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Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance (2018)

Chapter: Chapter 3.0 Coatings for Spot Paintings

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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
×
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Suggested Citation:"Chapter 3.0 Coatings for Spot Paintings." National Academies of Sciences, Engineering, and Medicine. 2018. Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25089.
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NCHRP Project 14-30 10 3.0 COATINGS FOR SPOT PAINTING Coating selection is vital for successful bridge spot painting. Coatings have a wide range of properties that affect not only performance after they are cured, but also application constraints the agencies must deal with. Coating selection effects: • Worker safety procedures for field personnel • Surface preparation • Environmental protection requirements • Atmospheric conditions for application • Application methods • Durability of the repair. There can be negative effects of new repair coatings applied over existing ones. Coating selection should always address that prospect to prevent coating failures. Spot coatings can be used singly or in multi-coat applications (typically of different coating types). The latter are commonly used as it is difficult to combine all desired coating characteristics in one coating. Single coatings can be used in one- or two-coat applications, but they typically entail performance compromises that limit their effectiveness compared to multi-coat systems. 3.1 Structural Coating Types There are a variety of coatings used to protect steel structures: Those include • Acrylics (Waterborne) • Zinc-rich coatings • Alkyds • Polyaspartics • Epoxies • Polysiloxanes • Polyurethanes The latter two coatings are typically used in new construction or removal and replacement projects. They offer potential for use in spot painting as they can supplant conventional intermediate and finish coats with one coat. They will not be discussed due to their apparent lack of use on spot coating repair projects. General characteristics of typical bridge coating categories are provided in Table 1. There are wide variances in the performance of specific products due to formulation differences. Manufacturers should be consulted for data on characteristics/properties of their specific products in each category. Spot painting is typically performed where the highest coating/corrosion stresses exist or where geometric or access factors resulted in poor application. Those issues point to the need for care in the selection of coatings for spot painting work. The durability of a coating is related to several factors including the quality of the material and workmanship, but none of those is more important than surface preparation and severity of environment. The most extensive surface preparation actions will offer the longest coating service life. Factors to consider in selecting coatings are: • Compatibility and durability matching with existing coatings

NCHRP Project 14-30 11 • Surface preparation • Soluble salt contamination • Work environments/conditions • Surface tolerance • Application requirements • Painter skill/coating friendliness • Project costs.

NCHRP Project 14-30 12 Coating Type Uses Advantages Limitations Comments Acrylics Primer, Intermediate Coat, Finish Coat • one-component coatings • low VOCs • user friendly • durable topcoats • excellent UV resistance • can be used as a topcoat over other coating types • low temperature/humidity restrictions for application • not suited over salt contaminated steel • not recommended for immersion or severe industrial atmospheres or over old paints where good wetting is needed Acrylic systems can be used in regions with restrictive VOC regulations. Top coats are commonly used with other coating types. Acrylic primers provide inhibitive protection. Alkyds (Conventional) Primer, Intermediate Coat, Finish Coat • one-component coatings • good wetting characteristics • user friendly • good adhesion • good abrasion weathering and UV resistance • fair corrosion resistance, • some alkyds cure slowly • not used for immersion service • poor resistance to chemicals and solvents • limited durability in high-stress environments, Alkyd coatings can be used over a range of existing coatings. Alkyds are surface tolerant and can be applied on some oil- contaminated substrates. Alkyd primers provide inhibitive protection. Alkyds (Calcium Sulfonate) Multi-Coat • one-component coatings • can be applied wet-on-wet • user friendly • good adhesion • good corrosion resistance • fair UV resistance • not suitable over salt-contaminated steel • very slow curing • picks up dirt kicked-up by traffic • not suitable for walking surfaces • cannot be top coated with other coating types • not used for immersion service unless over blast cleaned (SSPC SP6) steel Calcium sulfonate coatings are tolerant of many application errors. Calcium sulfonate penetrating sealers are available. Calcium sulfonate sealers and coatings have been used to seal pack- rusted joints. Calcium sulfonate coatings primarily provide inhibitive protection. Epoxies Sealer, Primer, Intermediate Coat • excellent corrosion resistance, • excellent chemical resistance • excellent abrasion resistance • can be applied over marginally prepared surfaces, • can be immersed • low VOC formulations available • two-component coatings • may need induction (set) time prior to application • limited pot lives • poor UV resistance (chalking) • slow curing • finite recoat windows for top coating • may need to be abraded for top coating more prone to cracking than other coatings Epoxies have the good corrosion resistance over contaminated surfaces. They are widely used as primers and intermediate coats. Epoxy coatings typically provide barrier protection. Table 1. Common coating types – uses, advantages, limitations and comments.

NCHRP Project 14-30 13 Coating Type Uses Advantages Limitations Comments Epoxy Mastics Primer, Intermediate Coat, Finish Coat* • Surface tolerant • Good wetting properties • Can be applied to high builds in several coats • Low VOC formulations available • Excellent chemical resistance • Excellent moisture resistance • Slower drying than regular epoxies • Longer recoat times than regular epoxies • High builds over aged alkyds can result in disbonding. Problems have been encountered in the past with low-cost, poor performing epoxy mastics. Epoxy mastics can be applied over marginally prepared substrates. They can perform well if proper quality control is provided and surface contamination of soluble salts is limited. Multiple coats perform better than single coats. Polyurethanes (two-component) Primer*, Intermediate Coat*, Finish Coat • fast curing • excellent UV resistance • excellent chemical resistance • good abrasion resistance • good gloss retention (topcoat) • two-component coatings • need close control of coating thickness during application • susceptible to pin-holing • limited pot lives • not recommended for immersion service (topcoat) Some two-component polyurethanes can be used as primers and intermediate coats similar to epoxies/epoxy mastics. Different two-component polyurethane coatings are used as finish coats. Polyurethanes (moisture-cure) Sealer, Primer, Intermediate Coat, Finish Coat • one-component coatings • excellent adhesion to most substrates • low temperature application • excellent UV resistance** • excellent chemical resistance • good corrosion resistance • humidity tolerance for application • immersion service • need close control of coating thickness during application • susceptible to pin-holing • curing dependent upon humidity levels Some moisture-cure polyurethanes can be used as primers and intermediate coats similar to epoxies/epoxy mastics. Those are frequently top coated with other coating types. Moisture-cure polyurethanes can also be used as penetrating sealers. Different moisture cure coatings are used as finish coats. Those are typically used in complete moisture-cure systems to facilitate application in humid environments. *Infrequent applications. ** Aliphatic finish coats only Table 1 (Cont.). Common coating types – uses, advantages, limitations and comments.

