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

Chapter: Chapter 2 - Corrosion Prevention and Control for Steel Bridges

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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
Page 15
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
Page 17
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Suggested Citation:"Chapter 2 - Corrosion Prevention and Control for Steel Bridges." National Academies of Sciences, Engineering, and Medicine. 2018. Corrosion Prevention for Extending the Service Life of Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25195.
×
Page 18

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7 This chapter takes a holistic view of alternative approaches to corrosion for steel bridges. Much of the chapter is dedicated to the choices made by designers and maintainers of steel bridges based on the synthesis survey and supplemental literature. Subsequent sections of this report go into greater detail regarding uncoated steel (weathering grades and stainless steels) and coated steel (including liquid coatings, galvanizing, and metallizing). Corrosion-Prevention Options When one considers corrosion-prevention options for bridge structural steel, it is helpful to recognize the corrosion-prevention mechanism, degradation of that protection, and even- tual repair or maintenance needs. Table 1 provides an overview of four primary corrosion- prevention strategies. Later sections of this report discuss coated and uncoated steel options in more detail. Designing for Corrosion Prevention Figure 2 graphically presents the responses provided by designers regarding the most com- monly used corrosion-prevention technologies. One respondent answered that stainless steel was used “nearly all of the time”; however, follow-up revealed that the response pertained to bearing assemblies rather than structural steel. Based on the survey, zinc-based coating systems and weathering steel are the most common corrosion-prevention schemes. Figure 3 shows the average percentage of bridges designed with some use of each technology assuming the midpoint of the percentage range for each response category. [For example, the answer “in many instances (50%–85% of the time)” was correlated to 67.5% of the time. Note that the totals add up to more than 100% because users were not constrained to selecting options that added up to 100%. The resultant calculations were normalized such that the total frequency of all options equaled 100%.] The data suggest that steel with zinc-based coating systems is used slightly more frequently than is uncoated weathering steel. Roughly 10% of all steel bridges are designed with some galvanized or metallized structural steel, often secondary members or sections. Of the states responding to the design section of the survey, a slight majority addressed cor- rosion differently based on environmental or operating conditions. Roughly one-third of those who did so reported having written guidelines, including Florida DOT (FDOT), New York State DOT (NYSDOT), and Missouri DOT (MoDOT). Issues that are typically considered included average daily traffic, difficulty of access, climatic conditions (humidity), use of deicing salts, proximity to the coast, height above water, tunnel-like conditions, ownership (state versus C H A P T E R 2 Corrosion Prevention and Control for Steel Bridges

8 Corrosion Prevention for Extending the Service Life of Steel Bridges local), and importance of road (e.g., evacuation route). The following are some specific issues that were reported: • Galvanize or metallize for hard-to-access structures (e.g., over roads with high average daily traffic); • Environment may dictate the selected option (including whether deicing salts are used); • Ownership—higher corrosion prevention is used for state versus local bridges; • Weathering steel is used/coated in accordance with FHWA guidelines; and • Established zones within the state (e.g., one state had four zones: mild rural/industrial, severe industrial, mild marine, severe marine). Protection Mechanism Functional Mode of Degradation Repair Methods Coated steel Protective film isolates steel from corrosive elements Protective coating is breached, typically caused by mechanical damage, weaknesses in the applied film (e.g., thin edges), or extreme environmental issues such as ultraviolet light exposure or mold growth. Maintenance painting or coating removal and replacement Uncoated weathering steel Forms corrosion patina, which reduces steel corrosion rate to a tolerable level Corrosion patina becomes damaged or disrupted because of local exposure conditions or mechanical damage. Painting or steel replacement Galvanized steel Metallurgically bonded zinc protects steel through barrier and sacrificial mechanisms. Galvanizing corrodes, reducing the zinc thickness. Maintenance painting or thermal spray application Metallized steel Sprayed metal coating (zinc, aluminum, or alloys thereof) mechanically bonds to and protects steel through barrier and sacrificial mechanism Metallizing corrodes or undercuts at weaknesses in the applied film (e.g., blind spots, edges). Maintenance painting or thermal spray application Table 1. Representative corrosion prevention methods for steel bridges. Figure 2. Survey results for corrosion-prevention technology usage.

