ends are commonly painted. On painted structures, an extra coat of paint may be applied to beam ends and other corrosion- prone areas. On the other hand, the finish coat may be omitted from the interior members of painted structures in less-corrosive environments. Innovative approaches to provide better corrosion prevention include A1010 and UNS 32205 structural stainless steels and high-durability coatings, such as galvanizing, metallizing, or duplex coatings. Stainless steel bridges are relatively new, whereas the advanced coatings have been around for decades. These more robust corrosion-prevention technologies are more expensive than the current state of the practice. However, LCC analysis shows that these materials can

70 Corrosion Prevention for Extending the Service Life of Steel Bridges be attractive for structures in severe environments, with long life expectancies, or with limited access for future maintenance. For the Bridge Maintainer The current state of the practice for maintaining uncoated weathering steel bridges involves cleaning and inspecting on a regular basis. Structure zones are painted as required based on the observed condition. In some cases, a weathering steel structure may be abrasive blasted and have a three-coat, zinc-based liquid coating system applied. The current state of the practice for maintaining coated steel bridges is to repair or replace the coating when the extent of coating breakdown/rusting becomes objectionable. In some cases, the coating is repaired in isolated areas (spot painting) or areas of specific concern (zone painting). When spot painting is performed, a full overcoat may be applied to create a uniform appearance. Various coating materials are used for maintenance painting, including acrylics, MCUs, alkyds, calcium sulfonates, and epoxies. Localized coating repair is typically performed on structures with premature coating breakdown or when budgets do not allow coating replacement. Deteriorated coating that is near the end of its useful life (e.g., 15–40 years, depending on environment) may be completely removed and replaced. This process is more expensive but should yield a long service life. The current state of the practice is to abrasive blast the steel to a near-white metal finish and apply a three-coat, zinc-based coating system. Field metallizing is an innovative technique that has been used by several states, offering longer life at an increased cost. As discussed, bridge cleaning is becoming more commonplace for coated structures. The benefits of cleaning for corrosion control appear obvious, but they have not yet been quantified. Steel Bridge Corrosion Research Needs This synthesis has identified a range of materials and practices to protect steel bridges from corrosion. It is clear from the information obtained that selection of the optimum approach depends on a variety of factors, including service environment, desired service life, structure design, accessibility for maintenance, and available budget. Varying criteria may be used to deter- mine if a bridge has been “protected from corrosion,” such as aesthetic criteria (percent surface rust), structural criteria (section loss), or NBI condition rating. Whatever the criteria, the goal of an effective corrosion-prevention and mitigation system is to ensure that steel corrosion is not the life-limiting factor for the structure. Several areas of research needs have been identified as a result of this synthesis. They can be categorized in the following broad categories: • Optimize current corrosion-prevention schemes (e.g., improve weathering steels, improve coating processes). This may include developing design details to reduce corrosion, perform- ing field studies to document the real-world performance of current technologies, and making incremental improvements to current practices. This may also include establishing a corrosion- performance requirement rather than a compositional requirement for weathering grade steel. • Develop and demonstrate state-of-the-art approaches to corrosion prevention. These approaches include metallic coatings, duplex coatings, high-durability finish coats, and improved corrosion- resistant steel and stainless steel. Research projects would generate specifications and standards, training programs, case studies, and so forth. • Improved methodologies for service life design and prediction based on environmental condi- tions at the location of the structure. This would include projects to move corrosion-prevention design from “deemed to satisfy” approaches toward partial probabilistic approaches.

Conclusions 71 • Improved methodologies for life-cycle planning, risk management, and financial planning for the preservation of corrosion-prevention systems. These methodologies would help own- ers justify preservation needs, allow them to demonstrate the value of executing preservation activities, and help them optimize their preservation practices. Four specific research needs statements (RNSs) have been developed in these areas. At the time this synthesis was prepared, two RNSs had been submitted and were under consideration by NCHRP. Two of the RNSs were developed as a part of this synthesis. The following is a brief synopsis of the four RNSs. Preliminary drafts of the RNSs developed as a part of this synthesis are in Appendix B. An RNS titled Methodology for Collecting, Archiving, and Sharing Steel Bridge Corrosion Data has been drafted as a result of this synthesis. The objective of the research is to consolidate avail- able existing studies on steel bridge corrosion-prevention performance, develop procedures for future evaluations, and create an accessible location to archive data for researchers, designers, maintainers, and others. An RNS titled Life Cycle Cost Model for Alternative Approaches to Steel Bridge Corrosion Pre- vention has been drafted as part of this synthesis. The objective of the research is to develop a credible, useful, dynamic LCC model that can be used by all levels of a public transportation agency to make informed decisions regarding corrosion prevention of steel bridges. The model would move corrosion-prevention design from deemed-to-satisfy approaches toward partial probabilistic approaches. End users of the model may include policy makers, planners, designers, and maintenance departments. An RNS titled Corrosion Rates of Uncoated Weathering Steel Bridges has been submitted and is under consideration for funding by NCHRP. The objective of this research is to provide quan- titative performance data of uncoated weathering steel (UWS) bridges to assist with updating guidance for proper selection of UWS as a corrosion-prevention scheme. The research should specifically link known corrosion loss rates of exposure coupons in various macroclimates to the specific microclimates within a real bridge. It is not envisioned that exposure testing will be pur- sued; rather, the existing UWS inventory will serve as the pool of potential specimens for further evaluation and linking in situ performance to climate data, bridge geometry, and deicing usage. An RNS titled Effective Use of Duplex Coating Systems to Improve Steel Bridge Structure Dura- bility has been submitted and is under consideration for funding by NCHRP. The objective of the research is to collect technical data and develop practical guidelines that allow bridge engi- neers to position duplex coating systems among alternative corrosion-control options for steel bridge structures. The researcher shall use a combination of laboratory testing and field studies to accomplish the project objectives. The objective will be accomplished by developing three documents for the bridge design, maintenance, and construction community: • 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 mainte- nance painting and considerations when selecting between galvanizing and metallizing. • Guide specifications for duplex coatings. These guide specifications will be developed in AASHTO format and will address the proper procedures for high-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 that, if filled, would improve the state of the art.