NCHRP Project 14-30 14 Coating Type Uses Advantages Limitations Comments Zinc-rich Coating (Epoxies) Primer • excellent corrosion resistance • excellent for corrosive environments • self-healing/galvanic protection • two- or three-component coatings • limited pot lives • should be applied over profiled steel • not recommended for immersion service Zinc-rich epoxy primers are typically applied by airless spraying. They need to be top coated for maximum protection and durability. Zinc-rich Coating (moisture-cure polyurethanes) Primer • fast cure • low temperature cure • excellent application properties • excellent corrosion resistance • excellent for corrosive environments • self-healing/galvanic protection • one- or two-component coatings • should be applied over profiled steel • curing depends upon humidity levels Zinc-rich moisture-cure polyurethanes are typically applied by airless spraying. They can function without finish coats. They need to be top coated for maximum protection and durability. Table 1 (Cont.). Common coating types – uses, advantages, limitations and comments.

NCHRP Project 14-30 15 3.1.1 Penetrating Sealers Epoxies, moisture-cure polyurethanes and calcium sulfonates are available as penetrating sealers usually containing no or little pigmentation allowing them to readily penetrate into rust and crevices and seal those surfaces. As they are readily applied at minimal coat thicknesses, typically 1-3 mils (25-75 microns) dry film thickness (DFT), they are intended to be top coated with one or more additional coatings. Some sealers can act as tie coats allowing existing coatings to be top coated after they have exceeded their recoat windows. Epoxy sealers can be used “lock down” aged coatings that will disbond (lift) when top coated with coatings containing strong solvents. Penetrating sealers are useful when spot coating complex physical details such as riveted structural members. 3.1.2 Coating Pigmentation Many coatings employ pigments to enhance their corrosion resistance. Denser coatings, epoxies and polyurethanes, provide barrier corrosion protection. They prevent fuels for corrosion (oxygen, carbon dioxide, water and corrosive ions from soluble salts) from contacting the steel substrates and causing corrosion. Plate-like pigments such as leafing aluminum, micaceous iron oxide (MIO), mica, and glass flakes are added to those coatings promote barrier protection. In addition, these pigments can promote other properties such as edge retention and resistance to film cracking. Barrier properties can be improved by adding successive coats of barrier coatings. Acrylics and alkyds are less dense resins and both rely on inhibitive pigments. Inhibitive pigments react at the interface between the coating and steel to passivate the steel and prevent corrosion. They include calcium sulfonate, lead (no longer used), zinc phosphate and metallic borates/borosilicates. Epoxy and moisture cure polyurethane zinc-pigmented primers typically use metallic zinc as the primary pigment. Initially metallic zinc-pigmented primers provide galvanic protection to the steel. To be effective, zinc-pigmented primers must be applied directly to clean, bare steel. Over time, metallic zinc pigments deplete forming oxides that act as barrier pigments. Zinc-pigmented organic primers benefit more in terms of corrosion resistance from barrier topcoats than inhibitive ones. Typically, barrier and zinc-pigmented coatings perform better over surfaces contaminated with soluble salts than inhibitive coatings. All of those coating types must have the suitable film build, especially over rough, pitted surfaces. 3.2 Coating Systems for Prepared (Profiled) Steel Substrates Coating systems employing zinc-pigmented primers are used by highway agencies when: • Seeking maximum durability from a spot painting repair • Performing painting operations in severe (industrial, marine, and immersion) environments • Repairing existing coatings with zinc primers (often with soluble salt contamination). Coating systems with organic zinc primers can be used for spot painting. For maintenance, painting organic zinc-rich primers (epoxy zinc and moisture-cure urethane zinc coatings) are used. They must be directly applied to steel cleaned to a minimum standard with a profile cut in

