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42 Steel is used for several elements of bridges, including towers, piers, girders, and trusses. Steel components in bridges often are protected from corrosion with protective coatings. Coatings provide aesthetic appeal and can be designed to provide resistances to several degradation mech- anisms, such as abrasion, moisture, or ultraviolet (UV) radiation. Coatings protect the steel through one or more of the following protection mechanisms: ⢠Providing a barrier between the metal and the corrosive environment; ⢠Providing sacrificial protection; or ⢠Providing corrosion inhibitors. For decades, the predominate coating system for bridge preservation was lead-containing âred leadâ alkyds. This paint system was inexpensive and provided adequate protection when proper maintenance was implemented. The red lead paint systems were phased out for several reasons, including ⢠The system contained lead and chromate. The use of both lead and chromate is discouraged or banned for environmental health considerations (Chang and Chung 1999; Kogler 2015; NACE 2012). ⢠The system was deceptively tolerant of inadequate surface preparation and priming practices, which led to reduced coating lifetimes in the long term (Chang and Chung 1999). ⢠The system, at its best, provided a maximum paint life of 20 years. Longer coating lifetimes are available in other coating systems (Chang and Chung 1999; Kogler 2015). In the 1980s and 1990s there was a great deal of research into alternatives to the lead-based alkyd coating systems. NCHRP Synthesis 257 documented the state of the steel bridge coat- ings toward the end of this evolution (Neal 1998). Although a number of alternative systems are available, the industry has gravitated toward abrasive blasting of the steel and application of a three-coat system consisting of zinc-rich primer with an epoxy intermediate coat and polyurethane finish coat; this system is preferred in part because of the conclusions of several FHWA-funded studies (Appleman et al. 1989a; Appleman et al. 1989b; Kogler et al. 1997; Kogler and Mott 1992; Smith and Tinklenberg 1995). This coating approach is common for new and existing structures. However, many bridge owners still used the âovercoatingâ option to deal with aged, lead-based coatings when cost, accessibility, condition, or other constraints precluded complete removal. Several alternative surface preparation approaches and coating chemistries have been evaluated for this approach (Ault and Farschon 2008; Farschon et al. 1997). There is renewed interest in spot repair, zone painting, and overcoat- ing because the new, nonlead coating systems require maintenance (Hopwood et al. 2018; KTA-Tator 2014. C H A P T E R 4 Corrosion Control of Coated Steel Bridges
Corrosion Control of Coated Steel Bridges 43 Coating Material Alternatives Figure 31 shows the popularity of various coating systems as indicated by a 2014 survey of state bridge coating maintenance practices (KTA-Tator 2014) and the survey portion of this synthesis. Both surveys show significant reliance on zinc-based liquid coating systems. Because the 2014 survey was for maintenance, galvanizing was not considered, but the data suggest that metallic coatings are increasingly being used for protection of steel bridges. Nonzinc-based liquid coating systems are occasionally used for new structures and often used when performing spot painting or overcoating to maintain an existing coated structure. The following sections discuss four categories of coatings: liquid coatings, galvanization, metallization, and duplex coatings. Liquid Coatings Although simple in concept, liquid coatings encompass a wide range of materials with nomen- clature that can be confusing. Liquid coatings can be classified by generic resin type (e.g., epoxy, alkyd, siloxane, acrylic, urethane), generic pigment type (zinc-rich, inhibitive), or other charac- teristics (high-build, fast-cure, high-durability). Furthermore, coating systems often are made up of multiple coats, each of which serves a different function (e.g., primer, intermediate, finish). A complete discussion of coating composition is beyond the scope of this synthesis but can be found in Chapter 6 of the Design Guide for Bridges for Service Life (Azizinamini et al. 2014). Bridge designers and maintainers should recognize the need to understand more than the generic type of coating being used. Coating selection includes the type of surface preparation, number of coats of paint, function of each coat, and so forth. The current state of the practice for protecting new steel bridges from corrosion with pro- tective coatings is to apply a three-coat liquid system with a zinc-containing primer over an abrasive-blasted steel surface. This is also the state of the practice for maintenance coatings where the strategy is to remove and replace the existing coating (KTA-Tator 2014). Figure 32 shows that most agencies surveyed prefer a three-coat system with an inorganic or organic zinc primer. One owner reported using a multicoat alkyd system, and three owners reported using a multicoat moisture cure urethane (MCU) system (it is not noted whether the moisture cure primer contained zinc). Nine agencies reported using metallizing. Figure 31. Relative popularity of bridge steel coating systems.
44 Corrosion Prevention for Extending the Service Life of Steel Bridges A significantly wider range of coating chemistries is used for spot touch-up and overcoating. Figure 33 shows the various types of coatings used for spot touch-up and overcoating as reported by agencies in a 2014 survey (KTA-Tator 2014). In addition to corrosion control and durability, these coating materials must be compatible with the existing coating. The overcoating must not chemi- cally attack the existing coating or impart curing stresses that may cause the existing coating to lift at the edges. The issue of compatibility is not an exact science, but knowing the existing coating chem- istry, thickness, and adhesion can help mitigate the risk of overcoating failure (Farschon et al. 1997). Three-coat System Containing a Zinc-rich Primer The most popular bridge coating system is a three-coat system consisting of an inorganic/ organic zinc primer, epoxy midcoat, and polyurethane topcoat. In this system, the primer, Figure 32. Popularity of alternative removal and replacement coating systems (data from KTA-Tator 2014). Figure 33. Relative popularity of alternative maintenance coatings (data from KTA-Tator 2014).