Corrosion Prevention and Control for Steel Bridges 9 Four respondents (from Kansas, New York state, Connecticut, and Maryland) reported using specific corrosion allowances in new bridge design for weathering steel structures; one of those respondents (Maryland) also indicated corrosion allowances for painted steel structures are used. Examples of corrosion allowances included adding 1/16-in. to the webs of exterior girders where drainage is allowed over the edge, including 1/8-in. additional thickness on weather- ing steel structures, and “increasing flange thickness.” A corrosion allowance of 1/16-in. would accommodate a corrosion rate of slightly less than 0.001-in. per year (1 mil per year) for a 75-year service life. The most common innovative practice reported by bridge designers was the use of metallizing or galvanizing. Florida reported a great interest in coatings with improved gloss and color retention. The most common reported research need by designers included issues related to maintenance decision making (when to replace coating system, how to cost-effectively maintain, how much corrosion can be tolerated when aesthetics are not a concern). A better understanding of the effects of deicing salts and the need for less-corrosive deicing materials was also a reported research need. Service Life Implications of Corrosion on Steel Bridges Of the designers responding to the survey, 40% indicated that life-cycle cost (LCC) consid- erations are formally considered either qualitatively or quantitatively during the design stage. Nebraska reported using a tool developed by the FHWA called RealCost. Alternatives with sig- nificantly (e.g., 15%) lower LCCs are preferred; if all options have comparable LCCs, the least initial cost is selected. Generally, if an analysis shows that a strategy is cost-effective in one situ- ation, it is presumed to be cost-effective in similar situations. Most of the states did not require corrosion maintenance procedures as part of the design, although one state (Rhode Island) indicated designers are required to provide recommended corrosion maintenance procedures for coated steel structures. Figure 4 shows how frequently designers and maintainers consider corrosion as the limiting factor for the service life of steel bridges. Maintainers consider corrosion to be a more significant Figure 3. Most popular corrosion-prevention technologies.

10 Corrosion Prevention for Extending the Service Life of Steel Bridges limiting factor for steel bridge service life than do designers. Roughly one-third of the designers responding said corrosion was the limiting factor for design service life of steel bridges most of the time versus roughly one-half of the maintainers. Warranties for Corrosion Control Corrosion-control warranties are sometimes used to help ensure the quality of industrial paint- ing projects (including bridges). A 1999 report discussed warranty clauses for bridge paint con- tracts in considerable detail (Chang and Georgy 1999b). As a practical matter, warranties are usually written for a time period considerably shorter than the anticipated coating service life (e.g., a 1- or 2-year warranty on a coating that might have a 15- to 30-year service life). Warranties generally require painting contractors to repair coating breakdown above some level during the warranty period. There appears to be a wide range of details behind the warranty requirements. For example, some warranties have relatively detailed criteria clarifying the type of unacceptable defects and the affected area thresholds that invoke the need for repair. Others have more gener- alized statements. Some warranties explicitly call for an inspection by the DOT and contractor, whereas others are silent on the issue. Figure 5 shows that most of both maintainers and designers do not believe warranties effec- tively extend the life of steel bridge corrosion prevention. Only one designer respondent (Maryland) reported using warranties specific to corrosion prevention in new bridge construction contracts. The respondent reported the warranties require that the “work shall be considered defective if visible rust or rust breakthrough, paint blistering, peeling, cracking, chalking, shadow-through, scaling or scaling conditions as noted in the Performance Criteria Table occurs during the warranty period.” The warranty period is typically 2 years. Most respondents (67%) reported that corrosion is not a concern during new construction or would be covered by a performance bond (17%). One state specifically mentioned the use of warranties and reported “few were successful but not all.” Figure 4. Survey results regarding corrosion as a life-limiting factor for steel bridges.

Corrosion Prevention and Control for Steel Bridges 11 Many maintainers responding to the survey (44%) reported that corrosion is not a warranty concern during maintenance. Roughly the same number of respondents indicated that corro- sion is a concern addressed through a warranty clause with corrosion criteria (21%) or a per- formance bond (24%). Warranty durations were reported as 1 to 3 years, with an emphasis on visual detection of rust/rust-through, blistering, peeling, scaling, or unremoved slivers. Corrosion-Control Maintenance Corrosion-control maintenance primarily involves cleaning and painting bridges. The use of galvanizing and metallizing topped the list of reported innovations. Also included were removal of pack rust with ultrahigh-pressure water jetting, innovative surface preparation methods (vapor blasting and laser paint removal), and innovative coating chemistries (including fluoropolymer, calcium sulfonate, and waterborne acrylic). One respondent reported that the agency requires inspectors to perform an electrical holiday test on representative coated areas to reduce the num- ber of pinholes and defects. Maintainers reported success with drain extensions, bridge washing, and preventive maintenance programs. Challenges exist with overcoating and the cost of inspect- ing spot-coating work. Maintainers identified many issues for steel bridge corrosion research, including improve- ments for scuppers and deck joints, noncorrosive deicers, innovations in protective coatings (better primers, better color/gloss retention, single coat systems, galvanizing, and metallizing), understanding the effects of salts and other contaminants on coating performance, methods for maintaining the protective patina on weathering steel, and addressing pack rust in joints. The decision to perform maintenance is most commonly a condition-based decision. For both painted and unpainted steel bridges, users identifying as maintainers reported that inspec- tion recommendations and coating condition code most commonly drive corrosion mainte- nance. The survey results suggest that routine inspections are generally adequate for identifying corrosion maintenance needs on both coated and uncoated steel structures. Seventeen percent of respondents reported that they use supplemental inspections for uncoated steel structures, and 11% reported that they use supplemental inspections for coated steel structures. In some cases, the supplemental inspections were used to populate a condition database separate from Figure 5. Survey respondents’ impressions on corrosion warranties.