NCHRP Project 14-30 16 the steel. This requires either abrasive blasting (typically SSPC-SP 6, “Commercial Blast Cleaning,” SSPC-SP 10, “Near-White Blast Cleaning,” or SSPC-SP 14, “Industrial Blast Cleaning”) or power tool cleaning to a high standard (SSPC-SP 11, “Power-Tool Cleaning to Bare Metal.” or SSPC-SP 15, “Commercial Grade Power-Tool Cleaning” to provide a minimum 1-mil profile in the steel). Abrasive blasting will generally require containment and should be selected when larger contiguous areas are to be painted. It may also be required for structural members consisting of complex shapes such riveted, laced box beams. For soluble salt remediation of profiled substrates, washing can be employed prior to surface preparation. Another method would be to employ re-blasting on previously abrasive blasted surfaces. Power-tool cleaning can be used for smaller areas (< 3% or several hundred square feet for smaller bridges). For spot painting, it is more amenable to use by in-house painting crews than abrasive blasting. Power tool cleaning may not work well with zinc primers when used on severely pitted surfaces. Coating systems for profiled steel substrates include zinc-rich primers and one or two additional top coats. Intermediate coats are typically of barrier coatings (epoxies or moisture-cure polyurethanes). Finish coats are usually those with good UV/weathering characteristics. Common generic coating systems for profiled steel substrates are shown in Table 2. Coating System Primer Intermediate Coat Finish Coat Purpose Corrosion protection Barrier protection UV protection and barrier protection 1 Coat Moisture-cure polyurethane zinc* 2 Coat Organic zinc 2-component polyurethane or Moisture-cure polyurethane or, Acrylic 3 Coat Organic zinc Epoxy, Aluminum pigmented epoxy or MIO pigmented epoxy or MIO pigmented moisture-cure urethane 2-component polyurethane Or Moisture-cure urethane or Acrylic 3 Coat Low VOC Organic zinc primer Epoxy or Acrylic Acrylic *At least one coating manufacturer has a zinc-rich aliphatic moisture-cure primer that it suggests can be exposed to direct sunlight (i.e., used without a finish coat). That coating will not provide the durability of comparable multi-coat systems. Table 2. Common coating systems for use on profiled steel substrates. 3.3 Coatings for Marginally Prepared Substrates Highway agencies may use low levels of surface preparation (SSPC-SP 2, “Hand Tool Cleaning,” and SSPC-SP 3, “Power Tool Cleaning” and, by definition, SSPC-SP 7, “Brush-Off Blast Cleaning”) for spot painting work. In those cases, the substrates are of variable consistency and content with the exception that they are to be clean and free from loose material. Limits of soluble salt contamination may be higher than for prepared (profiled) steel substrates, though that is not necessarily the case. The resulting substrates may include tightly adherent coatings, mill scale, tight rust and steel. Heavily rusted or pitted surfaces may contain high levels of soluble salt

NCHRP Project 14-30 17 contamination. Low-level surface preparation may not adequately prepare the surface or reduce the level of soluble salt contamination. That may result in decreased coating performance as compared to higher levels of surface preparation. Coatings for marginal surface preparation are either barriers or inhibitive in their resistance to corrosion. They may be applied singly or in coating systems with conventional coating top/finish coats. Common coatings/systems for marginal surface preparation are listed in Table 3 Typically, the coatings used for such applications are termed “surface tolerant” as described in SSPC Technology Update 1, “Surface Tolerant Coatings for Steel.” That document discuses those coatings, their uses, and limitations. Surface tolerant coatings are selected for a variety of reasons including: • Projects where spot repairs using abrasive blasting with containment is not economically feasible • Environments that are not highly aggressive (i.e. avoiding heavy chemical fumes, high temperatures, frequent immersion) • Substrates that are not highly contaminated (i.e. avoiding high concentrations of soluble salts, built-up grease or other concentrated contaminants) • Areas where abrasive blasting cannot be performed effectively (e.g., back-to-back angles or riveted laced structural members) • Locations where persistent moisture is present in the form of fog or condensation • Areas where oily deposits are present and difficult to treat by SSPC SP 1 Solvent Cleaning. Some epoxy, oil, and alkyd coatings may be tolerant over oil-contaminated substrates while moisture-cure polyurethanes and some epoxies may be tolerant of surface moisture. Surface tolerant coatings offer the advantage of use over marginally prepared substrates. That capability also results in those coatings being limited in their performance compared to the coatings placed over profiled steel (discussed above). Surface tolerant coatings are prone to premature failures where: • High levels of surface contaminants are present (e.g. >50 µg/cm2 surface chlorides) • Incompatible or aged existing coatings are substrates • Severe environments are present. One key in using these coatings is to apply them with suitable film builds on rough, pitted surfaces. 3.3.1 Tinting When using multi-coat systems, problems sometimes arise when applying successive coating applications that have the same appearance. This can occur when using the same system or systems of identical appearance for multiple successive coats (e.g., striping). This poses difficulties for painters and persons inspecting the work. It can be minimized by using a tint in one coat to provide visual contrast between successive coatings. Tints can be provided either pre- mixed in the coating (preferred) or added at the jobsite during mixing operations. Tinting generally will not affect the field properties of coatings, but it may limit the potential for re-use of coatings after the tinting material is added.

NCHRP Project 14-30 18 Coating System Coating Materials Comments 1 coat • Epoxy mastic • excellent corrosion protection (barrier) • good resistance to soluble salt contamination • not durable for direct UV exposure • commonly pigmented with aluminum or MIO 2 coat • epoxy primer • two-component polyurethane or acrylic finish coat • excellent corrosion protection • good resistance to soluble salt contamination • excellent UV resistance • moisture-cure polyurethane primer • moisture-cure polyurethane or, two-component polyurethane or acrylic finish coat • good corrosion protection • good resistance to soluble salt contamination • excellent wetting properties for application (aluminum pigmented primer) • rapid cure (all moisture-cure polyurethane system) • excellent UV resistance • alkyd primer • alkyd finish coat • good wetting properties for application • good corrosion protection • fair resistance to soluble salt contamination • calcium sulfonate alkyd (2 coats) • coats applied wet-on-wet • good wetting properties for application • good corrosion protection • fair resistance to soluble salt contamination 3 Coat • penetrating sealer or epoxy primer • epoxy intermediate coat • moisture-cure polyurethane or two-component polyurethane or acrylic finish coat • primer and intermediate coat commonly pigmented with aluminum or MIO • excellent corrosion protection • good resistance to soluble salt contamination • excellent UV resistance • better performance than 2-coat systems • moisture-cure polyurethane primer • moisture-cure polyurethane intermediate, two-component polyurethane or acrylic finish coat • primer and intermediate coat commonly pigmented with aluminum or MIO • excellent corrosion protection • good resistance to soluble salt contamination • excellent UV resistance • rapid cure (all moisture-cure polyurethane system) 3 Coat Low VOC • acrylic primer • acrylic intermediate coat • acrylic finish coat • low VOCs • good corrosion resistance • fair resistance to soluble salt contamination • excellent UV resistance Table 3. Common coatings/systems for use on marginally prepared substrates. 3.4 User-Friendly Coatings There are several coating characteristics that influence whether a coating is considered “user” friendly. User friendliness pertains not only to the workers (especially the applicators) but also to supervisors and the highway agency. Those characteristics include:

NCHRP Project 14-30 19 • Surface conditions acceptable for coating (e.g., tolerance of limited salt contamination) • Compatibility with existing coatings • Atmospheric conditions that permit painting • Application by brush, roll, or spray • Environmental friendliness • Number and severity of worker safety issues • Ease of coating preparation for painting (e.g. need for mixing, agitation, induction time, pot life, weight of coating) • Minimal overspray problems • Ability to coat complex substrates and crevices • Application tolerance (e.g., high build, sag resistance) • Rapid curing after application • Ease of coating repair/touch up • Ease of clean-up Coatings (and coatings systems) vary in the number of specific characteristics that they possess. No coatings are completely user friendly. A generalized chart for the coatings types is provided in Table 4. The ratings are 1) Excellent, 2) Good, 3) Fair, 4) Poor, and 5) Not Applicable. Factors that favor ease of use, tolerance for variations or contamination or, limited problems/concerns are rated better (i.e., Excellent or Good) while those that don’t are rated lower (i.e., Fair or Poor). Among generic coating types, specific products can have, different properties/ characteristics from those indicated in the table.

NCHRP Project 14-30 20 Coating Type surface cleanliness acceptable for coating compatibility with existing coatings atmospheric conditions that permit painting application by brush, roll or spray environmentally friendliness number and severity of worker safety issues ease of coating preparation for painting minimal overspray problems ability to coat complex substrates and crevices application tolerance speed of curing ease of coating repair ease of clean- up Acrylic primers F G G E E E E E F G F G E Acrylic finish coats G G G E E E E E N/A G F G E Alkyd primers G G G G G G E G E G F G G Alkyd topcoats G G G G G G E G N/A F F G G Calcium sulfonate alkyds F G G F G G E G E E P G G Epoxy primers/intermediate coats G F G G G F F P G G G F F Two-component polyurethane primers G F E G F F F P G F E F F Two-component polyurethane finish coats G N/A E G F F F P N/A F E F F Moisture-cure polyurethane primers/intermediate coats G F E G F F E P E F E F F Moisture-cure polyurethane finish coats G N/A E G F F E P N/A F E F F Zinc-rich epoxy primers F N/A G F F F F P N/A G G F F Zinc-rich moisture-cure polyurethane primers F N/A E F F F F P N/A G E F F Note: E=Excellent, G=Good, F=Fair, P=Poor, N/A= Not Applicable Table 4. User Friendliness of various generic coatings.

NCHRP Project 14-30 21 3.5 Service Life Expectations of Spot Coating Systems Spot painting repairs using 1- and 2-coat systems over marginally prepared substrates are expected to last approximately 5 to 7 years based upon the quality of the repair and the severity of the micro- and macro-environments. Use of 3-coat systems using zinc primers and profiled steel substrates may extend the expected service life up to about 15 years. Ideally, the durability of a spot repair should match the performance of the existing coating to the point that it warrants rehabilitation or replacement. New bridge coatings typically last 20-30 years in most inland exposures (Figure 1). A typical bridge maintenance-painting model incorporates the use of coating repair by spot or zone painting starting when an existing bridge coating is in relatively good condition (perhaps 15-20 years). The existing coating could then be repaired by spot/zone painting of failed/heavily stressed areas. Eventually, those would be supplemented by rehabilitation/full repair of the entire coating using spot repairs and one or more complete topcoats to reinforce the existing coating (i.e., overcoating). In that model, spot/zone painting and overcoating are expected to provide an additional 10-20 years of service until the coatings system reaches a point of terminal serviceability. The existing/repaired/reinforced coating will probably show overall breakdown at about 35-40 years. At that point, it would be removed — commonly by abrasive blasting — and replaced by a new coating system beginning a new maintenance painting cycle and extending the service life of a steel bridge possibly until its replacement. The service lives of spot repairs will depend upon the level of surface preparation and soluble salt remediation as well as the quality of the repair. Most of the coating systems listed in Tables 2 and 3 are anticipated to provide a minimum of 5 years of service if properly applied on acceptable, relatively uncontaminated surfaces. Coating systems listed in Table 2 applied on profiled steel substrates should last at least 5 years for atmospheric exposures in severe industrial and marine/coastal environments. In less severe exposures, those coatings should last up to 2 to 3 times longer. Coating systems listed Table 3 applied on marginally prepared substrates should last 5 years for atmospheric exposures in mild rural and moderate industrial environments. A key to the durability of spot repairs is the level of soluble salt contamination. All things being equal, in severe environments, an equivalent level of soluble salt contamination on surfaces prior to painting is more problematic than in mild or moderate environments. There are no national standards for acceptable levels of soluble salt contamination. Figure 1 Idealized Life Cycle of a Steel Bridge Coating

