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19 Alternatives The steel industry has developed corrosion-resistant steels as an alternative to protective coatings for uncoated use in bridges and other highway structures. Uncoated steel bridge designs are considered cost-effective because the maintenance/replacement of paint coatings is eliminated. Uncoated steels in current use for bridges include weathering steel grades (ASTM A588, ASTM A709), martensitic stainless steel (ASTM A1010), and lean duplex stainless steel (UNS32205). Although it is theoretically possible to build bridges of other corrosion-resistant steels, other alloys have not yet been accepted as cost-effective options. History of Weathering Steel in Bridge Applications Weathering steel was developed in the 1930s by the United States Steel Corporation and given the trademark name COR-TEN. Similar weathering steel alloys were developed by other companies, including Bethlehem Steel Corporation (alloy Mayari R) and Youngstown Steel (alloy Yolloy). In 1968, ASTM A588 Grade 50W was introduced to describe weathering steel alloys with a minimum yield strength of 50 ksi. In 1974, ASTM A709 Grade 50W was introduced to describe weathering steel for bridge applications. In general, weathering steels have alloy compositions similar to typical mild steel. However, by alloying with small amounts (less than 2 weight percent) of copper, phos- phorous, chromium, nickel, and/or silicon, the corrosion resistance can be increased by around four times that of structural carbon steel without copper (Crampton et al. 2013). The additional alloying elements in the weathering steel, particularly copper, stabilize a protective oxide layer, called the patina, which inhibits moisture, ion, and oxygen diffusion to the surface, thereby lowering the corrosion rate (Hara et al. 2007). During the 1960s and 1970s, weathering steel was regarded as a cost-effective alternative to coated steel. The first weathering steel bridges were built in Michigan, Iowa, and New Jersey in 1964. During the 1970s, evaluations of some bridges in Michigan revealed that the weathering steel was not per- forming well, was experiencing increased corrosion rates, and required remedial painting (Albrecht and Naeemi 1984). The increased corrosion of the weathering steel bridges was attributed to long times of wetness and heavy exposure to deicing salts (Albrecht and Naeemi 1984; Crampton et al. 2013; Fletcher 2011). In response, the use of weathering steel for bridge applications became limited. In 1989, FHWA issued a technical advisory that provided recommendations for the use, maintenance, and inspection of weathering steel bridges (FHWA 1989). In general, the advi- sory highlighted environmental conditions in which weathering steel structures may experience increased corrosion rates. The release of this technical advisory renewed interest in using weath- ering steel for bridge applications. It is estimated that 40% to 45% of bridges are being built with some form of weathering steel (Crampton et al. 2013). This is consistent with the survey results presented in Figure 2. C H A P T E R 3 Corrosion Control of Uncoated Steel Bridges
20 Corrosion Prevention for Extending the Service Life of Steel Bridges Stainless Steels Stainless steel refers to a family of ferrous alloys containing a minimum of 10.5% chromium. Alloys containing this minimum amount of chromium tend to form a passive chromium-rich oxide in oxidizing environments (Baddoo 2013; Gedge 2007). This passive film provides the stainless steel class of alloys their characteristic high resistance to corrosion. The stability of the oxide layer depends on the alloy composition, surface treatment, and environment. Stability increases with increasing chromium, molybdenum, and nitrogen contents. Stainless steels are generally classified in accordance with the prevalent phases in their micro- structures, which are austenite, ferrite, and martensite. Stainless steels generally can be classified into five categories: austenitic, ferritic, duplex, martensitic, and precipitation hardened. The microstructure of each alloy is carefully controlled by selecting an appropriate chemical com- position and heat treatment to achieve the desired material properties. Table 4 shows general properties of stainless steel families. Although these properties can vary widely from alloy to alloy within each stainless steel type, the table provides broad comparisons among the classifications. History of ASTM A1010 Stainless Steel in Bridge Applications ASTM A1010 describes a dual-phase martensitic stainless steel (Fletcher 2011). This steel is intended to meet the structural performance requirements of grades 50W or 70W of ASTM A709 while providing sufficient corrosion resistance for application in uncoated structures that undergo long periods of time of wetness or in chloride-exposure environments that are too corrosive for weathering steel alloys. ASTM A1010 has been used mostly in rail cars that carry coal. In 2004, the Fairview Road Bridge over the Glenn-Colusa Canal in Colusa, California, was constructed of ASTM A1010 grade 50 steel (Fletcher 2011). The first ASTM A1010 steel plate girder bridge, the Dodge Creek Bridge, was constructed in Oregon in 2012, and another Oregon bridge, the Mill Creek Bridge, was built in 2013 (ArcelorMittal 2016). Bridges have been constructed of ASTM A1010 in Oregon, Iowa, Virginia, and California. ASTM A1010 steel has a thin, continuous chrome oxide film that protects the steel surface from ions and water in the environment. Laboratory and natural exposure tests have shown ASTM A1010 steel to exhibit significantly lower corrosion rates than weathering steel and struc- tural carbon steel (Fletcher et al. 2003). Industry is currently developing a guide specification for highway bridge fabrication with ASTM A1010 steel. ASTM A1010 covers the standard for martensitic stainless steel; welding and fabricating bridge elements may require more controlled processes than those provided by typical high-strength, Stainless Steel Type Hardening Mechanism Corrosion Resistance Ductility Alloy/Tensile Strength (ksi) Weldability Austenitic Cold work High Very high UNS S30400/72 UNS S31600/72 Weldable Ferritic Cold work Medium Medium UNS S40900/65 UNS S43000/65-80 Hard to weld Duplex Cold work Very high Medium UNS S32101/94 UNS S32205/94 Weldable Martensitic Quench and temper Medium Low UNS S41000/70-204 UNS S42000/95-235 UNS S44000/109-294 Hard to weld Precipitation hardening Age hardening Medium Medium UNS S17400/135-190 Weldable Tensile strengths are given for popular grades in each stainless steel type. Tensile strengths are approximate and intended only as a comparison of functionally similar materials (AZoM 2001, Baddoo 2013). Table 4. General properties of stainless steel types.
