National Academies Press: OpenBook

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

Chapter: Chapter 2.0 Opportunities to Use Spot Painting

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Suggested Citation:"Chapter 2.0 Opportunities to Use Spot Painting." 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 2.0 Opportunities to Use Spot Painting." 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 2.0 Opportunities to Use Spot Painting." 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 2.0 Opportunities to Use Spot Painting." 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 2.0 Opportunities to Use Spot Painting." 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 2.0 Opportunities to Use Spot Painting." 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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Page 9

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NCHRP Project 14-30 4 2.0 OPPORTUNITIES TO USE SPOT PAINTING Bridge steel is composed of a variety of shapes which makes uniform coating application difficult, typically leading to insufficient coating thicknesses of one or more coats. Problematic features for painting include edges, faying surfaces, re-entrant corners, and fasteners. Coating failures often initiate where inadequate coating thickness is present. Excessive coating applications or buildup of periodic maintenance overcoats can also result in premature coating failures and render some maintenance painting options problematic. As coatings age, they can become brittle and contract leading to disbonding failures, either completely detaching from the steel/mill scale or as inter-coat failures detaching from underlying coatings. Premature failures are usually promoted by a combination of poor application practices and exposure factors that stress coatings, causing them to fail and allowing corrosion to initiate and spread. Aged coatings undergo weathering processes that render them less resistant to corrosive attack and ensuing failures. Corrosion of Beam End, Cross Frame and Bearing Coating Failure Exposing Rusted Mill Scale Inter-Coat Failure

NCHRP Project 14-30 5 Bridge macro-environments refer to the conditions in which a bridge is situated (e.g., marine, inland, rural, urban and industrial). In addition to the impacts of macro-environments, various locations on bridges have different micro-environments about the bridge in which protective coatings and steel are exposed to a range of conditions that may be more severe than those provided by the macro-environments. Macro-environments can stress coatings. In bridge locations exposed to sunlight, UV degradation can lead to weakened coatings that chalk and erode. Locations exposed to fog or high rainfalls can have extended time of wetness (TOW), which promotes corrosion. Macro-environments can also provide exposure to harmful chemicals that deteriorate coatings and promote corrosion (e.g. marine and industrial environments). Micro- environments such as areas under leaking deck joints can collect de-icing salts — also called soluble salts (see inset). Locations at the ends of bridges are often sheltered from sunlight and have extended TOW that promotes corrosion more aggressively that areas exposed to direct sunlight. Those areas may be considered severe environments if they are under constantly leaking joints. Micro-environment corrosion commonly occurs at beam ends/bearings under joints. It can also occur at splash zones on steel exposed to salt-laden aerosols kicked up by traffic (e.g. overpass fascia girders and verticals and diagonals of through truss bridges). As a result of those factors, bridge coatings can experience small areas of localized failure. Typically, most of the existing coatings remain intact and capable of years of additional service. In these situations, the failed areas can be repainted. This can be done without affecting the balance of the intact coating (i.e., spot painting). Other types of bridge coating failures (and ensuing corrosion) may necessitate different maintenance painting options. 2.1 Maintenance Painting Options As previously noted, there are four maintenance-painting options that can be used to address various circumstances on a painted steel bridge — 1) total removal and replacement of the existing coating, 2) overcoating an existing coating with another coating/system, 3) zone painting, and 4) spot painting. Removal and Replacement The most common maintenance painting action is total removal of an existing coating and What Are Soluble Salts? Soluble salts are chemicals/elements that can be dissolved in water to form ions. Those can penetrate through coatings to steel and promote corrosion. They include chlorides (from marine environments and deicing chemicals), sulfates and nitrates (typically from industrial atmospheres). Chlorides are the most common for bridge exposures and damaging from a corrosion standpoint followed in severity by nitrates and sulfates respectively. Soluble salts can be present on the surfaces of bridges elements regardless of whether they possess intact paint or rust. If they are detected in sufficient concentrations, they must be removed prior to paint application to prevent premature coating failures.

