The Minnesota Department of Transportation (MnDOT) conducted a one-day seminar on bridge maintenance painting strategy and project design in May of 2013. One outcome of that seminar was the realization that MnDOT needed a more uniform method to rate the condition of coatings statewide during biennial bridge safety (in conjunction with the new AASHTO Manual for Bridge Inspection) as well as a process to select and prioritize maintenance painting strategies. MnDOT assembled a Technical Advisory Panel to address the needs identified during the seminar and launched a multi-objective study in October of 2013. This article describes the outcomes of efforts undertaken and MnDOT’s novel approach to improving statewide bridge maintenance painting operations.
OBJECTIVE 1: CONDUCTING A TRANSPORTATION RESEARCH SYNTHESIS
The first objective of the study was to conduct a Transportation Research Synthesis (TRS) of representative transportation agencies’ policies, guidance and manuals related to best practices for bridge maintenance painting operations. Maintenance painting may be performed by agency-employed personnel or by contracting for these services. The TRS provided a vehicle by which to determine other agencies’ operations and their effectiveness. A survey questionnaire was prepared and distributed to 52 transportation agencies to determine common practices they used for maintenance painting of steel bridges. Survey questions were developed for five topic areas including: 1) Coating Condition Assessments; 2) Bridge Coating Maintenance Strategies; 3) Surface Preparation Methods; 4) Coating Systems; and 5) Use of In-House Painting Forces versus Contractors.
The survey collected information from 42 agencies, an 81-percent response rate. The results of each topic area were presented in TRS 1404, published on mndot.gov/research.
OBJECTIVE 2: IDENTIFYING BEST PRACTICES
The second objective of the study was to identify the best practices appropriate for MnDOT bridge maintenance crews from the results of the survey in order to develop a more robust state-wide bridge maintenance painting program.
Best Practices for Conducting Coating Condition Assessments
Because the condition of an existing coating system drives the selection of the appropriate maintenance painting strategy, an accurate assessment of that coating condition is paramount. While SSPC-VIS 2 (Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces) and ASTM D610 (Standard Test Method for Evaluating Degree of Rusting on Painted Steel Surfaces) can be used to rate the type and quantity of rusting present on a bridge structure, the amount and distribution of corrosion across multiple bridge elements can vary, is often not uniform, and can include coating deterioration in addition to corrosion. The preparation of a custom photographic guide containing actual images of bridge elements in various conditions is integral to consistently communicating the condition of the existing structure, selecting a maintenance painting strategy and prioritizing the work. While creating such a guide sounds rather straightforward, considerable planning was required to obtain the correct images.
As recommended by the study, MnDOT developed a standard coating assessment guide containing digital images of representative steel bridge elements depicting the four condition state categories (good, fair, poor and severe) for the new AASHTO bridge management element (BME) 515 Steel Protective Coating. The images contained in the guide include steel elements such as beam ends and bearings, cast-in-place pilings, steel elements beneath deck expansions, fascia beams, interior girders, hinge joints, pin and hanger assemblies, truss members, weathering steel and duplex coated railing. This approach was chosen to improve the consistency of the visual coating condition assessment data gathered across the state. The use of more detailed coating condition assessments may be warranted to establish condition thresholds for which maintenance painting strategies are appropriate, and determining risk of premature coating failure. Further, it aids in priority planning in relation to condition options and establishing bridge maintenance painting priorities.
Best Practices for Selecting Bridge Maintenance Painting Strategies
There are several strategies available for performing maintenance painting on steel bridges, such as spot touch-up, spot touch-up and overcoating, zone painting, and removal and replacement of the existing coating system. Apart from full removal and replacement, these maintenance painting strategies can serve to extend the service life of existing coating systems, postpone major painting projects and address aesthetic issues separate from corrosion. The condition of the existing coating system and underlying steel primarily drives the selection of an appropriate maintenance painting strategy.
Spot touch-up and overcoating can be a cost-effective maintenance strategy to prolong the life of the existing coating system. In this manner, the funding required to perform removal and replacement can be carefully budgeted and planned five-to-seven years ahead. However, the existing coating condition as well as the condition of the substrate beneath must be carefully assessed to reduce the risk of failure. The existing coating must also be analyzed (or historical records accessed) to determine the generic coating type for compatibility with the overcoat system. An Approved Products List (APL) for overcoat systems must be established, complete with surface preparation requirements and surface soluble-salt testing when an overcoating strategy is considered. Spot touch-up and overcoating could then be performed by contract or by using an in-house workforce. If the amount of coating deterioration is 10-to-15 percent of the total coated area, then removal and replacement of the coating system is a preferred strategy, as the amount of spot touch-up will likely not be economical, as per SSPC-PA Guide 5, “Guide to Maintenance Coating of Steel Structures in Atmospheric Service.”