Corrosion Control of Uncoated Steel Bridges 21 low-alloy steels. In initial experiences with ASTM A1010 materials during bridge construction, both the Virginia and Oregon DOTs found welding to be a challenge (Seradj 2015; Sharp et al. 2017). However, both states developed satisfactory welding procedures and successfully utilized ASTM A1010 in new bridge construction. Both studies concluded that ASTM A1010 may be suitable for bridge construction (Seradj 2015; Sharp et al. 2017). Duplex Stainless Steel for Bridge Applications Duplex stainless steels have a mixed microstructure of austenite and ferrite, which is achieved through an appropriate combination of chemical composition and heat treatment. This micro- structure gives the duplex stainless steel alloys corrosion resistance similar or superior to that of austenitic stainless alloys with greater strength than UNS S31600 (Atlas Steels Technical Depart- ment 2013; Baddoo 2013). This desirable combination of properties makes duplex stainless steels an attractive option for bridge applications. Several duplex stainless steel alloys are commercially available. Three common alloys that are considered suitable for bridge applications are UNS S32205, UNS S32304, and UNS S32101, although several other alloys may be suitable for use. Superduplex alloys, such as UNS S32750, are also available if corrosion resistance superior to the duplex stainless steels is required. Selecting the most suitable alloy for use is best approached by considering the environment, machinability, and available experience with stainless steels in similar applications and environments. Table A.1 in EN 1993-1-4, reconstructed here as Table 5, gives general guidance for alloy selection for external applications. Steel Grade UNS Designation Type of Environment and Corrosion Category Rural Urban Industrial Marine Low Mid High Low Mid High Low Mid High Low Mid High S41003 S43000 YI X X YI X X X X X X X X S30400 S30453 S32109 S30153 Y Y Y Y Y (Y) (Y) (Y) X Y (Y) X S32304 S31600 S31603 S31653 S31635 O O O O Y Y Y Y (Y) Y Y (Y) S31726 S32205 N08926 N08904 O O O O O O O O Y O O Y Corrosion conditions (aggressiveness of environment): Low, least corrosive conditions for that type of environment (e.g., low humidity or low temperatures); Mid, fairly typical for that type of environment; High, corrosion likely to be higher than typical for that type of environment (e.g., increased by persistent high humidity, high ambient temperatures, or particularly aggressive air pollutants). Designators (assessment of steel usability): O, potential overspecification from a corrosion point of view; Y, probably the best choice for corrosion resistance and cost; YI, indoor applications onlyâthe use of ferritic stainless steels for cosmetic applications should be avoided; (Y), worth considering provided that suitable precautions are taken (i.e., specifying a relatively smooth surface and then carrying out regular washing); X, likely to experience excessive corrosion. The European Standard (EN) designations for stainless steel grades found in Table A.1 in EN1993-1-4 (1993) were replaced with comparative UNS designations in accordance with International Organization for Standardization (ISO) 15510:2010. Table 5. Suggested grades of stainless steel for atmospheric applications.
22 Corrosion Prevention for Extending the Service Life of Steel Bridges Duplex stainless steels have been used in bridge structures for a wide range of applications. Table 6 shows a list of several bridges constructed between the years 1999 and 2011 with duplex stainless steel structural elements (Baddoo and Kosmac n.d.; Kuchta and Tylek 2013). Duplex stainless steel is attractive for bridge applications for several reasons, including ⢠The bridge can have high profile or high architectural content; ⢠Complex fabrications may have high fabrication and construction costs, which can be offset by the longer lifetimes attainable with duplex stainless steel; ⢠Duplex stainless steel offers a desirable architectural appearance; and ⢠Duplex stainless steel provides long-term corrosion resistance, which leads to reduced maintenance. Table 6 demonstrates a recent interest in the use of duplex steels for bridge construction and represents a growing portfolio establishing a performance track record for these alloys. Duplex stainless steel alloys are regarded as specialty materials to be used only when particularly high architectural or environmental requirements are present. However, their high strength, corro- sion resistance, and fabrication/forming properties make the alloys a potential candidate for future applications in more common bridge structures for which lower maintenance or longer structure lifetimes are desired. Corrosion-Prevention Design Issues for Uncoated Steel Bridges General Considerations The rate of corrosion attack on a bridge varies according to environmental conditions, struc- tural design details, cleanliness, and maintenance history (Kulicki et al. 1990). As such, no two bridges have identical corrosion behavior. Moreover, different locations on the same bridge will likely show different rates of corrosion attack. Therefore, the corrosion behavior and mainte- nance should be considered individually for each bridge and design detail (Kulicki et al. 1990). The use of ASTM A1010 for the construction of bridges did not begin until recently, so most of the experience on corrosion prevention for uncoated steel bridges has been on weathering steel bridges (Crampton et al. 2013; Fletcher 2011). Uncoated weathering steel has been used for construction of thousands of bridges for nearly 50 years (McConnell et al. 2014). Four ASTM A1010 bridges were identified in this literature review, with the earliest being built in 2004 (ArcelorMittal 2016; Kogler 2015). Therefore, many of the design recommendations available for corrosion prevention of uncoated steel bridges are based on the corrosion behavior of weathering steel bridges. Crevice corrosion, uniform corrosion, pitting, and galvanic corrosion have been identified on weathering steel alloy bridges and should be mitigated using bridge design (Kulicki et al. 1990). General corrosion may not be a concern because weathering steel is designed to corrode homogeneously across the surface, forming a protective patina (Crampton et al. 2013). The for- mation of a well-adhered patina layer is indicative of weathering steel performing satisfactorily. Similarly, ASTM A1010 steel will develop a tan patina under atmospheric conditions that may not be a sign of poor material performance. This patina will usually form much more slowly than occurs on weathering steel (ArcelorMittal 2016). Galvanic corrosion is caused when two dissimilar metals are placed in electrical contact in the presence of an electrolyte. The corrosion rate depends on the difference in corrosion potential between the two metals, the environment, and the exposed surface area of each metal (Kulicki et al. 1990). When dissimilar metals are to be joined, insulating materials are often placed between
