overcoating bridges and other structures...measured in accordance with astm d4214, “standard test...

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jpclPAINTSQUARE .COM JOURNAL OF PROTECT IVE COAT INGS & L IN INGS

OVERCOATING BRIDGES AND OTHER STRUCTURES

A JPCL eResource

Photo courtesy Ernst Toussaint, Ross Sherwood, Scott Stotlemeyer


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Copyright 2009 byTechnology Publishing Company2100 Wharton Street, Suite 310Pittsburgh, PA 15203

All Rights Reserved

This eBook may not be copied or redistributedwithout the written permission of the publisher.

OVERCOATING BRIDGES AND OTHER STRUCTURES


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iv Introduction

1 Overcoating: Maintenance or Mayhem? by Richard A. Burgess

6 The Vincent Thomas Bridge: A Study in Overcoating and QC by William H. Hansel, Caltrans, Terminal Island, USA

12 Preventing Overcoating Failures by Clive H. Hare

19 Overcoating the Caruthersville Bridge: How the Owner, Inspector, and Contractor Met the Challenges from System Selection to Application and Protection by Ernst Toussaint, Ross Sherwood, and Scott Stotlemeyer

24 Overcoating Challenges While Removing Lead Paint from the Hal Adams Bridge by Don Buwalda, Stephen Haney, and Greg Richards

30 Overcoating Lead-Based Alkyd Paint on Steel Penstocks: Practical Experience by Mike O’Donoghue, Ph.D.; Peter Roberts; Vijay Datta, M.S.; and Terry McManus

Contents

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IntroductionThis eBook features articles from the Journal of Protective Coatings & Linings (JPCL)

about overcoating bridges and other structures. All information about the articles is

based on the original dates of publication of these materials in JPCL. Please visit

www.paintsquare.com for more articles on these and other topics.

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Overcoating: maintenance or mayhem?Best Practices for Writing an Overcoating Specification

By Richard A. BurgessKTA-Tator, Inc., Series Editor

Fig. 1: Coating compati-bility is a bigger problem for overcoating than many believe. All photos courtesy of the author.

Whether a bridge, tank, pipeline, vessel, man-ufacturing facility, architectural structure or amusement park ride, at some point there will be a need for maintenance painting. The “paint guy” will share responsibility with

those involved in general maintenance, funding, prioritizing, public relations and safety. Those familiar with maintenance painting may recall the four maintenance painting strategies:Do nothing: The structure, including the coating system, is in good or very good condition. Maintenance painting is simply not necessary or needed. A second “do nothing” option in-volves the “extreme condition,” when the coating and corro-sion breakdown of the structure or item is so poor that it will be demolished, retired or decommissioned. Spot repair: Cleaning and coating only a few areas or sections of a structure will enhance corrosion protection, is relatively inexpensive and the aesthetics are not a major consideration. Localized areas are cleaned and spot rust and degraded or poorly adhered coatings are removed. One or more protective coating layers are applied to the area.

Spot repair and overcoat: Coating deterioration and break-down in one or more of the existing coats is providing some protection from corrosion, but erosion, weathering or other environmental conditions have compromised the barrier pro-tection and/or detracted from the desired aesthetic condition. Overcoating may be applied to the entire surface or be limited to specific areas such as those visible to the public.Remove and replace: This maintenance option is employed when existing coating systems need to be entirely removed and replaced due to extensive breakdown and corrosion of the structure or item. Depending on the service environment and other considerations, this approach may apply to all or only sections (zones) of the surface area. This column focuses on overcoating, which nearly always includes some degree of spot repair and touch-up applica-tion. Whether to enhance corrosion protection, restore and improve aesthetics or both, pre-planning and providing effec-tive, straight-forward specifications are critical to success. Considerations appropriate for overcoating, or any other scope of maintenance painting for that matter, are briefly described simply to provide some context for determining if overcoating is a reasonable maintenance option. Recommen-dations for project specifications follow these considerations.

Pre-PlanningMaintenance painting is most frequently triggered by some visual condition, be it corrosion and rusting, loss of gloss

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or color and at times for regulatory purposes (safety color codes). This may result in execution of a maintenance plan or program that already exists (for example, in petrochemical facilities), or alert the owner that maintenance painting will be needed, and planning and preparing work orders or contracts should be initiated. The impact on operations, safety and ac-cess are evaluated to determine the possible implications that may be imposed by maintenance painting activities, including shutdown times, lane restrictions on bridges and contractor access.

Coating Condition AssessmentA coating condition assessment may be limited to a visual assessment or involve a closer inspection of the coating system. The physical and chemical characteristics of the ex-isting coating system, including adhesion, coating layers and thickness, degree of rusting or corrosion, topcoat resin type and the presence of heavy metals (e.g., lead or chromium) may all be measured or evaluated when extensive assessments are conducted. Refer to to SSPC “Technology Update No. 3, Overcoating” (SSPC-TU 3) for discussion on the use and pos-sible interpretation of such collected data. When overcoat-ing appears to be a viable option for maintenance painting, the use of test patches to evaluate surface preparation and candidate coatings should be considered early in the project planning phase.

Project PlanningThe scope of the maintenance painting to be performed may be established based on historical experience, the coating condition assessment (general or detailed), available funding or test patch and other testing results. However, once the scope has been established, the details of the project and its impact on operations, the public, budget, duration, season,

timing and details for execution can be used to establish the details for contracts, work orders, special provisions and specifica-tions. Any additional aspects of the overall scope of the project should be detailed. This may include repair or replacement of steel elements or components, identification where specific procedures may be required (coating tie-ins), remediation of soluble salts, specific cleaning agents or processes to neutralize or sequester surface con-taminants, and limitations on the surface preparation and/or application procedures permitted. When abrasives are used, restrictions, such as the use of silica sand, should also be considered. Safety, health and environ-

mental issues are also part of the project planning. Lead paint is lead paint, whether it is on a process vessel, bridge or water tank. Compliance with all regulatory requirements must be included in planning, as well. Seemingly mundane details can escape consideration if not included in the detailed plan, such as who will supply power, compressed air, water, scaffolding and controls access (con-tractor or owner).

Contract Documents and Work OrdersPreparation of the contract or work order documents should include general and specific conditions, responsibilities of the contractor, owner and other parties that may be part of the agreement. Insurance, bonding, definitions, schedule and time allotment are also included. The key component of interest here are the technical specifications for overcoating that are a part of the contract requirements. If you intend on using an existing maintenance painting specification, make sure that it is up-to-date and not simply “copy-and-pasted” pieces of specifications or based strictly on coating manufacturers’ recommendations. Because the specifications are contractual, a critical review of the draft specification should always be performed by some-one with the experience and credentials to do so. This can be the internal “paint guy (or gal),” a trusted vendor or a consultant.

Technical Specifications for OvercoatingUltimately, in the eyes of a contractor, the responsibility for the specification will rest with the owner, and rightly so, as the specification is part of the owner’s contract documents. When you are tasked with preparing a specification, whether you work for or are retained by the owner, where do you think the content responsibility will lie in the eyes of the owner? Correct! The engineer, consultant or coating manufacturer that prepared them. The quality of the contract and specifica-

Fig. 2: Edge coating should include a stripe coat, even for spot repairs.

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tions can contribute to a relatively smoothly running project or can lead to Mayhem (you know, that guy from the insurance commercials).

Specification for Overcoating Structural SteelGeneralThe general section of the specification should include the scope of work to be performed; definitions for words and terms used in the specification; a list of published reference standards, test methods, procedures and regulations and other pertinent informational documents relevant to the work. Safety requirements that the contractor is responsible for, as well as the qualifications of the contractor and personnel may be included here unless addressed elsewhere in the contract. Project documentation and submittal re-quirements should also be included here. For the purpose of this article, the scope will involve a small truss bridge where rust is to be removed, tightly adhered coating is allowed to remain and the surfaces are to undergo spot repairs and overcoating.

MaterialsThe materials section allows the owner or specifier to establish the quality, type and source of materials used for the project. Among the items that may be included in this section are abrasives (required, permitted or excluded); cleaning agents (soaps, alkaline, degreasing); water (potable, non-potable); additives (chloride remedia-tion, corrosion inhibitors); equipment (type, size) and, of course, coatings. Proprietary or similar products that are to be used should also be listed. If substitutions or alternates will be considered, state what documentation or reasoning must be submitted with the request for substitution. The general rule used when selecting an overcoat product is that the same resin can be applied over the existing resin. Use of test patches can confirm compati-bility. Tie coats of penetrating epoxy sealers or a “universal” primer may be needed and also evaluated by test patch.