12 Corrosion Prevention for Extending the Service Life of Steel Bridges the NBI database. In other cases, the supplemental inspections were intended to define maintenance needs for planning and execution more thoroughly. Based on the survey results, a majority (58%) address corrosion similarly on all steel bridges. For those who addressed corrosion differently based on environmental or operating conditions, most did so based on assessments of the individual bridge rather than through written policies. Issues that drive corrosion maintenance decisions included • Weathering steel bridges washed at frequencies between 1 to 10 years, depending on the environment; • Different coating materials in different environmental zones; • Different maintenance strategies on different areas of the structure (e.g., zone painting or targeted debris removal); • Galvanizing or metallizing steel in congested areas; • Zone versus full repainting of weathering steel bridges based on exposure and current condition; and • Maintenance done differently based on the severity of corrosion or paint degradation. Figure 6 shows the various ways that agencies determine budget needs for corrosion mainte- nance. Most users reported that corrosion maintenance competes with other bridge maintenance needs for funding. “Other” responses included • A corrosion-control budget is established for in-house paint crew projects. For bridge painting by contract forces, steel bridge corrosion maintenance competes with other bridge maintenance needs. • A yearly defined amount of painting money is allocated. • Corrosion maintenance is included with other bridge-preservation activities. • Preventative maintenance is budgeted only for major bridges. Figure 7 illustrates the issues that affect what corrosion-related maintenance actions are taken. Lowest LCC and the availability of federal funding were more likely to drive corrosion-related maintenance decisions than were user impact or lowest construction cost. Figure 6. Survey responses regarding budgeting for corrosion maintenance.

Corrosion Prevention and Control for Steel Bridges 13 Figure 7. Survey responses regarding corrosion-maintenance decision drivers. Maintaining Coated Steel Bridges Figure 8 shows the percentage of responding agencies that use each of the various corrosion- control technologies when maintaining a coated steel bridge. For coated bridges, the most commonly used steel bridge maintenance methods are application of a zinc-based paint system (78%) and spot-coating repairs (54%). Spot repairs with a full overcoat appear to be less com- mon than spot repairs. About half of the states responding performed washing or rinsing to remove contaminants as a maintenance technique. Spot painting and washing were performed in-house by a significant percentage of the states, but contracting work using state specifications was the most common mechanism for performing steel bridge maintenance. Table 2 shows the range of estimated service life or recurrence interval provided by the respondents. Figure 9 illustrates how frequently various conditions dictate the need to repaint a coated steel bridge. The primary driver is condition-based (inspection recommendation or poor coat- ing condition rating) versus time-based. It does appear common that corrosion maintenance is performed at least in part to coincide with other work. Maintaining Uncoated Steel Bridges Figure 10 shows the percentage of responding agencies that use each of the various corrosion- control technologies when maintaining an uncoated steel bridge. For uncoated bridges, the most commonly used steel bridge maintenance method is zone painting (68%). Thirty-eight percent of the respondents reported that they had cleaned and repainted an entire weathering steel structure, although one respondent noted that this occurs infrequently. Fewer agencies reported that they wash/rinse contaminants on uncoated steel versus coated steel (41% versus 51%) as a maintenance technique. Washing is performed in-house by a significant percentage of the states, but contracting work using state specifications was the most common mechanism for maintenance painting of uncoated steel. Table 3 shows estimated service life provided by the respondents. Figure 11 illustrates how frequently various conditions dictate the need to perform corro- sion maintenance on an uncoated steel bridge. The primary driver is condition-based (inspec- tion recommendation, poor coating condition rating, or observed steel section loss) versus

14 Corrosion Prevention for Extending the Service Life of Steel Bridges Figure 8. Agencies reporting that they use various corrosion-control technologies for coated steel bridge maintenance. Strategy Reported Life or Recurrence Interval Removal of all coating and corrosion and application of a zinc- based paint system 15–50 years Removal of all coating and corrosion and application of a nonzinc- based paint system 15–35 years Removal of all coating and corrosion and application of metallizing (with or without additional coats) 25 years Spot prime and overcoat entire structure 5–25 years Spot repairs of corrosion and delaminated coating 5–25 years Washing/rinsing/cleaning to remove contaminants 1–5 years Table 2. Reported service life for coated bridge maintenance strategies.

