Concrete attack of aluminum.

[kenvlach]

Hi,
In another thread, I warned of concrete attack of aluminum due to high pH:
Aluminum column base plates
thread507-77669

I wonder if anyone in this forum has experience with concrete corrosion of aluminum, and whether a concrete sealer would offer sufficient protection. Feel free to advise, offer references, etc.

 

[dik]

For my thesis on crack propogation through concrete beams in torsion, the first samples used aluminum foil for the conductor... bad choice!

 

[ishvaaag]

Here sometimes aluminum frames to be left in sight are placed where mortars are yey to be applied. Knowing this the aluminum -likely the extrusion- providers put shapes within a plastic film, about 0.2 or 0.2 mm thick. This uses to protect enought the alumninum if there's not abrasion of the film.

Even if the galvanic pair always will form with different metals, it seems then that the more worrying part of aluminun corrossion comes when the cement is fresh, and so efficient isolation from fresh mortar and concrete is the more important thing. For the long term proximity, cathodic protection must be always feasible with a corrossion of so definite origin, yet keeping order between parts and not joining uncompatible things always helps.

 

[jimbo]

When I was much younger, I intended to use an aluminum railing on concrete stairs.

An older structural engineer said "never use aluminum in/on concrete". No explanation, just NEVER.

So I didn't and don't. I did use stainless steel.

PS I am not a structural engineer. I was using a "canned" stair design by a reliable state agency.

 

[ishvaaag]

Really in more than the corrossion problem one would have to consider how the aluminum railings used to be made ... and were quite ubly and maybe far more prone to be dismembered than steel ones, even if not stainless steel. So it was good advice.

But there are lots of windows, curtainwall etc on aluminum and live the whole life of the buildings unharmed, in fact much less harmed than many other items.

 

[structuresguy]

Hi all. Just a follow-up here to the other thread which kenvlach is mentioning. One of the other engineers at my company told me he spec'd coal tar epoxy coatings for the aluminum to protect it from concrete and other nasty chemicals at water and sewage treatment plants for many years.

After doing a little web research, I decided that this would be a good way to protect my aluminum columns. Simply coat the bottom few inches with this coal tar epoxy prior to installation. That way the aluminum is separated from the fresh grout.

 

[kenvlach]

An interesting and serious case of concrete attack on aluminum is posted at

http://www.finishing.com/261/89.html

The contractor took the following precautions:
1) Using Al coated with a baked-on polyester enamel “Morton Poyceram 1400.” However, no sealant at the post base where set into the grout.
2) Setting the aluminum posts in "a polymer modified shrinkage compensating hydraulic cement-based product…formulated not to contribute to the corrosion of Aluminum," Bonsal Commercial Anchor Cement.

The problem: corrosion of the posts is causing spalling of concrete from 8th floor balconies. “The coating has become a white powder and has separated from the post and is attached to the grout. The posts are pitted at the corroded areas.”

The contractor thinks salt corrosion may be involved – site is 2 miles from the Gulf of Mexico. My opinion is that effect of the salt is minor – the real cause is fresh concrete attack of the both the coating and the aluminum. However, I am not familiar with either branded product, so rather than offer an iffy opinion, I hope to learn something. Can the concrete experts here explain?
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