NCHRP Project 14-30 6 application of new coatings (removal and replacement). It a replacement action used when the overall condition of the existing coating is such that coating repair and rehabilitation options are not feasible or when existing coating condition plus other considerations (e.g., traffic volumes, life cycle costs or aesthetics) dictate its use. Removal of the existing coating usually entails abrasive blasting with expendable or recyclable abrasives. When used with a multi-coat system including a zinc primer, it is the most expensive maintenance painting option from a first cost standpoint. Depending upon the structure painted, the first cost can range from about $10/ft2 for a rural deck-girder bridge to over $30/ft2 for a major bridge in a city (e.g., New York). It is also the most durable maintenance-painting option. The anticipated service life of a removal and replacement project is typically 20 to 30 years depending on structure complexity, location, extent of corrosion damage/surface contamination (e.g., chlorides) prior to surface preparation/efficacy of contaminant removal, level of surface preparation, care in paint application and quality of the applied coatings. It provides the best aesthetic results of any maintenance painting option. Overcoating Overcoating is the other approach to total bridge painting, but it qualifies as a rehabilitation action, as the existing coating is still used and relied on for some steel protection. It typically consists of washing the intact existing coating to remove surface soils and minimize surface contaminants, spot repairs of areas where the existing coating has failed, application of spot coatings on the areas where corrosion/inter-coat failures have occurred, and application of one or more topcoats over the existing coating (typically barrier intermediate coats along with wreathing-resistant topcoats). Total and unit costs vary, but can be expected to be half to two-thirds the cost of removal and replacement. Project durability is usually assumed to be 10 to 15 years in mild or moderate environments. The limiting factors are typically the condition of the existing coating, the existing substrate and the surface preparation used. Many older painting projects used lead-based alkyds applied over mill scale. Those present the greatest risk of premature failure when overcoated. If the existing paint system possesses a zinc-primer on a blast-cleaned substrate, an overcoating project can last 15 years, with 20 years a reasonable expectation in moderate environments. Feathering of repair areas along with spray applications of spot repairs and topcoats offers the best aesthetics when using overcoating. Zone Painting Zone painting is a localized coating repair option that typically involves totally removing and replacing broad portions of existing coatings where there are discrete environments that are aggressive and require more frequent maintenance painting (e.g., beam ends, splash zones and fascia girders of overpass bridges). It typically used on larger areas than spot painting. It can also can be performed as a localized form of overcoating, using the same procedures, on limited portions of a bridge with coating deterioration. From the standpoint of unit costs, zone painting is more expensive than either removal and replacement or overcoating. Since zone painting is performed on highly stressed locations, at best the life expectancy is anticipated to be 10 to 20 years for atmospheric exposures in mild or moderate environments depending on the surface preparation and coatings employed. The aesthetics of zone painting projects is usually poor. Maintenance coating of weathering steel typically involves zone painting and will not be covered further in this guide.

NCHRP Project 14-30 7 Spot Painting Spot painting is a coating repair option used to address localized coating failures and corrosion. It can employ a range of surface preparation methods and coatings. From the standpoint, of unit costs, spot painting is the most expensive coating option, with costs several times that of removal and replacement. Highway agency survey data indicates spot painting service life expectations of 7 to 15 years in mild or moderate environments depending on surface preparation and coatings used for repairs. Since spot painting is typically performed on highly stressed locations, conservative performance expectations may be lower — 5 to 7 years. The aesthetics of zone painting projects is usually are poor. Spot painting is the maintenance paint option most amenable for use by in-house forces. In this guide, spot painting will incorporate the use of power tool cleaning for surface preparation as it best addresses dispersed localized coating failures that are commonly encountered on bridges and usually eliminates the need for expensive containment. It is amenable for implementation by both in-house forces and by contract. 2.2 Coating Condition Assessments Prior to making a maintenance painting decision, existing coatings on bridges should be assessed to determine what action best fits agency needs/circumstances. An initial review may be obtained from agency asset management data. That includes coatings ratings and perhaps pictures. That review could identify additional issues not related to coatings that might impact the maintenance painting decision making process. In addition to asset management data reviews, field coating condition assessments can be triggered by the age of a structure or the age of its coating. Field coating condition assessments can be performed on candidate bridges to identify existing bridge coating conditions/materials and bridge environs that may impact maintenance painting What is spot painting? Unfortunately, there is no universally accepted definition of spot painting. There is general agreement that it is a repair process for failures of existing coatings. However, among highway agencies there seems to be some differences in the scope of repairs. Some use the same procedures employed for removal and replacement to repair bridge coatings with “spot painting” to replace 10 to 25 percent of the existing coating. The context of “spot painting” used by most highway agencies addresses coating repair work on a much smaller scale. Guidance in the SSPC Paint Application Guide 5, “Guide to Maintenance Coating of Steel Structures in Atmospheric Service” recommends the use of “spot painting” as a coating repair options under the following circumstances: • Repairs are hidden and unimportant to aesthetics • Owner maintenance crews are available for this work • Structures are small and repair locations are readily accessible • Corrosion and degradation are limited to isolated areas and small sections which amount to less than 1% of the total area • Several of those circumstances are overly restrictive compared to typical highway agency “spot painting” practice. They vary conditions/practices described by this guidance document.