Corrosion Prevention and Control for Steel Bridges 15 How frequently does each of the following conditions contribute to the need for repainting a coated steel bridge? Figure 9. Conditions dictating the need to repaint a coated steel bridge. Figure 10. Agencies reporting that they use various corrosion-control technologies for uncoated steel bridge maintenance.

16 Corrosion Prevention for Extending the Service Life of Steel Bridges Strategy Reported Life or Recurrence Interval Removal of all corrosion and application of a protective coating system to the entire structure 12–35 years Removal of all corrosion and application of a protective coating system in zones to mitigate corrosion (e.g., beam ends) 3–35 years Removal of all corrosion and application of a protective coating system in zones solely for aesthetics (e.g., improve fascia appearance or prevent running rust stains) 10–30 years Washing/rinsing/cleaning to remove contaminants 1–10 years Table 3. Reported service life for uncoated bridge maintenance strategies. Figure 11. Conditions dictating the need to maintain an uncoated steel bridge. time-based. It does appear common that corrosion maintenance is performed at least in part to coincide with other work. Relative Cost The cost of various corrosion-prevention schemes is dependent on multiple factors. Con- sequently, there is not a clear option for the lowest installed cost. Maintenance considerations vary considerably as well, making LCC calculations even more complicated. For the purposes of perspective, a 2017 TRB presentation provided the data in Figure 12 (Medlock 2017). The chart provides the relative cost of a 230-ft continuous two-span bridge with varying corrosion- prevention schemes. The structure consists of 30, 52-in. plate I-girders installed in 10 lines with 99 cross frames and no bearings. This is the fabricator’s free-on-board (FOB) cost for steel delivered 130 mi from the shop. The cost does not include abutments, site work, installation, and so forth. Fabricator capabilities vary; these costs represent only one fabricator’s perspective. The data show weathering steel has the lowest delivered cost, whereas a painted steel bridge has a 19% to 24% premium. Galvanizing and metallizing options have a 41% to 57% premium.

Corrosion Prevention and Control for Steel Bridges 17 Figure 12. Relative fabricator cost for a hypothetical as-delivered steel structure with various corrosion-control strategies. The A1010 premium is 130%, but that is likely to decrease as the material gains market accep- tance. Of course, each material alternative will have different LCCs that should be considered when designing a bridge. Another study looked at the relative cost and performance of various coating maintenance strategies on a New Jersey bridge over saltwater (Ault and Farschon 2008). Figure 13 presents the 2007 inspection data (ASTM D610: 10 = best, 0 = worst) versus the cost of the coating system (dollar per square foot in 1986 dollars). The data suggest a trend toward increased performance with increasing cost, but the relationship has considerable scatter. Cost alone would not be a good basis for assessing the overall value of a coating system, simply because there are so many other criteria that play into the success of a coating system. The authors grouped systems together by generic coating type and applied definitions of success, failure, and premature failure to the condition observations over 20 years. The authors defined success as a system that was “good” at 8 years and only a “maintenance candidate” at 20 years. Failure was defined as a system that required complete replacement after 8 years. Figure 13. Cost versus 20-year performance for various maintenance coatings.

18 Corrosion Prevention for Extending the Service Life of Steel Bridges Premature failure required replacement before 8 years. A coating was considered in “good con- dition” if the average inspection rating was an ASTM D610 rating better than 7 (less than 0.3% rusting); a maintenance candidate had an average ASTM D610 rating of 4 to 7 (from 0.3% to 10% rusting); and a remove/recoat candidate had an ASTM D610 rating of less than 4 (more than 10% rusting). Using these definitions, Figure 14 shows the cost versus performance for each of the coating system groups. The organic zinc-based systems appear to provide a reasonable balance of cost and performance based on this set of data. Figure 14. Cost versus 20-year performance risk for various maintenance coatings.

Next: Chapter 3 - Corrosion Control of Uncoated Steel Bridges »
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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 517: Corrosion Prevention for Extending the Service Life of Steel Bridges documents and describes the current practice for corrosion prevention of steel bridges. This report provides information on choosing materials and coatings to prevent corrosion, and documents ways to develop an effective maintenance plan for newly constructed and in-service bridges and transportation structures. This report does not prescribe a practice or set of practices, as might be expected in a guidebook or manual.

The scope of this synthesis is limited to corrosion of the atmospherically exposed superstructure elements of steel bridges. The report is accompanied by the following appendices, available online:

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