NCHRP Project 14-30 22 Soluble salt contamination levels are typically stated in units of micrograms per square centimeter (µg/cm2) and are often ion specific (chloride, sulfate, nitrate, etc.). In very basic terms, corrosion science requires the combination of an anode, cathode, metallic pathway and electrolyte in order for corrosion of a steel substrate to occur. Steel, by its very nature, is a metallic pathway that inherently contains anodes and cathodes. An electrolyte (liquid capable of carrying a current) is the only additional requirement necessary for corrosion to begin. As a soluble salt, chlorides, sulfates, nitrates, and others will form electrolytic solutions when dissolved in water. The use of conductivity measurements in micro-Siemens per centimeter (µS/cm) may simplify acceptable soluble salt contamination levels. A conversion equation for surface concentration (µg/cm2) to conductivity (µS/cm) is provided in Formula 3 of SSPC Technology Guide 15, “Field Methods for Extraction and Analysis of Soluble Salts on Steel and Other Nonporous Substrates.” Conductivity measures an electrolyte’s ability to carry a current and is not ion specific. Some generalizations about levels of soluble salt contamination for atmospheric service of spot coatings are: Soluble salt surface contamination levels <10 µg/cm2 (~83 µS/cm) will probably be satisfactory for atmospheric exposures in severe exposures. Soluble salt surface contamination levels of 10 to 30 µg/cm2 (~83 to 250 µS/cm) will probably be satisfactory for atmospheric exposures in mild to moderate climates. Soluble salt surface contamination levels of >50 µg/cm2 (~415 µS/cm) will probably result in coating failures for any atmospheric exposures. 3.6 Spot Coatings Systems and Serviceability of Existing Coating Systems The model presented in this section should properly address serviceability issues for most existing coating systems if handled in a timely manner. For older existing coating systems, some additional considerations are needed if spot painting is identified as a viable option for coating life extension. Three common circumstances exist: 1) The existing coating system is an aged alkyd applied over mill scale 2) The existing coating did not use a zinc-rich primer but was applied over abrasive blasted steel 3) The existing coating used a zinc-rich primer applied over abrasive blasted steel. In Case (1), the existing coating system is probably 30+ years old and has possibly been overcoated. It may be thick and brittle with disbonding, either within the existing coating system or between it and the mill scale substrate. In the former case, the intact coating can be lightly sanded and top coated with one or more repair coats depending on how many coats of the existing coating were lost. In the latter case, if the mill scale was recently exposed, it may exhibit little rust. It may also be found to be cracked or completely/partially disbonded from the steel. Significant What is meant by “service life”? Generally, service life is assigned in years. Spot coatings are primarily intended to repair an existing bridge coating where it has failed and prevent continued deterioration/corrosion in the repair areas. From a practical standpoint, a 5-year service life could mean that the spot coatings can have between 3-5% failures measured by repair surface area and still be considered acceptable. The acceptable level of deterioration is left to the highway agency.

NCHRP Project 14-30 23 corrosion may also be present at locations having coating failures. Under those circumstances marginal surface preparation and use of coating systems in Table 3 are recommended. The corroded areas may need soluble salt remediation prior to painting. The remaining service life of the existing coating is probably low, especially with disbonding evident and those spot coatings/systems and level of surface preparation should be sufficiently durable to last until the existing coating is replaced. In Case (2), the existing coating is a barrier/inhibitive system. The age of the existing coating system will vary based on the bridge owner. Some highway agencies began using abrasive blasting in conjunction with lead-based alkyds in the late 1970s/early 1980s and continued with the emergence of inorganic zinc/vinyl coating systems in the mid-1980s. At least one highway agency used abrasive blasting in conjunction with multi-coat systems for maintenance painting, but deferred from using zinc primers. These situations are different than Case (1) as the existing coatings may disbond internally, but will not readily disbond from blast- cleaned steel. Partial coating failures should be addressed as noted for Case (1). However, if the overall condition of the existing coating is satisfactory, the failed areas can be repaired using the barrier coating systems in Table 2. The repair areas may need soluble salt remediation prior to painting. If the condition of the existing coating is good enough to warrant spot painting, it may last 10+ years without requiring rehabilitation or replacement and the barrier systems in Table 2 coupled with the profiled substrate should provide sufficient durability to match the performance of the existing coating during that period. For Case (3), the existing coating has either inorganic or organic zinc primer on a blast cleaned steel substrate. Zinc in early inorganic zinc/vinyl systems may have reacted and turned to a tight white coating that acts as a barrier. In some cases, the inorganic zinc may not have been top coated. In other cases, both inorganic and organic zinc primers are used in existing coating systems with other top coats. In any of those situations, complete coating failures result in areas where exposed ferrous corrosion is present. As noted with Cases (1) and (2), partial failures of the coating systems can be repaired by light sanding and replacing the disbonded coatings. For complete coating failures, the zinc-based systems in Table 2 coupled with a profiled substrate should be chosen in nearly every case. With timely repairs and rehabilitation, the existing zinc-based coatings may provide 50+ years of service in all, but the most severe exposures. 3.7 Compatibility Issues between Spot and Existing Coatings Occasionally, problems are encountered in coating repairs due to incompatibilities between the existing and repair coatings. Spot painting entails placing repair coatings over existing coatings that are probably of different generic types made by different coating manufacturers (many of whom may no longer be in business). Coating deterioration may pose compatibility problems that do not exist with undamaged coatings. Most existing coatings may be severely weathered, causing them to become embrittled, eroded, or chalked. Those circumstances may be aggravated by periodic overcoating that increases the film build, resulting in further deterioration and weakening of cohesive or adhesive properties in the existing coating(s). Coating compatibility problems may be due to several factors: 1. Aged, thick, poorly adherent existing coatings (e.g. lead-based alkyds over mill scale) - Partial coating failures should be lightly abraded feathering the edges of the completely intact coating followed by coating with low stress coatings such as alkyds.

