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Clay unintentionally included in concrete mix

Clay unintentionally included in concrete mix

Clay unintentionally included in concrete mix

I have a unique situation that came up in the last week, that I'd appreciate some opinions on.

We have a wastewater tank that has bolted steel glass lined walls with a concrete floor. The slab was poured about a year ago, and there are several concrete spalls 1"-2" in diameter dispersed randomly across the tank floor with no obvious external cause. We had some cores taken of the concrete, including directly at a couple of the spalls, and the preliminary tests have found a small chunk of bentonite clay at the base of the spall. Concrete quality appears to be normal other than the clay.

We theorize that the concrete ready-mix plant was scaping the bottom of the aggregate pile and let some clay get into the mix.

The tank has an aeration grid installed across the slab, which complicates the placement of some sort of coating, but the aeration grid can be removed if needed. We are discussing recommending a poly-urea coating, but if water were able to find its way under the coating, say at the seal between the coating and the glass lined wall, and swell more of the bentonite, it could possibly delaminate the coating. I'm not too concerned with the structural capacity of the floor, since its primary function is water-tightness, although we do suspect that the perimeter ring-wall also has clay mixed in, but it would be well contained by reinforcement (except for the outer 2" clear cover). We suspect that the underside of the slab is also covered by random spalls as well.

Has anyone come across this problem before? What sort of things might I not be considering? Does anyone have any suggestions for repair of this problem or opinions on the effectiveness of a poly-urea coating?

Thanks in advance!

RE: Clay unintentionally included in concrete mix

On one of the few projects where I chose not to use vapor barrier under the grade slab, numerous popups showed through the vinyl tile flooring in rather a large number of locations. They were deemed to be a result of coal particles in the aggregate which expanded when wet, but according to the testing agency, the percentage of coal particles was within allowable limits so no action was taken against the concrete supplier.


RE: Clay unintentionally included in concrete mix

I have seen one or two driveway slabs which had similar popups of about one inch in diameter. The owner was not pleased, but no action was ever taken so far as I know to correct the problem.


RE: Clay unintentionally included in concrete mix

I've heard of every kind of coating in the world (not really). Every couple years, a new one comes out, promising to cure the world's ills. Polyureas are hard to apply, but once they're on correctly, they're great.
But I'm not sure the coating will help. Is it supposed to seal the slab, to prevent the pop ups from, well, popping up. There's probably enough trapped moisture already in the concrete to activate the clay. And the polyurea won't help that. And if the project has any kind of groundwater, the moisture will come up from the soil.
I'd go with the laborious approach. Get those pop ups, dig out the clay chunks and repair them one-by-one with a Sika product, like Sikatop 123. This might be the owner's issue for a long time. The GC should be responsible for some of the costs, but if it's more than a couple of years, good luck with that.

RE: Clay unintentionally included in concrete mix

Locate the pop-outs and chisel out with a low velocity chipping hammer and patch with Sikatop 123, as suggested... That's about the best you can do without replacement... I've used 123 for almost decades... and it's an excellent patching material.


RE: Clay unintentionally included in concrete mix

Thanks for the responses... the hangup I have with chipping out each of the pop-outs is the shear amount of labor that would be involved. There are two tanks that are affected with each one being about 115' in diameter, and these pop-outs are roughly spaced two or three feet apart on average, so we're talking about repairing a few thousand pop-outs. At that point, I would think it could be less expensive to break out the entire slab and repour everything inside the walls of the tank. I'm also concerned about the possibility of leaks occurring at the pop-outs that most likely exist at the bottom side of the slab against soil.

Since these pop-outs are the result of rain water, I'm not convinced that there are not more pieces of clay that haven't yet swelled and created pop-outs. Once the tank is put into service, it will be exposed to water close to 100% of its lifetime, and leaks from the floor of the tank are not always obvious or even detected. This is why I was initially leaning towards a coating to restore the water-tightness of the floor. Hope this helps you understand my dilemma.


RE: Clay unintentionally included in concrete mix

What is the cost penalty (high!) of doing that, compared to the loss of volume of shot-creting a 1-2 inch "wall" of new concrete inside the tanks? The spalling pieces would push into the new shotcrete inside wall "liner", but not by a great enough force to break the "wall".

The original wall has spalls, but ought to be still strong enough to serve as a gravity retention wall.

RE: Clay unintentionally included in concrete mix

It seems that you have a clear case of defective construction (materials), I would suggest that you ask the contractor to replace the slab.


RE: Clay unintentionally included in concrete mix

Here is a defect I have seen before or more than one occasion....

On residential jobs, I have seen builders pour slabs with the use of a pump truck. They will often have leftover concrete or maybe a little pump time leftover if the pump price was negotiated at a flat fee.

Anyway, they will pour flatwork (driveway sections & walkways) with the extra concrete and use the pump truck.

There are no vapor barriers under the flatwork sections, and often no leveling sand or base material other than natural earth. When concrete blasts out of the pump's elephant trunk and hits the ground; it can displace large blobs of clay (in Texas) into the wet concrete. Some of these mud blobs will end up near the surface and create large holes. I've seen 1"-2" golfball sized holes all the way up to 3-4" baseball size holes.

RE: Clay unintentionally included in concrete mix

CVG - That will definitely be an option presented to the client, but I want that to be the client's call, because it would probably lead to pretty severe push-back from the contractor if we ask them to rip it all out. I'm planning on taking the position that I'm only going to lay out the options I see available and the impacts each option has.

RE: Clay unintentionally included in concrete mix

well, if you are representing the owner, than the most prudent thing to do is recommend reconstruction since there are limited options for repair, no guarantees and none are very good. Even if you are able to fix the swiss cheese on the top of the slab, you may still have issues with the bottom and with the ringwall with accelerated corrosion of rebar at the popouts. Another option would be to have the contractor purchase a bond to cover potential future repairs (maybe for 10 or 20 years). If the repair works, than good for the contractor, if not than the owner cashes in.

RE: Clay unintentionally included in concrete mix

To add what cvg wisely said, have the contractor suggest the fixes. You can review them. If you suggest a polyurea or a Sika product, they might do it, and if it doesn't work, it's your fault. Let them fix their problem.

RE: Clay unintentionally included in concrete mix

Thanks everyone once again for the advice. In case anyone is interested, further testing has revealed that at the base of each pop-out was found a piece of andesitic tuff course aggregate which are very absorbtive.

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