Corrosion Control of Coated Steel Bridges 45 intermediate, and topcoats all provide different functions. The zinc-rich primer serves as a bar- rier but more importantly provides corrosion prevention at defects where the steel is exposed (Ault and Farschon 2008). The epoxy midcoat provides additional barrier protection as well as mechanical durability. The midcoat is also more compatible with zinc primers than poly- urethanes and thus serves as a link between the two materials. The topcoat provides chemical resistance, abrasion resistance, UV resistance, and color and gloss retention. The AASHTO/National Steel Bridge Alliance collaboration (2014) has developed the Guide Speci- fication for Application of Coating Systems with Zinc-Rich Primers to Steel Bridges, which represents a consensus on best industry practice for shop application of zinc-rich coating systems to previ- ously uncoated bridge steel and includes the proper preparation of the steel. The guide specification addresses a three-coat system consisting of primer, intermediate coat, and topcoat but is also appro- priate for application of a two-coat system or primer only. The guide includes a series of charts that simplify the application parameters for a system based on zinc-rich primers on new steel bridges. Zinc-rich coatings are used by many industries for the corrosion prevention of steel sub- strates. These coating materials have demonstrated improved atmospheric corrosion perfor- mance when compared with carbon steels protected only by spray-applied organic coatings (such as an epoxy-urethane coating system). Society for Protective Coatings (SSPC) Paint 20 is an industry specification that categorizes zinc primers according to four vehicle types: Type I-A includes water-soluble inorganic post-curing vehicles such as alkali metal silicates, phosphates, and modifications thereof that must be subsequently cured by application of heat or a curing solution. Type I-B includes water reducible inorganic self-curing vehicles such as water-soluble alkali metal sili- cates, quaternary ammonium silicates, phosphates, and modifications thereof. Type 1-B coatings cure by a reaction among the zinc, silicate, steel substrate, and naturally occurring carbon dioxide during and after evaporation of water from the coating. Type I-C coatings include solvent reducible inorganic self-curing vehicles such as titanates, organic silicates, and polymeric modifications of these silicates. These systems are dependent upon moisture from the atmosphere to complete hydrolysis, forming the titanate- or polysilicate-zinc reaction product. Type II coatings involve organic vehicles, which may be chemically cured or may dry by solvent evapo- ration (heat may also be used under certain conditions). Common vehicles for Type 2 coatings include epoxies and moisture cure urethanes. SSPC Paint 20 defines three levels of zinc in the dried film: Level 1 is equal to or greater than 85%; Level 2 is equal to 77% up to 85%; and Level 3 is equal to 65% up to 77%. By this defini- tion, coatings with greater than 65% zinc by weight in the dried film are considered âzinc rich.â It is important to note that zinc content is weight percent. Volume percent will be much lower because the binder is considerably lighter than the zinc. Zinc content is determined in the dried film, so the wet product will likely have a lower zinc content. Finally, note that zinc particle size and purity are also issues that may affect performance but are not addressed in the specification. Inorganic zinc primers are sensitive to surface preparation and curing conditions. As a result, they are typically used for new steel coated in a shop rather than field maintenance. Inorganic zinc-rich coatings consist of powdered zinc in a silicate solution. These coatings are reactive coatings that chemically interact with the environment. These reactions begin when the solvent evaporates and ends when the zinc in the coating is inactivated by an accumulation of zinc salts on the coatings surface (Chang and Chung 1999). Inorganic zinc coatings are available in water-based (alkali silicate) and solvent-based (ethyl silicate) formulations. There has been some interest in the water-based formulations for complying with volatile organic compound (VOC) regulations in states such as California. Unfortunately, the water-based inorganic zinc coatings are sensitive to curing conditions and may exhibit freckle rusting, topcoat blistering, and softening if not properly cured (Ault and Farschon 1999). A higher zinc content makes the coating electrically conductive, allowing the coating to cathodically protect the steel substrate.
46 Corrosion Prevention for Extending the Service Life of Steel Bridges In addition to cathodically protecting the steel, inorganic zinc primers form a hard barrier layer upon reaction with the environment. This barrier mechanism works in conjunction with the cathodic protection afforded by the zinc particles. Organic zinc primers are mixtures of zinc dust or pigment in an organic vehicle; usually the vehicle is an epoxy resin or polyurethane. As with inorganic zinc primers, organic zinc primers are conductive. This can be achieved by either using sufficient zinc loading to ensure particle-to- particle contact or by using conductive fillers in the organic vehicle. In addition, the organic filler should be alkali resistant because reactions involving the zinc particles cathodically protecting the steel produce alkaline conditions (Chang and Chung 1999). Intermediate coats are commonly two-component epoxies that react with either a polyamine or polyamide hardener. The epoxies used tend to be medium molecular weight Bisphenol-A epoxy resins with polyamide curing agents (Chang and Chung 1999). The epoxy intermediates provide a barrier layer to protect coating defects, such as missed spots, in the primer layer. The intermediate coat is also designed to provide a good surface for adhesion of the topcoat. Polyurethane topcoats can be hydroxylated acrylic or hydroxylated polyester. The acrylic polyols have slightly better UV resistance, whereas the polyesters may have better chemical and abrasion resistance (Chang and Chung 1999). The main purpose of the topcoat is to provide aesthetic appeal through color and gloss retention and provide the coating system with UV pro- tection (Chang and Chung 1999; NACE 2012). Other zinc-rich primers have been developed, including zinc-rich MCU coatings. These coatings are more tolerant of humid and damp conditions than are the epoxy or polyurethane organic zinc-rich coatings. Zinc-rich MCU primers have recently been utilized on bridges with MCU midcoats and topcoats (KTA-Tator 2014). Moisture Cure Urethane Coating Systems MCU coatings come in single-component packaging and cure through reaction with mois- ture, usually atmospheric moisture (Chang and Chung 1999; Chong and Yao 2006). MCU coat- ings have been used in Europe and the United States. Because these coatings cure through a reaction with moisture in the surrounding environment, the coating cure is dependent on the residual humidity during application. If the humidity is too low, the cure can be too slow and the coating can crack and split. If the humidity is too high, the cure can be too fast, which may result in trapping of the carbon dioxide generated during the curing process and subsequent bubbles in the cured film. This would lead to bubbling of the coating (Chang and Chung 1999). Applying MCU coatings in the recommended environmental conditions is important to ensure proper coating performance. MCU coatings are organic coatings based on a urethane resin. They include zinc-rich for- mulations, penetrating sealer formulations, formulations optimized to function as barrier coatings (typically used as a midcoat), and formulations with varying degrees of weather- ability. Thus, it is possible to have a three-coat system with a zinc-rich primer that is made up entirely of MCU coatings. Galvanizing Galvanizing is the application of metallic zinc to a steel surface for corrosion prevention of the steel. The zinc initially protects the steel by providing a barrier between the steel and the environ- ment. When the barrier is breached (e.g., through mechanical damage or by corrosion) and zinc and iron are exposed to an electrolyte (e.g., surface wetness), the zinc cathodically protects the steel. The zinc coating protects the base metal from corrosion by providing both a barrier from
Corrosion Control of Coated Steel Bridges 47 the environment and sacrificial cathodic protection. If the surface becomes scratched, exposing the base metal, the zinc coating is slowly consumed while the substrate remains protected. Galvanized bridges have been in limited use since 1966. There are currently over 200 hot-dip galvanized bridges in the United States, most of which are short span bridges or bridge compo- nents that are small enough to fit into industrial zinc baths. However, as the industry looks to build more durable steel bridges, galvanizing is becoming more common and finding use on larger structures. During rehabilitation of older steel structures, replacement steel is often hot-dip galvanized. In most cases, structural steel for bridges is galvanized by means of a hot-dip process per ASTM A123, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products. Figure 34 is a schematic of the galvanizing process. The material to be hot-dip galvanized should be free of welding slag, mill scale, paints, varnishes, oil, and grease. This is often accomplished in multiple steps, which may include degreasing and acid pickling or abrasive blasting. The cleanli- ness of the steel surface is critical to the successful application of a galvanized coating because the molten zinc fuses with the base metal to form a metallurgical bond. Skip welds, crevices, and other areas may trap pickling acid. The trapped acid can vaporize rapidly during hot-dipping, possibly damaging the workpiece, or it may leak and stain or deteriorate the coating. During hot-dip galvanizing, the zinc forms an alloy with the steel, providing maximum adhe- sion between the metals. There is a gradual progression of zinc and iron alloy from pure zinc to pure iron, as shown in Figure 35. When a steel article is dipped in a molten zinc bath, the alloy- ing process between the steel and the zinc begins immediately. Four distinct layers or zones are produced during the alloying process: a 100% zinc outer layer and three distinct ironâzinc alloy layers. Note that the alloy layers increase in iron content and hardness as they get closer to the steel. The zincâsteel alloy layers are measurably harder than the steel substrate, which provides some degree of protection from abrasion. The integrity of the zinc coating and its thickness are influenced by a number of factors, including the composition of the steel, geometry of the steel part, condition of the steel substrate, the length of time in the bath, the rate of withdrawal from the bath, and the cooling rate. If the steel is thoroughly cleaned, a continuous coating forms over the surface, encompassing rivets, welds, and edges of complicated fabricated structures. Sharp corners, recesses, and inaccessible areas are all effectively coated by hot-dip galvanizing. Because all surfaces are coated during immersion, galvanizing also provides both inside and outside protection for hollow structures. Posttreatment may be performed to reduce coating thickness, improve properties or appear- ance, or otherwise alter coating character. Inspection of the galvanized product is normally Figure 34. Schematic of a hot-dip galvanizing process (Courtesy of the American Galvanizers Association).
