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October 13 - October 17, 2014

A specification for a bridge to be built of high-performance weathering steel requires an SSPC-SP 10, a profile of 3 to 5 mils, and metallizing with a final DFT of 10-12 mils. What challenges, time, and costs are associated with obtaining the profile?

Selected Answers

From Kevin Hahn-Keith of STV on October 30, 2014:
For the specifications I am familiar with, there are no requirements for the anchor profile on blasted weathering steel. I was told the anchor profile cannot be measured accurately on blast-cleaned weathering steel. The surface standard for these areas is the appearance of SSPC-SP10 when viewed at 3 feet. The contractors I have inspected on bridge rehabilitation projects have been able to achieve this standard, but it takes about twice the time as removing paint from previously painted surfaces. An additional layer of intermediate paint is used to cover the peaks on the rough surface. The areas being painted are the stringer ends and fascias exposed to salt air and road salts. In general, I have found that the painted surfaces last much longer than the weathering steel in the Northeast. The weathering steel just looks better, because you don't see the contrast between the paint and the rust.

From William Feliciano of NYS Dept. of Transportation on October 30, 2014:
Not to answer your question with a question, but is there a chance that a 3-5 mil profile is excessive for a 10-12 mil thermal spray coating? If you reference AWS C2.18 93, it specifies a 2-4 mil profile for TSC up to 12 mils thick. Beyond 12 mils, it recommends using a profile depth approximately one-third the TSC thickness. This may be a somewhat dated spec, but its technical recommendations appear sound. As important is the angularity of the profile. I advise caution, because many fabrication shops use a mix of steel shot and grit to reduce wear on their wheel blast machines. TSC coatings, since they depend on mechanical adhesion, requires strict adherence to angularity of profile. If this is the case in your shop, you may have to consider blasting the steel manually with grit alone rather than a mix of grit and shot. Be aware that SSPC-CS 23.00 as well as AWS C2.18 require the use of bend plates to verify adhesion of whatever blasting procedure/materials you settle on. I would think that blasting new weathering steel would not be much different than normal, new carbon steel, since both have millscale, unless the weathering steel is of higher hardness.

From Tom Schwerdt of TxDOT on October 27, 2014:
The answer to these salty areas may well be ASTM A1010 structural stainless steel. Yes, the plate cost is much higher than weathering steel, but the difference in fabricated cost is less shocking and the difference in actual installed cost isn't that bad when you look at the cost increase for the bridge as a whole.

From Karen Fischer of Amstar of Western New York, Inc. on October 21, 2014:
It escapes me why an owner in the snow belt states or near salt water areas would ever specify weathering steel. De-icing salts and salt water are detrimental to weathered steel. In our area, many of the owners have found that the bridge never stops rusting unless you apply a sealer immediately upon erection and every year or two, which  defeats the purpose of the weathering steel (to reduce the cost associated with regular painting schedules). Once that salt gets into the profile, it is near close to impossible to "wash" it all out. But getting back to the original question, why would you metalize a weathered steel bridge if the weathered steel is supposed to be the solution to an issue? A profile of 3-5 mils can easily be accomplished with 40 grit but you are likely to achieve an SSPC-SP 5.

From Lydia Frenzel of Advisory Council on October 18, 2014:
Why are you coating weathering steel? Is the bridge to be constructed over salt water or marshy land, where weathering steel might not be the best metal for the project. If you blast to SSPC-SP 10 and then coat, are you getting any benefit from using weathering steel?

From Tom Schwerdt of TxDOT on October 15, 2014:
Warren, FHWA does recommend painting weathering steel beam ends under joints. In heavy road-salt areas, this makes sense. Salt exposure and high time-of-wetness are the main failure modes for weathering steel. In the vast majority of Texas, you do not need to paint beam ends. All you need is proper detailing (drip tabs, no pooling, etc) and some little trays under the bearings to divert water and prevent column/pier/cap staining. If everything works right, you don't even need the trays if you have proper detailing, but I prefer to have both as the cost is tiny if they are installed during construction. As an aside, I have NEVER seen painted ends properly color-match weathering steel, and the color difference gets worse every year they are in service - the weathering steel darkens and the paint fades.

From Warren Brand of Chicago Corrosion Group on October 14, 2014:
Is the application being done in the field, or shop? If in a shop, you had better be able to control the environment and  hold the blast. Abrasive blasting to these strict guidelines will be a function of staging logistics, blast nozzle size, number of blasters, CFM of air and abrasive. Obtaining a 3 -5 mil profile will likely provide an SSPC-SP 5. An excellent reference would be "Joint Standard" SSPC-CS 23.00/AWS C2.23M/NACE No. 12 Specification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel" And, are we sure we want to be coating weathering steel?

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Tagged categories: Asia Pacific; Bridges; Dry Film Thickness (DFT); EMEA (Europe, Middle East and Africa); Latin America; North America; Specification; Surface Preparation; Surface profile

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