Pack Rust on Steel Bridges 1

[bridgebuster]

I'm working on a bridge rehabilitation project for a viaduct constructed in the late 40's/early 50's. Some built up floorbeams - photos attached (one photo is an extreme case) - have been affected by pack rust/rust pact/impacted corrosion – call it what you will.

The client is looking for specific remedies to this problem. Their opinion is that the traditional remedy of blast cleaning and painting won't work. In the 80's I worked on a project in Ohio with the same problem. The ODOT spec, in addition to blast cleaning and painting, called for the installation of an acrylic caulk between the steel plies prior to painting.

I would like to know if others have any successful methods of dealing with this, in terms of a coating or sealant.(From a structural standpoint there are locations where we may have to remove rivets; then remove the rust; heat straighten; then install bolts.) There's not a lot of information out there; although I came across differing opinions. One article recommended "meticulous cleaning" (that's not going to happen) followed by an acid neutralizing paint. Another said water blasting is the worst thing since it will only entrap the water

 

[hokie66]

Bridgebuster, didn't you post this before in the Bridge forum? I gave an opinion that some of it, e.g. the bottom flange pictured, will have to be taken apart to remove the pack rust.

 

[bridgebuster]

Hokie - I did have it there; then decided to repost it here. Alas, no response. Maybe the next place for it will be the painting forum.

 

[TVP]

Bridgebuster,

I don't recommend re-posting this to the paint/coatings forum. Any of us that are familiar with coatings, corrosion, etc. will be on this forum and have already read your post. If the others are like me, you haven't received any responses because this is a complicated issue that has a lot of implications relative to acceptable design practices, approval to industry standards/codes, acceptance by local/state/federal governments, etc. I concur with hokie that it appears some of this needs to be disassembled to remove pack rust. I also concur with the advice you were given relative to water blasting.

 

[bridgebuster]

TVP - thanks for the reply. I agree it is a complicated issue that just keeps getting kicked to the side.

I met with the client today & this was one of our discussions. Right now we're leaning towards some type of power tool cleaning followed by heating the affected area to approximately 250 F to dry it out, then sealing it. There won't be any disassembling of the member other than an individual rivet here & there - it's just that there too many potential complications - and no attempt to heat straighten anything.

 

[hokie66]

If you go that route, I wouldn't use an acrylic sealant. My choice would be one of the Sika polyurethane sealants. Suggest you talk to Sika to determine the best one.

 

[bridgebuster]

hokie - we we're discussing sealant types today. One person thought a urethane would absorb water and wouldn't work; another said it was the right material. The ODOT spec I saw from 20+ years ago called for acrylic (I don't know if it was successful); but one of the acrylic products they listed - by Koppers - was discontinued and re-introduced as a urethane. Just wondering why you wouldn't use acrylic. I would imagine urethane has better bonding capability - e.g. Gorilla Glue is urethane based and that stuff really works well.

 

[hokie66]

The acrylic would absorb water moreso than the urethane. Acrylics breathe, urethanes don't. And a polyurethane sealant will last far longer than an acrylic in an external environment. I would stick with one of the big companies like Sika, and ask for their advice. Sikaflex 11FC is the one I prefer, as adhesion is excellent and it is readily paintable. That is the product number in Australia...it may be different where you are. Make sure the sealant joint design is appropriate in regard to width vs. depth, backing rod, and primer...and of course, that the design is actually followed on site.

 

[bridgebuster]

Thanks for the advice hokie. The Sika product is available in the US as well.

 

[TomDOT]

I've never found a solution to pack rust which truly satisfies me. The typical "good enough" answer has been to power tool a minimum 1/2" depth, remove any remaining loose or debonded material, penetrating sealer on the pack rust (possibly with caulk after) - then go ahead with painting the structure.

Urethane is a better choice than acrylic, though I suspect the old ODOT spec was a solvent-reduced acrylic, not the common waterborne acrylic latex. Solvent-reduced acrylic wouldn't be a bad choice, urethane's just better. I wouldn't use a waterborne acrylic.

Pack rust is rarely "solved" for the reason mentioned earlier - you have to disassemble the plates to do so. I have seen one project where pack rust was fully removed in some limited locations because gusset plates were being removed and replaced with beefier ones.

 

[bridgebuster]

TomDot - Thanks for the reply. I agree that the best way to fix the problem is disassemble plats, but that's not going to happen on my project because these are floorbeams and girders - way too expensive. One question - do you believe the "good enough" answer is good enough; does it hold up over fairly under a typical painting cycle of 15 or 20 years?

 

[TomDOT]

Bridgebuster - ask me again in 10 years or so? I have a couple of nice big coastal bridges which were recently repainted. I am certain of the surface prep, we used this approach and they had a lot of pack rust.

I would actually like to see more aggressive pack rust removal, but the structural guys have expressed significant discomfort at having more non-contact area.

 

[bridgebuster]

Thanks Tom for the reply; interesting point about the non-contact area.

In 10 years I probably won't be interested in pack rust anymore.

 

[TomDOT]

What, not going to drive over any more steel bridges?

The first areas painted on the earliest project don't seem to have any issues with the pack rust areas yet. We do have some limited areas on the steel with minor breakthrough on some edges and bleed out of back-to-back plates. Not associated with pack rust. Not surprising to see them, given the complexity and size of the structure. Not enough for the driving public to even notice.

 

[hokie66]

Non-contact areas? I would worry more about the condition of the rivets.

 

[TomDOT]

Rivets in poor condition are routinely replaced with galvanized A325 bolts.

 

[hokie66]

But how do you determine which rivets are in poor condition? All of them that are stretched by the products of corrosion?

 

[bridgebuster]

thanks for the reply tom.

hokie - There's a guideline we use for rivet replacement based on the head size (been floating around on NYSDOT projects for about 40 years) but stretched is a different case; I've never seen it addressed. This leads to the concern about non-contact areas because there's a limit to how much separation a bolt(s) can suck up. It looks like we're thinking the same thing.

 

[TomDOT]

hokie66: My method to determine which rivets (or whatever) need replacing as follows:

1) I notice rivets which look significantly corroded (metal loss) or deformed.

2) I call or email one of the structural guys and ask them to come evaluate and to "use their engineering judgement."

My focus is paint and corrosion. I believe they have guidelines as well, but I haven't gotten into that.

 

[hokie66]

I would think that where there is pack rust, the rivets are corroded as well. The cover plates depend on the rivets to develop the horizontal shear, so if the rivet size is reduced, the bending capacity of the girder is compromised. This could be determined by removing some rivets for examination.