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Publication Number: N/A
Date: January 1997
FHWA Bridge Coatings Technical Note: Overcoating (Maintenance Painting)
From: Bridge Coatings Technology Outreach Team
Topic: Overcoating (Maintenance Painting)
Description: Over the past several years, "overcoating" has become the common term used to describe bridge maintenance painting operations which only partially remove existing paint and apply new coatings over a mixed substrate of existing paint, bare steel, and rusted surfaces. The practice of overcoating encompasses and is very similar to traditional maintenance and touch-up painting that has been conducted for many years. Overcoating differs from traditional maintenance painting in the following ways:
Overcoating operations vary widely depending on the condition of the existing steel and paint system and the specified surface preparation and new coating system, but generally these operations have the following components:
Key Variables for Overcoating Success
Several variables have a significant effect on overcoating risk for bridges. These factors are described in detail below:
Cost is the most common driver for the selection of overcoating versus full removal and replacement of existing paint systems. Due to the dramatic increase in costs for full maintenance painting operations spurred by environmental and worker safety regulations, overcoating has become increasingly popular. Present costs for overcoating lie in the range of $1 to $3 per square foot while full removal and replacement of paint systems is $5 to $20 per square foot. This large difference in initial cost of maintenance has made the choice of overcoating quite attractive to bridge owners. While overcoating may be a viable maintenance and economic option in some cases, the feasibility of overcoating a particular bridge should also be based on the relative potential performance of each maintenance option and the risk associated with potential early failure of overcoating systems. While an analysis of initial costs of various options will almost always point to overcoating, a life cycle analysis will often show full paint removal and application of a high durability coating system to be the most cost effective option, particularly for bridges or areas of bridges in marine environments or exposed to significant deicing salt application.
As detailed above, the performance of a newly applied overcoating is expected to be highly dependent upon the condition of the existing coating over which it is applied. Overcoating over existing aged paints that are often brittle and loosely adherent can pose a significant risk of early failure and large scale disbondment. The applied overcoat applies added stress to the existing coating in various ways; an additional coating adds physical weight to the existing coating; as the new coating drys or cures it shrinks which adds stress to the existing coating; as the ambient temperature cycles, the two paint systems expand and contract at different rates, adding stress to the existing system; and, the solvent contained in the overcoat can soften the existing coating and reduce its mechanical properties.
The risk of early failure cannot be avoided, but the following steps can minimize this risk:
Several investigators have assessed the viability of overcoating and the relative performance of various overcoating materials in the recent past. Some evaluations have been performed in laboratory accelerated tests, but for the most part, patch or area tests on in-service structures have been used. Evaluations have brought mixed results. Several materials have proven to provide good service for several years in overcoating applications, but no paint material marketed as an "overcoating" or "surface tolerant" paint has proven to be a direct substitute for good surface preparation. Although the specific coating material chosen for a particular maintenance painting operation is certainly of importance, it is quite often other critical variables which determine the success of failure of an overcoating job. These factors include: the condition of the existing paint, the extent of corrosion on the substrate, the level of surface cleanliness achieved, and the environment of exposure. A recently completed FHWA-sponsored study of various overcoating materials applied to bridge structures in various parts of the country resulted in the following general results: (a) Multicoat systems, overall, performed better than single coat systems, with three coat systems showing generally better performance than two coat systems. This result is thought to be directly related to the occurrence of holidays and pinholes during brush application of maintenance coating materials; however, it shows a measurable benefit of the application of multiple coats in a realistic scenario. Other studies have shown similar results.
(b) Coating materials performing well in the study were 3-coat moisture-cured urethane systems, and three-coat epoxy based systems using a penetrating low-viscosity sealer as a primer. In addition, two separate 3-coat low-VOC alkyd systems also performed well. Of the single coat systems, the coatings based on calcium-sulfonate alkyd resins did best. Similar generic results have been found by other investigators.
(c) In general, coatings that did well at any one test site did well at all four, diverse test sites (indicating some measure of surface tolerance, or "overcoating acceptability" for specific paint materials). Also, those materials that failed badly and early at any one site generally failed at more than one site. This would lend support to the use of patch tests as screening for acceptability of a particular overcoating material on a particular structure.
In the subject testing, failures were of two varieties, (1) early coating disbondment due to incompatibility of the overcoating material with the existing paint, and (2) rust through of the newly applied overcoating material at areas where the coating is over bare steel or existing rust.
Case (2) is well documented and not surprising. Virtually all coatings tested over "less-than-ideal" surface preparations will have a service life shorter than the same coating material applied over a clean surface., With respect to case (2) type failure, a considerable database of material performance results exists. Testing of paints on steel panels prepared to and SSPC SP-3 or SSPC SP-11 surface preparation (i.e., power tool-cleaning) has yielded enough results to use these data to judge the expected service life of maintenance coatings (overcoatings) when no gross incompatibility failures occur.
Case (1) failures are more difficult to quantify and almost impossible to prepare for as a bridge owner. For this reason, patch testing of a material and weathering for at least one or two (recommended) seasonal cycles on the structure of interest is recommended as a conservative approach to avoiding early failure of the overcoating application. Having done this and used existing data to predict a reasonable service life for the applied maintenance paint in the actual environment of the bridge over bare SP-3 steel, a reasonable prediction of the service life of the overcoating system as a whole can be made. This life prediction can be used to determine the life cycle cost of the overcoating application versus various other candidate maintenance painting options for the structure.
Worker Protection - Although overcoating operations are intended to minimize the removal of existing paint from a structure, if the existing paint contains lead, contractors must abide by the provisions of the OSHA Lead-in-Construction Standard (29 CFR 1926.62) for worker protection.
Environmental Compliance - Overcoating jobs do not generate the volume of lead-containing waste that abrasive blasting operations do; however, the paint chips removed and often the wash water used on the bridge must be collected and disposed of properly. Local environmental protection requirements will vary for jobs of this type and size.
For further information, please contact a member of the Bridge Coatings Technology Outreach Team: Ron Andresen, FL-Cen.; Dan Brydl, IL Div.; Mark Clabaugh, FL-East; Dr. Shuang-Ling Chong, HNR-20; Carl Highsmith, Region 3; Joe Huerta, HNG-20; Bob Kogler, HNR-20; Mike Praul, ME Div.; Larry O’Donnell, MA Div.