FHWA Bridge Coatings Technical Note: Epoxy Mastic Bridge Coatings
From: Special Projects and Engineering Division, Office of Engineering Research & Development, HNR-20
Topic: Epoxy Mastic Bridge Coatings
Description: Epoxy mastic, or high-solids epoxy coatings have received a recent growth in popularity for application to steel bridge structures as a corrosion control coating. These coatings are based on one of several epoxy resins and are usually cured using a polyamide or amine curing agent. These coatings are two-component and have a finite workable life before curing after mixing. Epoxy mastic coatings promise high build in fewer coats, environmental compliance due to low solvent content, and inherent "surface tolerance" requiring less-intensive and costly surface preparation prior to application. Many of the commercially available versions of these materials are heavily pigmented with aluminum and are marketed as direct-to-metal primers and as "overcoating" paints.
Cost Impact: Epoxy mastic bridge coatings have gained popularity in part due their relative low cost. Information from paint manufacturers shows material costs for epoxy mastic systems approximately 30% lower than for other, more durable coating systems. However, as the cost of paint material generally represents less than 10% of the cost of a bridge repainting job, the savings obtained using the less expensive coating material are insignificant compared to the savings obtained in future maintenance expenditures by using more durable coating materials.
Performance Experience: Recent and ongoing FHWA-sponsored test programs have found that epoxy mastic type coatings have performed relatively poorly as a generic class. While there is certainly variability between different formulations within this generic class, epoxy mastic coatings have tended to fail by underfilm corrosion and primer-substrate disbondment. This failure may initiate at the site of defects in the coating film, or at thin spots such as at edges or corners of members. Failures underneath the intact film following penetration of moisture and oxygen to a contaminated steel surface have also occurred. Like most epoxies, epoxy mastic type coatings show poor weathering resistance as a topcoat; ultraviolet radiation from the sun tends to degrade epoxy resins over time. It must be noted that the referenced testing has been performed in marine or salt (chloride)-rich environments; therefore, the exposure conditions have represented a harsh condition on a steel bridge.
Summary of Supporting Data: In marine atmospheric exposure testing, epoxymastic type coating systems showed significant failures after 3 to 5 years even over near-white blast cleaned (SSPC SP10) steel.1, 2
Over prerusted, hand tool and power tool cleaned steel, these coatings showed significant failure after less than two years.3
On a steel bridge in Central New Jersey used for testing 47 various coating systems, only 3 of the 12 epoxymastic type systems tested graded at 8 or above according to ASTM D610 after eight years of exposure. In the same test, 10 of the 14 systems tested with zinc-rich primers scored 8 or better, and 13 of 14 scored 7 or better.4 (ASTM D610 "8" = 0.1% rust; "7" = 0.3% rust)
In marine exposure testing, epoxymastic coatings from various vendors showed significant underfilm corrosion from intentional defects after as little as 18-months. Similar underfilm corrosion results were found on SSPC SP-5 test panels exposed to laboratory accelerated tests.5, 6
In the FHWA-sponsored "PACE" study, the epoxy primer coating systems tested showed poor performance compared to other generic types of coatings evaluated under similar conditions. This study showed epoxy primer systems failing due to underfilm corrosion.7
In various FHWA-sponsored test programs, the low solvent content and high viscosity of several epoxymastic type coatings coupled with their sometimes short workable potlife after mixing, has caused application difficulties. Whereas this has not been the case with all products of this generic class, many of those with aluminum pigmentation have shown application difficulties in both brushing and conventional air spraying.
Recommendation: The test results and field experiences detailed above lead to concern over the use of epoxymastic type coatings on steel bridges in salt-rich environments. These coatings have gained popularity due to their compliance with environmental regulations, their ability to achieve high build in fewer coats than traditional bridge paints (e.g., alkyds), their relatively short dry-to-recoat times, and their low cost per gallon. In harsh environments these advantages are offset by a high risk of underfilm corrosion initiating at defects or structure edges. In addition, the low solvent content and high viscosity of many of these products can lead to excessive holidays and inconsistent paint thicknesses, especially if less than diligent quality assurance is provided at the jobsite.
References:
- "Environmentally Acceptable Materials for the Corrosion Protection of Steel Bridges - Task C, Laboratory Testing," FHWA Publication No. FHWA-RD-91-060.
- "Environmentally Acceptable Materials for the Corrosion Protection of Steel Bridges - Final Report," (draft) Final publication in progress.
- "Environmentally Acceptable Materials for the Corrosion Protection of Steel Bridges - Final Report," (draft) Final publication in progress.
- "Structural Coatings Performance Evaluation (Mathis Bridge Study)," New Jersey Department of Transportation, Bureau of Materials Engineering and Testing, Revised October 1995.
- "Evaluation of Volatile Organic Compound (VOC)-Compatible High Solids Coating Systems for Steel Bridges," FHWA Publication No. FHWA-RD-91-054.
- "Comparison of Laboratory Test Methods for Bridge Coatings," FHWA Publication No. FHWA-RD-94-112.
- "Performance of Alternative Coatings in the Environment (PACE); Volume II: Five-year Field and Bridge Data of Improved Formulations," FHWA Publication No. FHWA-RD-89-235.
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