Corrosion Control of Uncoated Steel Bridges 23 Date Name and Location Type of Bridge Duplex Stainless Steel Components and Grade 1999 Punt da Suransuns Bridge, Switzerland Stress ribbon pedestrian bridge; 131-ft span Four structural ribbons S32205 2001 Millennium Bridge, York, United Kingdom Tilted box girder arch pedestrian bridge; 262-ft main span Arch S32205 2002 Bridge Apatê, Stockholm, Sweden Tied-beam pedestrian bridge Main girder S32205 2003 Kungalv Bridge, Kungalv, Sweden Arch rail bridge, upgrade Replacement of corroded carbon steel deck hangers (after 8 yearsâ service) S32205 2003 Pedro Arrupe Bridge, Bilbao, Spain Box girder pedestrian bridge; total length 459 ft Box girder with carbon steel internal structure S32304 2004 Likholefossen Bridge, Forde, Norway Lightweight pedestrian bridge; 79-ft span All except concrete columns S32101 Continuous prestressed concrete road bridge; length 3,465 ft, 459-ft maximum span 2004 Viaduct CÌ rni Kal, Slovenia Wind barrier from tubular sections (110 tons) S32101 2005 Cala Galdana Bridge, Menorca Arch road bridge; 148-ft main span Main structure, including the two arches (160 tons) S32205 2005 Arco di Malizia, Siena, Italy Single arch road suspension Arch S32304 2006 Siena Bridge, Ruffolo, Italy Cable stayed pedestrian bridge; 197-ft span Load-bearing structure S32101 2006 Piove di Sacco Bridge, Padua, Italy Dual-arch road suspension Arches, deck, and casing (110 tons) S32304 2006 Celtic Gateway Bridge, Holyhead, Wales Arch pedestrian bridge; total length 525 ft, main span 230 ft Load-bearing arch (220 tons) S32304 2008 Zumaia Bridge, Spain Pedestrian bridge; length 92 ft with a 16-ft wide deck 428 components and three plates (20 tons) Composite GRFP and S32205 2009 The Helix, Marina Bay, Singapore Tubular pedestrian bridge; total length 280 m Main structure (400 tons structural pipes; 200 tons other structural parts) S32205 2009 Stockfjarden Outlet, Flen, Sweden Road bridge Load-bearing I-beams S32101 2009 Meads Reach, Bristol, United Kingdom Stressed skin arc pedestrian bridge; 180-ft span Stressed skin arc (75 tons) S32205 2009 Sant Fruitos Bridge, Spain Pedestrian arch bridge All load-bearing structural elements S32101 2009 Stonecutters Bridge, Hong Kong Cable-stayed road bridge; 5,236 ft total length, 3,340 ft longest span Outer skin of the towers (1,800 tons plate, 200 tons pipes) S32205 2010 Second Gateway Bridge, Brisbane, Australia Road bridge over river Reinforcing bar in concrete pile caps S32101 2011 Harbor Drive Pedestrian Bridge, San Diego, United States Pedestrian bridge; 531-ft curved span S32205 Table 6. Bridges using duplex stainless steel.
24 Corrosion Prevention for Extending the Service Life of Steel Bridges the metals to prevent galvanic corrosion (Kulicki et al. 1990). Zinc (galvanizing) and aluminum components experience galvanic corrosion when attached to steel structures. Crevice corrosion is a common form of localized corrosion on steel bridges where access to the outside environment is limited. Differences between the environment in the crevice and outside the crevice drive corrosion. Examples are oxygen cells or metal ion cells. Crevice corrosion has been observed in small gaps between mating surfaces and under debris where elevated chloride or hydrogen ion concentrations can occur (Crampton et al. 2013; Kulicki et al. 1990). Corrosion under accumulated debris (sometimes called poultice) is another common example of crevice corrosion on steel bridges. A thorough cleaning program helps reduce this type of corrosion. 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 (Crampton et al. 2013; FHWA 1989). In FHWA Technical Advisory 5140.22, the FHWA recommended considering the following design details for uncoated steel structures (FHWA 1989): 1. Eliminate bridge joints where possible. 2. Expansion joints must be able to control water that is on the deck. Consider the use of a trough under the deck joint to divert water away from vulnerable elements. 3. Paint all superstructure steel within a distance of 1½ times the depth of girder from bridge joints. 4. Do not use welded drip bars where fatigue stresses may be critical. 5. Minimize the number of bridge deck scuppers. 6. Eliminate details that serve as water and debris âtraps.â 7. âHermetically sealâ box members when possible, or provide weep holes to allow proper drainage and circulation of air. 8. Cover or screen all openings in boxes that are not sealed. 9. Consider protecting pier caps and abutment walls to minimize staining. 10. Seal overlapping surfaces exposed to water (to prevent capillary penetration action). In 2013, 31 weathering steel bridges in Iowa built in conformance with this technical advisory were visually inspected on behalf of Iowa DOT. The inspections found that many of the causes of premature deterioration evidenced in earlier (pre-1989) weathering steel bridges were elimi- nated. The report attributed the improved weathering steel performance to the implementa- tion of design details based on guidance in FHWA Technical Advisory 5140.22. Specifically, the report calls out the beneficial use of the following (Crampton et al. 2013): 1. Narrow flange widths (Figure 15); 2. Narrow splice plates to prevent ponding water at the leading edge (Figure 16); 3. Coped stiffeners that do not trap water (Figure 17); 4. Minimal use of transverse and longitudinal stiffeners (Figure 15); 5. Water diverter plates installed on bottom flanges (Figure 18); 6. Elimination of joints by use of integral abutments (Figure 18 and Figure 19); 7. Painting ends of steel members below joints even in integral abutments (Figure 18 through Figure 20); 8. Minimal use of scuppers; and 9. Elimination of bottom flange lateral bracing, even on curved girders (Figure 15).