48 Corrosion Prevention for Extending the Service Life of Steel Bridges conducted at the galvanizerâs plant before shipment. Because the service life of a galvanized coating is directly related to the zinc coating thickness, thickness and coverage are critical checks of galvanized coating quality. Design and fabrication of the workpiece can influence the quality of hot-dip galvanizing. If not thoroughly cleaned, skip welds, crevices, and other areas may become stained or have defec- tive coating. Pockets or air bubbles may prevent the molten zinc from coating certain areas. Large workpieces may have to be dipped in two orientations to coat all surfaces. The overlap line may be a darker gray, although the integrity of the coating on the overlap area is not compro- mised. Parts with unequal thickness or nonsymmetrical design may be warped if not properly constrained during dipping. Ideally, a workpiece to be hot-dip galvanized should be made of a single steel alloy because different steel alloys develop different coating characteristics. The galvanized coating thickness is influenced by steel thickness and geometry, galvanizing bath temperature, steel chemistry, bath immersion time, surface condition of the substrate, and rate of withdrawal from the bath. Generally, thicker steel will produce thicker coatings. The basic finish requirements of a galvanized coating are that it be relatively smooth, con- tinuous, lustrous, and free from gross surface imperfections (e.g., cracking, peeling, bare spots, lumps, blisters, and inclusions of flux, ash, or dross). These are subjective requirements that should nonetheless be addressed by the job specifications. The galvanized coating should be continuous to provide optimum corrosion prevention. Chain slings, wire, or other holding devices may be used to immerse an article into the galva- nizing kettle if it has no suitable lifting fixtures. These items may leave marks on the galvanized surface. If required, these areas can be repaired. Differences in the luster and color of galvanized coatings generally do not significantly affect corrosion resistance. The presence or absence of spangles (zinc crystals) also has no effect on coating performance. Wet storage stain is the name given to the porous, bulky deposit that may form on the surface of closely stacked, freshly galvanized articles that become damp (from exposure to the elements or condensation) in poorly ventilated conditions during storage or transit. Wet storage stains should be removed before installation to prevent premature coating failure. Figure 35. Alloy layers of a hot-dip galvanized coating (Courtesy of the American Galvanizers Association).
Corrosion Control of Coated Steel Bridges 49 Metallizing Metallizing is a process in which molten metal or ceramics are sprayed onto a substrate. There are several metallizing technologies, including arc wire, flame, plasma, and high-velocity oxygen fuel (HVOF). Twin wire arc spray is the most common metallizing technology for bridge steel. Common thermal spray materials are pure aluminum, pure zinc, and aluminumâzinc alloys. These coatings protect the steel through both barrier and sacrificial mechanisms. The coating alloys have different cathodic protection efficiencies and self-corrosion rates that affect their performance. Correctly applied, metallizing can provide excellent corrosion prevention and be a cost-effective corrosion-protection strategy. There is increasing interest in and use of metallizing for bridges and bridge components. Several transportation departments have used metallization on bridges and been pleased with the results (KTA-Tator 2014). A notable metallized bridge in the United States is the Memorial Bridge between Portsmouth, New Hampshire, and Kittery, Maine (Zoller 2014). The bridge design employed structural innovations to minimize gusset plates and relocate remaining splice plates for ease of maintenance, and used employed simplified rolled shapes for coatability. Thermal spray zinc with a clear sealer was applied in the shop over a white metal blasted surface. The thickness was nominally 10 mil above the roadway and 14 mil below the roadway. Based on the presentation, the bridge designers estimated a 33% cost premium for an anticipated double service life. At the time, there were at least four other metallized bridges in New England. Historically, state DOTs have written their specifications based on industry standards, such as NACE No. 12/AWS C2.23M/SSPC CS 23 [âSpecification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corro- sion Protection of Steelâ]. The standard includes requirements for surface preparation, coating application, repairing coating defects, measurement of coating thickness, adhesion testing of the applied coating, and application of sealers and topcoats over the thermally sprayed metal coating. This standard is intended for use by facility owners and specifiers who develop project specifications for the application of metallizing for the preservation and maintenance of steel structures and components. The standard also may be used by metallizing inspectors to assess the quality of surface preparation and coating application and by metallizing contractors to develop project work plans. The AASHTO/NSBA Steel Bridge Collaboration is developing a document that represents a consensus on best practice for the application of metallizing to steel bridges and other highway structures. To help ensure coating quality, applicator training and certifications are available. Metallized coatings are usually porous. To varying degrees, the coating corrosion products may fill this porosity. Sealers such as acrylic urethanes, polyester urethanes, vinyls, phenolics, epoxies, or thermal-sprayed polymers can enhance the service life by providing barrier protec- tion to the steel substrate (Chang and Georgy 1999a; Ellor et al. 2004; Lau 2015). An FHWA study (FHWA 2014) documents slip coefficient testing of both sealed and unsealed zinc and 85/15 zincâaluminum alloy metallizing. The testing demonstrated that unsealed metallizing can meet Class B slip requirements, whereas sealed metallizing does not. When used in joints, thermal spray coatings should not be sealed. Duplex Coatings Duplex coatings is a concept that capitalizes on the synergetic effects of conventional paints to provide a protective barrier coat to metallic coatings such as galvanizing or metallizing. In this case, the metallic coating replaces the primer of the typical three-coat, zinc-rich coating systems. The term âduplex coatingâ originally referred to organic coatings over galvanizing. An organic coating applied to metallizing has historically been referred to as a âsealer,â in part because such
50 Corrosion Prevention for Extending the Service Life of Steel Bridges coatings originally were relatively thin and intended to seal any porosity that might exist in the metallizing. However, full-thickness organic coatings have taken the place of sealers, and the terminology âduplex coatingsâ now is commonly applied to systems with either galvanizing or metallizing. Properly applied metallic coatings are more durable than zinc-rich liquid coatings because they are thicker, have a better bond, and can provide more sacrificial protection. However, if the metallic coatings are exposed to the environment they will corrode over time. Applying a barrier coat over the metallic coating will prevent the metallic coating from corroding. The two coats work synergistically. Top coats typically fail at holidays, where bare metal becomes exposed and eventually undercuts as the metallic coating corrodes. Metallic coatings are more resistant to the formation of holidays, and more-sacrificial metal (e.g., zinc) is available. Additionally, the metallic coating will not begin to corrode until the barrier coating breaks down. The total life of duplex coatings can conservatively be estimated to be the sum of the individual lives of each of the coatings. However, one author has shown through observational evidence that the synergetic life is 1.5 to 2.3 times the conservative summation of the individual lives (Van Eijnsbergen 1994). Innovative Corrosion-Prevention Coatings for New Structures Self-healing Coatings Nanotechnology is being used to develop coating systems that will heal after mechanical damage. Research is being completed to improve the ability of the coating to deliver corrosion inhibitors to corroding areas of the substrate (NACE 2012). Single-component Coatings Single-component, polyurethane, zinc-rich coatings and MCU coatings are available that perform comparably to the corresponding two-component systems. However, in the case of zinc-rich polyurethanes, single-component systems may have shorter pot lives than their two- component counterparts (Chang and Chung 1999). Thus, the two-component systems may be desired because of their greater storage stability. One state transportation department reports the use of single-component MCU coatings for overcoating (KTA-Tator 2014). California has developed waterborne acrylic latex formulations in its chemistry division. Its latest formulation is an acrylic latex-fluoropolymer resin blend. This coating has improved gloss and color retention and the potential to last significantly longer than previous formulations. Single-coat and Two-coat Systems To reduce cost, single-coat and two-coat systems have been developed. Research completed by the FHWA on single-coat systems identified some promising single-coat systems that do not yet perform as well as the zinc-rich three-coat systems in laboratory tests or natural exposures (Yao et al. 2011). A recent survey of the state transportation departments suggests few instances in which one-coat systems were selected for use (KTA-Tator 2014). Research completed by the FHWA on two-coat systems identified two-coat systems that per- formed comparably to the zinc-rich three-coat system in laboratory tests and a natural exposure (Chong and Yao 2006). Specifically, these two-coat systems were zinc-rich epoxies or MCUs top- coated with polyurethanes or polyaspartics. However, later testing on a zinc-rich MCU/aliphatic polyurea urethane two-coat system exhibited poor coating performance in laboratory and natural exposure testing. Several transportation departments report using two-coat systems. The Con- necticut Department of Transportation has applied two-coat systems on four bridges and reports lower apply times, lower contractorâs labor costs, and reduced travel delays compared with the application of three-coat systems (Chong and Yao 2006; KTA-Tator 2014).