Best Practices for Selecting Surface Preparation Methods
Best practices with regard to surface preparation are based on the maintenance coating strategy selected. The active strategies include spot touch-up, spot touch-up and overcoat, and total removal and replacement. (An inactive strategy is to elect to do nothing.)
Surface preparation methods include cleaning and degreasing (SSPC-SP 1) and hand or power tool cleaning (SSPC-SP 2/SSPC-SP 3). If a greater degree of surface cleanliness (and roughness) is desired, such as when heavy rust, pitting and pack rust are present, commercial-grade power tool cleaning (SSPC-SP 15) may be performed. The prepared areas should be transitioned (feathered) into the existing sound coatings. Chloride remediation may be performed during the surface cleaning procedure followed by retesting. The current MnDOT threshold of 7 µg/cm2 water-soluble chloride was determined to be reasonable for Minnesota bridges.
SPOT TOUCH-UP AND OVERCOAT
Surface preparation methods are identical to that described in the Spot Touch-Up section above. However, regarding overcoating of existing sound coating, additional surfaces to be coated are subject to cleaning and degreasing.
TOTAL REMOVAL AND REPLACEMENT
Prior to abrasive blast cleaning, grease and oil contamination must be removed as per SSPC-SP 1 followed by chloride testing on representative surfaces. Chloride remediation testing should be performed following abrasive blast cleaning if chloride levels exceeded 7 µg/cm2 prior to blast cleaning. Abrasive blast cleaning is performed to achieve an SSPC-SP 10/NACE No. 2, Near White Metal Blast Cleaning prepared surface and achieve the specified surface profile depth.
Industry experts recommend power washing as a key component of cleaning and chloride remediation. However, this method can be prohibitive to agencies such as MnDOT due to environmental regulations. Regardless of the surface preparation method selected, paint removal practices must be performed in accordance with the agency’s environmental guidelines.
Best Practices for Selecting Coating Systems
Similar to best practices for selecting methods of surface preparation, the best practices for selection of coating systems are based on the maintenance painting strategy.
SPOT TOUCH-UP OR SPOT TOUCH-UP AND OVERCOAT
Epoxy mastic and polyurethane finish coats are often specified for this method, either with or without an epoxy penetrating sealer. When overcoating is not carried out, the visibility of the spot touch-up approach may impact whether or not a color match finish spot coat is applied. With overcoating, some additional barrier (and atmospheric) protection is provided as well as a uniformity of appearance.
REMOVE AND REPLACE EXISTING COATING SYSTEM
Typically, an organic (epoxy) zinc primer and epoxy mid-coat with a polyurethane or polysiloxane finish coat, or moisture-cured urethane (MCU) zinc primer with two coats of MCU finish are used for this method.
Best Practices for Determining Whether to Use In-House Crews or Contract Painting
Best practices relating to the use of in-house crews verses contract painting are based on whether bridge structures contain lead and/or other toxic metals as well as the total surface area requiring maintenance coating and the maintenance strategy selected.
Beyond worker safety and environmental protection, in-house crews should receive formal training on proper surface preparation techniques as well as proper coating mixing, thinning and application procedures. Assessment of environmental and safety considerations, worker skill sets, square footage, access and agency decisions regarding work planning and management of risk are appropriate to judge whether or not to use in-house crews or contractors.
OBJECTIVE 3: PREPARING A BRIDGE MAINTENANCE PAINTING MANUAL
The information obtained in Objectives 1 and 2 was used to establish a decision process and ultimately a revised Bridge Maintenance Painting Manual (BMPM) for MnDOT bridge maintenance crews. The BMPM addressed four subject areas including conducting a coating condition assessment, selecting maintenance painting strategies, establishing priorities and executing the work.
Conducting a Coating Condition Assessment
Visual coating condition assessments are typically performed during the biennial or annual bridge inspection but may also be performed at the discretion of the MnDOT District that owns and manages the bridge. During the visual assessment, the steel bridge elements are assessed and classified into one of the four bridge element condition state categories explained earlier. Condition state guidance is provided for the protective coating system, the steel superstructure elements themselves and the section loss of steel elements. However, some deficiencies may not be visually apparent. When that is the case, a physical assessment may be performed to determine properties of the coating system such as adhesion, thickness and number of coating layers, and substrate condition.
