United States Department of Transportation
i
Comparison of Laboratory Testing Methods for Bridge Coatings
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1995-06-01
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[PDF-859.82 KB]
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Abstract:This study was performed to investigate the effect of combining a freeze cycle, an ultraviolet/condensation cycle (QUV), and a salt-fog plus pollutant/dry cycle (Prohesion) in an accelerated laboratory weathering test on the performance of coating systems for steel bridges. The test results were compared with those obtained from the salt-fog test, the Prohesion test, and natural marine exposure. The coating systems selected were water-based systems of acrylic, acrylic epoxy, inorganic zinc alkali silicate, vinyl, and zinc-rich epoxy, and solvent-based systems of calcium sulfonate/alkyd, high-solid epoxy, zinc-rich polyurethanes, epoxy mastic, epoxy urethane mastic, and low-VOC epoxy. All of these coating systems contained volatile organic compound (VOC) content less than 340 g/L. Evaluation parameters for coating performance included coating film thickness, coating gloss, hardness, adhesion strength, blistering, rust, and creepages at scribe. The failure modes of these coatings are discussed and the coating performances are compared. Low-VOC solvent-based zinc-rich polyurethane/polyurethane/polyurethane coating systems outperformed the rest of the coatings tested. The epoxy mastic system and the epoxy urethane mastic system developed serious undercuttings at the scribe. The waterborne acrylic system and the waterborne acrylic epoxy system did not protect steel effectively and they blistered rapidly at the scribe. Zinc-rich primers were highly resistant against corrosion without developing any undercutting on steel, but their water-based topcoats exhibited extensive delamination due to the poor adhesion to the zinc primers. Waterborne vinyl blistered badly in all the laboratory tests, but performed fairly well after 28 months of outdoor exposure. The test results obtained in this study were also analyzed by a statistical variance method to determine the difference among the test methods, and coating systems. The cyclic freeze/QUV/Prohesion test results were found to generate a much more promising performance trend when compared to the natural marine exposure results than did the Prohesion test alone. However, salt-log test results exhibited large deviations from those of the natural marine exposure, indicating that salt-fog testing is not a reliable predictor of field performance of coatings.
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