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

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3 According to the 2015 National Bridge Inventory (NBI), there are over 180,000 steel bridge structures in the United States—nearly 30% of the bridge inventory. Corrosion is one of the most often-cited problems with steel bridges. To varying degrees, it affects both the appearance and structural integrity of a bridge. In some instances, the appearance of a corroded bridge may not be an issue, but in others, it can cause significant public concern. Similarly, the structural effects of corrosion can range from limited weakening of a structure to sudden bridge collapse, depending on a variety of details. A 1990 report provides guidelines for evaluating the structural effects of corrosion in steel bridges (Kulicki et al. 1990). A 2001 FHWA study identified the annual direct cost of corrosion for highway bridges in the United States to be $6.43 billion to $10.15 billion (Koch et al. 2002). That cost includes $3.79 billion per year to replace structurally deficient bridges and $0.50 billion per year for the maintenance painting of steel bridges. A number of studies have shown that the indirect user costs (e.g., traffic delays and lost productivity) are an order of magnitude higher than the direct maintenance costs. Although there has been much research performed to develop corrosion-resistant steels (to be used without a coating) and cost-effective coatings, it has generally been done in a somewhat isolated and uncoordinated fashion. Furthermore, the considerations for corrosion control vary based on geography; the optimum approach will likely be different in areas of significant deicing salt usage, coastal areas, or the arid regions of the southwestern United States. Some states encounter more than one of these situations, potentially complicating their approach to corrosion control. Synthesis Topic 48-03 gathered information on practices to extend steel bridge service life by protecting such bridges from corrosion. Scope Until the late 1970s, most steel bridges were protected from corrosion by the application of three coats of a lead or chromate-based coating system directly over the mill scale that had formed on the steel. Additional coats of the same paint were applied as necessary when corrosion had progressed to an unacceptable level. As the direct and indirect costs of painting increased, the industry looked to alternative corrosion-control practices. These alternatives primarily fell into two categories: corrosion-resistant steels for uncoated use and cost-effective coating sys- tems for structural steel. Although most of the literature considers one or the other of these two approaches, this synthesis addresses both approaches. It is challenging to make the optimal choice among the right material, coating, and actions while considering the local environment. There is no single comprehensive document that C H A P T E R 1 Introduction

4 Corrosion Prevention for Extending the Service Life of Steel Bridges provides both qualitative and quantitative information on the various approaches to corro- sion control. Owners are replacing and/or maintaining a large number of bridges and building significant numbers of new bridges and transportation structures. This synthesis is intended to help them choose material and coatings in an informed manner and develop effective mainte- nance plans for implementation for newly constructed and in-service bridges and transportation structures. This synthesis documents and describes current practice for corrosion prevention of steel bridges. It is not intended to prescribe a practice or set of practices, as might be expected in a guidebook or manual. The scope of this synthesis is further limited to corrosion of the atmo- spherically exposed superstructure elements of steel bridges. Although similar principles may be applicable, the synthesis does not intend to specifically consider other steel bridge elements, including the bridge deck (reinforcing steel), buried or immersed areas of the substructure, signposts, suspension cables, bearings, and so forth. Approaches to Corrosion Prevention of Steel Bridges Corrosion-resistant steels (“weathering steels”) have been developed for uncoated use in bridges and other highway structures. The first significant use of uncoated weathering steel on highway bridges in the United States was in the mid-1960s, when weathering steel was used in New Jersey and Michigan. Under the right conditions, weathering steel will form a protective oxide (sometimes called a patina) that slows corrosion sufficiently to eliminate the need for painting. In the 1980s and 1990s, a number of research projects investigated weathering steel performance based on perceived problems and agency reservations. This research led to design guidance, such as detailing to prevent water accumulation and painting portions of weathering steel structures in zones where the patina may not form properly (e.g., adjacent to expansion joints). A 2014 survey indicated that despite some drawbacks, most states had a positive perception of the performance of unpainted weathering steel (McConnell et al. 2014). Recently, there have been efforts to fabricate plate girder bridges from highly corrosion-resistant steels (e.g., ASTM A1010, UNS32205, UNS32316). These alloys exhibit corrosion resistance in environments where weathering steels are susceptible to corrosion, such as those with long periods of wetness or high levels of chlorides. Ideally, any increased material cost would be offset by the savings associated with longer service life, simpler detailing, or the elimination of the need to paint certain areas (i.e., zone painting). In addition to new steel materials, the industry has developed improved coating systems for corrosion control of structural steel. Currently, the most common coating system for new construction and existing coating replacement is a three-coat system with a zinc-based primer applied over abrasive blasted steel (KTA-Tator 2014). There are several details to this generic system that vary by state and application. Most states also employ overcoating, spot painting, and/or zone painting strategies for maintaining existing coating systems. The risks and benefits of these strategies vary depending on the specific circumstance and thus are the subject of much discussion. Although efforts are ongoing to improve on the value proposition of traditional paint sys- tems, another approach is to use a hot-dip galvanized or metallized coating 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.” The synergetic life of duplex coatings may be 1.5 to 2.3 times the sum of the individual lives; this means they may result in service lives in excess of 75 years (Van Eijnsbergen 1994). Unfortunately, bridge owners have had mixed experience with duplex coatings (including the galvanizing and metallizing