Corrosion Control of Uncoated Steel Bridges 25 Figure 15. Typical framing for weathering steel bridges in Iowa. Note narrow flange width, minimal use of transverse stiffeners, and lack of longitudinal stiffeners and bottom flange lateral bracing (Crampton et al. 2013). Figure 16. Typical detailing for weathering steel bridges in Iowa. Note the narrow flange splice plate that prevents ponding water at the leading edge (Crampton et al. 2013). Figure 15 through Figure 20 are re-creations of Crampton and colleaguesâ Figures 10 through 15 in their paper on the assessment of weathering steel bridges in Iowa showing examples of the beneficial design details that can help prevent premature deterioration in weathering steel struc- tures (Crampton et al. 2013). During this project, Iowa DOT representatives reported that the agency requires three water mist applications to initiate patina development on weathering steel. Similar to Iowa DOT, the Florida Department of Transportation (FDOT) uses detailing to ensure satisfactory performance of weathering steel bridges and control possible staining of
26 Corrosion Prevention for Extending the Service Life of Steel Bridges Figure 17. Typical detailing for weathering steel bridges in Iowa. Note the coped stiffener that allows water to pass (Crampton et al. 2013). Figure 18. Typical detailing for weathering steel bridges in Iowa. Note the water diversion plate (arrow), the painted ends of the girder, and the use of integral abutments (Crampton et al. 2013). bridge elements beneath the superstructure. FDOT requires the following details on weathering steel bridges (FDOT 2017): 1. Provide drip tabs on the bottom flange of all box girders and I-girders up grade from each pier/bent to divert runoff water. 2. Provide drip strips along the outside edge of exterior I-girders to channel runoff water past pier/bents or to pier/bent troughs adjacent to girder ends. 3. Slope the caps at all end bents and at piers located at intermediate deck joints. Provide troughs or other means to drain water from the cap to a drain pipe embedded in the end bent or pier. At end bents, extend the pipe drain through the embankment and out of the adjacent retain- ing wall or slope pavement. 4. Provide a ½-in. thick sacrificial end plate at the ends of all I-girders to protect girders from leaky joints.
Corrosion Control of Uncoated Steel Bridges 27 Figure 19. Typical detailing for weathering steel bridges in Iowa. Note the use of integral abutments and the painted ends of the girder (Crampton et al. 2013). Figure 20. Typical detailing for weathering steel bridges in Iowa. Note the painted ends of the girder (Crampton et al. 2013). 5. Use sealed expansion joints. Avoid any type of open joint that allows runoff to reach the steel. 6. Provide details that take advantage of natural drainage. Eliminate details that retain water, dirt, and other debris. 7. Provide a stainless steel drip pan at the top of each column supporting steel straddle pier caps. Show the drip pan connected to a drain pipe embedded in the column. Size the drip pan sufficiently so that it will capture water dripping from the straddle pier cap and pre- vent it from staining the sides of the column. Coordinate the design of the drip pan with the bearing. Sketches of these details for I-girders and box girders are given in the FDOT Structures Detailing Manual in Figures 16.12-1, 16.12-2, and 16.12-3. These figures are re-created here in Figure 21, Figure 22, and Figure 23, respectively (FDOT 2017).
28 Corrosion Prevention for Extending the Service Life of Steel Bridges Figure 21. FDOT weathering steel I-girder details (1 of 2) (FDOT 2017).
Corrosion Control of Uncoated Steel Bridges 29 The Texas Department of Transportation (TxDOT) requires the use of drip plates on all weathering steel beams and girders to reduce or eliminate staining on concrete substructure. TxDOT provides a sample drip plate detail in Figure E-11 in the agencyâs Bridge Detailing Guide; that detail is re-created in Figure 24 (Tagnoli 2016). The North Carolina Department of Transportation (NCDOT) has used similar details, called drip beads and shown in Figure 25, which are also meant to control water drainage; however, the use of drip beads is not explicitly specified in the agencyâs design manual (NCDOT 2016). Service Environment Considerations The corrosion performance of uncoated steel bridges may be negatively affected by certain environmental conditions. In the mid-1980s, inspections performed by the Michigan Depart- ment of Transportation (MDOT) on weathering steel bridges found instances where the patina formation did not slow the corrosion rate as desired (Albrecht and Naeemi 1984). When heavy corrosion was encountered, the normally well-adhered brown patina was black and came off in large flakes (Hara et al. 2007; Albrecht and Naeemi 1984). The unsatisfactory performance of weathering steel was connected to certain environmental conditions that led to long periods Figure 22. FDOT weathering steel I-girder details (2 of 2) (FDOT 2017).