Corrosion Control of Coated Steel Bridges 51 Specifications Based on the survey conducted for this synthesis, all the respondents said their state has a qualified products list for protective coatings. Five agencies (states of Michigan, Minnesota, Wisconsin, and Florida, and New York City) reported that they have a performance-based specification for coatings. The California DOT (CALTRANS) develops waterborne acrylic latex formulations in the agencyâs chemistry division. The National Transportation Product Evaluation Program (NTPEP) was established to mini- mize the amount of duplicative testing of transportation materials performed by state laboratories. NTPEP currently has programs for 19 product types, including structural steel coatings (SSC). The SSC program evaluates protective-coating systems intended for use on new and existing structural steel prepared by abrasive-blast cleaning. The program provides the end user with test results that can be used to make performance judgments on coating systems for long environmental exposures. The testing program was developed around a three-coat system consisting of a zinc primer, epoxy or urethane intermediate, and an aliphatic urethane finish coat; however, coating systems are not required to meet any specific compositional requirements for submission and testing. NTPEP exists purely for testing materials and providing test results. The evaluation of the test results is left up to each member department. More information about NTPEP is available at www.ntpep.org. The SSC program evolved in part from the Northeast Protective Coating Committee (NEPCOAT). NEPCOAT is an affiliation of nine northeastern states with the purpose of develop- ing acceptance/testing criteria of protective coating for use on highway bridge steel. NEPCOAT currently uses NTPEP test results to establish a qualified product list. More information about NEPCOAT is available at www.nepcoat.org. According to the NTPEP website, the 26 states highlighted in Figure 36 currently use NTPEP results. In addition, seven NEPCOAT states that are not highlighted on the map indirectly use Figure 36. State DOT usage of NTPEP structural steel coating system test results, updated November 2015 (used with permission).
52 Corrosion Prevention for Extending the Service Life of Steel Bridges the NTPEP data. Therefore, two-thirds of the states use the NTPEP program to inform their structural steel coating selection. Table 11 shows how the survey respondents specify corrosion prevention during design (note that respondents could select more than one option, so the categories do not add up to 100%). Agencies generally have their own standard specification or utilize a special specification to contract for coating systems. Agencies using nonzinc-based systems rely on a qualified product list. The responses suggest an opportunity for AASHTO to develop standard specifications for protective coatings. Currently, the AASHTO/NSBA collaboration is developing standard guide- lines for protective coatings on steel bridges. For weathering steel bridges, agencies use either AASHTO or agency standard specifications. Table 12 shows how the survey respondents specify corrosion prevention during maintenance of coated steel structures (note that respondents could select more than one option, so the categories do not add up to 100%). Agencies tend to perform work with in-house crews when using a nonzinc-based paint system. About half of the agencies use in-house crews to perform spot repair of corrosion/delaminated coating and bridge washing/rinsing. Few states use AASHTO standard specifications for corrosion maintenance activities. Agency standard specifications are the most common for contracting painting activities. States use special specifications to contract for met- allizing and overcoating work. The responses suggest an opportunity for AASHTO to develop standard specifications for maintaining protective coatings. AA SH TO S ta nd ar d Sp ec ifi ca tio n Ag en cy S ta nd ar d Sp ec ifi ca tio n Sp ec ia l Sp ec ifi ca tio n Q PL Pe rf or m an ce -b as ed Re qu ire m en t Zinc-based paint system 50% 100% 44% 53% 14% Nonzinc-based paint system 0% 25% 0% 100% 0% Metallized structural steel 8% 31% 85% 8% 8% Galvanized structural steel 31% 62% 38% 23% 0% Weathering steel QPL = qualified product list. 66% 55% 8% 3% 8% Table 11. How agencies specify corrosion prevention during design. In -h ou se W or kf or ce AA SH TO S ta nd ar d Sp ec ifi ca tio n Ag en cy S ta nd ar d Sp ec ifi ca tio n Sp ec ia l Sp ec ifi ca tio n Clean to bare metal and apply a zinc-based paint system 14% 21% 100% 52% Clean to bare metal and apply a nonzinc-based paint system 75% 0% 100% 0% Clean to bare metal and field metallize 0% 0% 50% 100% Spot repair and overcoat entire structure 38% 15% 100% 100% Spot repair of corrosion and delaminated coating 50% 5% 80% 35% Wash/rinse/clean to remove contaminants 53% 0% 37% 16% Table 12. How agencies specify corrosion control during maintenance of coated steel structures.