ExecutionThe execution section provides the detail regarding pre-cleaning requirements (pressure washing); surface con-taminants (diesel fumes); surface imperfections (weld spat-ter, sharp corners, steel defects); items to be protected, and control and containment of the work area. Certain procedures or processes can be excluded (including prohibiting abrasive blast-cleaning) but otherwise leave the means and methods to the contractor. Use performance-based language for surface preparation, coating application, quality control and repair

procedures. Note that it is a responsibility to state what is to be done, but ensure there is a performance and/or acceptance criteria and state what methods or standards will be used to measure and test. Pre-clean all surfaces to be prepared and painted with low-pressure water at no less than 4,000 psi to remove sur-face contaminants. Prior to pressure washing, remove bulk de-posits of dirt, soil, leaves and bird droppings manually. Place all collected debris in containers for disposal. Supplement cleaning with brushes, brooms, rags and wiping to produce no less than a chalk rating of 8 on remaining coating when measured in accordance with ASTM D4214, “Standard Test Methods for Evaluating the Degree of Chalking of Exterior Paint Films.”

Surface Preparation Spot RepairsClean all areas of visible rust and corrosion in accordance with SSPC-SP 15, “Commercial Grade Power Tool Cleaning.” Remove pack rust and crevice corrosion using impact tools to the satisfaction of the project engineer. Prepare areas of blistered, cracked and delaminated coating in accordance with SSPC-SP 3, “Power Tool Cleaning.” Surface tempera-ture should be a minimum of 5 F above the dew point before performing surface preparation. Spot-abrasive blast-cleaning to SSPC-SP 6/NACE No. 3, “Commercial Blast Cleaning” would only be justified if there were numerous spots or locations where the size of spots were quite large or abundant. In such an instance, full removal and replacement may be a more de-sirable maintenance option. Intact, adherent coating surrounding rusted and/or degrad-ed areas must be feathered back such that the repair coating will extend at least 2.5 to 5 centimeters (1 to 2 inches) onto the intact coating.

Fig. 3: Overcoating does not need to be a “one-and-done” strategy.

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Soluble Salt MitigationWhen removal of soluble salts is neces-sary, the requirements are included in the surface preparation section because the criteria must be achieved prior to coating application. Additional surface prepara-tion and cleaning may be required. Use of proprietary products may be used when salt removal is difficult and salt levels are high. The locations and frequency of testing are important. In the writer’s opinion, the most important surfaces to test are those that have already exhibited corrosion and are the most difficult to prepare. Random testing should also be used to demonstrate absence of soluble salts.

Coated SurfacesCoated surfaces must be cleaned using low-pressure water during the pre-cleaning process. However, additional cleaning may be appropriate if the surfaces have become contaminated from other opera-tions, such as removal of blistered, cracked or delaminating coating. Glossy and slick surfaces may require scarification to aid in bonding of the overcoat. If SSPC-SP 7, “Brush-Off Blast Cleaning” is warranted, the abrasives should be fine and the nozzle pressure should be kept low. When thick coatings are present and brittle, SSPC-SP 7 may damage otherwise sound coating. Use of abrasive blast-cleaning, even brush-off blast-cleaning, can significantly add to time and cost for an overcoating project and should be carefully considered.

Coating Application Ambient ConditionsRelative humidity (RH); the temperatures of the air, surface and material (Ta, Ts, and Tm); and the dew point (DP) are all to be considered before performing coating application. Minimum and maximum values can be established by the specification. Alternatively, requiring that the ambient conditions conform to the manufacturer’s published requirements is certainly appropriate. This can also avoid confusion or “push-back” by applicators when the manufacturer has more liberal limits than the specification. Yet, if there are sound reasons for being more restrictive, then by all means specify the limits appropri-ate to the situation.

Mixing and ThinningComplying with the manufacturer’s instructions is recom-mended. Thinning may be prohibited, but when permitted, it should comply with the manufacturer’s recommended prod-ucts for the temperatures and/or VOC limits of the workplace. Induction time and pot life should be considered based on the ambient conditions and the quantities to be mixed.

Spot ApplicationWhen application begins, the surface to be coated should meet the surface preparation standard required by the spec-ification. The coatings selected for spot application should lend themselves to brush and roller application and should be applied so that the film extends onto the feathered area of the intact coating. These are typically surface-tolerant coatings such as epoxy-mastic or moisture-cured urethane, among others. Penetrating epoxy sealer (100%-solids) can be used as a first coat where pack rust, crevice corrosion or difficult-to-remove corrosion is present and the spot primer has been applied over it. Stripe coating should be performed with the spot primer and/or tie coat when used. A stripe coat of the finish coat may also be applied, particularly where edge corrosion exists.

Overcoat ApplicationFollowing adequate cure of the applied spot coat, a full coat is applied. This may include a tie coat and finish coat or a finish coat compatible with the existing coating resin. Test patches will help to determine which overcoating systems perform better than others. The products used should provide the desired finish (gloss, semi-gloss) and color(s). Brush and roller or spray application can be used depending on surface area, location and how protection of other surfaces from splatter, spits and overspray will be accomplished. It was mentioned earlier that spray application may be prohibited by some owners or cir-cumstances. It is worth noting here that a full overcoat does not necessarily mean to all surfaces. Non-visible surfaces (under deck beams and diaphragms) may not need an additional coat when exposed steel is being overcoated. This is particularly true when the major factor in maintenance painting is aesthetics.

Fig. 4: Spot repairs on the underside of this bridge seemed sufficient, but later proved to be unsatisfactory.

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Repairs and Deficient Coating A process for addressing coating repairs from mechanical damage and defects due to preparation or application should be established. In particular, edges of the old coating system may lift when overcoated if thick and not feathered, or if not tightly adhered. These require correction which may be as simple as re-applying the spot coat and/or overcoat, or in the worst cases, removal, preparation and re-application of the spot primer and overcoat.

Quality ControlQuality control must be exercised to achieve a completed project compliant with the specifications, including refer-enced standards and performance criteria when tested or examined by the methods called for in the specification. We teach that quality control is the responsibility of the contrac-tor. It is; however, the contractor should realize that control over quality is actually in the hands of the craftsmen doing the work (be it surface preparation, mixing or applying the paint) rather than, or in addition to, the foremen and superintendents at the project. The owner and engineer are obligated to clearly state the performance required, the criteria and the manner by which it will be measured. A requirement to submit quality control reports on a regular basis should also be included.

DemobilizationDemobilization is just as much a part of the project as mobili-zation. It has been included as part of the overcoating speci-fication as a place to house requirements for leaving the site. Requirements typically include cleaning the work and laydown areas, collection and proper disposal of all wastes (including wastewater from low-pressure water cleaning, if applicable) and ensuring no reusable materials leave the site with con-tamination when toxic materials are involved.

ConclusionThe specification guidance provided herein cannot address every situation or circumstance; no static document can. The complexity associated with what, on the surface, may appear to be a simple spot-and-overcoat solution, has also hopefully been conveyed. Overcoating specifications and methods that are tried-and-true are the progeny of the mistakes, errors, screw-ups and mayhem that came before. Future overcoating specifications will be superior to the ones in use today simply because over time we will notice the scars, warts and short-comings.

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Quality control (QC) for a bridge overcoating op-eration is very similar to both shop blasting and coating of new steel and a full coatings removal job. All three types of coating QC have the same elements, which can be summarized as obser-

vation, measurement, comparison, and documentation. Common items covered under this blanket statement are environmental conditions, such as air and surface tempera-tures, relative humidity, dew point, and surface preparation to specified standards such as SSPC-SP 1, SP 10, SP 5, etc. However, once the coating operation reaches the coatings ap-plication stage, the overcoating operation diverges from shop and full removal. The latter two operations have complete access to a prepared substrate before coating application, so, for example, applying the specified coating system and measuring its dry film thickness (DFT) according to SSPC-PA 2 are relatively straightforward. In contrast, with overcoating—especially when multiple existing paint systems are involved—measuring DFT accurately can be challenging. This article describes the painting of the Vincent Thomas Bridge’s east tower to highlight expected and unexpected

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By William H. Hansel, Caltrans,Terminal Island, USA

The Vincent Thomas Bridge: A Study in Overcoating and QC

issues encountered and overcome during a spot prime, full overcoat paint project in 2011. It focuses on how multiple coating systems can affect parts of the work plan and QC plan of an overcoating project.

