Prospective single coat products are to be tested later this year in a study being carried out by the US Federal Highway Administration and the Connecticut Department of Transportation. Tests will be carried out on candidate materials with performance characteristics matching the 'coating performance wish list' that has been put together. Properties include slip coefficient, salt fog resistance and other weathering properties, adhesion, abrasion resistance, edge retention and so on, application criteria such as dry time to handle, temperature range for curing, sag resistance, chip resistance, cure time to measure DFT, pot life, recoat and overcoat with details of the type of tests that should be carried out and the suggested acceptance criteria.
The goal of this programme is to identify high-performance corrosion protection systems that are designed to maximise quality and minimise the impact of the painting process on the delivery schedule of the steel. Actual coating testing is planned for autumn 2004.
Historically, the industry has relied on the use of lead pigmented coating materials to provide corrosion protection to steel bridge structures from the time of the earliest use of steel in bridges in the late 1800s.
Only in about 1970 was there a switch to multiple-coat systems based on the use of metallic zinc in various coating resins to provide in situ cathodic protection to the steel. While there was a wide variety of resin binder possibilities, the systems most widely used in the US were based on either inorganic zinc silicate or epoxy zinc-rich materials in combination with a variety of mid-coat and topcoat materials (zinc/vinyl/vinyl, zinc/epoxy/urethane, zinc/epoxy mastic/urethane, zinc/zinc/urethane, and so on). The current system commonly in use employs an epoxy mid-coat and a polyurethane topcoat. While these materials have performed very well, they are laborious, require three separate coats, and are generally costly and time consuming to apply. The performance of these systems indicates that they provide decades of service in a wide variety of industrial and marine exposure environments.
In the mid to late 1990s, an effort was undertaken to develop the 'next step' in coatings technology: a system that would provide better performance while being applied in only two coats. This was dubbed 'Rapid deployment', a process which anticipates the use of fast dry-to-recoat materials so that the entire coating system can be completely installed above a 3.6m-wide roadway lane in one eight-hour shift.
Since 1996, several iterations of two-coat paint systems have been formulated and applied to bridge structures. Paint suppliers, raw material suppliers, and public bridge owner agencies have subjected these systems to accelerated tests in parallel with traditional three-coat systems with promising results. At least one supplier's material has been submitted to the comprehensive set of tests required by the AASHTO National Transportation Product Evaluation Program with results due soon. Use of these systems is particularly attractive to bridge owners in metropolitan areas where traffic congestion caused by work zones is a critical factor. Recent research presented at the 2003 annual conference of the Society for Protective Coatings indicates a 35% cost savings on a Connecticut Department of Transportation project using the two-coat system on existing bridges. Moreover, savings in a shop application during fabrication of new steel are expected to equal or exceed those experienced in the field.
These materials, while representing a great leap forward, still fall short of the 'system of the future' for the bridge of the future. The logical progression is to develop a single coat system that can be applied in the shop when a new steel bridge is under construction. This material will be as thick as the two-or three-coat systems now proposed (12 to 15 mils - 305 to 380 microns) and exhibit performance properties equal to those outlined in the 'coating performance wish list' that has been put together for the testing programme.
Eric Kline works for KTA-Tator
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TEST DESCRIPTION/PROPERTY
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SUGGESTED ACCEPTANCE CRITERIA
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| Slip Coefficient and Tension Creep for Slip-Critical Bolted Connections | AASHTO Class B (slip coefficient of not less than 0.5) |
| Salt Fog Resistance(unscribed specimens) | After 5000 hours of exposure, the coating shall exhibit blistering no worse than #6 Few and no spontaneous delamination |
| Salt Fog Resistance(scribed specimens) | When X-scribed using a straight shank tungsten carbide tip lathe cutting tool (ANSI B94.50, Style E), undercutting (per ASTM D1654) shall not exceed 4mm. |
| Cyclic Weathering Resistance(unscribed specimens) | After 15-336 hour cycles, the coating shall exhibit no blistering and no spontaneous delamination. Color shift (from baseline) shall be £3 DE and gloss reduction shall be a maximum of 50% of baseline. |
| Cyclic Weathering Resistance (scribed specimens) | X-scribed using a straight shank tungsten carbide tip lathe cutting tool (ANSI B94.50, Style E), undercutting (per ASTM D1654) shall be reported. |
| Taber Abrasion Resistance | CS-17 wheels, 1000 gram load, 1000 cycles. Results (in milligrams of weight loss and the Wear Index) shall be reported. |
| Tensile (pull-off) Adhesion | Coating shall exhibit a minimum pull-off value of 4.1 MPa (~600 psi) when tested per ASTM D4541 using a self-aligning, pneumatic adhesion tester. |
| Freeze Thaw Stability (via Tensile [pull-off] Adhesion) | After 30 thermal cycles, adhesion testing shall indicate that there has been no reduction in the adhesion values, when compared with those obtained in Test No. 5, (which exceed the allowable test variation in ASTM D 4541). |
| Coating Composition | There are no acceptance criteria. The purpose of the compositional testing is to establish baseline values of the tested batch for future batch verification, as required. |
| Two-Year Atmospheric Testing (Oceanfront: Kure Beach, NC) | No creepage from the scribe, no bold surface rust and no blistering. Color shift (from baseline) shall be '3 DE and gloss reduction shall be a maxi
Important Update: Bridge Design & Engineering MagazineFor more than 31 years, Bridge Design & Engineering has been a trusted source of new insights, knowledge, and connections for the global bridge community. With the rapid shifts in the publishing landscape and increasing costs, sustaining the printed magazine at the high level of quality our readers expect has become increasingly challenging. After careful consideration, Hemming Group Ltd has made the difficult decision to discontinue the print publication. You will continue to have full access to the Bridge Design and Engineering website - including the complete archive. Although the site will no longer be updated, the archive remains a rich resource of engineering knowledge and project insight. You can also continue to access the Bridge Directory, IBIG, through the website or directly at: https://www.bdeibig.com/ While this marks the end of an era, it also opens a new chapter, one focused on strengthening the connections, collaboration, and technical excellence that our community thrives on. Our energy and investment will now be directed toward expanding and enhancing our industry leading events that bring the bridge community together. Our flagship events Bridges and Bridges Scotland will continue to thrive as essential annual gatherings for bridge owners, managers, designers, engineers, and suppliers. These events play a vital role in sharing innovation, showcasing best practice and emerging technologies, and fostering meaningful collaboration across the industry. We are excited to build on their success and will further develop these events in the years ahead. For more information, please email enquiry.bridge@hgluk.com |