Corrosion Control of Coated Steel Bridges 53 Table 13 shows how the survey respondents specify corrosion prevention during maintenance (note that respondents could select more than one option, so the categories do not add up to 100%). Agencies generally use standard specifications or special specifications to contract painting of weathering steel structures. Washing/rinsing of weathering steel structures is often performed by in-house crews, although sometimes is contracted using agency specifications. AASHTO standard specifications are not used for maintenance of weathering steel. Corrosion-Prevention Design Issues for Coated Steel Bridges There are several issues that the designer should consider when selecting and specifying cor- rosion control coatings. The most obvious, coating selection, includes whether to use metallic or liquid primers, whether to use inorganic or organic zinc (when using a zinc-rich primer), galva- nizing versus metallizing, whether or not to topcoat metallic coatings, and what liquid coating chemistry to use. In addition to selecting a corrosion control coating, the designer must incorpo- rate design details that will be less prone to damage and/or easier to coat. A designer may specify a more-durable coating in areas more prone to damage, such as the exterior fascia or steel near expansion joints (a concept referred to as âzone paintingâ). The need for enhanced corrosion prevention is driven by the corrosivity of the environment. Finally, the designer may consider adding warranty language to the specification to ensure a high-quality protective coating system. Each of these issues is elaborated on in this section. Coating System Selection Liquid versus Metallic Coatings When considering coatings for steel bridges, most designers default to liquid coatings. Although metallic coatings (galvanizing and metallizing) have been used on bridges since the early 1900s, they have been viewed as specialty or niche corrosion-prevention strategies that require too much additional effort for most circumstances. In the United Kingdom, a multicoat system with thermal-sprayed aluminum was common in the 1990s (FHWA Study Tour Team et al. 1997). Galvanizing and metallizing do require some specific considerations during the design phase that can increase the cost of the coating system. Life-cycle cost analysis can be used to justify the higher initial cost if the agency is amenable to such considerations (Farschon et al. 1997). Additional design considerations include detailing for producibility. In the case of galvanizing, this includes ensuring the part can be accommodated by the galvanizerâs bath, adequate drainage provisions, and design geometry that will not distort unacceptably during galvanizing. For In -h ou se W or kf or ce AA SH TO S ta nd ar d Sp ec ifi ca tio n Apply full coating 15% 15% 100% 62% Zone paint corrosion-prone areas 26% 9% 100% 65% Paint for aesthetics 0% 0% 75% 50% Wash/rinse/clean 86% 0% 36% 7% Sp ec ia l Sp ec ifi ca tio n Ag en cy S ta nd ar d Sp ec ifi ca tio n Table 13. How agencies specify corrosion control during maintenance of uncoated steel structures.
54 Corrosion Prevention for Extending the Service Life of Steel Bridges metallizing, the design details include ensuring all surfaces are accessible for proper surface preparation and thermal-spray metallizing. Organic Zinc versus Inorganic Zinc Primers Zinc-rich, three-coat systems entail either an organic or inorganic zinc-rich primer, a bar- rier midcoat, and durable topcoat. The choice between an inorganic and organic zinc primer deserves some consideration. Table 14 compares characteristics of the two materials. As a gen- eral rule, most bridge owners favor the organic zinc for field application. Historically, inorganic zinc primers were favored for shop application. However, many states now allow organic zinc primer in shop applications because it can shorten the coating time. Modern organic zinc coat- ings with a high zinc load can perform comparably to inorganic zinc coatings. Metallized versus Galvanized In general, galvanized and metallized coatings have performed well in the field. They can pro- vide long service life, especially when top-coated with a liquid coating to form a duplex coating (Chang and Chung 1999; Kogler 2015; Lindsley 2015). Galvanizing or metallizing may provide coating lifetimes ranging from 15 to 20 years in aggressive environments (e.g., marine environ- ments) and may exceed the 100-year structure design life in benign environments. The coating life depends on several factors, including surface preparation, application quality, bridge design, use of a sealer or topcoat, the exposure environment, and application material. Table 15 lists some of the advantages and limitations of galvanized and metallized coatings. When selecting Advantages Limitations ⢠Ease of application in a wide variety of environments Organic Zinc ⢠Ease of top-coating without bubbling or blistering ⢠Can achieve film builds in excess of 4 mil without mud cracking ⢠Resistant to saltwater or freshwater splash and spray ⢠Comparable performance to inorganic zinc when applied as part of a three- coat system ⢠Can meet Class B slip resistance ⢠Not recommended where direct contact with acids is possible ⢠Limited service life in immersion environments ⢠Temperature resistance limited by the organic polymer (typically 250°Fâ 450°F) ⢠Solvent resistance limited by the organic polymer ⢠Not as effective as inorganic zinc without a topcoat Inorganic Zinc ⢠Excellent corrosion resistance in aggressive environments ⢠Can be used as a single-coat system ⢠High temperature resistance (750°F) ⢠Excellent resistance to organic solvents ⢠Abrasion resistant ⢠Will not burn or support combustion ⢠Can meet Class B slip resistance ⢠Requires greater skill to apply than organic zinc ⢠Tends to mud-crack at thicknesses over 3 mil ⢠Porous nature can result in blistering or holidays of organic overcoats if not properly applied ⢠Tends to adsorb contaminants during fabrication ⢠Limited service life in immersion environments ⢠Not recommended for prolonged exposure to acids below pH 5.0 or alkalis above pH 10.5 ⢠Depends on moisture from the atmosphere to complete hydrolysis; low moisture levels in shops may slow cure Table 14. Comparison of organic and inorganic zinc primers.
Corrosion Control of Coated Steel Bridges 55 whether to use metallizing or galvanizing, the main deciding factors are usually the size of the pieces to be coated and the complexity of their detailing. Duplex Coatings versus Metallic Coatings As described, duplex coatings consist of metallizing or galvanizing coatings top-coated with a urethane or other polymer. Duplex coatings have several advantages, some of which include ⢠The protection mechanisms of the galvanizing or metallizing primer and the topcoat are synergistic, resulting in a multiplicative increase in coating service life (Kogler 2015; Lindsley 2015); ⢠The topcoat or sealer can have a more aesthetically pleasing appearance than metallizing or galvanizing alone (Chang and Georgy 1999a; Kogler 2015); and Advantages Limitations ⢠May provide extended maintenance-free service, around 20 years in marine Galvanizing environments and longer in less-severe exposures ⢠In rural environments, pure zinc galvanizing may provide satisfactory protection for the life of the bridge ⢠Has good abrasion resistance ⢠Has uniform thickness over corners and edges ⢠Provides cathodic protection and barrier protection to the steel substrate ⢠Covers interior surfaces and threaded parts of components ⢠Requires the removal of ash and other surface contamination following hot- dip application ⢠Can be accomplished only on components that fit in available galvanizing kettles ⢠The thermal shock associated with the galvanizing process can cause cracking in areas of high constraint or thermal expansion mismatch, such as along welds or in areas of abrupt topography change ⢠Components with sealed spaces can explode from thermal expansion of the air inside the sealed space ⢠Is sensitive to surface preparation ⢠Welded details that are galvanized tend to have a lower fatigue life than ungalvanized welds ⢠Thickness is limited by the electrochemical properties of the substrate Metallizing ⢠Can be done using a variety of materials, with each material having its own subset of properties ⢠May provide extended service life in marine environments, especially when used in conjunction with a sealer and/or topcoat ⢠Has good abrasion resistance ⢠Can provide cathodic protection to the steel substrate if a metallizing material is chosen that is anodic to the substrate material ⢠Does not require cure time ⢠Can be used on components of any size ⢠Can provide barrier protection to the substrate if used in conjunction with a sealer ⢠Can be applied at any specified thickness, unlike galvanizing ⢠Can be applied over a broad range of temperatures ⢠Does not coat interior spaces; it is a line-of-sight application akin to spray painting ⢠May have thickness differences in corners or on component edges ⢠Is usually porous and is not a good protective barrier coating on its own ⢠Is sensitive to surface preparation ⢠Tends to have lower adhesion to the substrate than galvanizing Table 15. Advantages and limitations of galvanizing and metallizing.
