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of Bridge Structural Steel
By Skip Pendry, Focus Market Manager
The Sherwin-Williams Company
The 90,000 bridges built in the U.S. during the 1930s are nearing the end of their useful lives, while another 223,000 interstate bridges built between 1965 and 1975 need major repairs, including painting. Currently, as the Federal Highway Administration is spending $217 billion to repair and rebuild the nation's transportation infrastructure, coatings will continue to play a key role in protecting bridge structural steel from corrosion and extending bridge life.
These coatings are subject to harsh conditions, especially in certain areas. For instance, the geometry of bridges makes bridge bearings susceptible to collecting deicing salts and debris. The bearings are also exposed to a high degree of friction and stress that can lead to premature coating deterioration and corrosion of the underlying steel substrate. Expansion joints are another problem area, because improper design or insufficient maintenance can lead to expansion joints that leak, thus spilling deicing salts and sand directly onto the coated steel structures below.
For many years, highway agencies from a majority of U.S. states have specified in-shop application techniques, mostly of a primer coat to new structural steel, with additional coats of paint applied after the steel was shipped to a job site and erected. Increasingly, it is becoming common practice for a bridge builder to have not only the primer, but often a system's second and third coats applied in the fabricating shop. In fact, many officials report that they expect to convert to total in-shop painting.
IN-SHOP PAINTING ADVANTAGES
In-shop painting of structural steel used in bridge construction has several advantages over field painting:
- the coatings are not subject to weather conditions during application;
- the entire coatings application and inspection process can be more easily controlled;
- there are cost-savings when the processes of fabrication, blast cleaning and painting are all done under one roof, and
- the difficulties of field painting, such as working around other trades, etc., are eliminated, so in-shop painting helps to meet demands of accelerated construction schedules.
MAXIMUM LONGEVITY
Possible problems with in-shop painting can be minimized by taking certain measures up front. Here are some ways to maximize the longevity of the coatings applied in shop:
Design Projects for In-Shop
Painting. Whenever possible, back-to-back angles should not be used in a project because they create surfaces inaccessible for coating. Or, if the angles are set flush and tack-welded, crevice corrosion may occur. If back-to-back angles must be used, the surfaces should be blast cleaned to a minimum commercial blast (SSPC-SP 6), and the fabricator should apply at least one coat of primer prior to bolting up the angles. Normally a zinc-rich or rust inhibitive polyamide epoxy primer is recommended.
Whenever it is cost-efficient, continuous welding of structural members is preferred. Tack, or stitch welding often creates areas of crevice corrosion that lead to problems in aggressive environments. The owner and structural engineer must do a cost-benefit analysis to determine the point at which continuous welding economies are necessary or cost-efficient, since it does add to the cost of the fabrication process.
Bridge coatings often fail around expansion joints, because sand and debris often clog drainage systems and cause standing water in and around the joints. Designing drainage pipe with sufficient pitch can help to alleviate this problem.
APPLICATION AND CURING AREAS MUST BE ABLE TO HANDLE SURGES IN PRODUCTION |
Select Coatings for Bridge
Structural Steel. Coatings applied in-shop range from primer only to the complete, multi-coat paint system. Coatings for bridge structural steel typically must meet the specifications set forth by the Department of Transportation for the state in which the bridge is to be built.
Because of the trend toward in-shop painting of structural steel, coatings manufacturers have been responding to fabricators' needs by developing new products that dry and cure in much less time and at lower temperatures than standard coatings, to help fabrication shops improve production. They are also developing products to meet increasingly strict government regulations through water-based and higher solids coatings that eliminate or reduce the emissions of VOCs. As a result, a number of coatings systems are available that meet all forthcoming environment regulations, provide excellent long-term corrosion control, and can be easily applied in-shop with the proper equipment and know-how.
Most shops/states are using an inorganic zinc system as the primary coating system on steel to replace the traditional red-lead and basic-lead silico-chromate systems previously used. The systems are often comprised of an inorganic zinc primer, a polyamide epoxy intermediate coat and an acrylic polyurethane topcoat.
Recent tests and studies sponsored by the Federal Highway Administration (FHWA) have found that in marine atmospheric exposure testing, multicoat paint systems comprised of an inorganic or organic zinc-rich primer, epoxy intermediate coat and urethane topcoat showed excellent long-term performance and corrosion control.
While applying these coatings is slightly more difficult than the very forgiving alkyd paints of the past, they provide significant performance advantages in harsh exposures.
Zinc-Rich Primers. The function of the primer is to prevent corrosion. Zinc-rich primers are typically specified because of their outstanding corrosion protection properties.
Zinc-rich coatings for steel bridges fall into two general categories: inorganic zinc and organic zinc. Both types of coatings are used as direct-to-metal primer coats in a multi-coat paint system.
USE A MIXTURE OF SHOT AND GRIT, SINCE GRIT ADDS ROUGHNESS TO THE PROFILE |
Inorganic zinc coatings are comprised of zinc metal powder mixed into an inorganic silicate paint binder. This binder can be either solventborne (ethyl silicate) or waterborne (alkali silicate). High solids solventborne zinc-rich primers and waterborne zinc-rich primers have been developed to enable fabricators to reduce VOCs. The concentration of zinc powder in the mixed coating is about 80% by weight for the best performing inorganic zinc paints.
Organic zinc coatings contain zinc metal pigment mixed into an organic paint resin such as epoxy or urethane. The high zinc metal concentration in these coatings creates high electrical conductivity that allows the primer to provide sacrificial protection to any areas of exposed underlying steel.
