We have retrofitted out home with in floor heat in a 1.5-2" suspended slab. I had planned to used self compacting concrete but we had to reject the batch delivered by the cement company as it was completely segregating. Turns out the company that said they knew how to make scc didn't have a clue. So we poured conventional concrete. However, I did not think to removed the forms I had placed at doorways to stop the scc from flowing from room to room. There are six breaks in the concrete throughout the house and now that the slab has cured the concrete is lifting/moving under weight at the joint in the slab. I am thinking I need to break out the concrete a foot or so on either side of these dams/forms, remove the dam and then try to tie concrete together by pouring in Quik Level or some similar self levelling, quick curing concrete (glue down flooring is scheduled for next week) with primer. Is this a good idea or does anyone have a better idea? I should add that the floor in the house was all over the place so I used varying thicknesses of under slab insulation to keep the slab at 1.5 to 2" thick. I suspect this foam board insulation is part of why the slab is moving at the joints.

Zad - what was used for the dams? If it is narrow and removable, try pulling it out and filling the gap with an epoxy. This should bond the concrete to each other and to the floor.

Don't be in a rush to glue things to fresh concrete. Let it dry and cure.

The dams are 2x4 on edge, ripped down to 2". The floor was poured a month ago and the radiant heat has been on for the last week. The finishers come next Tuesday however I could have them start with crown moulding trim in the rooms with tile. I plan to do a CaCl test over the weekend or use a meter if I can get ahold of one. What type of epoxy would you recommend using? Hadn't thought of that at all and I like the idea.

I spoke with the concrete placer and he called it "concrete curl" where the top part of the slab dries faster than the lower part forcing the slab to curl upward. He has seen it many times when plastic is used under concrete, as we did. He suggested cutting the top of the slab where the curl starts and letting it fall. The problem is that there is radiant tubing in the slab and in some areas it is really close to the surface. I still like the idea of using epoxy, just need to find a source in bulk as it will take a fair bit to fill the spaces left by the dams.

Zad - check with Hilti. Also any of the large basement repair people will use epoxy to fix basement cracks.

Some of the flooring (ceramic) place may also carry it.

If it is turned up at the ends, chances are you can jump on it to crack it. This would not hurt the pex.

Another product that would work is Ardex Feather finish. We have used it to repair thin slab cracks. It sticks solid.

With poly under the slab, I would not be gluing down anything for at least two months and probably three. Concrete reaches its design strength in about 28 days so it is unlikely that yours has total cure and released its excess moisture yet. With glue on top and poly under, there is no place for the extra moisture to go.

Fortunately, concrete doesn't need to dry and can remain damp forever without any issues. In fact, it makes it stronger. Adhesion between the concrete and epoxy might be an issue, but I expect that here it is a mechanical bond, not influenced by moisture (unlike something like garage floor paint).

Jon - you are right, the concrete doesn't care about moisture. However, most glues are somewhat to very susceptible to moulds when trapped against wet concrete. As are many floor coverings. Most glues tend to get rejected from the concrete by the moisture, just like the garage floor paint.
Even ceramic if installed with modified thinset with pop loose while hardwoods will swell and shrink.
Unless there is some real need for speed, its just not worth it. If necessary, some primers will allow for early installs but the installer better know what he is doing.

Is the poly underneath the slab the cause of these problems?

I hear that a lot from contractors that polyethylene under slabs causes problems with curing. I actually witnessed a contractor purposely tear and rip holes in the poly barrier before the pour of the slab. The homeowner wanted poly and they put it in but they basically ripped it apart and poked holes everywhere prior to the pour.

 

Posted By Lbear on 16 May 2014 11:20 AM
Is the poly underneath the slab the cause of these problems?

I hear that a lot from contractors that polyethylene under slabs causes problems with curing. I actually witnessed a contractor purposely tear and rip holes in the poly barrier before the pour of the slab. The homeowner wanted poly and they put it in but they basically ripped it apart and poked holes everywhere prior to the pour.

 

I have witnessed Visqueen being punctured with rakes during the slab placement to let out trapped air. That is why I think clean, compacted gravel is probably better than Visqueen to stop capillary action of water.

that is great, except for RADON. Cheers, Eric

I believe that the problem is maybe NOT in the poly but in the concrete mix they are using. The contractors still use the same mix whether or not you have poly. While the mix worked well with non-poly applications, it doesn't work well when poly is used. Therefore the contractors hate the poly because it causes problems with their "tried and true" concrete mix.

In commercial applications one pours floor slabs and they are vapor/moisture impermeable on the bottom, just like a house floor slab with poly would be. Yet they don't have the problems that residential contractors are having with poly. Maybe something needs to be learned from the commercial side? Like what kind of mix they are using and the procedures they use to cure and float it?

It's not the poly that is causing the problem. It's the different curing rates of the exposed concrete surface compared to the polyed surface (bottom) All concrete wants to shrink as it dries so the top goes into tension because it is drying, lifting the edges.

• spray the top with water to prevent it from drying and let it cure.
• lay poly over the top to prevent it from drying and let it cure.
• spray the top with a commercial seal/cure agent to prevent it from drying and let it cure.

Posted By FBBP on 16 May 2014 03:03 PM
It's not the poly that is causing the problem. It's the different curing rates of the exposed concrete surface compared to the polyed surface (bottom) All concrete wants to shrink as it dries so the top goes into tension because it is drying, lifting the edges.

• spray the top with water to prevent it from drying and let it cure.
• lay poly over the top to prevent it from drying and let it cure.
• spray the top with a commercial seal/cure agent to prevent it from drying and let it cure.

I would assume the above applies for InsulDeck floor work also, correct?

What's nice about ICF walls is that it creates the "perfect" curing environment for concrete.

In this case the slab was covered for the first three weeks with building wrap while the drywall and painting was done. The curling only started once the cover came off the radiant heat turned on. My mistake was bringing the slab up to temp too quickly. Now that I have had some time working with this problem I find the the curling only occurred where there was no tubing (mostly in corners) and the worst was in one room. I have broken the corners down and in all but the worst room I will just install the wood over top. In the worst room I have broken out the concrete where it lifted badly and will use quik level and primer to fill up to finished depth. I am using Bostik's Best Glue and they allow placement over 12 lbs moisture per 1000 square feet per 24 hours, which as I understand it is quite high.

In this case the slab was covered for the first three weeks with building wrap

Something vapor impermeable (like poly) would make more sense to me.

Drywallers hate poly as a surface to work with stilts, it is too slippery. For them, had to go with building wrap and it seems to work well. Any feedback on using Level Quik (fast setting underlayment) to patch a 4 square foot hole in 2" concrete slab? We plan to lay an inch at a time to patch hole in slab, using primer as well.