Difference Between QC and QAFor the purposes of this article, QC and QA are defined as fol-lows. “Quality control (QC) is a procedure or set of procedures intended to ensure that a manufactured product or performed service adheres to a defined set of quality criteria or meets the requirements of the client or customer. QC is similar to, but not identical with, quality assurance (QA). QA is defined as a procedure or set of procedures intended to ensure that a product or service under development (before work is complete, as opposed to afterwards) meets specified require-ments. QA is sometimes expressed together with QC as a single expression, quality assurance and control (QA/QC).”1

Overcoating and Maintenance Painting“Overcoating” has become the common term for bridge maintenance painting operations that only partially remove

East tower of Vincent Thomas BridgePhoto and Figs. 1-8 courtesy of the author


existing paint and apply new coatings over a mixed substrate of existing paint, bare steel, and rusted surfaces. Traditionally, maintenance painting was either accomplished as part of routine lifecycle maintenance (for larger bridges) or with other maintenance activities (e.g., steel component replacement, etc.). Overcoating is now viewed as an alterna-tive to full removal and replacement of a failed existing paint system, and because of increasing cost constraints, some bridge owners now view overcoating as a primary bridge reha-bilitation option.2

Vincent Thomas Bridge In its publication, “The Vincent Thomas Bridge,” the San Pedro Bay Historical Society gives the following description of the structure. “The bridge is 6,060 feet long. The main suspended span is 1,500 feet long and its main side spans are 500 feet long. There are 10 approach spans at each end with a total length of ~3,500 feet. The towers are 365 feet tall.”3 Construc-tion of the Vincent Thomas Bridge (VTB) began in May 1961 with work on the substructure. No rivets were used in con-structing the bridge, making it the first in the United States to be constructed entirely with welding.4

The original coating system was a vinyl wash applied over blasted steel. The specification called for the application of “vinyl wash primer and minimum of 4.0 mils in at least 4 applications on undercoats and minimum 2.0 mils additional in at least 2 applications on finish coats. Main susp. Cable given vinyl wash primer, one coat S.Q.Dr.R.L (58G53), one coat of white traffic Paint, 61G95, and two applications of vinyl green, 59G78” (system 1). The DFT of the system over blasted steel is not clear from the spec, which was dated 1959 and on file at Caltrans’ lab in Sacramento. Based on DFT readings of just the original coating system, it appears to be a 10-mil system: an 8-mil primer and a 2-mil finish coat. For the main suspension, the generic type of S.Q. Dr.R.L. (58G53) is not clear from the spec, and no other record of it could be found. It could be a quick-dry red lead coating. The traffic paint was an alkyd. Various other systems have been used to coat the VTB but none have been used on the entire structure. All were over-coat systems, except for those applied to new steel added during earthquake retrofits. Among the systems were the following.• 1976–1983: Vinyl Wash, Zinc-Rich Primer, Organic Vehicle Type, Iridescent Green Vinyl Chloride (system 2)• 1983–1999: Paint Water-bourne Primer (PWB), Red and Pink, formulas PWB 145A and PWWB 146 A, PWB Finish, Light & Dark Green formulas PWB 82 and PWB 83 These were waterborne acrylic latex coatings that were formulated to comply with changes in EPA and SCAQMD air quality regulations. The dark green finish coat matched the Federal Green color used extensively on bridges throughout America (system 3).5

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Fig. 1 left): Unspecified coating from south leg face J, Leafing aluminium finish coat. Fig. 2 (right): Coating system 2, zinc-rich primer delaminating from coating system


• 1999–present: Paint Water-Bourne Primer (PWB), Red and Pink, formulas PWB 145C & PWB Finish, Light Green PWB acrylic latex primer (PWB 145C and 146C) and finish, Light Green (PWB 82B 82 and Iridescent Green (PWB 169F)5 (sys-tem 4)

Initial ConditionThe east tower had numerous coating systems applied to various members during the structure’s 48-year service life to date. Coating system 1 was applied to all of the tower’s bridge members, except new steel added during the earth-quake retrofit, on the both the upper and lower tower. Coating system 2 was applied to all tower bridge members except the upper tower’s cross bracing. Coating system 3 was applied to the entire lower tower. Coating system 4 was applied to both tower legs and caps. The earthquake retrofit was specified to utilize coating system 3 and was applied to numerous stiffener plates added to both tower legs. This collection of coating systems exhibited various de-grees of visible degradation. The tower caps had large areas of pack rust and areas of intercoat delamination. The upper tower cross brace’s topcoat was very faded, with large areas of primer exposed; the gusset plates were rusted; and areas of pack rust were found on lateral members. The upper tower legs appeared to be in perfect condition. The lower tower legs also appeared to be in perfect condition, except for the panel designated south leg (face J). This panel exhibited numerous blisters. A knife cut was made to a blister, and a coating not specified for the section (PWB 139 Leafing Aluminum) was found between the light green and iridescent finish (Fig. 1). The lower cross braces exhibited large areas of blistering, especially on the gusset plates. Closer examination revealed that the zinc primer from coating system 2 was delaminating from coating system 1 and taking all subsequent systems with it (Fig. 2). In addition, the upper chord of the cross brace

directly beneath the finger joints was severely corroded with numerous areas of pack rust. A coating plan was therefore developed to address these various problem areas.

Coating PlanBecause of the mixture of surface deterioration and coating failure, Caltrans decided to use a targeted approach. The entire tower was to be pressure washed to an SSPC-SP 1, Solvent Cleaning, with 5,000 psi maximum pressure, utilizing a zero degree rotating nozzle. After surface preparation, coating system 4 would be applied to the entire structure. A stripe coat of each primer coat was specified to precede both prim-er coats. The primer coats were specified for application at 3–6 mils DFT each. Before the application of the finish coats, caulking was specified for all gaps greater than 6 mils. The finish coats were specified for application at 1.5–3 mils each coat. Each problem area would be addressed as follows. The areas of pack rust on the lateral members and tower caps were all in areas of intermittent water immersion, where acrylic latex does not perform well. A waterborne epoxy coat-ing was selected as the primer for these areas to be applied over an SSPC-SP 10, Near-White. After priming, the areas of section loss that would retain water were to be brought level with a two-component epoxy filler typically utilized for auto body repair. The entire coating system 4 was to be applied over the filler and epoxy primer. All rusting vertical members, including gusset plates, were to be abrasive blasted to an SSPC-SP 10 or power tool cleaned to an SSPC-SP 15, Commercial Grade Power Tool Cleaning, and spot primed. The remaining bridge members would be cleaned in a variety of ways. Power tool cleaning to SSPC-SP 3, was used for rusted areas, and hand tool cleaning to SSPC-SP 2 would be performed on existing coatings. Areas with the zinc system were cleaned by whatever removed it completely. For example, gusset plates were abrasive blasted and accessible flat areas were blown with an air wand and scraped. If any zinc system still remained, a special power blasting tool was then utilized. Crevices were cleaned with a needle gun. Immersion zones were abrasive blast cleaned and coated with the epoxy primer. Any exposed steel substrate was to be spot primed. In all cases, the entire coating system 4 was to be applied. Thus, spot priming would be followed by stripe coating and a full coat of PWB 145 red, a stripe coat and full prime coat of PWB 146, and the rest of coating system 4.

QC PlanWhile SSPC-PA 2 is the guiding document for measuring DFT, it works best for coatings applied over completely blast-cleaned steel (in the shop or field), with no existing coatings or rust. But measuring the DFT of the VTB overcoating sys-tem proved to be more challenging because of the multiple

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Fig. 3: Accepted SP 1 on web of diagonal member


existing coatings in various parts of the bridge. The existing systems could have caused measurements of the overcoat system to vary, possibly giving unreliable data. Destructive testing could yield accurate dry film measurements but it also would present coating integrity issues and was deemed inap-propriate for this project. Magnetic thickness gages, one of the gages allowed in PA 2, would be used for QC, but faced with a wide divergence of existing coating thicknesses, Caltrans decided to map the existing systems’ coating thickness and keep typical areas

as baselines. Areas abrasive blasted to an SSPC-SP 10 finish would serve as the new coating system’s baseline because the typical areas would yield only approximate new coating thick-nesses. The mapping was to be conducted after the pressure washing was completed (Fig. 3). Hold points were established for the environmental condi-tions, pressure washing, surface preparation, spot coats, stripe coats, and each individual full coat. DFT readings were then made in accordance with the mapped areas, with the distri-bution based on the different numbers of existing coating systems in various areas. Thus, given the wide disparity in baseline readings of existing coatings, the gage and spot readings were taken as closely

as possible to the requirements of PA 2. A daily bridge report form (Fig. 4) would be completed by the lead structural steel painter. The report form contained areas for recording environmental conditions, surface prepa-ration methods, coatings utilized, comments, unusual events, personnel, and equipment utilized.

