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NewGuy2005

Is There a Concrete Finisher in the House?

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I am adding a small 100 sq ft sun room to my house. The concrete was poured over my existing patio in order to raise the slab to the same level as the rest of the house. Plastic was laid down on the existing slab before the new 6" deep concrete was poured on top.

The new concrete took a several hours to cure. The finishers seemed to think the long cure was due to the vapor barrier that was laid between the old concrete and the new.

Now, when you walk on it, the surface of the new concrete is ground into dust. In fact, you can just run a broom across it and brings up dust.

We are going to be laying tile on top of this so we will not be walking on the surface much longer.

Questions:
Will laying the tile solve the problem?
Does the surface of the concrete need to be ground down to where it is more solid?
Should we pour on an epoxy top coat?

The contractor is a good guy and will do whatever is needed, but I want to know what is needed from an independent source.

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Taking several hours to start to tighten up is not that big a deal. Yes, placing the concrete on plastic does slow the hydration/initial set of the concrete, since none of the water could be absorbed by the lower surface. Often, when a vapor barrier is used, a 2" sand cushion is placed on the barrier, but it is not required.

The fact that the top surface is "dusting", sounds more like the the concrete had too high a slump to begin with, and the cement to water ratio was too low and the top surface which is only sand and cement is not that strong. If it is just being used to set tile, I would take a high pressure washer (3000psi minimum) with a "turbo tip" and blast the concrete. It will eat away anything that is not solid. That should expose the aggregate which provides a good surface to set the new tile. Concrete is often considered cured at 28 days after placement. Depending on the mortar/thinset being used to set the tile, some require a minimum cure time prior to setting tile. The concrete can crack during curing and these cracks can telegraph though your new tile.
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Let it cure for at least 20 days, more would be better. Just because you can walk on it doesn't mean it's cured!

Since you can't pressure wash it, I would get a floor buffer with a stiff brush and use that to clean away any loose material. Then sweep and vacuum as necessary before tiling.

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I would not have put the plastic down, no need because existing concrete is not going to absorb much water.

Probably the concrete was way too wet when poured and not finished properly or poor concrete mix. Was this truck, bagged
concrete mix, or self mixed?

You can rent concrete floor sanders but you would have to seal the room off with plastic. Wetting the floor would help with the dust.

With the small area I would just wire brush the floor.

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I disagree with the absorbtion comment,,i have an older home that does not have plastic under the basement slab or the garage slab and when it is very humid both are's sweat,,,while my brother has a newer home ( new code makes plastic a must in our area ) his area's do not sweat at all...( illinois location )
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If the humidity is high and the slab cold, sweating can be condensation. Yes, that much. I have a family cabin on a lake with the slab poured over where there are some underground springs (nearly 50 years ago).

We have a channel around the edge in case there's incursion through the concrete retaining wall; I put in waterproof underlayment (schluter-ditra), and porcelain tile. Ain't no water coming through that. But it still sweats on high humidity days, because the floor is cold from the springs. I control some of that with rugs; they absorb a little bit of water, but more importantly they insulate the change in temperature between air and tile.

No mildew or water collection (including the after effects of super storm sandy) yet. Damp sometimes, yes. A fan and dehumidifier control them. We used to have standing water under the floor (on furring strips).

But I have no inputs on the original concrete question :ph34r:. Haven't played with it.

Wendy P.
There is nothing more dangerous than breaking a basic safety rule and getting away with it. It removes fear of the consequences and builds false confidence. (tbrown)

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I think you need Concrete Rebound Hammer. It's on this site, a quick search ought to tell you everything you need to know about it. And there is a -lot- to know about it.
-B
Live and learn... or die, and teach by example.

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lurch

I think you need Concrete Rebound Hammer. It's on this site, a quick search ought to tell you everything you need to know about it. And there is a -lot- to know about it.
-B



I dunno.

Concrete Rebound Hammer seems to know everything about everything...

Except concrete.
"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

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You have a weak layer of concrete that needs to be removed. Have your contractor use a 7" grinder with a diamond blade and lightly grind to remove this layer. There are grinders available with vacuum attachments that will reduce the dust by 95%. After this layer is removed you can proceed. I have been in the business for over 25 years and this will solve your problem.

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Yes the Poly added to the issue. If they were worried about the existing slab being to "hot"(dry and thirsty), they could have sprayed it with water before pouring.

Mix could have been too wet.

But more I suspect that because of the aforementioned issues the the finishers simply got on it to soon mixing the bleed water into the surface. now just let it cure awhile and there are several types of sealers out there if need be after scraping the surface for loose concrete.

You do not mention location, humidity, ventilation ect
That spot isn't bad at all, the winds were strong and that was the issue! It was just on the downwind side.

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