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Aluminum in Concrete - no good

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Aluminum in Concrete - no good

Postby sru_tx » Tue Sep 05, 2017 12:27 pm

While selling furniture in my booth, I am often asked about placing some of my steel/wood pieces outside. Typically I say that I don't recommend it. at all. ever. But seeing how often I get the question I have started working on a new idea for a table that can be placed outdoors.

My thought process went something like this:
What metal? Aluminum? doesn't rust (check) already have some (check)
What to make the top out of? Cast concrete? Weather tolerant (check) Artsy (check)

Marriage made in heaven... or so I thought.

So I put together a prototype and take the unfinished piece to this weekend's market just to gauge response. Interestingly a vendor friend dropped by and over the course of conversation I find out that aluminum and concrete don't mix. The alkali nature of concrete attacks the aluminum. The resulting corrosion doesn't stop once the concrete has cured. It just keeps going, corrosion expanding, until the concrete breaks. Doing research online I now find that it's commonly known issue (electrical conduit in slabs, fence posts in concrete) and just isn't done unless it's coated with a bitumen (asphalt) material or possibly some other coatings.

Question: has anyone here (fence/gate builders?) successfully placed aluminum in concrete? recommended coating material?

The simple solution is to switch from aluminum to steel or stainless but I like the look and have a frame ready to be cast.

thanks.

steve

here's a pic of the rough prototype. no cleanup no polishing.
concrete_proto.jpg
concrete_proto.jpg (83.42 KiB) Viewed 558 times
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Re: Aluminum in Concrete - no good

Postby cj737 » Tue Sep 05, 2017 12:33 pm

You can certainly treat the concrete with a sealer or stain/paint to separate it from the aluminum. You can also treat the aluminum with paint, powder coat, or epoxy. But raw-raw won't last (as you discovered).
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Re: Aluminum in Concrete - no good

Postby sru_tx » Tue Sep 05, 2017 1:03 pm

Sealing the concrete would only help if the curved support was mounted on the concrete pads. In this case an aluminum frame is embedded inside the concrete on each end to support the piece and the concrete. I'm testing a roof patch material but it's such a mess to work with. I'll check into an epoxy paint. thanks
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Re: Aluminum in Concrete - no good

Postby Mattwho777 » Tue Sep 05, 2017 2:33 pm

sru_tx wrote:While selling furniture in my booth, I am often asked about placing some of my steel/wood pieces outside. Typically I say that I don't recommend it. at all. ever. But seeing how often I get the question I have started working on a new idea for a table that can be placed outdoors.

My thought process went something like this:
What metal? Aluminum? doesn't rust (check) already have some (check)
What to make the top out of? Cast concrete? Weather tolerant (check) Artsy (check)

Marriage made in heaven... or so I thought.

So I put together a prototype and take the unfinished piece to this weekend's market just to gauge response. Interestingly a vendor friend dropped by and over the course of conversation I find out that aluminum and concrete don't mix. The alkali nature of concrete attacks the aluminum. The resulting corrosion doesn't stop once the concrete has cured. It just keeps going, corrosion expanding, until the concrete breaks. Doing research online I now find that it's commonly known issue (electrical conduit in slabs, fence posts in concrete) and just isn't done unless it's coated with a bitumen (asphalt) material or possibly some other coatings.

Question: has anyone here (fence/gate builders?) successfully placed aluminum in concrete? recommended coating material?

The simple solution is to switch from aluminum to steel or stainless but I like the look and have a frame ready to be cast.

thanks.

steve

here's a pic of the rough prototype. no cleanup no polishing.
concrete_proto.jpg


Maybe try powder coating
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Re: Aluminum in Concrete - no good

Postby Poland308 » Tue Sep 05, 2017 7:05 pm

Cast the concrete with galvanized or ss bolt threads sticking out. Then just bolt the top on with some plastic shims between the concrete and aluminum.
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Re: Aluminum in Concrete - no good

Postby MarkL » Wed Sep 06, 2017 10:43 pm

Put a heavy garbage bag over the aluminum before you pour the concrete. As long as the bag doesn't rip during the pour, the aluminum will never be exposed to the chemistry of the concrete. The point where the aluminum enters the concrete will be a problem because if you just cut the bag flush with the concrete, there will be a small bit of contact between the two. Maybe wrap the point where that interface occurs with electrical tape which will extend beyond the concrete.
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Re: Aluminum in Concrete - no good

Postby Arno » Thu Sep 07, 2017 2:12 am

MarkL wrote:Put a heavy garbage bag over the aluminum before you pour the concrete. As long as the bag doesn't rip during the pour, the aluminum will never be exposed to the chemistry of the concrete.


True, but a secondary problem can occur where water (condensation, seepage) gets between the plastic and the aluminium and gets trapped shutting sections off from oxygen. The outer portions still remain exposed to oxygen and this sets up a kind of galvanic internal 'cell'.

This then sets off a crevice corrosion process on the aluminium which basically 'eats away' all the aluminium and likely will crack the concrete with the resulting aluminium salts that are deposited and take up more space.

Coating the part of the aluminium and probably up to an inch or so above the level of the concrete with a tough, chip-resistant paint or surface treatment (anodizing is no use. it gets eaten too) that fully seals the surface from any later water ingress is the key.

In this case with a ornamental piece something like a silver coloured epoxy or powdercoat on the area that gets cast into the concrete will probably be a good option and should not be obvious once it's all set.

For dry/indoor constructions you can actually have 'bare' aluminium cast into concrete as long as the concrete cures and dries out completely. This then basically removes the caustic environment as there's no more water to facilitate the ion transfer and it won't continue eating away the alu. Once water gets into the equation it starts to go again though..

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Re: Aluminum in Concrete - no good

Postby sru_tx » Thu Sep 07, 2017 9:46 am

Thanks for the responses.
Powder coating is a good idea but I'd like to find something I can do in-house to keep the costs low. I realize that I will need to change the internal foot design to be much cleaner and easier to ensure that all surfaces are coated. My prototype has so many nooks and crannies it will be impossible to fully coat.

Arno, I didn't realize the chemistry required moisture. I found lots of info regarding aluminum electrical conduit buried in concrete slabs. From all of my reading I didn't get the impression that moisture and curing had any effect. Ultimately I need to clean up the design and coat coat coat.

I will clean up the design and then try a thick coating of epoxy paint or possibly a spray on plastic coating like a tool-dip product.

thanks
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Re: Aluminum in Concrete - no good

Postby MarkL » Thu Sep 07, 2017 3:23 pm

Arno wrote:True, but a secondary problem can occur where water (condensation, seepage) gets between the plastic and the aluminium and gets trapped shutting sections off from oxygen. The outer portions still remain exposed to oxygen and this sets up a kind of galvanic internal 'cell'.

This then sets off a crevice corrosion process on the aluminium which basically 'eats away' all the aluminium and likely will crack the concrete with the resulting aluminium salts that are deposited and take up more space.

If this occurs, how can aluminum be used in marine applications where part of the aluminum is submerged and part is above water?
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Re: Aluminum in Concrete - no good

Postby Poland308 » Thu Sep 07, 2017 11:59 pm

Galvanic corrosion is a big issue with boats. But aluminum is lighter than ss so it's often used despite this fact. It's worse in salt water than in fresh.
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