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Publication Number: FHWA-HRT-11-047
Date: May 2011
Performance Evaluation of One-Coat Systems on New Steel Bridges
Paul Virmani, HRDI-60, (202) 493-3052,
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This document is a technical summary of the Federal Highway Administration report, Performance Evaluation of One-Coat Systems on New Steel Bridges (FHWA-HRT-11-046).
The current state of practice in bridge coating usually involves multilayer coating typically consisting of a zinc-rich primer over an abrasive blast-cleaned surface and two additional coating layers on top of the primer. Although this current coating technology provides a comprehensive solution for better corrosion protection of steel bridges, the overall cost involved is relatively higher than its lead-based predecessors. The purpose of this study is to evaluate the performance characteristics of various commercially available high-performance coating materials that can be applied as one-coat systems to steel bridges in shop application.
Eight one-coat systems and two controls, a three-coat system and a two-coat system, were chosen, and their performance was evaluated using accelerated laboratory testing (ALT) and outdoor exposure conditions.
Performance of these coating materials was evaluated on the basis of variations in color and gloss, changes in adhesion strength, changes in pencil scratch hardness, and the development of surface defects (holidays, blisters, and rusting) and rust creepage. Regression analysis was used to identify correlations among the various performance parameters, and a comprehensive system was developed to rank the coating systems based on overall performance.
Eight one-coat systems and two controls that performed well in the field and in earlier Federal Highway Administration (FHWA) studies were evaluated in this study.(1,2) Table 1 lists all of the 10 coatings systems.
Test Panel Preparation
Steel test panels of two sizes were used in this study. The small panels were 4 x 6 x 0.2 inches (10 x 15 x 0.48 cm), and the large panels were 6 x 12 x 0.2 inches (15 x 30 x 0.48 cm). All test panels were blast cleaned to Scientific Society for Protective Coatings Surface Preparation 10 standard, and coatings were applied on the cleaned test panels using airless spray. Half of the total test panels (111 out of 222) were scribed diagonally following the instructions specified in American Society for Testing Materials (ASTM) D1654-08.(3) Panels were scribed to study the potential performance of the coating systems at local film damage. The other half of the panels were left unscribed to characterize undamaged conditions and physical properties such as gloss, color, pencil scratch hardness, etc. Two additional panels of each coating system were prepared exclusively for initial adhesion strength and Fourier transform infrared spectroscopy; they were not used in any of the tests.
Table 1. Summary of coating systems.
* One-coat systems contain one coat of paint that acts as the primer/top coat and do not contain an intermediate coat.
Note: The blank cell indicates that the two-coat system does not contain an intermediate coat.
ALT and outdoor exposure conditions were used to test the coating systems. For ALT, 19 accelerated test cycles (each test cycle = 360 h) were conducted for a total test period of 6,840 h. This method is similar to ASTM D5894-.05, with the addition of a freeze cycle for 24 h.(4,5)
Outdoor exposure conditions involved the following:
Performance Evaluation Techniques
Coatings were evaluated before and after exposure for the following parameters:
All coating systems were evaluated for color, gloss, rust creepage, and holidays every 360 h in ALT and every 6 months in outdoor exposure conditions. Adhesion strength was evaluated once before testing and once at the termination of testing.
Correlation Among Performance Parameters and Exposure Conditions
Correlation among test parameters, such as color or gloss, for various coating systems can help researchers better understand interactions among test variables. This correlation would be specific to the type of exposure condition such as ALT or outdoor exposure testing. Linear regression analysis was performed to identify relationships between the various performance characterization parameters. The objective of this analysis was to observe whether any correlation(s) existed among performance parameters. Regression analysis was also performed to examine if any correlations existed between the exposure conditions. Panels with a GFP coating system were not available for outdoor testing. As a result, the GFP system was excluded from the regression analysis.
Based on final performance data in ALT and the outdoor exposures, all one-coat systems and the two controls were ranked. A comprehensive numerical analysis was used to assign weighted coefficients to the four exposure conditions. The calculated coefficients for the four exposure conditions are as follows:
Coefficients were also assigned to the performance parameters based on the authors' knowledge and past experience with their overall impact and significance in evaluating a coating system. Weighted coefficients of the various performance parameters are as follows:
Final performance ranking of all coating systems is shown in table 2.
Table 2. Comprehensive rank of one-coat and control systems.
Topics: research, infrastructure, structures
Keywords: research, structures, One-coat, Two-coat, Three-coat, Steel bridge coatings, Corrosion protection, Accelerated testing, Outdoor exposure, and Coating performance evaluation
TRT Terms: research, infrastructure, Facilities, Structures