Historically, inorganic zinc coatings have been specified for in-shop painting of bridge structural steel. However, more states are now allowing organic zinc coatings as the primer.
Intermediate and Topcoat Systems. The function of the intermediate coat is to protect the zinc-rich primer. The function of the topcoat is to provide color and gloss retention, and to protect the intermediate coat. Acrylic polyurethane topcoats provide outstanding color and gloss retention for bridge applications.
Coatings manufacturers are also introducing other intermediate and topcoat systems that have bridge coating applications. Consider the applications for a new edge-retentive, multipurpose epoxy formulated using a modified phenalkamine epoxy resin that cures in temperatures as low as 0 degrees F, and for moisture cure urethane coatings that can be applied at high humidity and cure at temperatures as low as 20 degrees F.
IN-SHOP PAINTING GUIDELINES
Physical Layout. Setting up the physical layout and workflow of a paint shop for fabricated steel is a significant challenge, because unlike production line operations, metal fabrication involves widely varying output levels, and the parts produced are often quite large.
To achieve an efficient shop painting environment, application and curing areas must be able to handle surges in production. This includes establishing areas for blasting, applying the primer, intermediate coat and topcoat, and storing coated steel that has not cured sufficiently for shipping. Controlling shop conditions, such as heat and humidity, and controlling contaminants using air purifying methods is also important.
Surface Preparation. Surface preparation is critical in assuring a smooth finish with good adhesion. Degreasing and solvent cleaning is necessary prior to blasting. Laminations, burrs and weld spatter must be removed and rough welds smoothed. Sharp edges should be ground.
When using centrifugal blast machines, keep abrasives totally free of grease and oil. Once metallic abrasives become contaminated, they impart a thin film of oil and metal fines to the blast-cleaned surface, which may cause adhesion failure of the coating system. It is also preferable to use as the blast medium a mixture of shot and grit rather than shot only, since grit adds roughness to the profile which enhances adhesion.
Storage and Mixing of Coatings. Proper storage and mixing of the coatings to be applied is necessary. Manufacturers generally recommend protected storage in the range of 40 degrees F to 100 degrees F. The ideal application temperature is about 70 degrees F.
Power agitation, even for single-component materials, is recommended. Mix coatings in whole units, not partial batches, because incorrect ratios may lead to premature failure of the system.
Use all the prescribed components of the same manufacturer's system (thinner and clean- up solvent, for instance) to ensure compatibility. Avoid excessive thinning, because it can increase costs and reduce film build.
Handling Steel During Application. Careful handling of heavy, large steel beams is required. Steel should be spaced to allow easy access to each member. Ensure sufficient layout space so rushing is not required to keep up with the flow of steel that passes through.
Skids (or "bucks," used as supports) should be clean so they do not contaminate freshly blast cleaned or painted steel during turning. Skids should be grounded, and topped with an inverted steel angle to keep the contact point between the paint skid and the fabricated steel to a minimum.
Application. Careful application of in-shop coatings can reduce paint failures in the field. The zinc-rich primers now preferred are generally considered more difficult to apply than the traditional red-lead and basic-lead silicochromate systems previously specified for steel bridges, particularly the new waterborne zinc-rich coating systems. The early oil-alkyd coatings were quite forgiving. Waterborne inorganic zinc-rich coatings are best applied only where proper air flow and humidity controls are in place.
Zinc-rich primers can cause spray tips to wear rapidly, recognized by changes in the spray pattern or by film build problems such as sagging. Tips should be periodically inspected and replaced.
Edges of beams and angles should be properly striped prior to application for the full spray coat. Applicators should not drag paint hoses over freshly coated steel, walk on the surfaces or handle the steel before the coating is sufficiently cured. Sharp edges, hinges, the bottom flanges of I beams, and troughs are other areas that must be carefully coated.
Prior to spraying the primer, the steel should be inspected for areas that will be difficult or tight to spray paint (end clips, gussets near a flange, or nuts and bold heads.) These tight areas should receive a brush coat prior to the spray application.
Both primer and topcoat should be specified with minimum and maximum thicknesses. Too little coating usually causes premature failure in service, while excessive thickness may lead to "mudcracking" sometimes associated withzinc-rich primers or to coating delamination. Fabricators painting structural steel must be careful to avoid too much film thickness of flat and web areas of structural pieces and too little thickness on the vertical and flange areas.
Sometimes, the spraying technique used when T-braces are coated leaves uncoated areas on the edges. This occasionally occurs on the outside edges of flanged steel. The painter should be made aware of this potential problem so that the edges will be properly coated.
The first and last few feet of I beams or other structural members tend to have a lighter or lower coating thickness than the midsection because the painter usually coats the first few feet then moves down the beam, overlapping as he goes. Overlapping does not occur with the first and last few feet.
Curing. Allow sufficient time for curing under controlled conditions; usually, this is 8-24 hours. For waterborne inorganic zinc rich coatings, sufficient air flow is critical, and high humidity during curing must be avoided. Where topcoats are applied in-shop, proper curing of the primer must be assured.
Moving and Storing Coated Steel. Nylon slings or chain hooks should be used to minimize abrasion when steel is handled shortly after coating. Stacked steel should be protected with softeners (such as carpet remnants) to minimize damage. During transit, the steel should be securely anchored to truck or rail trailers to prevent damage to the coating.
CONCLUSION
Bridge replacement and repair is on the rise, and in-shop painting of bridge structural steel is increasing. It is important to remember that careful shop setup, design of bridge steel, and coatings selection and application can improve productivity for fabricators and extend service life for bridge coatings.
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