Selecting a Maintenance Painting Strategy
The results of the visual coating condition assessment are used to determine whether maintenance painting is warranted and which strategies are likely to provide adequate preservation of the structure. There are five maintenance painting strategies that may be selected for any given painted steel bridge, including painted weathering steel. These include: do nothing, spot touch-up, spot touch-up and overcoating, removal and replacement in zones, and removal and replacement of the existing system for the entire structure. For weathering steel, maintenance options include abrasive blast cleaning, allowing the patina to reform, and coating application.
When the existing coating system will be overcoated, there are risks that must be assessed based on the condition of the existing coating system. The risk can be categorized as minimal (nil), low, moderate or high depending on the coating property being considered (Table 1).
Table 1. Risk Assessment for Overcoating Based on Coating Physical Properties
|>400-500 psi||>10-20||Clean & Profiled|
|10-20%||2A||200-400 psi||20-30||No Active Rust|
|<200 psi||>30||Rust, Flaking
Table 2. Maintenance Coating Systems: Years of Anticipated Service Until 3-5% and 5-10% Coating Breakdown Reoccurs
|One-Coat Touch-Up||One-Coat Surface-Tolerant
|One-Coat Surface-Tolerant Polyamide Epoxy
One-Coat Aliphatic Polyester Polyurethane
|Three Coats – Zinc-Rich Epoxy,
High-Build Epoxy and High-Solids Polyester
Determining when maintenance painting will be performed can be dependent on the visual threshold (tolerance) established for the degree of rusting on a surface (Figure 1). There are also performance expectations that can aid in selecting the maintenance painting strategy, influenced by the service environment, coating products selected and degree of surface preparation.
|Rust Grade 5-G 1% Rusted||Rust Grade 5-G 3% Rusted||Rust Grade 4-G 10% Rusted|
|Fig. 1: General Rusting. Images from SSPC-VIS 2 show the appearance of a painted steel substrate exhibiting 1% general rusting (top), 3% general rusting (center) and 10% general rusting (bottom).|
Figure 2 identifies the generalized expected service life performance of different maintenance painting strategies based on three coatings systems. Table 2 provides estimates of the projected service life of the three coating systems until 3-to-5-percent coating breakdown and 5-to-10-percent coating breakdown occurs. Either of these scenarios may represent the visible threshold at which maintenance painting is performed.
The expected performance of various protective systems can influence the selection of a maintenance painting system when more than one option exists. An example is provided in the idealized service life (deterioration) curve for a coating system in Figure 2.
|Fig. 2: Generalized coating deterioration curve based upon expected service life.|
The importance of the deterioration curve in decision-making is related to the anticipated additional service life that may result from coating touch-up (TU), touch-up and overcoat (TU+) and from full removal and replacement (RR) is the appropriate maintenance painting strategy when the deterioration has progressed to that point. If touch-up occurs at the curve and TU lines intersect, the curve can be moved to the right (X number of years) since further coating breakdown is postponed. The same example exists when the curve and TU+ line intersect; the curve is moved to the right (X number of years) if touch-up and overcoating is performed. The slope of the curve is a function of the surface preparation performed, the coating system applied (products and layers) and the service environment (mild, moderate and aggressive).
An example of the influence of maintenance painting on service life based on Figure 2 is illustrated in Table 2. After initial painting, the practical service life is 18 years (intersection of 10 percent and the curve). Touch-up will be performed approximately six years later (at 24 years), 10-percent rusting occurs again and this time touch-up and overcoating is performed. Approximately 9 years later (33 years from the initial painting), 10-percent rusting will occur again.
The service life can also be dependent on service environment. Aside from the general service environment for the bridge, steel protective coating systems on specific elements generally begin to fail first under leaking expansion joints where the steel is exposed to chlorides, frequent wet and dry cycles and high humidity. Addressing underlying issues is crucial to preserving the protective coating system.
Developing a priority list for a group of bridges that require maintenance painting may be easy if the extent of coating breakdown is widely dissimilar. The worst bridges are addressed first. However, there are other factors that should be considered when establishing priorities including structure integrity; social impacts (detours); safety and health (worker, public and environment); scheduling of other bridge work; remaining service life and distribution of budget. Alternatively, the priority ranking for maintenance painting may simply be based on the general rating of coating breakdown and degree of rusting.
Executing the Work
The MnDOT BMPM contains six flow diagrams that guide the user through the maintenance painting processes described in the Manual and guide the assessor through a series of questions, the answers to which lead them to the following series of questions, or even to another diagram.