NCHRP Project 14-30 24 - Full coating failures should receive marginal substrate repairs feathering the intact existing coating followed by repair using a coating/system provided in Table 3. See Factor 2. 2. Curing method of repairs coating (co-reactive coatings such as epoxies or 2- component polyurethanes cross-linking and causing disbonding of aged alkyds, acrylics or vinyls — (Figure 2) - Minimize the overlap of co-reactive coatings over existing coatings. - Avoid the use of co-reactive coatings for repairs, especially for existing coatings described in Factor 1. 3. Reaction of existing coatings with solvents in repair coatings resulting in lifting — (Figure 3) - Some epoxy and polyurethane coatings contain strong solvents that can cause disbonding of the existing coating typically during curing. This can be prevented by applying a monomer epoxy penetrating sealer or alkyd as a tie coat between the existing and repair coatings if problems are encountered. 4. Chemical incompatibility of the existing coating with repair coatings - Chemical incompatibilities include alkyds over zinc and a broad range of conventional coatings applied over calcium sulfonate alkyds. 5. Hardness of the existing coating - Repair coatings that exceed their recoat window typically harden to the point that they need to be abraded or covered with a tie coat (typically epoxy penetrating sealers or alkyds) before being top coating. Some acrylics require that epoxy coatings be abraded before they can be applied regardless of the age of the epoxy relative to its recoat window. Glossy polyurethane topcoats typically should be abraded before they are topcoated. When highway agencies have concerns about painting over existing bridge coatings with specific products, they can apply a test patch on a bridge using the intended topcoats. This work can be performed per ASTM D5064, “Standard Practice for Conducting a Patch Test to Assess Coating Compatibility.” The minimum duration of the test is the time required for the applied coating to cure, but experts have recommended longer test periods (for a year, or at least over a winter). Figure 2 Disbonding caused by epoxy/ polyurethane overcoating system applied over an aged brittle alkyd coating over mill scale. The failure occurred two years after overcoating. Figure 3 Lifting failure on polyurethane system applied over aged alkyd paint system. The failure was detected several days after application.

NCHRP Project 14-30 25 3.8 Climate Restrictions for Spot Coatings Climatic conditions play a significant role in the selection of coatings. They affect: • Conditions under which a coating can be applied — Coatings should not be applied in rain, wind, snow, fog, or mist, or when the substrate temperature is less than 5o F above the dew point. Coatings should not be applied on wet or damp surfaces unless so stated in a manufacturer’s product data sheets (PDSs) and no coating should be applied on frosted or ice-coated surfaces. Coatings can be affected if relative humidity is either too low or too high, and the coating manufacturer should provide those limits. Generally, the minimum and maximum temperatures of concern are 35o-40o F (1.7o-4.4o C) and 120o- 125o (48.9o-51.7o C), respectively (either surface or atmospheric temperatures). The painting crew should consult the PDSs to determine allowable ambient and steel surface temperatures for the coatings being used. Besides relative humidity, some coatings (e.g., moisture-cure polyurethanes) require minimum humidity levels (about 6%) to cure properly, possibly necessitating humidity measurements to ensure that a satisfactory amount of moisture is available. Other coatings may have specified maximum humidity levels. Typically, the maximum relative humidity is 85%. Moisture-cure polyurethanes usually have reduced minimum and maximum temperature thresholds of about 20o F (6.7o C) and 100o F (37.8o C), respectively. They can commonly be used at relative humidities up to 99%. Painting activities should cease when the wind velocity reaches 25 mph, either in gusts or in steady state. • Pot lives of multi-component reactive coatings (epoxies and 2-component polyurethanes) —After mixing co-reactive coatings, painters have limited time to apply them — termed pot lives. Those vary significantly with temperature. At 40° F (4.4° C) epoxies may have pot lives about 8-10 hours. At higher temperatures about 75° F (23.9° C) and 100° F (37.8° C) they decrease precipitously dropping to 4 hours and 2 hours respectively. Some epoxies will have pot lives about half of those values. 2-component polyurethanes have equivalent pot lives at those temperatures. • Post-application cure intervals — After coatings have been applied, they begin to solidify during the curing process. For most conventional coatings, cure times are important as they impact when a coating can be nondestructively inspected for DFT per SSPC PA 2. That should be performed when the coating has cured to the “dry-to-handle” state. The other important milestone is the “recoat” window, which denotes the minimum and maximum time that a coating can be recoated. For most paint applications the minimum time is most important. It generally coincides closely with the “dry-to-handle” time (within 2 hours). Those are typically temperature dependent. At 40° F (4.4° C) epoxies may have a minimum recoat time of 48 hours. At higher temperatures of about 75° F (23.9° C) and 100° F (37.8) they decrease precipitously to 8 hours and 4 hours respectively. The 2-component polyurethanes have faster recoat times at 40° F (4.4° C) - about 18 hours - but similar cure times to the epoxies at about 75° F (23.9° C) and 100° F (37.8° C). 3.9 Non-Traditional Coatings/Materials Nontraditional coatings and other protective materials offer potential advantages in supplanting or supplementing protection provided by conventional coatings. They are typically applied over minimally prepared substrates, commonly SSPC SP 2, “Hand-Tool Cleaning”, complex surfaces, salt-laden environments, substrates exposed to extended time of wetness and areas of poor physical access.