56 Corrosion Prevention for Extending the Service Life of Steel Bridges ⢠Extended service lifetimes can be achieved in marine environments (Chang and Georgy 1999a; Kogler 2015). Duplex coatings have limitations, including ⢠The application limitations of galvanizing or metallizing, whichever is chosen for inclusion in the coating system; and ⢠Unique application requirements, which can cause performance issues if applicators do not follow, or are not aware of, those requirements (Kogler 2015). Galvanization and metallization used in duplex coatings may provide long coating service lifetimes even in aggressive marine environments. However, in the presence of water, the coating life depends on several factors, including pH, agitation of the solution, aeration, temperature, and wear from cavitation or flow. Types of Liquid Coatings. The zinc-rich, three-coat system consisting of an inorganic/ organic zinc-rich primer, epoxy midcoat, and urethane topcoat has been received well by its users when proper surface preparation is achieved (Chang and Chung 1999; KTA-Tator 2014; NACE 2012). The stringent surface preparation requirements of the zinc-rich primers usually require the complete removal of the old coating. The life expectancy of this system ranges from 20 to 40 years. This service life estimate has been successfully achieved by the coating system on several bridges in multiple exposure environments (Chang and Chung 1999; Kogler 2015). The coating service life depends on several factors, such as exposure environment, application environmental condition, application quality, and maintenance practices. Shop versus Field Application The most popular bridge coating systems for new construction are zinc-rich, three-coat systems and metallizing with a sealer (KTA-Tator 2014). The performance of these coating systems depends heavily on the surface preparation and application conditions. Therefore, maintaining surface cleanliness after surface preparation and providing a clean application envi- ronment are key to ensuring satisfactory performance of the coating system. For new bridge construction, the complete process of surface preparation and coating application can be carried out in the shop on individual bridge components. Application in a shop environment is more conducive to satisfying the stringent application requirements of the zinc-rich primers and met- allizing than field application, although shops sometimes have humidity control issues that can cause curing time issues for inorganic zinc coatings. In 2012, a 30% increase in application cost in field application over shop application was reported (Kogler 2015). This suggests that shop application may be less expensive than field application and may lead to more consistent service life. After the coating is applied and sufficiently cured, the bridge components are transported to the bridge construction site, the bridge components are erected, and field touch-up is performed to repair damaged areas. The uncontrollable nature of field application may make surface preparation for inorganic zinc primers or metallization more difficult or costly to achieve and maintain. Therefore, consid- eration should be given to move toward coatings that are more tolerable of surface contamina- tion. For example, if a zinc primer is still desired, an organic zinc primer may be a better choice than an inorganic zinc primer because the organic zinc primer is more compatible with organic material on the surface than is the inorganic zinc. In the case where field application is necessary, more surface-tolerant coatings may be con- sidered for use. For example, MCU coatings have a humidity window during which successful
Corrosion Control of Coated Steel Bridges 57 application can be achieved. As such, care should be taken to ensure the weather conditions are as specified for the coating application and during the coating cure or field containment may be necessary to control the environment conditions, which can increase the application cost (Chang and Chung 1999; Kogler 2015). Design Detailing The FHWA recommends using design details to prevent avoidable corrosion on both painted and unpainted steel bridges (FHWA 1989). Most of the recommended design details are meant to reduce the overall time of wetness. Specifically, design details are meant to divert runoff water, avoid water ponding, and minimize collection of dirt and debris (FHWA 1989). For coated bridges, design features can have a direct effect on coating performance, application costs, and maintenance schedules (Kogler 2015). Important design variables include Complexity. Bridges with complex surface features, such as box beams, rivets, bolts, and tight clearances, are more difficult to coat well and clean during maintenance or repair (Kogler 2015). Height and access. Difficult-to-reach areas may require equipment to properly access during maintenance or coating application. Moreover, the bridge may have to be closed while such areas are addressed. This increases cost and labor (Kogler 2015). Large and unique structures. Bridges with unique structures may require separate approaches for painting and maintenance. Bridges with moving parts may require coatings with increased flexibility and compatibility with lubricants around the moving areas of the bridge (Kogler 2015). Utilities. Often, bridges serve as crossing points for utilities. The existence of live utilities on the bridge can complicate coating application and maintenance and may prevent access to the steel underneath the utilities (Kogler 2015). Rail sharing. Bridges may share their capacity with rail traffic. Bridge maintenance can therefore lead to delays in the rail traffic schedules. In addition, the existence of a high-voltage third rail may preclude certain coating maintenance practices, such as high-pressure water jetting (Kogler 2015). There are several areas on each bridge that should be examined separately from the stand- point of localized corrosivity. These include deck joints, drainage areas, lateral and vertical splash zones, fascia elements, bottom flanges, cables, gratings/bearings/curbs, and built-up members. Deck joints. Deck joints tend to trap water, leading to long periods of wetness and corrosion damage (Kogler 2015; FHWA 1989). Use of integral abutments rather than expansion joints at the ends of spans can significantly reduce the maintenance needs of a bridge. When joints must be used, minimizing the number of joints and using closed or sealed joints can reduce corrosion activity. Experience shows that troughs tend to fail or become clogged with debris over time, limiting their effectiveness (Kogler 2015). Small spacings. Closely spaced girders, superstructure elements, or other steel can lead to long periods of wetness and collection of debris, both of which are conducive to high corrosivity (Kogler 2015). Moreover, small spacings have little accessibility to perform maintenance or repair. Therefore, small spacings should be avoided. For example, parallel girders may be removed from the design by using a continuous span instead (Kogler 2015). Grid decks. Open grid decks can expose the entire superstructure to deck water. Although free draining, experience dictates that robust protection schemes should be used to protect superstructure under grid decks (Kogler 2015).
