Recondition or Overcoat?
From JPCL May/June 2024
By Daniel Zienty, Bolton & Menk, Inc.; James Connor, Bolton & Menk, Inc.
Often times, the answer on when to consider the option of a complete exterior reconditioning or an overcoat for a water tank is quite simple: it’s visual. Sometimes, the need for a complete reconditioning is clear, such as when the tank exhibits extreme rusting along with excessive chalking and other traits associated with an ending to the protective coating’s longevity and serviceability. Other times, however, it’s not quite so clear. Areas exhibiting rusting and corrosion are minimal, as are inherent signs of cracking, delamination and the thinning of the protective coating. Of lesser consideration regarding the decision to overcoat, and one that may not be thought of until after the project: appearance.
On the southeastern quadrant of the tank, numerous spots were abrasive blast-cleaned to a Commercial finish. ALL PHOTOS: COURTESY OF THE AUTHOR
Prior to work taking place, readings were taken on the tank column referencing milage and cracking failure (a), while spot adhesion testing and milage readings were taken on the roof radial plate (b). Corrosion was widespread on the interior dry catwalk and access table (c) and the immersion compression ring (d). There were additional indications of a thinning coating system on the roof radial plates (e and f).
In 2017, the City of Isanti, Minnesota, had just that type of situation regarding its 750,000-gallon fluted style water tower, which was originally built in 2003. The city contracted with an infrastructure engineering firm for a complete assessment of the facility inclusive of the interior (immersion), interior dry, exterior surfaces and tank appurtenances. The results identified a need for monitoring of the interior (immersion) surfaces where moderate rusting along isolated areas of the roof support system were identified, along with interior dry areas subject to condensation such as the bottom bowl and access tube.
However, the exterior after 14 years appeared to be performing to expectations, with areas exhibiting only minor rusting. No maintenance was prescribed, though as mentioned, the tank should continue to be monitored, aligning with AWWA guidelines of 3 to 5 years as it draws closer to its life expectancy range of 15 to 20 years. While some systems can last significantly longer, the recommendation would allow the City of Isanti to anticipate the pending project. At that time, a life-cycle cost comparison of spot repair and overcoat versus a full removal and replacement would be conducted.
The contractors performed power washing on the entirety of the fluted column before spot abrasive blasting followed.
Reevaluation
In June of 2022, the City of Isanti requested an inspection, and updated evaluation of the original 2017 report. The field assessment identified minor to moderate corrosion in the interior immersion area, mainly at the weld seams and structural steel support edges, with portions of the edges experiencing a “stratified” or “pack rust” condition. The interior dry condition was like the previous inspection, but with the addition of pinpoint corrosion along the walkway and catwalk.
The surfaces of the exterior accessible without special rigging, such as the tank base, fluted column and roof, were evaluated for coating thickness, adhesion and visual conditional failures. Millage ranged from approximately 10 to 17 mils on the roof to approximately 7 to 11 mils at the base column. Crosscut adhesion testing, following ASTM D3359 Method A, was performed on both the roof and column base, resulting in readings (3A) that would justify the tank as a candidate for overcoating.
Spots at the baseplate and bottom section of the fluted column were Commercial Blast Cleaned (a), with a closeup of feathered edges shown (b). Also, Near-White Blast Cleaning was performed on the roof support angle of the interior immersion surfaces (c), and along the outer painters’ rail (d).
Visual examination revealed the presence of “micro-cracking,” on both the roof and column; however, in the areas where present corrosion was not visible. Though minor rusting was observed on the tank’s exterior, it appeared to be limited to less than 5% of the total surface.
Based on the above assessment two options and associated costs cut presented to the city for their consideration, they included the following options.
Option 1: Exterior Overcoat
- Complete coating removal (abrasive blast) and replacement of the interior (immersion) coating system;
- Complete coating removal (abrasive blast) and replacement of the interior (dry), limited to the access tube and bottom bowl, and spot repairs to remaining surfaces (<5%); and
- Pressure wash, spot repair (<10%), and overcoat exterior (two-coats).
Option 2: Exterior with Complete Removal and Replacement
- Complete removal and replacement of the interior (immersion) coating system;
- Complete removal and replacement of the interior (dry), limited to the access tube and bottom bowl, and spot repairs to remaining surfaces (<5%); and
- Complete removal (abrasive blast with full-containment) of the exterior coating system and replacement with a three-coat zinc/epoxy/urethane system.
The life cycle for an overcoat is approximately 10 years, whereas complete replacement has an approximate life cycle of 20 years.
With the two options determined, cost estimates for both were developed based on the overall scope, including consideration for appurtenances identified with the city, and a cost/life cycle analysis was completed. Those estimates were $475,000 and $825,000 for options 1 and 2, respectively.