30 Corrosion Prevention for Extending the Service Life of Steel Bridges of wetness or exposure to high chloride concentrations (Crampton et al. 2013; Albrecht and Naeemi 1984; FHWA 1989). This suggests the development of the protective patina in weather- ing steel may be hindered by environmental factors, such as high humidity, high rainfall, or exposure to deicing chlorides. The potential impact of the service environment on the extended performance on uncoated steel bridges should be considered. Weathering Steel Because of the early cases of unsatisfactory performance observed on weathering steel bridges in Michigan and other states in the 1980s, the FHWA sponsored a forum on weathering steel with the goal of developing guidelines on the proper use and maintenance of weathering steel. This forum resulted in Technical Advisory 5140.22, which was released in 1989. This advisory urges caution when considering weathering steel for bridge construction in marine coastal areas; areas of high rainfall, humidity, or persistent fog; or industrial areas where the bridge may be exposed to high concentrations of waste fumes (FHWA 1989). Figure 23. FDOT weathering steel box girder details (FDOT 2017). Figure 24. Sample drip plate detail as required by TxDOT on weathering steel bridges (Tagnoli 2016).
Corrosion Control of Uncoated Steel Bridges 31 Bridge sites in marine coastal areas may contain air with higher-than-normal chloride con- centrations that may prevent the protective patina from forming (Crampton et al. 2013). The Atlantic, Pacific, and Gulf coastlines contain atmospheres with high airborne salinity that may be transported to bridge sites inland by prevailing winds. Therefore, the chloride concentrations at a specific location may depend on the wind direction, speed, distance from shore line, and topography of the area (FHWA 1989). The FHWA recommends against the use of uncoated weathering steel in areas where the chloride deposition rates exceed 0.5 mg/100 cm2/day, when measured by wet candle method (FHWA 1989). Guidelines for conducting the wet candle test can be found in ASTM G140-02, Standard Test Method for Determining Atmospheric Chloride Deposition Rate by Wet Candle Method. Areas of excessive condensation may lead to unsatisfactory weathering steel performance (Crampton et al. 2013; FHWA 1989). Climate conditions that lead to excessive condensation include high rainfall, high humidity, low water clearance, or persistent fog. The condensation leads to long periods of wetness, which prevents a protective patina from forming (FHWA 1989). The FHWA recommends caution when using weathering steel in areas of yearly average time of wetness exceeding 60% when measured in accordance with ASTM G84. The FHWA also recommends minimum water clearances of 10 ft over stagnant water or 8 ft over running water (FHWA 1989). Figure 25. NCDOT sample drip bead details (NCDOT 2016).
32 Corrosion Prevention for Extending the Service Life of Steel Bridges In heavy industrial areas, the air may contain chemicals such as nitrogen or sulfur compounds that can deposit on and decompose uncoated weathering steel surfaces (FHWA 1989; Albrecht and Naeemi 1984). For example, sulfur oxides can dissolve in moisture on the steel surface. Once on the surface, the sulfur oxides can be oxidized to sulfuric acid, which attacks steel (Albrecht and Naeemi 1984). The proximity to pollutant sources can alter the performance of weathering steel. Other factors specific to bridge location and geometry may affect weathering steel perfor- mance. These include direct salt spray from an adjacent structure or tunnel-like conditions (FHWA 1989; Crampton et al. 2013). Tunnel-like conditions result when steep abutment walls, low clearances, and/or wide overpasses combine to prevent roadway spray from being dissipated. This can lead to heavy salt deposition on the underside of the bridge. The FHWA recommends against the use of uncoated weathering steel if tunnel-like conditions are present in a bridgeâs design in areas where winter deicing salt is used (FHWA 1989). ASTM A1010 Stainless Steel In cases where the exposure is too corrosive to allow the use of uncoated weathering steel, ASTM A1010 stainless steel may provide a successful alternative. Such severe microclimates typically have long periods of wetness or high salt contamination levels (Crampton et al. 2013). Laboratory and exposure testing suggest that ASTM A1010 stainless steel would per- form significantly better than weathering steel alloys in such conditions (Fletcher 2011; Fletcher et al. 2005). ASTM A1010 stainless steel has been successfully used in corrosive environments in applications such as coal rail cars and coal processing equipment (Fletcher et al. 2005). Although several bridges have been constructed from A1010 stainless steel in the past few years, it is too early to perform a systematic, documented corrosion investigation. Anecdotal observations on their appearance within a short time after installation have been reported in the literature. Zone Painting The goal of uncoated steel bridge design is to use the appropriate application of corrosion- resistant steel to limit painting to maintenance touch-up or zone painting only in the most corrosive areas (Kogler 2015). Painting of the end of steel members below joints was observed during Iowa DOT inspections of weathering steel bridges and deemed beneficial to bridge performance by Iowa DOT (Crampton et al. 2013). The FHWA recommends painting all superstructure steel within a distance of 1.5 times the depth of the girder from bridge joints (FHWA 1989). After a review of weathering steel research, Iowa DOT concluded that painting, if done properly, is an effective measure to protect areas where heavy corrosion has occurred or is expected (Crampton et al. 2013). Corrosion Performance Specifications Neither weathering steel nor ASTM A1010 steel have corrosion performance standards. Instead, their corrosion resistance is assumed based on the alloyâs material composition. Weather- ing steel alloys for structural components of bridges are ASTM A709 Grade 50W and 70W. The single corrosion-related requirement for the steel is a material composition requirement. These steels must have an atmospheric corrosion resistance index of 6.0 or higher when calculated in accordance with Method B of ASTM G101-04. Method B of ASTM G101-04 predicts the relative atmospheric corrosion of low-alloy carbon steel when alloyed with copper, nickel, chromium, silicon, and phosphorus based upon the concentrations of those elements. ASTM A1010 standard does not contain a corrosion requirement or discuss corrosion resistance.