58 Corrosion Prevention for Extending the Service Life of Steel Bridges Drainage areas. Drainage areas see increased time of wetness, larger amounts of dirt and debris, and may be exposed to deicing salts. All of these factors lead to increased corro- sion activity (FHWA 1989). Drainage areas should be designed to ensure fast water flow and mitigate the tendency for drainage systems to clog or create standing water (Kogler 2015). Experience suggests that drain pipes and scuppers receive little attention during maintenance, leading to short service lives of only a few years before clogging renders them ineffective (Kogler 2015). Splash zones. If tunnel-like conditions are present over a roadway, the traffic activity underneath the bridge can affect the service life. Higher traffic activity, higher traffic speeds, and higher ratios of trucks to cars can increase salt deposition in splash zone areas or increase the splash zone size (Crampton et al. 2013). Bridge components that are close enough to traffic to be within the splash zone are susceptible to increased corrosion. If bridge components must be within the traffic splash zone, consideration may be given to provide more-robust protection in these areas (Kogler 2015). Cables. Coatings for suspender cables must have adequate flexibility and resistance to sun damage. Historically, thick-film waterborne alkyds and calcium-sulfonateâmodified alkyd coatings have been used with success (Kogler 2015). Water-based acrylic coatings that remain flexible at film builds of 10 to 20 mil are becoming more common on suspen- sion cables. Gratings, bearings, and curbs. Historically, these elements have proven inefficient to protect with liquid coatings. Consideration should be given to galvanizing, metallizing, or constructing the elements of corrosion-resistant materials (Kogler 2015). Water traps. Effort should be taken to identify water traps in bridge elements and eliminate them (FHWA 1989). Common water traps include horizontal areas, bottom flanges, and transverse stiffeners. If water traps cannot be eliminated through design, consideration should be given to providing robust protection, such as duplex coatings, in these areas (Kogler 2015). Inaccessible details. Details that cannot be accessed for inspection and maintenance should be avoided (Kogler 2015). Box and tubular members. âHermetically sealâ box and tubular members when possible or pro- vide weep holes to allow proper drainage and circulation of air (Kogler 2015; FHWA 1989). The insides of the members are often coated as well because these spaces are not easily accessed for field inspection and maintenance (Kogler 2015). Slip resistance. The AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications require bolted connections be designed as âslip-criticalâ if the connection is subjected to âstress reversal, heavy impact loads, severe vibration or located where stress and strain due to joint slippage would be detrimental to the serviceability of the structureâ (AASHTO 2012). AASHTO LRFD Bridge Design Specifications provide three different categories (Class A, B, and C). Class A surfaces have a minimum slip coefficient of 0.33, which can be achieved with unpainted, clean mill scale. Class B surfaces have a minimum slip coefficient of 0.50, which can be achieved with unpainted, blast-cleaned surfaces. Class C surfaces have a minimum slip coefficient of 0.33 but are applicable only for hot-dip gal- vanized coatings. Because crevice corrosion leads to pack rust problems, it is desirable to coat the mating sur- faces of bolted connections. Coatings are certified to meet one of the slip coefficient classifi- cations by the testing described by the Research Council of Structural Connections (RCSC) document Specification for Structural Joints Using High-Strength Bolts (RCSC 2014). Certification requires that the coating meets criteria for both slip coefficient and tension creep. Two tests are performed to certify a coating for either Class A or B slip resistance: a short-term compression test and a long-term tension creep test.
Corrosion Control of Coated Steel Bridges 59 Service Environment Considerations Bridge location and exposure environment can affect bridge service life. Proximity to chloride or industrial chemical sources can also be important. Because of the wide variety of environ- ments and climates present in the United States, it cannot be expected that all coating systems will perform equally across the country. The exposure conditions of a specific bridge involve its macroenvironment, the general corrosion-affecting factors associated with the bridgeâs location, and its microclimate, which is associated with each bridge element (Ault et al. 2000; Kogler 2015). The macroenvironment of a structure has historically been categorized into three categories: mild (rural), moderate (industrial), or severe (marine). The definitions are as follows (Kogler 2015): Mild (rural)âLittle to no chlorides or pollution in the form of sulfur dioxide, low humidity and rain- fall, absence of chemical fumes, and usually an interior (inland) location. Moderate (industrial)âSome (occasional) airborne salts or intermittent deicing salt runoff. A loca- tion with low or no salt may still be classified as moderate if it is directly downwind of industrial pro- cessing with corrosive airborne contaminants (e.g., sulfur dioxide), in a heavily polluted urban area, or moderate to high humidity. Severe (marine)âHigh salt content from proximity to seacoast or from frequent deicing salt, high humidity and moisture. The microenvironment is defined for each bridge element and affected by the construction material, orientation, proximity to splash zones or drainage areas, exposure to sunlight, prevail- ing wind direction, and so forth. These factors affect the corrosion activity of the bridge element because they affect the time of wetness. Sunlight can also cause direct degradation of the coating through ultraviolet damage. Defining the corrosion environment is important because the durability of protective coat- ings is directly affected by their exposure environment. Thus, in mild or moderate locales, there may be several corrosion-prevention options appropriate for the exposure, whereas aggressive environments may necessitate the use of more-robust protection systems than are required for other areas on the same bridge. Zone Painting Zone painting is a maintenance practice in which the old coating is completely removed and replaced with a new coating system in targeted areas across the bridge structure. These targeted areas are those that experience more-severe corrosion attack than the majority of the bridge. The goal for coating selection may be to select a coating that is estimated to last the life of the bridge. This would limit future maintenance actions associated with corrosion prevention to touch-up or zone painting (Kogler 2015). Use of Warranties Warranties are a tool that can protect the bridge owners, such as the state departments of transportation, from poor workmanship during coating application. Without a warranty, the contractors responsible for coating application may not be accountable for defects in their work. The basic idea of the warranty is that the contractor warrants that methods used are considered acceptable in the industry and that the work will perform satisfactorily for a certain amount of time (Chang and Georgy 1999b). Commonly, warranties have two parts: the scope of the warranty and the warranty period. The scope of the warranty describes what defects the warranty covers. A well-written scope will define the type of defect(s) to be warranted, how the severity of the defect will be measured, and
60 Corrosion Prevention for Extending the Service Life of Steel Bridges at what point the contractor becomes accountable for the defect (Chang and Georgy 1999b). The warranty period is the length of time the work performance is warranted. Further discussion of corrosion warranties was presented in Chapter 2. Economics An economic analysis of alternative protective coating options needs to consider both the initial installed cost and the LCC of the coating option. Many of the factors required to perform the LCC analysis are not easily determined values but can be estimated based on experience and available studies. LCC analysis requires a knowledge of the initial installed cost of the coating system as well as the maintenance frequency and cost over the life of the bridge. A FHWA study included a detailed analysis of LCCs for various bridge painting scenarios (Farschon et al. 1997). Anticipated service lives and cost projections were determined for various coating systems as part of the project. Eight options were evaluated in detail: 1. Alkyd overcoat applied to a structure in a corrosive environment every 3 years for 60 years (20 applications). 2. Epoxy overcoat applied to a structure in a corrosive environment every 4 years for 60 years (15 applications). 3. Metallizing applied to a structure in a corrosive environment every 30 years for 60 years (two applications). 4. Inorganic zinc/epoxy/urethane applied to a structure in a corrosive environment every 15 years for 60 years (four applications). 5. Inorganic zinc/epoxy/urethane applied to a structure in a corrosive environment with a 15-year life and maintenance of roughly 20% of the surface every 5 years for the remaining 45 years (nine maintenance coatings). 6. Epoxy/urethane applied to a structure in a corrosive environment every 10 years for 60 years (six applications). 7. Alkyd overcoat applied to a structure in a mild environment every 10 years for 60 years (six applications). 8. Metallizing applied once to a structure in a mild environment with a 60-year service life. For each option, the equivalent uniform annual cost (EUAC) was calculated over a 60-year service life. Figure 37 shows the initial cost and EUAC value for the six coating options in a cor- rosive environment. The graph shows an inverse relationship between initial cost and EUAC. Examples 1 and 2 (overcoating) have the lowest initial cost but the highest EUAC. Conversely, example 3 (the metallizing option) has the highest initial cost but the lowest EUAC. Figure 38 shows that in a mild environment (where the coatings would be expected to last longer), despite costing nearly three times as much, the metallizing option has a life-cycle cost comparable to that of the less-expensive alkyd coating. Corrosion Maintenance Issues for Coated Steel Bridges In general, there are five bridge-coating maintenance practices: do nothing, remove and replace, do zone painting, do spot painting, and do overcoating. The decision of which practice to implement is usually made following a condition-based assessment of the bridge. Such assess- ments are triggered based on the age of the structure, age of the coating, or when complaints are received, or the assessments are completed on a calendar-based schedule (KTA-Tator 2014). An LCC analysis can be completed to determine which maintenance strategy is the most appropriate for a given situation.
