concrete filled column

[london95]

I have a concrete filled column that is weeping and rusting from the inside out. Can calcium in concrete cause the steel to rust from the inside out? If so can I stop it?

 

[hokie66]

If the concrete contained chlorides, including calcium chloride, it possibly could be causing the corrosion. Are you sure the tube is filled where the rust is showing? If there is a void filled with water, it could be the culprit. Is this an isolated instance? You may have to make a hole in the tube to find out, and then you can take a sample of the concrete for testing. If concrete is in contact, and the alkalinity is high enough, the steel should be protected.

 

[london95]

I am sure the tube is filled. I have more than one area doing this. I also have friend that this is happening to. Could the calcium be wicking water from the ground or is this just water from the concrete?

 

[civilperson]

Concrete is usually thought of as a superior coating for the protection of steel from corrosion. (NO ADDED CHLORIDES ALLOWED and a chloride limit of 0.1 or 0.08 from materials/water). Concrete design always assumes a path for escape for free water, the inpermeable steel column does not allow any escape except out the top or bottom contributing to a constant wet condition and thus a corrosion problem.

 

[hokie66]

When you say you have a concrete filled steel column, do you mean a small column like a Lally column or a big, concrete filled tube column for a high rise building? Size and thickness? And what is the external coating?

 

[london95]

It is a small column 7'-6" and 3 1/2" round, I don't know the steel thickness. The coating is a red paint.

 

[hokie66]

You mentioned calcium. If the concrete contained calcium chloride, which is sometimes used ill advisedly as an accelerant, then that would be your problem. If so, I don't think there is any way the corrosion can be arrested.

If the tube was sealed both ends with plates after the concrete filling, as civilperson said, no venting for moisture would be available, and that could be your problem.

 

[apsix]

A photo could be very helpful.

I'm struggling to picture a filled tube with internal corrosion bad enough to cause weeping through the wall, unless there is something very wrong.
Is it subject to salt water?
Does it have an internal leaking rainwater downpipe?
Is it outside, in contact with soil?
Is the concrete full of voids due to poor compaction/low slump? (I don't expect an answer for this one?)

 

[Ron]

When steel is embedded in concrete, there is a corrosion passivating effect caused primarily by the high pH. That is not necessarily true of steel in contact with concrete but not embedded, with or without excessive chlorides in the concrete.

The interface of concrete with steel in a pipe column will corrode from the inside, particularly if the excess concrete moisture was not allowed to vent as noted by others.

If you have perforation of the steel and weeping, then the corrosion is advanced (obviously); however, it is also likely that the concrete is relatively high in chlorides, either inherent to the concrete materials that were used in the mix or through the addition of calcium chloride as an accelerator (ill advised as noted by others).

You might want to create some additional drainage to allow the moisture in the concrete to escape. Check the moisture content and let it get down into the range of 5 to 7 percent by weight. That is pretty much an equilibrium moisture content.

 

[apsix]

Hokie, Ron
If the pipe was sealed enough to prevent moisture venting wouldn't this limit the oxygen supply, and therefore also limit the corrosion?

 

[hokie66]

apsix,

I think there would be sufficient air in the concrete to facilitate corrosion. It is not easy to expel all the air when placing concrete in a 3 1/2" tube. That said, we only have the OP's opinion that the rusting is from the inside rather than the outside.