These diagrams require the user to:
- Examine the steel protective coatings;
- Determine the maintenance painting strategy for painted steel elements;
- Determine the maintenance painting strategy for unpainted weathering steel elements;
- Risk assessment for overcoating;
- Determine who will perform the work; and
- Surface preparation and paint system options.
Maintenance Painting Procedures for MnDOT Personnel
The final section in the BMPM includes the maintenance painting procedures for agency bridge maintenance crews. Topics include:
- Safety and environmental protection;
- Quality control;
- Pre-cleaning and cleaning;
- Procedures for touch-up maintenance painting;
- Procedures for touch-up and overcoat maintenance painting;
- Procedures for zone removal and replacement of the existing coating system; and
- Weathering steel bridges.
OBJECTIVE 4: NEXT STEPS
The TRS and the updated BMPM served as the initial steps in the process to identify best practices for MnDOT’s Bridge Maintenance Painting Program. The next steps included developing a custom coating assessment guide, conducting training and pursuing a bridge maintenance painting test site.
|Fig. 3: Sample photos from the MNDOT “Steel Bridge Coating Condition Assessment Photographic Field Guide,” developed to assist with performing coating condition assessments.|
Training was conducted over two days in mid-April of 2015. Participants were introduced to the content and utilization of the BMPM and the Steel Bridge Coating Condition Assessment Photographic Field Guide (Figure 3). Beginning in 2016, MnDOT will perform visual coating assessments using the coating assessment guide and rate element 515 Steel Protective Coating. As inspectors become more familiar with the rating system and decision process, the BMPM will be evaluated and updated. Ongoing training, including field training on implementation of these practices, is paramount to the success of a bridge maintenance painting program.
Selecting optimum coating materials and corresponding levels of surface preparation are also critical in order to protect bridges from corrosion, especially in states with severe climates. The TRS identified maintenance painting systems that are typically used by other agencies (Figure 4).
|11%||Moisture Cure Urethane|
|9%||Calcium Sulfonate Alkyd|
|9%||Epoxy Penetrating Sealer/Epoxy Mastic/Polyurethane|
|9%||Epoxy Penetrating Sealer/Epoxy Mastic/WB Acrylic|
|Fig. 4: The TRS-identified maintenance painting systems that are typically used by other agencies.|
To be able to more effectively select proper surface preparation methods and coating products for bridge maintenance painting performed in Minnesota, MnDOT initiated a bridge maintenance painting test site research project. In August of 2015, five maintenance coating systems, which were recommended by paint manufacturers based on MnDOT’s goals and objectives, were applied to steel beam ends at a test site. The coating systems will be evaluated over a four-year period. Objectives of the study include identifying generic coating materials that can be effectively applied by MnDOT bridge crews to successfully extend the service life of the existing coating systems and provide a means of determining anticipated deterioration rates for maintenance painting strategies employed in Minnesota.
Ultimately, the data gathered from all of these efforts will provide valuable information and lead to better budgeting and planning of painting operations in Minnesota. JPCL
ABOUT THE AUTHORS
Sarah Sondag is a senior engineer with the Minnesota Department of Transportation Bridge Office. In her current position she supports state-wide bridge maintenance operations in the areas of training, research, best practices, data tracking and reporting, performance measures and asset management. Previous employment includes soils, construction and traffic engineering positions with the Minnesota Department of Transportation in District 1 as well as a design engineering position with Mark Thomas & Company in San Jose, Calif. Sarah has a Bachelor of Civil Engineering and a Master of Science in Civil Engineering from the University of Minnesota. She is also a registered Professional Engineer in the State of Minnesota.
Rich Burgess is a senior consultant for KTA-Tator, Inc. where he has been employed for over 23 years. He is a member of SSPC and NACE and an active committee member for joint standards. Burgess is an SSPC-Certified Protective Coatings Specialist, a NACE-Certified Coating Inspector Level 3 (Peer Review) and an SSPC C-3 Supervisor/Competent Person for Deleading of Industrial Structures. In his current position, he performs coatings evaluations, coating failure analysis, specification preparation, expert witness and project management services for clients in the transportation, power generation, water/wastewater, shipping, marine and aerospace industries. Burgess is a principal instructor for the SSPC C-1, C-3, and C-5 courses, for the NACE CIP Program and a variety of KTA-offered training seminars. He holds a Bachelor of Science degree in Environmental Science from Rutgers University and a Master of Science in Operations Management from the University of Arkansas.
As seen in JPCL January 2016
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