paint system coating thicknesses 1

[doganarslan]

Are there any references at AASHTO that give ideas of paint systems' and coating layers' thicknesses for steel bridges? I found some SPCC and FHWA guides but couldn't see this thickness info like summarized attached specification table for an Korean bridge.

Dogan

 

[STrctPono]

We follow NEPCOAT. It's a consortium of North East States that got together and established some pre-approved bridge coating systems. These are all standard polyurethane topcoat type systems.

Link

We typically follow List B (3 coat system) for large elements such as 6ft deep plate girders that cannot be practically zinc dipped in any kettle in the US. Paint specs are typically very comprehensive documents. I think ours is something like 20 pages so there is so much more to it than just following this paint system. We are currently looking at incorporating fluoropolymers in our specs to replace polyurethanes.

 

[dik]

Good Great link... I have a couple of the Carboline systems in my current Coating Types...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[doganarslan]

Thanks for the replies STrctPono and Dik.

I found a paragraph regarding this topic like copied below in FHWA's "Steel Bridge Design Handbook - Corrosion Protection of Steel Bridges - FHWA-HIF-16-002 - Vol. 19" guide. And it is also refering to NEPCOAT. I think it will be OK to use this study.

Most agencies maintain their own unique set of qualification factors for proprietary coatings.
Sometimes these factors are state-specific. Cooperative regional working groups have also
shared resources to develop common qualification and approval lists (e.g., the Northeast
Protective Coatings Committee, NEPCOAT). These systems employ a battery of standard
accelerated “torture tests” which attempt to mimic years of harsh exposure over the period of a
few thousand hours in a test cabinet. In recent years, the bridge community has established a
nationwide cooperative testing program for bridge paint performance. This program, the
National Transportation Product Evaluation Program (NTPEP), is maintained by the American
Association of State Highway and Transportation Officials (AASHTO) and provides the first
national level clearinghouse for bridge paint performance data under the Structural Steel
Coatings branch of the program.

 

[dik]

I wasn't aware of NEPCOAT... a useful, and IMHO correct data link... no exceptions to it. STrct deserves a BPS for the link.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

Thanks dik.

Yes, NEPCOAT is well respected and accepted. I did speak incorrectly, earlier, when I said that it was all polyurethane based topcoats. They actually do have some polysiloxane and polyaspartic topcoat systems, but for whatever reason, they are only approved with their 2 coat paint system. The 3 coat paint system is pretty much the gold standard but polyurethane topcoats are popular because they are relatively inexpensive, not because they are the best.

One more thing, and I tell you this because I learned this when I did my first steel bridge and it is not very obvious if you read AASHTO.... In order to achieve the slip coefficient that you desire for slip critical bolted connections, the faying surface can ONLY be primed. No topcoat will pass the slip coefficient test. So you will need to provide a paint blocking detail in your specs or on your plans or at least stipulate it with words which connections are considered slip critical and how to block the paint. You will notice that in the footnote of the NEPCOAT website.

 

[Settingsun]

STrctPono, that faying surface bit is an engineering conundrum. On the one hand, the risk of corrosion is so great that a fancy coating is needed. On the other hand, critical connections are just too difficult.

Top coats may not pass reliably, but I once had a headache that crane rail clips were delivered fully painted including friction surfaces. After the contractor trialled removing it and made a giant mess, some were sent for lab testing. Friction was found to be adequate.

(That rail is working hard after the contractor got the anchor bolt projection wrong and only got the nuts half on.)

 

[STrctPono]

steve, Interesting! You guys actually paid to have the samples tested for friction!? Sounds like this was a major error and you must have had lots of rail clips for testing to be worth it.

 

[Settingsun]

The details are a little hazy but there would have been 1200-1400m of rail and it was one of those situations where the daily cost of not operating is pretty high.