NCHRP Project 14-30 8 decisions. Those assessments can be sufficient in scope to provide information necessary to plan and perform follow-up maintenance painting work resulting from agency selection of a maintenance painting option. Guidance for conducting field coating condition assessments is provided in Appendices A-C. Field Coating Condition Assessment Data Impact on the Use of Spot Painting Field coating condition assessments provide data that can support the use of spot painting. In the AASHTO “Manual for Bridge Element Inspection,” Appendix E, Steel Protective Coatings, spot painting (i.e. repair) is listed as a feasible maintenance option for coatings rated as Fair, Poor or Severe. Data resulting from field coating condition assessments contribute information to the decision making process that support the use of spot painting or the other painting options. Service Environment — Inland exposures, given as SSPC 2A in SSPC commentary, “Using SSPC Coating Material Standards” (Table 1), are typically suitable for spot painting. Aggressive environments (e.g. marine SSPC 2B and industrial SSPC 3A-3D) are more problematic and would require higher-order surface preparation, soluble salt remediation, and the use of multi-coat repair systems. Rusting — Key issues related to the use of spot painting are the extent of coating failures and their disposition. SSPC recommends the onset of maintenance painting when rusting exceeds 0.3% or greater. Typically, spot painting would be considered for 0.3% to 3% rusting (rust grades 5 and 6) though the percentage may be higher for small bridges (perhaps a maximum repair area of 500 ft2). Spot painting is a viable option when the rust is localized/concentrated (e.g. rust grades 5-S and 6-S). Coating Condition and Coating Thickness and Adhesion — Spot painting typically will have limited interaction with the existing coatings unless they have experienced partial coating failures. These may have a greater impact on the use of other coating options. Soluble Salts — The presence and concentrations of soluble salts may affect the surface preparation method and coating(s) used and the maintenance painting option selected. If a wet method is required for remediation, it may extend the work time and require an additional waste treatment and disposal step. Aesthetics — Signature bridges or other high-visibility bridges are usually candidates for removal and replacement or overcoating to preserve aesthetics. While spot and zone painting can be used to address localized coating failures and rusting, from an aesthetics standpoint, they are less desirable than removal and replacement or overcoating. Environmental and Worker Safety – Environmental issues such as disturbing existing coatings with hazardous materials, impacting endangered species, etc. may affect the decision to use spot painting, especially if that work is restricted to in-house forces. Most painting work will require workers to use PPE and have some level of training and testing. Those requirements will be amplified when hazardous materials, working at heights or special access requirements are involved. If spot painting is a viable option based on other criteria, lack of in-house capabilities can be offset by performing the work by contract. Other Considerations – Access to bridges carrying or over high ADT roads or those in urban areas may be limited and more durable coating options such as overcoating or

NCHRP Project 14-30 9 removal and replacement may be suitable. 2.3 Coating Decision Making Circumstances influencing which option is selected include: 1) The overall condition of intact portions of an existing coating (e.g. thickness and adhesion) 2) The disposition and extent of deterioration of an existing coating 3) Structural corrosion and severity (along with criticality of affected structural members) 4) Surface contamination by corrodants such as sulfates and chlorides 5) Bridge type, size and importance 6) Work site/environmental factors 7) Traffic management issues 8) Painting budgets. These factors can interact to complicate decision making about maintenance painting. The attendant Do Nothing option is always a potential outcome of any maintenance decision. That option may relate to a coating condition that does not warrant any action or to agency budget or work capacity shortfalls that prevent their execution.

Next: Chapter 3.0 Coatings for Spot Paintings »
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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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