NCHRP Project 14-30 26 3.9.1 Tapes Tapes are typically thick (up to several hundred mils). They can be cloth sheets impregnated with petrolatum or uncured viscous resin sheets. Their application is more like applying wallpaper than painting. However, environmental conditions (temperature and relative humidity) must be assessed prior to application to ensure proper adhesion of the tape on steel. Tapes act as barriers to reduce the availability of oxygen, water, and soluble salts after a spot coating is placed in service. Due to their extreme thicknesses, they have superior barrier properties and can be used for difficult applications such as ground-to-atmosphere transitions and splash zones of steel bents/piles or beam ends under leaking joints. Some of these can be used to repair steel wrappings on suspension and bridge stay cables. Depending on type, tapes are self-adhering, heat-cure applied as wrappings, or attached by gluing. Some tapes perform acceptably in sunlight while others need to be topcoated with a UV-resistant coating (e.g., acrylic topcoat). Tapes have not been widely used on highway bridges. 3.9.2 Greases Greases, waxes, and other petroleum-based products and sealers can be applied over marginally prepared SSPC SP 2 and SP 3 substrates. Greases and petroleum-based (e.g., Cosmoline) products have been used by some DOTs for decades to provide supplementary protection and perform spot repairs on failed coatings. Greases have been commonly used by highway agencies on beam ends, bearing plates, and rockers. Those materials can be applied by hand or scraper on complex and poorly accessible surfaces, simplifying application (Figure 5). Typically, there is little control on film build, though it is desirable to establish a minimum build (e.g., 15-30 mils or 375-750 microns) and require applicator inspections with a wet-film gage. Greases can last a minimum of 5 years in mild to moderate environments. Figure 4 Petrolatum impregnated tape covering a diaphragm (arrow) Figure 5 Grease applied on a rocker bearing

NCHRP Project 14-30 27 3.9.3 Conversion Coatings Conversion coatings used in highway applications are typically rust convertors containing tannic acid combined with an acid-tolerant binder, such as a vinyl emulsion. The tannic acid converts the rust to iron tannate, a stable blackish corrosion product. The binder forms a coating for subsequent application of primers and other topcoats. Issues are sometimes encountered with the conversion process that can prove problematic for long-term service. Some highway agencies use rust convertors for minimal surface preparation spot repairs and leave them un- topcoated. SSPC “Surface Preparation Commentary for Metal Substrates” notes, “So-called “over-rust primers” (also referred to as “rust converters”) do not perform as well as conventional coatings applied over clean steel, and the effectiveness of rust converters is unproven.” 3.10 Coating Manufacturer Product and Safety Data Sheets Coating manufacturers provide PDSs and safety data sheets for their coatings. PDSs are useful to assess when selecting coating types for use in spot painting and including them in approved product lists. They are also vital for use in the shop to determine proper storage procedures for coatings and their shelf lives. In the field, they are vital for guidance in the preparation of the surfaces that will receive them; determination of the material, atmospheric, and surface conditions to apply them; application of the coatings. including mixing, pot life, application method(s), additions of thinners, application thicknesses; various stages of curing (dry to the touch, recoating time/window, and final cure); equipment clean-up materials; and repair methods. Key highway agency personnel (materials personnel, field supervisors, mixers, painters, and inspectors), as well as third-party inspectors and contractors, should read and understand the PDSs. Safety data sheets (SDSs), previously termed material safety data sheets, are also vital documents and are mandated to be available to all parties when working in the field. They are necessary to protect workers and the public from hazards posed by coatings as they are to be applied. All parties directly involved in using protective coatings need hazard awareness training to become familiar with general safety issues related to coatings and specifically to those related to the coatings used on the projects at hand. All major coating manufacturers have web sites and, commonly, PDSs and SDSs for their coatings and other coating-related products are available on them. Coating manufacturers will typically provide separate SDSs for each component of multi- component coatings. If highway agency personnel have questions about either PDSs or SDSs, they should contact the manufacturer for clarification. 3.10.1 Product Data Sheets There are no standards or formal requirements for formatting or content of PDSs. PDSs typically are structured as follows: Figure 6 Rusted beam treated with a conversion coating

NCHRP Project 14-30 28 • Product Description — This describes the type of coating (e.g., primer, intermediate, and finish coat), its binder/resin, and any further definitions that would assist the user in determining its function (e.g. universal primer). Beneficial features of the coating are described though they may not be technical (e.g., “easy to apply and recoat” or “abrasion and chemical resistant”). • Product Characteristics — Finish (flat, semi-gloss/satin, or high-gloss), Color (e.g., gray), Volume Solids (in percent), Weight Solids (in percent), VOC (g/L and lb./gal), and for zinc primers, percent by weight. • Recommended Spreading Rate — Maximum and Minimum in both wet film thickness (WFT) and DFT in mils or microns and theoretical coverage (ft2/gal or m2/L). • Drying Schedule — Given at some prescribed WFT (mils or microns) usually at a range of temperatures near the minimum (e.g., 40o F or 4.4o C), maximum (e.g., 100o F or 37.8o C) and some intermediate standard temperature, usually 75o F or 77o F(23.9o or 25o C). Cure times are given for dry to touch, to recoat (minimum), to recoat (maximum), and to cure fully. The latter may include times for atmospheric and immersion service. Times may be given in minutes, hours, days, or months, as appropriate. • Storage Data — Shelf life (months) along with maximum and minimum storage temperatures. Flash point and reducer information may also be provided. • Recommended Uses — Includes bridge and other services (e.g., potable water). May note that the coating meets an SSPC paint standard or other standards not applicable to bridge service. • Recommended Systems — Various recommended systems incorporating the coating (may be for different services such as immersion and atmospheric), • Surface Preparation — Recommended surface preparation treatment, including cleaning/degreasing and level of SSPC surface preparation standard (e.g., SP 6 abrasive blasting/2-mil profile). Also recommended/minimum surface preparation requirements. • Tinting — Conditions under which tinting is allowed and tint material. • Application Conditions — Ambient, surface, and material minimum and maximum temperatures for application and maximum and minimum relative humidity. • Safety Precautions — Requires reference to that document for use of coating. • Application Bulletin — Provided as separate sheets including detailed instructions for surface preparation including a range of surface preparations (from minimum to preferred) and recommendations for timing of application after surface preparation is completed. It also provides application conditions (noted above), application equipment (e.g., brushes — types, rollers — nap, spray equipment — conventional, or airless), and recommended thinner/clean-up solvents. For airless spraying, pump ratio, pressure, hose ID, spray tip size, and permitted thinning. For conventional sprayers, sprayer model, spray nozzle size, atomization pressure, and permitted thinning. The application sheets provide the Spreading Rates and Drying Schedule, noted above, along with tips for supervisors and painters to get the best performance out of the coating including guidance on the use of accelerators. 3.10.2 Safety Data Sheets The OSHA Hazard Communication Standard (HCS) requires chemical (i.e., coating) manufacturers to provide SDSs to communicate the hazards posed by coatings (OSHA 2015). The format, which is uniform for all coating manufacturers, include the section numbers, headings, and associated information as follows: Section 1 — Identification includes product identifier; manufacturer or distributor name, address, phone number; emergency phone number; recommended use; restrictions on use.

