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Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)
3

Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

(OP)
Hi all. I've been doing a fair amount more pier and wharf structural design. Although it's similar to bridge design, there are some nuance differences that I am trying to understand especially with the materials. I am designing a small aluminum boarding pier and it is supported by a concrete pile cap. I've always been taught to avoid putting aluminum in contact with concrete. This boarding pier is in the ocean.

See attached detail. My aluminum transverse girder will be welded to a bearing plate that is anchored into the concrete pile cap via stainless steel anchors. Currently, I have shown a 1" non-shrink grout material between the bearing plate and top of pile cap. I'm not entirely happy with it. Ideally, I would like to replace that 1" grout with something more like a neoprene bridge bearing pad. However, my coworker doesn't' seem to think that will have much durability in that environment. In speaking with some of the aluminum pier manufacturer's, they mentioned Ultra High molecular Weight Polyethylene as a separation material. But I'm not sure of its ability to support weight. The aluminum pier weighs 23,000 lbs.

I need something durable that won't dry out, crack, delaminate, etc. Also something that can provide bearing support for the structure.

RE: Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

You can use a non-cementitious epoxy grout or place an asphaltic membrane below the base plate to separate.

RE: Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

Ron is on the right track about using an epoxy between aluminum and concrete but you should also examine your use of stainless anchors in concrete. Crevice corrosion is likely to be a major issue. Stress corrosion cracking may also be a problem. Use steel fasteners and protect them or (since your quantity is small) use a salt water tolerant material such as Monel.

RE: Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

UHWM Poly is very tough stuff, but this is a very difficult situation.
Their are elastomeric bridge pads you could use those, though epoxy sounds better.
TBE is right about SS. Threaded SS fasteners will fail either by crevice corrosion or chloride stress cracking.
The choices are either very high alloy SS (6%Mo superaustenitic or superduplex) or Monel (preferred for this).
Regardless make sure that this will drain well with no pockets or ledges.

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P.E. Metallurgy

RE: Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

(OP)
Thanks for the information all!

Ron, I do actually like the idea of the epoxy grout/mortar as opposed to the cementitious non-shrink grout. I don't use epoxy mortar often, only for the nosing of bridge deck joints. That stuff takes a serious beating from over 175,000 vehicles each day. so I know it to be durable. I guess the lower compressive modulus is actually not a bad thing as I am looking for something softer.

Tugboat,EdStainless.... now you guys have me worried. Can you help sort out my concerns. Where do you think that crevice corrosion will be an issue? Between the SS and the aluminum faying surface or the SS and the epoxy? Would it be better to cast the anchors in place instead of setting in epoxy?

In my past pier projects, I have specified 2205 stainless steel threaded rods, nuts, and washers. I know that this is not a readily available (if at all) item so we specify that they need to be machined from 2205 stock. The Contractors hated me and fought me on this but in the end complied. My understanding was the 2205 duplex grade was a good stainless steel type for both crevice corrosion and stress cracking. For this job, since it's small I was going to specify 316LN. It should be noted that none of these threaded rods are in direct tension/stress. Do I still have a concern for stress corrosion?

I have never heard of Monel. I will look into that. Is there an ASTM for it? Is Monel a proprietary trademarked name? Although my application is pretty lightly loaded (will need to account for some extreme event lateral seismic loads) can Monel be used for a structural fastener?

RE: Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

The locations of concern are the threads themselves.
2205 has enough SCC resistance that cracking will not be a concern. And it does have better corrosion resistance than 316 so that is helpful.
You should use a good thread compound in order to seal this area from salt ingress. the other approach is to use a synthetic grease and caps over the bolt/nut.
Where the SS is in the concrete isn't a concern, the high pH will suppress corrosion of the SS.
Monel used to be a trade name, but it is used generically now, the alloy is about 65%Ni35%Cu. The alloy is 400, and it is a common marine service alloy. Bolts and hardware are in stock different places. I even found online prices with a quick search. This material will be fully corrosion resistant and tough as heck, not super strong but should be fine for this application.

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P.E. Metallurgy

RE: Separation Material Between Aluminum and Concrete (UHMW Polyethylene?)

(OP)
All. Thanks for the help. I did decide to go with the epoxy grout beneath the bearing plate. I also am specifying a bitumastic coal tar epoxy to be painted on the backside of the remaining aluminum shapes in contact with the concrete.

Contacted some manfuacturers of both 2205 SS and Monel hardware to get information. Still haven't decided yet which one I'm going with.

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