Corrosion Control of Uncoated Steel Bridges 33 Decision Process for Selecting Corrosion-Resistant Materials Selecting the construction material for a bridge includes considerations for the environmental conditions, service environment, service life, economics (installed cost and LCC), and aesthetics. Service Life The service life of an uncoated steel bridge depends on several factors, including the construc- tion material, service environment, bridge location and geometry, and maintenance routine. The predominate material choice for uncoated steel bridges is ASTM A709 weathering steel. Weather- ing steel performs best when industrial environments, marine/coastal environments, and areas of high rainfall or humidity are avoided (Crampton et al. 2013; FHWA 1989). In conditions where weathering steel is not applicable, the higher corrosion resistance ASTM A1010 steel may provide a suitable alternative (Fletcher 2011). Bridge location and geometry may affect the service life. Proximity to chloride or industrial chemical sources can be important. In addition, tunnel-like conditions over roads exposed to deicing salts, rainfall, or water may lower service life (Crampton et al. 2013). 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). Maintenance of uncoated bridges is important to ensure an uncoated steel bridge reaches its intended service life (FHWA 1989). Uncoated bridges may not require remedial painting if used in appropriate conditions. However, experience suggests that inspections and cleaning practices can extend service life (Crampton et al. 2013). Economics The cost of a bridge can be broken up into the installed cost and LCC. Reports funded by TxDOT and U.S.DOT contained discussions on the cost of weathering steel and ASTM A1010 stainless steel uncoated bridges, respectively. The report discussing weathering steel bridges con- sisted of an overview of cost analysis available in the literature (McDad et al. 2000). The report discussing ASTM A1010 bridges contained an LCC analysis for a single example bridge girder made of ASTM A1010 steel and compared it to the same girder constructed of painted carbon steel in an environment of high salt concentrations (Crampton et al. 2013). The initial cost of a weathering steel bridge theoretically is less expensive than a painted carbon steel or painted high-strength, low-alloy steel because of the additional cost of paint- ing the carbon steel or high-strength, low-alloy steel bridges (McDad et al. 2000). Fabrica- tors reported a 10% to 15% savings in shop time by eliminating painting procedures during in-shop bridge construction (McDad et al. 2000). Weathering steel may provide significant initial cost savings. In more corrosive environments that would preclude the use of uncoated weathering steel, ASTM A1010 steel bridges may be considered for use. The material cost of A1010 steel is a little more than double that of conventional carbon steel (Fletcher 2011). However, the conventional steel would require painting during the initial construction. The cost analysis completed by Fletcher estimated the painted carbon steel girder to be roughly 80% the cost of the unpainted ASTM A1010 girder (Fletcher 2011). The results of a 125-year LCC analysis completed by Fletcher comparing an uncoated ASTM A1010 girder to a coated conventional carbon steel girder are given in Figure 26
34 Corrosion Prevention for Extending the Service Life of Steel Bridges (Fletcher 2011). Figure 26 shows that the ASTM A1010 girder has a lower LCC than the painted carbon steel girder after the first repainting of the carbon steel girder. The results show the total cost of the girder for three estimated repaint costs (calculated on a surface area basis for painted steel). This analysis assumes the uncoated ASTM A1010 girder to be maintenance free, which may not prove to be the case as experience accumulates with current structures. The analysis further assumes that the coated steel bridge will require repainting every 20 years, which may not be the case with some structures. However, the key point is to recognize that a greater initial cost should be acceptable if LCCs (e.g., repainting, public inconvenience) can be significantly reduced. Each individual structure requires its own costâbenefit analysis. Aesthetics Weathering steel and ASTM A1010 steel develop a light brown oxide upon exposure to the atmosphere; however, the ASTM A1010 steel weathers significantly more slowly than weathering steel (Crampton et al. 2013; Fletcher 2011; Kogler 2015). It is up to the designers to determine whether such an appearance is desirable. Inspection of weathering steel bridges in Texas revealed significant staining of concrete struc- ture beneath the weathering steel bridge components, an example of which is shown in Fig- ure 27 (McDad et al. 2000). Such stains may not be aesthetically acceptable and can create the opinion among the general population that the bridge is rusting away, even when the material is performing as desired. In the case of the bridges in Texas, several methods were employed to control staining, including drip pans and drip plates (McDad et al. 2000). Properly working design details for the control of staining are shown in Figure 28 (McDad et al. 2000). The Iowa DOT requires the use of wrappings, made of polyethylene or a similar material, to temporarily protect the substructure from staining during construction until the deck is placed. The Iowa DOT also requires the use of flange deflectors on the outside flanges of exterior weathering steel Figure 26. Change of the total cost with time assuming a repainting interval of 20 years (Fletcher 2011).
Corrosion Control of Uncoated Steel Bridges 35 girders to divert runoff ahead of abutments and piers to control potential staining (Iowa DOT Office of Bridges and Structures 2017). The use of such design details to control staining may be considered during bridge design. There are a few other aesthetic nuances to consider before deciding to use weathering steel or ASTM A1010 stainless steel. For one, incomplete blasting of weathering steel can cause a blotchy appearance, as shown in Figure 29 (McDad et al. 2000). Areas where mill scale is not removed may not form the light brown protective patina typical of well-performing weathering steel. Although this is not an issue from a corrosion standpoint, the aesthetic appeal of the bridge is negatively affected. Second, markings not removed after blasting can streak and stain the bridge as the patina grows, as shown in Figure 30. Third, no filler metal specific to welding ASTM A1010 stainless steel exists. Thus, more corrosion-resistant filler metal cathodic to ASTM A1010 is typically used during welding. This may make the weld areas shiny and bright throughout the bridge service life because the ASTM A1010 material can cathodically protect the weld areas under certain conditions (Kogler 2015). This literature review did not reveal instances of such galvanic corrosion limiting the bridge service life. Figure 27. Stained concrete caused by patina formation on weathering steel components (McDad et al. 2000). Figure 28. Effective control of staining on a weathering steel bridge (McDad et al. 2000).