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Fig. 4: Daily report form completed by lead structural steel painter

Fig. 5: Intercoat delamination, from MCU Zinc and lift oil

Fig. 6: Vacuum removal of debris entering from the finger joints


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Plan ExecutionThe work plan called for the upper and lower towers to be addressed in three increments each for a total of six work sections (WS). • WS1: The tower caps and top cross brace • WS2: the tower legs below the top cross brace and the mid-dle cross brace • WS3: the tower legs below the middle cross brace down to the cat walk • WS4: the tower legs from the roadway to below the catwalk and the cross member directly beneath the expansion joint • WS5 the tower legs from below the catwalk down to the low-er cross brace and the lower cross brace • WS6: the tower legs beneath the lower cross brace and the tower anchorage Work on ws1 went smoothly and was documented as con-forming in all areas. In WS2, however, unexpected difficulties were presented. During the pressure washing, areas around the earthquake retrofitted members had delaminated on both legs. Air wands were brought in, and the delamination was chased until all

remaining coatings were tightly adhered. In addition, five lev-els of the north leg had large areas of delamination on face J. These areas were also chased with air wands until only tightly adhered coating remained. The earthquake delamination was traced to another unspecified coating under the existing paint system. A zinc-rich moisture cure urethane (MCU) had been applied to the retrofit members, and wide areas of over-blast occurred on the existing coating system. All of the coating systems above the MCU delaminated, although the MCU was tightly adhered to the substrate. The MCU was roughened with 60 grit sandpaper and spot primed with PWB 145 (waterborne acrylic) before the application of coating system 4. The other coating delamination on face J was determined to be between coating systems 1 and 3, with some areas of intercoat delamination between the finish coats of coating system 4. The source of the delamination between coatings systems 1 and 3 was identified as poor surface preparation. Chalking on the surface of the topcoat from system 1 was exposed, and it readily wiped onto a dark, lint-free rag. Areas adjacent to the chalked area remained tightly adhered. The chalking was removed by power tool sanding with an 80 grit sandpaper, and, again, a prime coat of PWB 145 was applied. It was also necessary to feather edge the non-delaminated areas adjacent to the failed coating. The intercoat delamination between the topcoats was de-termined to be caused by deleterious material under the final finish coat. Lubricating oil, for example, from an air powered hoist, had been deposited on the first finish coat during the fi-nal finish coat application process, causing the coating failure (Fig. 5). The affected areas were re-washed and determined to be oil free by examination under black light. These areas were feather edged with power tools and spot primed. The divergence in coating thickness was extreme in the failed coating, making DFT readings even less reliable. Wet film readings were taken at representative areas during each overcoat to help ensure specified coating thicknesses were applied. Work in WS3 was unremarkable and determined to be con-forming. In WS4, which contained the largest areas to be blast cleaned, combined with the finger joints directly overhead, the work presented challenges in the elimination of deleterious materials. A perfect seal could not be obtained below the fin-ger joints because the wind from vehicular traffic moved the roof of the containment system constantly. Vacuuming imme-diately before application of the coatings and hand cleaning to remove debris deposited before cure was required before applying the subsequent coat (Fig. 6). Environmental challenges were also encountered as this work began in November. The overnight low was consistently less than 50 F, and it became necessary to heat the contained area to maintain a steel temperature above the specified

Fig. 7 (top): Accepted PWB 146 Pink primer applicationFig. 8 (above): Accepted PWB 169 Iridescent green final finish coat

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minimum. Whenever the barometric pressure in the outside environment dropped, more frequent environmental readings were required to ensure the internal conditions remained within the specified parameters. This challenge continued in all subsequent work stations. Work in WS5 revealed isolated areas of coating delamina-tion between coating systems 1 and 2, which were mostly confined to the gusset plate areas. Face J on the south leg required abrasive blasting to SP 6 to remove the unspecified coating. Work in WS6 had a continuation of the face j problem and was addressed as discussed above. Figures 7 and 8 (p. 52) show examples of conforming work.

ConclusionQuality Control is a daily process that combines observation, measurement, comparison, and documentation to ensure the successful delivery of a specified product. Overcoating existing coating systems can present unique challenges that multiply with the addition of each subsequent coating system. This particular project presented anticipated problems that were included in the work plan and surprises that required investigation and adjustment.

References1. whatis.techtarget.com/definition/0,,sid9_gci1127382,00.html.2. www.fhwa.dot.gov/publications/research/infrastructure/structures/bridge/overct.cfm.3. “The Vincent Thomas Bridge” San Pedro’s Golden Gate, San Pedro Bay Historical society 1988, p. 21.4. “The Vincent Thomas Bridge” San Pedro’s Golden Gate, San Pedro Bay Historical society 1988, p. 17.5. Specifications, www.dot.ca.gov/hq/esc/ttsb/chemical/specifications.htm.

William H. (Bill) Hansel has 37 years of experience in the steel coating industry, including offshore oil rigs, shipyards (maintenance and new construction), water treatment facilities, and refineries. Currently, he is a structural steel painter supervisor with Caltrans on the Vincent Thomas Bridge in the Port of Los Ange-

les, CA. Bill holds a BA from California State University. He is SSPC PCS, BCI-2, and C-3 certified.


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During the past 20 years, overcoating strategies in the maintenance painting of steel bridges have grown considerably in importance. The trend may be broadly attributed to the increased overall cost of the complete removal of lead-based

systems and the application of high-performance coating systems over completely renovated, properly prepared steel surfaces. The cost of replacing an old lead-based paint system with an inorganic zinc/epoxy urethane over a Near-White Blasted surface (SP-10) can range from $5 to $20/sq ft ($54 to $217/sq m). In 1970, the vinyl topcoats in similar systems cost approxi-mately $0.50 to $1/sq ft ($5.43 to $11/sq m). While working on one of the first structures to require containment in 1980, the author calculated painting costs for an SSPC-SP 10, three-coat, chlorinated rubber system to be about $2/sq ft ($22 sq m). The dramatically escalating costs of complete coating removal and replacement painting are not so much related to the cost of surface preparation as they are to the removal, handling, and disposal of old lead-based systems. Contain-ment of air-borne lead dust from blast cleaning, disposal of lead-based paint waste, and protection of workers and the public all figure prominently in the rising costs of bridge painting. It is hardly surprising that overcoating structures without removing existing lead-based systems has become so popular. Overcoating involves spot cleaning of rusted areas on the structure (usually employing power tool cleaning techniques), followed by repair of these areas and recoating the entire

By Clive H. HareCoating Systems Design Inc.

Preventing Overcoating Failures

Editor’s Note: This article appeared in JPCL in November 1997. The author presented an ear-lier version of this paper at the Fifth World Congress on Coating Systems for Bridges and Steel Structures, held February 3-5, 1997, in St. Louis, MO, and sponsored by the University of Missouri-Rolla.

This article will explain common failures on overcoating projects as well as describe some of the devices that mitigate failure.

surface with 1 or more finish coats. In most cases, the strate-gy has worked well, but such design carries its own baggage; there have been numerous failures of coating systems on overcoated bridges. Failures have been mostly of 2 types (Figs. 1a and 1b): recurring corrosion at spot-cleaned areas; and complete delaminations, often at the interface of the steel and original (lead-based) primer. Cohesive failures within the old system have also been encountered, again most often in the older coatings. Risks of both types of failures may be minimized if the nature of the problems is analyzed and the coating system (including coating materials, surface preparation, and applica-tion) is designed to mitigate these dangers. This article will explain common failures on overcoating projects as well as describe some of the devices that mitigate failure.

Aluminized Epoxy Mastic-Based OvercoatsVarious systems have been introduced for overcoating, in-cluding aluminized epoxy mastics, moisture-curing urethanes (often aluminized), calcium sulfonate wax systems, alkyd systems, and even latex paints. To understand why many over-coating products are aluminized, we must see what aluminum brings to protective coatings1:• high oxygen and moisture barrier properties; and • excellent ultra-violet (UV) and heat reflection abilities, com-bined with stress dissipation. The first group of properties is critical in preventing corrosion from recurring in spot-cleaned areas; the second group is


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critical for resisting the stress effects that cause delamina-tion. As aluminum pigmentations delay the evaporation of solvent systems, it is likely that they may also add to adhesion over pitted steel surfaces by prolonging the wet period during which penetration into the surface may occur.

Preventing Corrosion from Recurring over Spot Cleaning Anti-Corrosion MechanismFor the purist, the very fact that aluminized epoxy mastics can perform over rusted steel is somewhat surprising. Aluminized epoxy mastics are barrier coatings, and these coatings have been thought to rely on film impermeability to the transmis-sion of electrolytic solutions to the metal.2 Virtually all paint films will allow enough water through their continuum to sup-port the corrosion reaction, but electrolytic solutions, such as salt water, are another story. It is generally considered that barrier coatings work by resistance inhibition, acting like a fil-ter that screens out ionic material and allows only pure water through to the metal.