Corrosion Control of Coated Steel Bridges 61 Figure 37. Initial and life-cycle costs for six steel bridge coating scenarios for a 60-year service life in a corrosive environment (re-created from Farschon et al. 1997). Figure 38. Initial and life-cycle costs for two steel bridge coating scenarios for a 60-year service life in a mild environment (re-created from Farschon et al. 1997).
62 Corrosion Prevention for Extending the Service Life of Steel Bridges Do Nothing The decision to do nothing is made when no maintenance action is required. The existent coating may be in good condition. This may also happen if maintenance activities are delayed. Corrosion maintenance may also be delayed because of lack of funding, deferred to coincide with additional work, or not be performed if the bridge is planned for demolition. Remove and Replace During removal and replacement, the existing coating system is removed across the entire structure and a new coating system is put in place. This may be done for several reasons. Some of which include ⢠Widespread corrosion issues, ⢠Significant coating deterioration over widespread areas, and ⢠The existing coating system must be removed for reasons other than coating condition, such as for aesthetic reasons. Removal and replacement provides the longest duration of service life because all corrosion is removed and a new coating system is applied. High-performance coating systems that are sensitive to surface condition, such as systems including inorganic zinc-rich primers or metal- lizing, usually require removal and replacement. However, given the longer service life of these high-performance coatings, it may be economically advantageous in the long run to execute the complete removal and replacement of the coating system. Surface preparation for removal and replacement may be similar to the following: Prior to abrasive blast cleaning, degreasing per SSPC-SP 1 followed by testing representative surfaces for chloride or salt levels. If chloride/salt levels are unsatisfactorily high, remediation can be performed prior to abrasive blast cleaning using pressure washing. Once the chloride/salt levels are satisfactory, abrasive blast cleaning according to SSPC-SP 10, Near-White Metal Blast Cleaning should then be performed and the specified surface profile depth should be achieved (KTA-Tator 2014). There are a variety of local and federal environmental and worker health regulations that must be followed during coating removal and replacement. Coating removal and replacement gener- ally requires full containment of the structure being coated. For abrasive-blasting operations, the containment is usually designed to contain the dust and support the negative air pressure created by a dust collection system. During coating application, the containment should be designed to contain overspray and facilitate ventilation. SSPC (2015) describes methods of paint removal, containment systems and procedures for minimizing or preventing emissions from escaping the work area, and procedures for assessing the adequacy of the controls over emissions. Waste from coating removal is a combination of spent (used) abrasive, old paint, and metal- lic particles. Recycled abrasives are often used to minimize the amount of waste produced. The waste may be hazardous, depending on the coating material being removed and local regulations. Zone Painting Zone painting is a maintenance practice in which the old coating is completely removed and replaced with a new coating system in targeted areas across the bridge structure. These targeted areas are those that experience more-severe corrosion attack than the majority of the bridge. The service life of the coating system can be greatly increased with zone painting because the corro- sion is removed in aggressive areas and a new coating system is applied. Surface preparation for zone painting may be similar to that of removal and replacement.
Corrosion Control of Coated Steel Bridges 63 Spot Painting Spot painting is a maintenance practice in which discrete areas are prepared and coated. These discrete areas may be where the coating deteriorated or localized corrosion occurred. This practice can extend the service life of coatings by 2 or more years and may be used in conjunc- tion with an overcoat to cover up the areas of spot painting. Spot repair and overcoating can be cost-effective maintenance strategies. However, spot painting may not be economical if 10% to 20% of the coated surface area needs repair (KTA-Tator 2014). Surface preparation for spot painting may be similar to the following: Pressure washing followed by degreasing per SSPC-SP 1 and hand or power tool cleaning per SSPC- SP 2/SSPC-SP 3. If a greater degree of surface cleanliness (and roughness) is desired SSPC-SP 15, Com- mercial Grade Power Tool Cleaning, may be specified. The prepared areas should be feathered into the existing coatings. Testing representative surfaces for chloride or salt levels should be completed. If chlo- ride/salt levels are unsatisfactorily high, remediation can be performed using pressure washing. Once the chloride/salt levels are satisfactory the coating can be applied (KTA-Tator 2014). Spot painting generally requires less-extensive containment and worker protection than coat- ing removal and replacement. Depending on local regulations and the extent of work being performed, vacuum-shrouded tools and/or permeable tarps may be used to capture debris. Coatings may be applied by brush and roller, eliminating overspray concerns. Overcoating Overcoating is the application of a new coating over an existing coating. It entails spot clean- ing and priming degraded areas, cleaning intact paint, and applying the new coating system over the existing system (SSPC 2004). This is often done in conjunction with spot repair to address discrete areas of corrosion damage. Overcoating and spot painting work best when surface- tolerant coatings, such as MCU and alkyd coatings, are used (Ault and Farschon 2008). Surface preparation for overcoating may be similar to that of spot painting. Decision Process for Selecting Among Alternative Maintenance Practices Several factors can affect the cost and performance of bridge maintenance performance and should be considered when selecting a maintenance practice. In a recent report on the current bridge maintenance painting operations completed on behalf of the Minnesota Department of Transportation, it was recommended that maintenance practices be based on a bridge condition assessment (KTA-Tator 2014). Other things that may be considered when deciding on a mainte- nance practice include bridge usage, possible release of hazardous waste during coating removal, exposure environment, evidence of potential compatibility problems between existing coating and new coating, evidence of early failure of previously applied coating system, limitations on accessibility, limitations on time to perform work, and any specific environment regulations (Chang and Chung 1999). Integration of Painting and Structural Work Major bridge rehabilitation projects may include painting of the steel superstructure in addi- tion to deck replacement, expansion joint improvements, bearing replacement, and so forth. The most obvious approach is to complete all of the construction work before the painting activities. However, abrasive blasting of existing steel can benefit structural rehabilitation by removing paint hazard in areas that will be disturbed during structural work, facilitating final
64 Corrosion Prevention for Extending the Service Life of Steel Bridges design details by revealing the extent of corrosion damage and resulting in a more-efficient overall project by taking advantage of access, lane closures, and other resources. New steel being installed as part of a project is most effectively coated in the shop before erection. However, it is likely that paint on prefinished components may be damaged during construction. The project team needs to establish the coating repair process and responsibility during planning. Of course, the relative scope of the painting and structural work affects the sequencing of the work. The value of painting on large rehabilitation projects can exceed the value of the structural work. One paper reviewed four projects to illustrate how planning the sequence of painting is a vital part of the overall bridge rehabilitation project design (Farschon et al. 2006). The examples each highlight a different approach to integrating structural steel repairs with protective coating application. A key issue is when in the construction sequence to remove existing coatings and apply the various components of the coating system. A well-thought-out design scheme mini- mizes external impacts, costs, and overall construction time. The lack of a plan can result in an illogical sequence of operations and wasted resources.