36 Corrosion Prevention for Extending the Service Life of Steel Bridges Corrosion Maintenance of Uncoated Steel Bridges Proper inspection and maintenance of uncoated steel bridges ensures the bridges meet their expected service life. The FHWA recommends the following maintenance activities (FHWA 1989): 1. Clean troughs of open joints and reseal âwatertightâ deck joints. 2. Clean deck drainage systems (scuppers, troughs, etc.) to ensure they continue to divert deck drainage away from the steel structure. 3. Periodically clean and repaint steel within 1.5 times the depth of the girder from bridge joints. 4. Remove dirt and debris that can hold moisture. 5. Maintain screens over access holes in box section to prevent entrance by animals. 6. Remove growth of vegetation that prevents the natural drying of surfaces wet by rain, spray, etc. In addition, a literature review by Iowa DOT states it may be necessary to periodically scrape off loose rust if present (Crampton et al. 2013). In the synthesis survey, one state indicated that the agency reestablished the patina on weathering steel structures through a 3,500-psi hot water or 5,000-psi cold water wash when necessary. The main inspection and maintenance activities on an uncoated steel bridge are briefly elaborated on in the following sections. Figure 29. Blotchy texture of weathering steel created by incomplete blasting (McDad et al. 2000). Figure 30. Different colors of staining on weathering steel caused by marking on surface after blasting (McDad et al. 2000).
Corrosion Control of Uncoated Steel Bridges 37 Inspection Effective inspections should be implemented to evaluate bridge performance and identify potential problem areas. Inspectors should recognize the unique nature of weathering steel and ASTM A1010 stainless steel and differentiate between the possible protective and unprotec- tive oxides (Crampton et al. 2013). The aspects of inspecting uncoated steel bridges discussed here are confined to weathering steel bridges because this review did not identify inspections of newer, ASTM A1010 steel bridges. Inspection of weathering steel bridges is focused on evaluating the oxide, or patina, layer on the steel surface. NCHRP Report 314 gives guidelines for color, texture, and flake size to aid inspectors in determining whether an oxide is protective (Albrecht et al. 1989). Table 7 and Table 8 show the guidelines for color and texture, respectively. NYSDOT has developed inspection techniques based on these guidelines. More recent research carried out in Japan resulted in a similar approach but assigned a numerical rating to describe the condition of the oxide layer (Hara et al. 2007). This rating system, shown in Table 9, ranges from 1 to 5 (Hara et al. 2007). In general, an index of 5 represents early exposure before complete oxide formation; indices of 3 or 4 can represent a pro- tective oxide, for which the corrosion rate has stabilized; and indices of 2 or lower may represent nonprotective oxide, for which the corrosion rate approaches that of traditional low-alloy steel (Hara et al. 2007; Crampton et al. 2013). These indices may be used to rate weathe ring steel com- ponents based on overall condition and appearance to prioritize structures for maintenance or repair. The Iowa DOT Bridge Inspection Manual contains a visual rating system that is peri- odically verified with a tape test. The rating system is shown in the manual in Figure 2.4.2.4, which is re-created here in Table 10 (Iowa DOT 2015a, pp. 2-22â2-23). Tape tests were used to help collect and evaluate oxide samples during inspection of weath- ering steel bridges in Iowa (Crampton et al. 2013, Iowa DOT 2015b). The samples may then be Color Condition Yellow orange Initial stage of exposure Light brown Early stage of exposure Chocolate brown to purple brown Development of protective oxide Black Nonprotective oxide Source: Albrecht et al. 1989. Table 7. NCHRP Report 314 appearance guidelines for color of the weathering steel oxide layer. Texture Condition Tightly adherent, capable of withstanding hammering or vigorous wire brushing Protective oxide Dusty Early stages of exposure; should change after a few years Granular Possible indication of problem depending on length of exposure and location of structure Small flakes, 0.24-in. diameter Initial indication of nonprotective oxide Large flakes, 0.48-in. diameter or greater Nonprotective oxide Laminar sheets or nodules Nonprotective oxide, severe corrosion Source: Albrecht et al. 1989. Table 8. NCHRP Report 314 appearance guidelines for texture of the weathering steel oxide layer.