With approval from the city specifications were developed, the bid packet was to identify the exterior scope as separate options or alternates, with Alternate 1 representing full exterior replacement and Alternate 2 exterior spot repair. This allowed the city, with its engineer, through the bidding process to determine the most cost-effective solution—overcoating.
After primer application, an initial tie-coat of epoxy was applied to the tank’s girder and shell plates.
Construction
The contractor received its notice to proceed on May 3, 2023, and mobilized on May 22. The project was on a tight schedule as this tank is Isanti’s only water storage facility. The city required completion within 60 days due to concerns that its distribution system would be operating directly off its well pumps during peak demand. This had the contractor working simultaneously on both interior and exterior surfaces as weather allowed.
Specific to the exterior, commencement of the project began following Option 1 per the follow-up assessment recommendation mentioned previously, which identified a power-wash followed by spot repairs of failed areas (<10%) and application of a two-coat system. This was addressed more specifically in the specification which called for a thorough water jet cleaning in accordance with SSPC-SP 12, WJ-4 of all exterior surfaces, using a rotating nozzle with a minimum temperature of 140 degrees F biodegradable detergent and water solution (1 to 2 oz. detergent per gallon of water) at a minimum of 4,000 psi with a flow rate of 3 to 5 gallons per minute.
The process was followed by a complete high-pressure rinse with clean water. Exposed metal and spot areas exhibiting rust and or identified microcracking were then abrasive blasted to an SSPC-SP 6/NACE No. 3, “Commercial Blast,” with edges feathered for a smooth transition into tightly adhered coating, with an angular profile of 2.3 to 3.0 mils.
A polyamidoamine primer coating was applied across the previously blasted spots of the tank’s fluted column.
At the onset of surface preparation, the southeast quadrant of the tank’s fluted column, the first two stage drops, required significant spot blasting. This brought initial concern as the specification, based on the field evaluation, indicated <10% of the entire surface as needing repair. However, this area would prove to be the worst with respect to exterior coating failure. The remaining surfaces, especially the girder and shell plates, saw few to no repairs required.
Following the surface preparation, the specification comprised a spot prime-coat (polyamidoamine) of the affected areas, followed by a complete tie-in coat of a similar product (differentiating color) and high-solids (polyurethane) finish coat. As discussed in the opening, this is where appearance comes into play regarding overcoating. The application needs to properly achieve overall film build especially over repair areas, so they are less noticeable, while also not creating stresses, cracking and or delamination, affecting coating performance and intended extension of service life.
Noted in the re-evaluation, dry film thickness (DFT) readings for the exterior surface ranges from 10 to 17 mils on the roof and 7 to 10 mils on the column. Roller application of the coating system by the contractor allowed for controlled film build at spot repairs, and about 1.5 mils each on the tie-in and finish coat, for a total of approximately 3 additional mils. The final considerations in assessing aesthetic appearance are color and gloss, as this can become a factor by visibly accentuating repaired areas. Specific to this tank, the city elected to go with a light color, white.
Conclusion
The City of Isanti had its tank reconditioning completed, on schedule, by the end of July. Completed, the tank had met the city’s expectations for appearance and completion time. The next assessment of the tank, the two-year warranty inspection, is scheduled for 2025, the first hurdle in what the city anticipates is a service life that meets or exceeds planned expectations.
As potential service life cycles become extended, in part, through the further development of protective coating systems, performance—including appearance—will still come down to a measure of due diligence by the engineer through upfront planning, and execution by the contractor.
References
- Helsel, Jayson and Lanterman, Robert. “Expected Service Life and Cost Considerations for Maintenance and New Construction Protective Coatings Work.” KTA-Tator, Inc. Presented at NACE CORROSION 2016, March 6–10, 2016, Vancouver, B.C.
ABOUT THE AUTHORS
Daniel Zienty
Bolton & Menk, Inc.
Daniel Zienty is a Coating Specialist with Bolton & Menk, Inc., who has more than 25 years of industry experience. He holds a bachelor’s degree in Construction Technology from Purdue University and is a NACE-certified Coating Inspector, as well as an SSPC-certified Protective Coatings Specialist and Supervisor for Deleading Industrial Structures. He has received numerous Engineering Excellence Awards from the Consulting Engineers Council of Minnesota for his work on water tower restorations, and has authored and presented related articles on protective coatings maintenance.
James Connor, Senior Protective Coating Specialist
Bolton & Menk, Inc.
James Connor is a Senior Protective Coating Specialist and American Welding Society (AWS) certified Weld Inspector with Bolton & Menk, Inc., with more than 36 years of industry experience. Connor is also a certified Senior Coatings Inspector with the Association for Materials Protection and Performance (AMPP), and he has also served on the development committee for Bridge Component Standards with the American Institute of Steel Construction.
Tagged categories: Features; Water Tanks