NCHRP Project 14-30 29 Section 2 — Hazard(s) identification includes all hazards relevant to the chemical; required label elements. Hazards are grouped into physical (e.g., flammable aerosols or simple asphysxiants) and health hazards (e.g., acute toxicity or carcinogenicity) and rated by severity (ratings 0 to 4 from least severity to greatest with sub-groups designated by suffixes A, B...). Also, label elements. Includes prevention methods, response to exposures, storage, disposal and special information for use. Section 3 — Composition/information on ingredients includes information on chemical ingredients; trade secret claims. Section 4 — First-aid measures includes important symptoms/effects, acute, delayed; required treatment. These include: eye contact, inhalation, skin contact and ingestion, and symptoms. Section 5 — Fire-fighting measures lists suitable extinguishing techniques, equipment; chemical hazards from fire. Section 6 — Accidental release measures lists emergency procedures; protective equipment; proper methods of containment, and cleanup. Includes actions for non-emergency personnel and emergency responders along with environmental precautions, containment, and clean-up. Section 7 — Handling and storage lists precautions for safe handling and storage, including incompatibilities. Includes protective measures for workers, occupational hygiene, prevention of storage near dangerous conditions (e.g., flame) or materials (e.g., solvent-based coatings near oxidizers). Section 8 — Exposure controls/personal protection lists OSHA’s Permissible Exposure Limits (PELs); Association Advancing Occupational and Environmental Health (ACGIH) Threshold Limit Values (TLVs); and any other exposure limit used or recommended by the chemical manufacturer, importer, or employer preparing the SDS where available as well as appropriate engineering controls and personal protective equipment (PPE). This includes both solid materials (e.g., pigments like titanium dioxide) and solvents (e.g., methyl n-Amyl Ketone) usually given in time- weighted average exposures at specific concentrations in air (e.g., 10 mg/m3 for a time-weighted average of 8 hours). Guidance is to be provided on appropriate engineering and environmental exposure controls and individual protection measures (hygiene, eyes, body/skin, and respiratory). Section 9 — Physical and chemical properties lists the chemical's characteristics. Physical state (liquid), boiling point, flash point, evaporation rate, flammability limits, vapor pressure/density, relative density, viscosity, and heat of combustion. Section 10 — Stability and reactivity lists chemical stability and possibility of hazardous reactions. Includes possibility of hazardous reactions, conditions to avoid, incompatible materials and hazardous decomposition products. Section 11 — Toxicological information includes routes of exposure; related symptoms, acute and chronic effects; numerical measures of toxicity. This includes a list of constituents and exposures for animals/humans with exposure results (e.g., skin — mild irritant) and symptoms. Section 12 — Ecological information. * Dosages of spills and wildlife/plants affected along with exposure periods. Section 13 — Disposal considerations. * General guidance for disposal. Section 14 — Transport information. * Transportation hazards (US DOT) UN number, shipping name (paint), transport hazard classification (e.g., flammable) and severity of hazard (e.g., 3) and determine if shipment poses and environmental hazards (yes/no).

NCHRP Project 14-30 30 Section 15 — Regulatory information. * Producer-required regulatory information (e.g., SARA 313) or special state hazard labelling (e.g., State of California) Section 16 — Other information, includes the date of preparation or last revision. The Hazard Material Information System prepared by the American Coating Association (ACA). A blue (health)/red (flammability)/yellow or orange (reactivity)/white (personal protect — to be filled out by user) label listing hazards rated 0-4 with 0 the least hazardous and 4 the greatest hazard. Categories that are rated include: flammable liquids, skin corrosion/irritation, respiratory sensitization, carcinogenicity, and specific target organ toxicity (single and repeated exposures). *Note: Since other agencies regulate Sections 12-15, OSHA does not enforce these sections. These documents must be readily available to employees and copies should be provided to in-house workers prior to the work and copies should be maintained and readily available at the jobsite.

Next: Chapter 4.0 Spot Painting of Steel Bridges »
Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 251: Spot Painting to Extend Highway Bridge Coating Life: Volume 1: Guidance provides approaches for employing spot painting in a cost-effective, safe, and environmentally compliant manner. Bridge coatings are the primary means of corrosion protection for steel bridges in the United States. Most bridge coatings tend to fail prematurely in localized areas and spot painting can be used restore the lost corrosion protection and extend the service lives of existing bridge coatings, often at a fraction of the cost of a complete bridge repainting. However, many state highway agencies do not perform spot painting primarily due to performance concerns and lack of familiarity with its proper utilization and execution.

The guidance is accompanied by NCHRP Web-Only Document 251: Volume 2: Research Overview provides the evaluation method for the guidance document.

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