38 Corrosion Prevention for Extending the Service Life of Steel Bridges Appearance Index Rating Oxide Layer Thickness Description Rust Flake Size 1 >800 µm (>0.031 in.) Large swelling and laminated flaky layer >25 mm (1 in.) 2 >400 µm (>0.016 in.) Partial swelling and flaky layer 5 to 25 mm (0.2 to 1 in.) 3 <400 µm (<0.016 in.) Nonuniform rust 1 to 5 mm (0.04 to 0.2 in.) 4 <400 µm (<0.016 in.) Adherent and uniform dark brown rust Fine, <1 mm (<0.04 in.) 5 <200 µm (<0.008 in.) Light brown rust Fine Source: Hara et al. 2007. Table 9. Appearance indices of the weathering steel oxide layer. stored for comparison during later inspections of the bridge to help determine if the oxide con- dition is changing over time. Several other tools have been used to inspect weathering steel performance in the field with varying degrees of success. Ultrasonic thickness gauges were used during inspection of weath- ering steel bridges in Texas in an effort to measure component thickness and corrosion rates. However, the majority of component thickness measurements on corroded components were higher than what was specified in the bridge designs. This was attributed to the plates normally being rolled to be thicker than specified and the expansion effects of oxide growth because oxide is usually thicker than the original steel material (McDad et al. 2000). Surface chloride tests were attempted in the field during inspection of weathering steel bridges in Iowa. The chlorides within or below the patina layer were not able to be measured consis- tently. Comparison of field measurement to lab measurement revealed that field measurement was extracting only a portion of the chlorides on the steel surface. In addition, there was difficulty in creating a seal on poorly performing patina surface because of the porous nature of the oxide and its poor adhesion to the surface (Crampton et al. 2013). Similarly, chloride tests using lab titration techniques on samples extracted from weathering steel bridges in Texas were deemed unreliable because of high test-to-test variation from the same locations (McDad et al. 2000). Cleaning/Washing The FHWA recommends cleaning and maintaining water drainage systems, design details, and other areas that can accumulate dirt and debris that may retain moisture on the steel surface (Crampton et al. 2013; FHWA 1989). High-pressure washing the bridges regularly may be a cost- effective way to accomplish such cleaning (FHWA 1989). High-pressure washing may have the additional benefit of removing chlorides from the surface, especially if the washing is carried out following a winter deicing season (Crampton et al. 2013). However, there is debate on whether washing to remove chlorides within and beneath the patina layer is cost-effective because of the high volumes of water needed to penetrate the patina and remove the chlorides (Crampton et al. 2013). At a minimum, high-pressure washing would remove dirt and debris and lower the sur- face chloride levels. The benefits of high-pressure washing on service life depend on the bridgeâs age, size, environment, and location. Thus, a costâbenefit analysis of performing high-pressure washing should be completed for all or parts of the bridge (Crampton et al. 2013). Zone Painting Areas where heavy corrosion is anticipated can be painted before installation to mitigate the corrosion damage (Crampton et al. 2013; FHWA 1989). The FHWA recommends painting
Corrosion Control of Uncoated Steel Bridges 39 Patina Rating Condition Description Example Condition in Field Example Tape Test Specimen 8 Very Good Uniform color pattern, generally dark brown with some lighter reddish-brown, metallic and purple-brown spots. May be difficult to see small rust product clusters. Texture may be dimpled or rough but uniform in pattern. Patina layer is thin but dense and adherent, indicative of very good protective properties. Superior adherence, tape test sparse with only very small flakes (<1 mm). 7 Good Uniform color pattern, generally dark brown with some lighter reddish-brown, metallic and purple-brown spots. Individual rust product clusters visible. Texture is dimpled or rough but uniform in pattern. Patina layer is thin but dense and adherent, indicative of good protective properties. Tape test easily removes very small (<1 mm) flakes. 6 Satisfactory Dark brown coloration but begins to show minor variation. 1â5 mm flakes loose on surface, easily removed with tape test. Underlying layer adherent, still relatively dense, thin and protective. Texture more granular, and loose flakes may be less protective, holding water and salts. Chalky poultice layer may be present but not significantly affecting performance (i.e., flake size). (continued on next page) Table 10. Iowa DOT weathering steel patina rating.
40 Corrosion Prevention for Extending the Service Life of Steel Bridges Patina Rating Condition Description Example Condition in Field Example Tape Test Specimen 5 Fair Dark brown with black and some color variation. Blotchy with some salty or rusty stains. Medium (5â25 mm) flakes over most of area loose and nonprotective, easily removed with tape test. Layer beneath flakes thicker and more permeable, with some pitting beginning. Nonprotective; contaminants penetrating. Elements with poultice may show significant associated flaking. 4 Poor Color is dark brown and black but nonuniform, with widespread blotchiness and staining. Nonprotective. Large (>25 mm) flakes, or layered delamination beginning in some areas. Thickness/permeability of rust increased, with pitting and section loss possible. Poultice areas have thin delamination sheets or very large flakes. Layer below loose poultice may appear similar but still somewhat adherent. 3 Serious Blackish, stained, blotchy appearance. Formation of laminar sheets with deeply pitted semiadherent layer beneath; chunks and sheets of rust product removable by hand. Aggressive advancement of pitting and section loss; can be up to 50%. Complete failure of patina to protect base steel. Source: Iowa DOT 2015a. Table 10. (Continued).
Corrosion Control of Uncoated Steel Bridges 41 overlap areas and all steel below roadway joints (FHWA 1989). Other areas may include embed- ded beam ends, small gaps or crevices, or areas where long periods of wetness are expected. Water Drainage Design details and drainage systems can be utilized to minimize standing water on the bridges. The FHWA recommends maintaining and cleaning drainage systems to ensure they drain water properly (FHWA 1989). Such practices can be considered to maximize bridge service life. Full Painting of Aged, Uncoated, Corrosion-Resistant Steel Structures Painting is an effective remedial option in cases where weathering steel does not perform as expected. For example, remedial painting on weathering steel bridges in Michigan was recommended in 1994 after 22 years of service (Crampton et al. 2013). Other bridges in Alaska, California, Iowa, Ohio, and Louisiana were remedially painted or scheduled for remedial paint- ing in the 1980s before the environmental limitations of weathering steel were realized (Albrecht and Naeemi 1984). With proper surface preparation, it is possible to protect weathering steel from further corrosion. However, it can be difficult to remove all the salt and rust before paint application. Failure to do so can shorten the paint life. ASTM A1010 stainless steel can also be painted (Fletcher et al. 2005). However, ASTM 1010 steel bridges are relatively new, so this review did not find a case where remedial painting was necessary.