Pure water will not readily conduct an electric current. Therefore, any water that accesses the metal surface beneath the film will have such a high electrical resistance that little or no corrosion current will flow. Hence, corrosion is prevented. To maintain corrosion protection, the steel beneath the film must be free of electrolytic salts. If “filtered” water that passes through the film finds and dissolves corrosive salts (chlorides and sulfates, etc.) beneath the film, an active electrolyte will be established and will undermine all possibility of barrier protec-tion. High levels of electrolytic salts are often found within the rust deposits in power tool-cleaned areas protected in the standard overcoating process. But aluminum epoxy mastics have a good record as anti-corrosive coatings over these bad surfaces, so they probably do not work by resistance inhibi-tion mechanisms. We must therefore seek out more plausible mechanisms for the ability of these coatings to prevent the recurrence of corrosion. While many researchers still cite resistance inhibition as the major mechanism of barrier protection3, a growing number of researchers have established that thick films of protective coatings may also control corrosion by restricting the amount of oxygen that gets through to the interface.4 Thomas5, measuring the oxygen transmission rate through thick, aluminized epoxy films, found that unlike conventional films of normal film thickness, the rate of oxygen transmis-sion through the aluminized epoxy mastic was lower than the amount of oxygen required to fuel the corrosion reaction. Judging from this and other research, it would appear that these coatings may work by oxygen deprivation (Fig. 2). This conclusion is thought to be largely related to the flat shape of the aluminum pigment particle. In the film, the particles align themselves parallel to the surface and create a sort of metallic armor to greatly impede the passage of water and oxygen.1,5 It is not possible to starve the corrosion reaction of water. Therefore, it seems likely that starving the reaction of the nec-essary oxygen is indeed possible. In this case, it matters little that there is a conductive electrolyte beneath the film; if there

Stress-induced delamination at vulnerable sites in recoated leaded paint systems

Corrosion through new system in rusted areas from which leaded system had previously been removed

Fig. 1a - Common failure types in overcoated systems. Figures courtesy of author

Fig. 1b -Stress-induced delamination of newly-applied overcoat on aged lead-based paint. Photo courtesy of Lloyd Smith, Corrosion Control Consultants and Labs Inc.

Thick films of higher barrier coating reduce passage of some water and most oxygen, thus depriving cathode reaction of fuel

Without necessary oxygen, the presence or absence of chloride ions at interface has less relevance to the rate of corrosion

Fig. 2 - Corrosion control by barrier coatings (oxygen deprivation)


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is not enough oxygen, the corrosion reaction cannot proceed. Oxygen ingress is minimized as film thickness increases. A key to the performance of the aluminized epoxy mastic over rusted areas is the high degree of thixotropy and sag control that is deliberately built into these systems by the formulator. This allows films to be satisfactorily applied to a dry film thick-ness of 5 to 10 mils (125 to 250 micrometers) in 1 coat. The danger here is, of course, that high thixotropy may lead to too little flow and pinholing (particularly over rusted surfaces from which occluded air must be released). Therefore, a key param-eter of formulation is a good balance of the rheological force that ensures high build properties and the surface tension that ensures flow. The presence of aluminum platelets within the film reduces the possibilities of pinholes that extend com-pletely through the coating, although there must be sufficient flow to allow the occluded air bubbles to break and heal over.

Role of Adhesion in Preventing Recurring CorrosionIt is also taught that for coatings to work properly, they must adhere to a film substrate that is free of foreign matter.6 Certainly, if the coating cannot access true metal, chemical adhesion (either primary valency or secondary valency) is not possible. Rust is a foreign material and will effectively prevent chemical adhesion because it separates the coating and the substrate. Loose rust may be particularly problematic, for the coating has to wet and bind the rust into its continuum during application and drying. It has, however, long been known that coatings stick better to an irregular or porous surface than they do a smooth one. If loose rust is removed from the steel, the resultant corrosion product is soundly anchored and the surface is firm, although in some cases it is pitted and uneven. If paint is able to pene-trate deeply into the crevices of these pitted surfaces, it may achieve an adequate degree of mechanical (but not chemical) adhesion to the substrate.

Overcoating Design for Maximum AdhesionThis type of adhesion may be optimized by appropriately en-gineering the primer formulation. The trick is to coat the pitted steel with a low molecular weight, low viscosity, wet coating that stays wet for as long as practical, and progresses from the low viscosity, uncured state to the cured state relatively slowly. Those who remember the way red lead, linseed oil primers behaved on rusty steel surfaces will at once recognize what is needed. Raw oil was, in fact, well suited to this purpose. It was a relatively small molecule, an excellent low energy, high wetting, monomer of very low viscosity, which took a long time to progress from the wet state to a dry film. Those familiar with the application of raw oil-based coatings in the recoating of old bridges will readily recall how the old binder wicked away

from the body of the coating into the rust or chalk of an older film. Pigment particles from the overcoat materials are too large to squeeze into the crevices between rust on corroded steel or chalk particles on the old finishes. Therefore, the binder wicks away from the pigment and into the substrate. This par-titioning of pigment and binder is a characteristic of coatings displaying good mechanical adhesion, and only occurs in slow drying, low viscosity, high wetting systems. These charac-teristics pertain as much to some well-designed epoxies, how-ever, as they do to traditional oil primers. Good overcoating design serves to emulate these characteristics of oil paints in the epoxy overcoats. Thus, neither the rust nor the chalk needs to be removed for good adhesion if the binder wets the steel well enough to penetrate and surround the chalk or corroded areas. In accessing the crevices of the steel surface, therefore, it is desirable for the binder solution of these coatings to partition from the pigmentation and soak into the crevices of the corroded steel. In short, the coating must lie against the rusted surface in a wet state for as long as possible. The lower the molecular weight and viscosity of the paint, and the slower its conversion from liquid paint to solid film, the more efficient will such partitioning and wicking effects be. Small molecules of low molecular weight cannot only squeeze into small spaces easier, but they are also more mo-bile and have greater energy to do so. The longer this mobility lasts, the better the opportunity for good mechanical adhe-sion to marginal surfaces. Binders having low viscosity, low conversion rates, and lots of slow evaporating solvents are desired. Low viscosity binders may be engineered from thermoset-ting systems (which build up molecular weight after appli-cation). Low molecular weight epoxies are ideal candidates. Rather than traditional medium molecular weight polymers (epoxies and epoxy curing agents, which are large, more bulky, oligomeric materials), smaller molecules should be sought. The molecular weight of the curing agent should not be too low, however, and its functionality should not be too high, for this will lead to excessively stressed films of high cross-link density. To prevent excessive and unwanted cross-link density in a system, curing agents such as polyamides and amidoam-ines, which themselves have low-energy, high-wetting proper-ties, will also maximize penetration of corroded surfaces.

Role of Aluminum Pigments in AdhesionAs mentioned above, increased adhesion may be obtained by reducing viscosity through large amounts of solvent. Such formulations, however, do not equate well with present trends in environmental regulations, and it is necessary to keep the film wet by other techniques. Aluminum also may play a role in achieving this goal. The overlapping lamella plates of


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aluminum delay the migration of solvent from the film. There-fore, they assist the wetting process by keeping the available solvent against the substrate for a longer time. The downside of this, of course, is that all of the solvent may have not left the film by the time the cure of the film approaches the glass transition temperature (Tg). This is the temperature at which the polymer changes from a rubbery state to a glass-like or brittle material. This reaction may result in some residual solvent entrap-ment. However, solvent entrapment does not have the conse-quences in bridge painting that it has in interior tank painting. Different applications have different design requirements and the need for different formulating strategies. The use of new exempt solvents (non-volatile organic compound (VOC), non-hazardous air pollutant (HAPs) solvents such as parachlo-robenzotrifluoride) may also be of value in enhancing wetness without building VOC, although this solvent is expensive.

Stresses and Delamination FailuresOrigins of Stress-Induced DelaminationIn addition to failures related to the recurrence of corrosion on spot-cleaned areas, the second type of failure that has haunted overcoating systems is stress-induced delamination that occurs in service. The entire overcoated composite, in-cluding epoxy and the old lead-based system, is delaminated. The germ of this failure is found not in the epoxy itself, but in the condition of the existing system after preparation for the epoxy application has been completed. Existing systems are most often made up of multiple layers of old oxidizing systems, usually oils or oil-alkyd combinations, although phenolic varnish-based paints and other modified systems have also been used. The nature of the individual films is less important than their mechanical properties. When originally applied, these oxidizing systems were slow drying, soft, flexible films of low modulus with high elongation at

break properties and virtually no internal strain. They were thought to be highly suitable for marginal, non-blast-cleaned surfaces, which were common in the 1950s and 1960s when many of the bridges were originally built and painted. Mechan-ical failure in these newly-applied systems was very rare, even when over mill scale. Unfortunately, such oxidizing systems are not mechanically stable. Over the years, in the presence of oxygen and UV light, they tend to undergo a variety of changes in structure (Fig. 3). Generally, these are oxidative changes leading to increased cross-linking, but other effects may take place. These chang-es include chain schism, hydrolysis and depolymerization, or even the volatilization of low molecular weight breakdown res-idues. The increasing cross-link density is most critical. The net effect of these changes is the gradual increase in modu-lus, a reduction of elongation at break properties, and a slow but highly significant build-up in internal strain. Mechanical viability of any system depends upon the maintenance of the condition where the adhesive strength across the interfaces and the cohesive strength within the layer is always greater than the cumulative stresses that act upon it. When total stress within the system is greater than the ad-hesive or cohesive strength in any element of the composite, the system must either deform or break at its most vulnerable site. Good deformation (i.e., high elongation at break) proper-ties are not the strong suit of aged, oxidizing paint systems, and brittle failure under stress (either adhesion loss or crack-ing) is their usual response. In most practical overcoats, the site at which brittle failure will occur is usually the interface between the first coat of oil primer (the oldest film) and the steel. The condition of this interface is one of the most critical aspects in overcoating. In a study done several years ago, the author found that when this interface had been blast cleaned, delamination after overcoating was rare, but when the inter-face was smooth (e.g., mill scale), a failure was likely.6 Bridges built before abrasive blasting became a standard practice (say the late 1960s) are therefore more vulnerable to stress-induced delamination than bridges built afterwards. It should be borne in mind that failures may also occur because of adhesive and cohesive insufficiencies elsewhere within the composite, and failures at these sites are less dependent on the original surface preparation. Strain development within a composite is a result of ap-plied stress, which may arise from any one or more of various sources. However, stresses from different sources are cumu-lative.7 Tensile stresses from different sources (internal stress from film formation and aging, hygrothermal stress from cool-ing or drying out) are all additive and increase levels of strain within the composite. Compressive stresses such as occur when the temperature of a film is raised are also additive, but are subtractive to tensile stresses. This effect tends to reduce the total strain within the composite, at least temporarily.