Introduction 5 process and/or finish coat application). Some departments of transportation (DOTs) have had success with the approach, whereas others have had issues with galvanizing and metallizing qual- ity, experienced adhesion problems with the finish coating, or have had to perform maintenance painting of the duplex system sooner than anticipated. Bridge cleaning is becoming more common as highway agencies focus on asset management and preservation practices. In addition to removing soluble salts, bridge cleaning helps keep drainage paths clear and removes poultice. Poor drainage and poultice buildup often contribute to accelerated corrosion on steel structures. Bridge cleaning typically involves pressure washing accessible portions of the bridge with water. Northern states often perform bridge cleaning in the spring after the end of the road-salting season. Study Approach and Report Organization This report is a compilation of knowledge on various corrosion-prevention strategies, includ- ing effective practices in use as well as new, innovative practices. The synthesis gathered relevant information through a literature review and a survey of highway bridge owners, including the state DOTs (voting members of AASHTO Subcommittee on Bridges and Structures and Sub- committee on Maintenance) and other highway bridge owners. The survey of highway bridge owners was performed using SurveyGizmo, an online survey software tool. In total, there were 82 respondents from 46 states and three agencies [New York City DOT, MTA Bridges and Tunnels (New York City), District of Columbia DOT]. Forty- two respondents addressed the “designer” section of the survey, and 40 respondents addressed the “maintainer” section. Appendix C contains a list of the survey respondents. (Appendices A through E can be found at www.TRB.org by searching for “NCHRP Synthesis 517.”) Figure 1 depicts the states represented by the respondents. Figure 1. Agencies responding to the survey.

6 Corrosion Prevention for Extending the Service Life of Steel Bridges This report is organized into the following sections: Chapter 2, Corrosion Prevention and Control for Steel Bridges, discusses the choices made by designers and maintainers of steel bridges based on the synthesis survey and supplemental literature. Chapter 3, Corrosion Control of Uncoated Steel Bridges, discusses alternative uncoated steel materials. Within this section, design and maintenance issues are discussed. Chapter 4, Corrosion Control of Coated Steel Bridges, discusses alternative protective coat- ings including liquid coatings, galvanizing, and metallizing. Within this section, design and maintenance issues are discussed. Chapter 5, Bridge Cleaning, discusses the state of the practice for removing contaminants from steel bridge structures using wet and dry methods. Chapter 6, Conclusions, presents a summary of the synthesis findings grouped by available technologies, design approaches, maintenance approaches, and research needs.