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"Fool Me Once..." The Case of the Recurring Disbonding Coating Failure on a Highway Bridge Structure
By David S. Leyland
A metropolis in the north east region of the United States contracted to have one of their major bridge structures repainted. The particular bridge structure was constructed around the turn of the century, and periodically over the last 90 years, oil and oil/alkyd coatings were applied to maintain the structural steel members. While the existing coating system was generally intact, localized areas of corrosion were present, thus the decision to repaint. XYZ Painting Company was contracted to blast clean the structure according to SSPC-SP7, "Brush-Off Blast Cleaning", then apply a three coat system consisting of an epoxy mastic primer and intermediate, followed by a urethane finish coat.
The contractor began the project during the spring and finished all coating application by late summer. Upon completion of the project, the bridge looked like new and the City was satisfied with the work performed. However, an inspection one year later revealed the newly applied coating system to be catastrophically disbonding from the bridge in large pieces, measuring 12 inches or more in size. The City notified the contractor of its findings and the contractor returned to the site to touch-up the areas of disbonded coating at no additional expense. The contractor did extensive touch-up in restoring the painting system to the original condition. Over 1,500 gallons of additional coating material was required.
Unfortunately, the story does not end here. One year later the scenario of disbonding paint reoccurred. Large pieces of paint were spontaneously disbonding in random areas. Once again the City notified the contractor of their findings and requested that the contractor return to the site and perform additional touch-up. The contractor visited the bridge site and was dismayed to find that extensive touch-up would be required to address the many areas where the paint had disbonded. The contractor decided that prior to additional touch-up time and expense, the cause of failure must be determined, as the contractor could not afford to perform touch-up work indefinitely. As a result, KTA-Tator, Inc. was contracted to visit the bridge site, independently determine the cause of the disbonding failure, and offer an unbiased repair recommendation.
Field Visit
Many observations were made during the field visit. Random disbonding of the coating system was occurring on the web areas of most of the large beams. Areas of disbonding often measured up to 1 square foot in size. All disbonding was occurring at the paint/steel substrate interface, and not between coating layers. It was quite evident that an extensive amount of touchup had been performed in areas where previous disbonding had occurred. Upon microscopic examination, many fine cracks were randomly located, 1 to 2 feet apart over the surface of the newly applied coating system. These fine cracks penetrated to the steel substrate beneath. No cracking was observed in areas that had been touched-up.
Adhesion tests were performed over or adjacent to areas where fine cracks existed in the coating. Adhesion was generally poor in these areas. Tests performed in areas away from fine cracks revealed only slightly better adhesion characteristics.
Coating thickness was measured using both a non-destructive magnetic dry film thickness gage and a destructive thickness instrument known as a Tooke Gage. The nondestructive magnetic coating thickness gage indicates total coating thickness of all layers, while the destructive Tooke gage reveals both the number of coats and the thickness of each, up to 50 mils. Dry film thickness measurements of the entire coating system, including all previously applied coats of paint, ranged from 20 to 40 mils. Observations made with aid of the Tooke Gage revealed over 12 layers of paint previously applied to the bridge structure.
Finally areas from which coating had disbonded revealed an underlying substrate containing intact, smooth mill scale, a blue oxide layer that forms on hot rolled steel during the rolling process. No surface roughness was evident.
Cause of the Failure
As a result of the extensive field observations and data collection made during the field visit, laboratory testing was not required in order to identify the cause of failure. It was determined that spontaneous disbonding occurred on the bridge structure for the following reasons:
- The steel surface had virtually no surface profile, due to the presence of smooth intact mill scale. Consequently, the adhesion of the originally applied prime coat was compromised.
- Numerous coats of paint had been applied since the bridge was first constructed nearly 100 years ago. Total coating thickness was extremely high, routinely measuring over 30 mils.
- Many of the previously applied coating layers were very old, heavily oxidized and brittle.
- The specification required that all areas of intact coating on the bridge be blast cleaned according to SSPC-SP7, "Brush-Off Blast Cleaning". This method of surface preparation fractured and embrittled the old, existing coating system.
- The application of additional coats of paint (two coat epoxy mastic/urethane topcoat) created additional stresses, by adding more thickness and weight, to an already burdened coating system. Additional stresses were exerted by the forces associated with solvent penetration and curing of the newly applied coatings.
- Additional stresses to the total coating system were exerted by vibrations resulting from the heavy volume of traffic supported by the bridge, and by warming/cooling, and freeze/thaw cycling. Remember extensive disbonding was only observed after the coating system had been exposed to a season of winter weather.
Discussion
According to "Beneficial Procrastination" by Eric S. Kline and William D. Corbett , upgrading an existing coating system by overcoating is a viable alternative to total removal, particularly if the existing system contains lead. This delay is designed to permit new technology to develop, which in turn can lower the costs associated with lead paint removal in the future. However, this approach to upgrading requires careful evaluation of the condition of the existing coating system. Not all structures, even those which "appear" to be in relatively good condition, are candidates for upgrading.
Visual assessment for the extent of corrosion and the substrate condition, combined with physical tests including total system thickness, number of layers and thickness of each, and finally the adhesion characteristics all play a significant role in establishing whether or not the structure is a candidate for upgrading.
In this case, if these common field tests were performed prior to preparing the specification, the owner would have discovered that the numerous layers observed, extensive total coating thickness measured, and overall poor adhesion characteristics culminated in making overcoating risky at best, and that test patches (described below) were a necessity, if total removal was not desirable at this time. Test patches mirroring proposed surface preparation and coating system recommendations are often used to determine coating compatibility. The test patches should be large (10 feet x 10 feet or greater), and all work should be performed in accordance with the proposed specification. In fact, ASTM D5064. provides a standard practice for conducting a patch test to assess coating compatibility. After the coating has had an opportunity to properly cure, an individual knowledgeable in coatings should evaluate the initial adhesion characteristics of the overall coating system. Once the test patch is allowed to weather, (ideally over the winter), the adhesion characteristics can be reevaluated and a judgement made as to whether or not full scale upgrading is feasible.
KTA recommended that the entire structure be blast cleaned to a Near-White
condition (SSPC-SP10) followed by reapplication of the coating system.
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