AGED OIL PAINT SYSTEMHigh modulusLow elongation at breakHigh internal strainVulnerable to brittle failureWill crack or delaminate instead of deforming

NEW OIL PAINT SYSTEMLow modulusHigh elongation at breakZero internal strainResistant to brittle failureWill deform before cracking or peeling

Fig. 3: Age-induced changes in mechanical response to stress of oil paint systems


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Unless film temperature exceeds the Tg (unlikely in these high Tg systems), it is doubtful that an increase in compressive stress will produce a permanent relaxation in internal strain. Continued cycling of a paint system through periods of com-pressive and tensile stress is more likely to fatigue the system eventually. Long-term cross-linking of oil paints produces high levels of tensile strain within the films, therefore weakening the system and limiting its capability to accommodate additional stresses that may be applied to the system during service. Unfortunately, the overcoating process and the presence of overcoats are major sources of additional applied stress on the already highly strained old coating system. The additional stresses arise from various sources, and in adhesive, coherent systems, they may be transferred from one coat to another throughout the composite. Swelling of the film from the absorption of solvents from the overcoating film is minimal in an old oil system, as is swelling from the absorption of water. The old coatings are very hard, and absorption of water and solvents is probably minimal. More important, these stresses, when they do affect the film, result in expansion of the film and compression of the mole-cules while the expanding film is still adherent. Shrinkage stresses arising from the drying and curing of the epoxy are of greater concern than the effects of water and solvents from the recoat. These are internal tensile stress-es that are transferred throughout the existing composite, increasing the level of total strain within the composite. Shrinkage stress further compromises the system’s ability to withstand additional stress. Within 7 to 10 days of application, epoxy cure is largely complete, and internal stress accretion (from application and curing) levels off. In fact, a slow stress relaxation process may begin. Further stresses, however, may originate in the form of ex-ternal stresses, such as impact or vibration. By far, the largest external stresses are hygrothermal stresses derived from changes in humidity and temperature. Tensile stresses from the cooling or desorption of water or desorption of entrapped solvent are the most problematic. Increasing temperature and the absorption of water from the air by the newly applied composite (producing compressive stresses) also occur, but are less of a problem. The tensile stresses are additive to the internal stress already applied, and compound the total strain within the coating. The applied stress may be particularly catastrophic when it is applied suddenly. This is why we get a spate of failures after sharp drops in temperature during winter. As noted earlier, brittle failure will occur at the weakest site within the composite. In old lead-based paint systems applied over steel that was not originally blast cleaned, such as those found on bridges built before 1970, this failure often occurs at the interface of the metal and original primer. Old oil paints are

badly affected by stress at this stage, for their high modulus and low elongation properties allow no deformation. When originally applied, oil coatings have ideal mechanical proper-ties for adhering to these smooth surfaces, but after aging they are very brittle and highly subject to tensile failure. Hence, the most frequent site of stress-induced overcoating failure is the interface of the old lead-based primer and the smooth, mill scale-bearing steel substrate. As with adhesion, the stress accretion can be minimized, thus reducing the burden of an overcoat on existing, aged coatings. The reduc-tion of stress can be addressed at the coating formulation or coating system design stages.

Stress/Strain Mitigation Techniques in FormulationIn formulation (Fig. 4), the major emphasis is in resin system design. Here, the emphasis should be to reduce cross-link density and Tg (exactly the opposite to the requirements for chemical resistance, tank linings, etc.). The result is a flexible system that incurs less internal stress during curing and more readily allows strain relaxation. Thus, curing agents should be selected from the low functionality polyamides, amidoam-ines, imidazolines, and polyglycol diamines, rather than from aliphatic amines, arylyldiamines, cycloaliphatic amines, and multi-functional amines. A greater molecular distance be-tween reactive groups on the curing agent yields a final film that is more flexible and stress dissipative. However, as mole-cules become larger, their viscosity increases. Highly reactive or functional cross-linking agents are not wanted because they increase the brittleness of the film. Flexibilizers such as the aromatic hydrocarbon resins, plasticizers, and reactive chain stopping monofunctional modifiers are all valuable ad-ditions to the formulation, because they reduce and decrease cure rates. These factors also improve adhesion but are not

1.) Maintain Cross-link Density and Tg as Low as Practical. a.) Low functionality resins b.) Slow curing agents with long, flexible chains between reactive sites (polyamides, amido-amines, polyglycol diamines; not alkylene amines, cycloaliphatic amines, etc.) c.) Mono-functional chain-stopping diluents d.) Non-reactive flexibilizers—plasticizers, aromatic hydrocarbon resins e.) High OH:NCO ratio and low functionality in urethanes

2.) Use Aging Stable Systems. a.) No oxidative systems b.) UV-stable systems c.) Reflect UV with aluminum pigmentations

3.) Use Stress-dissipative Pigmentations. a.) Flat platey pigments (aluminum flake, talc, mica, chloride, micaceous iron oxide)

4.) Formulate at PVCs Well Below the CPVC.

5.) Minimize the Moisture Imbibition of the System.

Formulation Design for Minimal Stress Accretion

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suitable for low temperature application. These systems also give high elongation at break levels and do not generate high levels of internal stress during cure as part of the aging pro-cess. High OH/NCO ratios are preferred for aliphatic polyure-thane finish coats, and these should be well laced with both UV absorbers and hindered amine light stabilizers. Aluminum flake and other flat, platey pigments are again beneficial, because they tend to dissipate stress.8 The ratio of pigment volume concentration to critical pigment volume concentration (PVC/CPVC) also plays a role. The general consensus is that internal stress increases to a point near the CPVC. This increase is much lower when aluminum (or other platey pigmentations) are employed. The use of platey pigments, particularly aluminum, in the finish coat will reduce internal strain by other effects. Not only is aluminum stress dissipative, but it also effectively reflects UV and heat. Therefore, it minimizes both stress cycling and the UV-induced changes in the finish. Such changes may lead to long-term stress build-up, here analogous to the oxidative changes in the lead-based paint system that led to aging-re-lated stress build-up in the first place. Finally, the barrier effects of platey pigments will also play a part in stress dissipation, because these properties reduce moisture penetration into the film. Therefore, the amplitude of hygroscopic cycling, which can also weaken the system, is reduced.

Poor Design Practice: All three coats are applied over existing film, maximizing the stress on the coating system.

Good Design Practice: Thin coat of aluminum system provides maximum resistance to UV light, maximum resistance to penetration of water and oxygen through the film, and maximum stress relief on vulnerable areas.

Poor Design Practice: All three coats are applied over existing film, maximizing the stress on the coating system.

Fig. 5 - Inappropriate system design for overcoating

Fig. 6 - Optimal system design for overcoating.

Stress/Strain Mitigation Techniques in System DesignAlthough internal stress can be minimized through coating system design, it is at this stage when error often occurs. For years, experts have told coating design engineers that addi-tional film thickness brings improved performance. This prin-ciple is true in those areas of the system where the overcoats are applied over bare steel. Unfortunately, internal stress in thermosetting systems (such as epoxies and well-cured oil films) increases with film thickness, and, consequently, with an increasing number of coats (Fig. 5). Thus, to minimize the amount of stress on the existing lead-based and oil-based coating system, the amount of paint applied over the existing lead-based paint must be minimized (Fig. 6). In areas where the existing coating is intact and providing sufficient protection, a lot of extra paint thickness for corro-sion protection is unnecessary. High builds of epoxy or ure-thanes over the old film can, however, wreak havoc in terms of stress accretion.

ConclusionThe keys to successful overcoating can be found in both sys-tem design and in coating formulation. Following are addition-al suggestions for coating formulation.• Specify high wetting, low viscosity, low molecular weight primers (i.e., thermosetting resins) over degraded steel. Pref-erably, the primers will contain large amounts of solvent, stay wet against the steel, and slowly convert to solid films.• Select only polymers that build up molecular weight after application. Do not use lacquers (thermoplastic resins). They do not allow good penetration into and adhesion on poor steel surfaces.• Whenever possible, use flat, platey pigments (optimally alu-minum flake) in both the primer and finish coats. If aluminum color is not acceptable as a finish, the use of other platey pig-ments, such as micaceous iron oxide, talc, chlorite, and mica, is possible, although performance may be compromised.• Use flexible, long-chained, low functionality curing agents (polyamides, etc., not polyamines).• Tg values should be low, and solvent evaporation rates should be low. • As finish coats, use resin systems that provide better dissi-pation of internal stress. System design factors that are key to successful overcoat-ing include the following.• Wherever possible, limit overcoating to bridges that were blast cleaned before the application of lead-based paint (i.e., bridges built after 1970).• Prepare the bare steel areas as well as possible, preferably with scarifying tools.• Apply spot primers in 2 or more coats to a film thickness of 10 mils (250 micrometers) total, but only over the corroded areas.


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• Minimize the number of coats applied to areas of the sub-strate still bearing intact lead-based paint.• Note that overcoating will be less likely to succeed where the system is subject to severe fluctuations in temperature and humidity (especially sharp falls in temperature).• Do not use the overcoating approach where there is clear evidence of existing widespread adhesive or cohesive defi-ciencies in the lead-based paint system.References1. C.H. Hare, “The Aesthetic and Engineering Attributes of Aluminum Pigmentation” American Paint Journal (October 12, 1992, and October 19, 1992), 78 and 36.2. J.E.O. Mayne, “Paints for the Protection of Steel— A Review of Research into Their Modes of Action” British Corrosion Journal (May 1970), 106.3. B.W. Cherry, Australias Corr. and Eng. (October 1974), 23.4. H. Haagen and W. Funke, “Prediction of the Corrosion

Protective Properties of Paint Films by Permeability Data” JOCCA (October 1975), 359.5. N.L. Thomas, “Coatings for Rusting Steel: Where are We Now?” JOCCA (March, 1991), 83. 6. C.H. Hare, “Aluminum Based Paints for Lead Encapsulation,” Presentation given at Washington Paint Technical Group Thirty-Third Annual Symposium, April 13-14, 1993, Tysons Corner, VA.7. D. Perera, “Stress Phenomena in Organic Coatings” Chapter 49 in Paint and Coatings Testing Manual, Fourteenth Edition, ed. J.V. Koleske (Philadelphia, PA: ASTM, 1995), p. 585.8. K. Sato, “The Internal Stress of Coating Films” Progress in Organic Coatings (Vol. 8, 1980), 143.

About the AuthorClive H. Hare is a JPCL Contributing Editor and the author of the monthly column Trouble with Paint.


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Overcoating the Caruthersville BridgeHow the Owners, Inspector, and Contractor Met the Challengesfrom System Selection to Application and Protection

By Ernst Toussaint, EIT;Ross Sherwood, EIT,Tennesee Department of Transportation; and Scott Stotlemeyer, PE, Missouri Department of Transportation

Editor’s Note: This arti-cle appeared in JPCL in May 2010 and is based on a paper presented at PACE 2010, the joint conference of SSPC: The Society for Protec-tive Coatings and the Painting and Decorating Contractors of Ameri-ca, held February 7–10 in Phoenix, AZ. At the time of the presenta-tion, Toussaint was with Greenman-Pedersen.

Fig. 1: Caruthersville Bridge–original coating system. Photos courtesy of the authors

Several thousand state bridges are scheduled for maintenance painting in both Tennessee and Missouri. On the Caruthersville Bridge (I-155 Mississippi River Bridge), the northernmost bridge over the Lower Mississippi River, overcoating

with a calcium sulfonate alkyd is an ongoing project. Although overcoating increases the risk of coating incompatibility (stress, premature failure, etc.), use of a calcium sulfonate sys-tem on this unique asymmetrical structure was a viable option that the Tennessee Department of Transportation (TDOT) spearheaded and the Missouri Department of Transportation (MoDOT) supported. This option provided both DOTs with an initial low cost and extended the coating life of the bridge, thus extending corrosion protection. Every type of generic coating system has its require-ments for surface preparation and application as well as its advantages and disadvantages in terms of performance and life-expectancy. Meeting the application and inspection challenges of a calcium sulfonate on this project has provid-

ed invaluable experience and knowledge of this system. The project-specific challenges have been evaluated, researched and successfully solved to the satisfaction of the respective DOTs. This article discusses the requirements of a calcium sulfonate system as learned from its use on the Caruthersville Bridge. The interaction among the DOTs, inspection firm and painting contractor is presented to assist in future design and construction contracts that specify a calcium sulfonate alkyd system.

BackgroundThe Caruthersville Bridge is a steel through-truss deck bridge that spans the lower Mississippi River from the outskirts of Dyersburg, Tennessee to Caruthersville, Missouri. Construc-tion of the bridge began in 1971 and was completed, including coating application, in 1976. The four-lane bridge is 78 feet wide and is approximately 7,102 feet long. The total length of the bridge consists of 1,030 feet of Missouri approach spans; a 2,150-foot main structure; 920-foot and 520-foot main spans; and 2,480 feet of Tennessee approach spans. The bridge has approximately 1,324,000 square feet of painted structural steel. The original coating system remained largely intact for over 30 years. The bridge is not symmetrical. The Tennessee side struc-ture is much longer than the Missouri side structure due to the natural curve in the river. Rip-rap (rock or other material) was placed on the outside bank to keep it from eroding further and bypassing the bridge. The bridge consists of inspection ladders, a catwalk under-


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neath the bridge, and 12 steel spans flanked by pre-stressed concrete approach spans. The structure is supported by reinforced concrete piers and abutments that rest about 6 miles from the New Madrid Fault. A 1993/1994 study found that the bedrock was 2,700 feet under the ground surface. The bridge also has two truss spans, eight plate girder spans, and two multi-girder spans. The bridge deck is approximately 99 feet above the Mississippi River. It is understood that the structure was abrasive blast cleaned before application of the original coating system. The Caruthersville Bridge had an original coating system consisting of a three- and sometimes four-coat alkyd system. A shop-applied alkyd primer was ap-plied to all of the structural steel. An additional two coats were applied in the field to floor beams and lateral braces, while an additional three coats were applied to members at the bridge deck level. Figures 1 and 2 depict the original coatings of the bridge.

Investigation of Existing Coating Condition and ContentAn initial field investigation of both approaches of the bridge revealed coating breakdown and minor surface corrosion on approximately 3% of the painted surfaces. Pinpoint corrosion

was also found on the underside of the bottom flanges of lateral bracing as well as the underside of the girder bottom flanges (Fig. 3). Corrosion along the edges of box beams and minor areas of coating breakdown were found on edges of the supporting members of the inspection walkway (Fig. 4). The webs, stiffeners, and top flanges of both girders and floor beams exhibited very little corrosion. Chalking was evident throughout the structure. Coating adhesion was assessed in accordance to ASTM D 3359, “Adhesion by Tape Test,” using Method A (X-Cut), and Method B (Cross-Cut). No adhesion results were below 3A and 3B, indicating that the majority of the original coating system still adhered well to the bridge. The original coating system along the floor beams and lateral bracing is a three-coat alkyd, with an orange primer and intermediate coat and a green top-coat. Based on the original report and the condition survey, its average dry film thickness ranges from 3.6 mils to 17.4 mils, with the low millage on the stiffeners. Thus, the initial investigation revealed that the majority of the painted structural steel is in excellent condition after more than 30 years of service, with some isolated areas of coating breakdown. The coating is in the beginning life cycle segment of coating degradation. Considering that there is approxi-mately 1% to 3% of coating breakdown of the total surface area of the steel, both DOTs have agreed that this is the opti-mal time to perform an over-coat maintenance painting repair to extend the life of the existing coatings for many years, thus reducing the bridge’s total life cycle maintenance costs. Coating samples were removed and sent to an independent laboratory to test for heavy metals. The laboratory test re-vealed that part of the alkyd system, presumably the red lead primer, has heavy metal content of 16% lead, 0.2% chromium, and less than 0.01% cadmium. Although the primer was not

Fig. 2: General side view of Caruthersville Bridge.

Fig. 3: Corrosion on bottom flange

Fig. 4: Edge corrosion


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expected to be disturbed during cleaning and preparation, appropriate worker and environmental protection measures were taken in case of accidental disturbance of any heavy metals.1

Containment In accordance with the Tennessee Department of Transporta-tion Standard Specifications for Road and Bridge Construc-tion, 2006 edition, Section 603.13 b-“Containment,”2 before beginning surface preparation, the contractor submitted drawings of the containment system to the engineer for review. The contractor also obtained all permits required for the project. The additional requirements for containment were given in the project site plans. To protect the traveling pub-lic and the environment from loose paint chips, wastewater, solvent, coating drips and overspray, the containment was designed in accordance with SSPC Technology Guide No. 6, “Guide for Containing Surface Debris Generated During Paint Removal Operations.”3

The containment system was thus designed to contain debris generated from SSPC-SP 3, “Power Tool Cleaning,”3 and SSPC-SP 12, “Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating.”3 That is, the framework was constructed with a chain link support system and a tarp impermeable to air and water. Following the design of an SSPC Class 2P containment,3 the system included these compo-nents: A2-Flexible, B1-Air Impermeable, Support Structure C2 –Flexible, D1-Full Seal Joints, Entryway E4-Open Seam, Air Make Up F2-Open and Input Air Flow G2 Natural. The containment system also followed the design of a Class 2W containment, with these components: A2-Flexible, B3a-Water Impermeable, Support Structure C2 –Flexible, D1-Full Seal Joints, Entryway E3-Overlap, Air Make Up F2-Open, and Input Air Flow G2 Natural (Figs. 5 and 6).

Protecting Workers from Exposure to Heavy MetalsIn accordance with 29 CFR 1926.62, “Lead,” the contractor and inspection personnel took precautions to prevent over-exposure to lead dust in case it was accidentally generated from power tool cleaning methods. SSPC-SP 12,“Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating,” was also specified to remove loose coating, dirt, and debris from the surface of the steel; it was determined that the SP 12 method posed no significant risk of elevated lead exposures, as described by OSHA in the preamble to the lead in construction standard. Before beginning cleaning operations, workers had their blood samples tested by an independent laboratory for lead and zinc protoporphyrin (ZPP). All of the workers used proper personal protective equipments (PPE) to ensure a minimal risk to lead. The workers used half mask respirators with high efficiency particulate air (HEPA) filters with appropriate car-tridges, gloves, goggles with side shield protectors, protective suits, steel toe boots and ear plugs to reduce the levels of noise below 90 decibels. The power tools were equipped with HEPA filters to collect debris generated from removing loose coating on the surface of the steel bridge. Wash and change stations were positioned inside of the contractor’s regulated area to promote worker hygiene and to keep all contaminat-ed articles of clothing at the jobsite for proper removal and cleaning.

Coating Selection and Inspection Concerns Overcoating maintenance painting and repair with 1% to 3% breakdown of the original system to extend its service life was expected to save the DOTs well over $15 million over the bridge’s total life cycle maintenance costs. When a coating starts to deteriorate, it tends to break down exponentially rather than linearly (Fig. 7), so the DOTs considered overcoat-ing optimal at the 1% to 3% point of breakdown. While overcoating is a viable alternative to a costly complete

Fig. 5: Containment chain link system

Fig. 6: Containment with tarpaulin


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removal and recoating of the bridge, both DOTs understood that overcoating presents challenges. Alkyds are susceptible to attack from coatings blended with a strong solvent or with a high solvent content. The best way to minimize the stress on the ex-isting coating was to overcoat the existing structure with another alkyd, and the DOTs selected a two-coat calcium sulfonate alkyd system, with the primer and topcoat differing only in tint base (red primer and beige topcoat). Prior to surface preparation of the areas showing some break-down, the entire structure was cleaned in accordance with SSPC-SP 12, “Low Pressure Water Cleaning,” with maximum pressures of 3,000 psi. Areas with loose coatings and rust were power tool cleaned in accordance with SSPC-SP 3, “Power Tool Cleaning.” There was no bare steel after surface preparation. A penetrating sealer was applied to areas of back-to-back angles. Bolts and crevices were stripe coated, followed by two coats of the alkyd on the entire structure, which exhibited at least a 3A adhesion rating. There are certain advantages to a calcium sulfonate alkyd, in-cluding easy mixing (single package), easy application, available with relatively low volatile organic compound (VOC) content, low

cost, relatively good corrosion resistance, and tolerant of many surface conditions as described in SSPC and NACE standards. This coating is typically used in mild service environments. However, because alkyd systems are flexible, dry slowly, and cure by reaction with oxygen from the air (oxidation), they pres-ent several inspection concerns: foreign particles can be embed-ded into the soft surface and the soft surface can accidentally be damaged during quality assurance measures. And unless the (relatively slow) curing mechanism is modified to prevent residual tackiness, the mechanism allows the ambient (humid-ity, temperature, etc.) conditions to play a major role during the curing process. In accordance with ASTM D3363, “Pencil Hardness Scale,” which determines the hardness of a coating on a scale from 6B to 6H, soft to hard respectively, the alkyd selected is a 5B, indicat-ing that the system is soft. This is a point of concern because if a coating is soft and slow drying, its abrasion resistance is poor. Bleeding or color migration can occur if the prime coat’s color drastically contrasts that of the topcoat once the topcoat has been compromised by abrasion. A bleeding effect can also occur in isolated areas of the bridge that do not have an ade-quate amount of sunlight. In such areas, at high pressures (3,600 psi), the airless spray gun partially impregnates the finish into the soft, tacky red primer and allows the primer to slowly show through the topcoat over several days. In these areas, it is under-stood that the red primer, which is still tacky, has interface layers that show through the topcoat even with adequate topcoat thickness of 4 to 6 mils, for example. This bleeding phenomenon occurred on the project (Fig. 8). Color migration and bleeding of the primer also occurred due to an undercured primer and low mils around the nuts and bolts of the structure while striping between coats (Fig. 9). The soft film also allows foreign particles (e.g., dirt) to become imbedded onto the surface and to a certain degree into the coating before it dries to touch (Fig. 10). Industry experience has shown that

Fig. 7: Corrosion curve

Fig. 8: Bleeding and color migration of the primer along the entire bottom flange.

Fig. 9: Bleeding and color migration of the primer through the topcoat during striping of the topcoat.


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this coating can remain soft for several months at a time. The amount of dirt that imbeds into the coating before its full cure can greatly affect its color and gloss retention. Although the surface contaminants affect the coating’s aesthetic appearance, its corrosion protection properties are not compromised and the expected life of the system should be achieved. To eliminate the bleeding effect from striping the nuts and bolts, the coating manufacturer recommended that the striping be performed with the topcoat instead of the red prime coat. The coating manufacturer also recommended that the contractor stop applying the red prime coat (over the existing coating) ap-proximately two feet from all striped areas to prevent overspray to those areas. The contractor was then to apply a final (beige) topcoat to all the areas to provide adequate coverage and film build and avoid color migration. Replacing the red primer with the beige topcoat during striping procedures would eliminate migration of the red to the beige coat, thus providing a uniform appearance to the bridge. The soft, flexible, surface tolerant calcium sulfonate allows a steel substrate to contract and expand without causing the coat-ing to crack. However, due to the slow curing and drying nature of this alkyd system, measuring the film thickness with a dry film thickness gauge proved to be unreliable. Therefore, the inspec-tion staff monitored the average film thickness with a wet film thickness notch gage. The film thickness of the final coat was also calculated daily with a notch gage, supported with square footage calculations and deductions of approximately 20% for overspray and waste.

Project Completion The Caruthersville Bridge Project was scheduled to be com-pleted by the end of painting season 2010. The first phase in painting season 2009 consisted of preparing the surfaces of the Tennessee and Missouri approaches to be primed and topcoat-

ed with the calcium sulfonate system. The second phase of the project was scheduled to begin early 2010 and to be completed by November 2010. At the time of this writing, the project is on schedule, with SSPC-QP 1- and QP 2-certified VHP Enterprises performing the surface preparation and coating work.

References 1. Tennessee Department of Transportation: “Coating Condition Evaluation of the Interstate 155 Bridge Over the Mississippi River.”2. Tennessee Department of Transportation Standard Specification for Road and Bridge Construction, March 1, 2006. 3. SSPC-SP 3, “Power Tool Cleaning,” and SSPC-SP 12, “Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating”; SSPC Technology Guide No. 6, “Guide for Containing Surface Debris Generated During Paint Removal Operations,” Pittsburgh, PA: SSPC: The Society for Protective Coatings.

About the AuthorsErnst Toussaint, EIT, has managed bridge coating projects, including several for the Florida and Tennessee departments of transportation. Currently self-employed as a consultant to the industry, he previ-ously worked for Greenman-Pedersen, Inc. and KTA-Tator, Inc. He also has worked as a chemical engineer for manufacturing

industrial process facility components. He is a member of SSPC: The Society for Protective Coatings. He holds a B.S. in Chemical Engineering.

Steven Ross Sherwood, Jr., EIT, has been with the Tennessee Department of Trans-portation for 11 years, working in the Con-struction Division. Current