I am getting ready to pour a concrete slab with radiant heat. I have the sand level and (sort of) flat. Then 6 mil visqueen. Then two inch rigid foam insulation. The plumber will soon put down the PEX tubing, after which I will put down 2″ of sand, then pour a 4″ slab. My question is that after the visqueen and foam went down, we got a good rain so I have water under the foam insulation board in the low places. With the 6 mil under there, it’s not draining. I’m wondering if that will be a problem either during or after the pour. I will be putting 2″ of sand over the foam so that may turn to muck. Anybody have any experience with this sort of thing???
Thanks very much, Lex.
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I work in the Seattle area, if we didn't pour concrete in wet conditions there would be no concrete in Seattle. ;-)
While not ideal, it's not really going to hurt anything unless it's a lake & the foam is floating.
Sand has moisture in it, the concrete will generally suck up all the moisture & evaporate it out, it makes for longer curing times on the concrete. Generally speaking the longer the cure the better the concrete.
That works for me. I'm out on the Olympic Peninsula, in Sequim so your info is good. It's sort of along the lines of what I was thinking.
Thanks a lot.
Hey I even know how to prononce Sequim & not call it seequim. ;-)
Know any good spots for winter steelhead over there?
“The Hand of providence has been so conspicuous in all this, that he must be worse than an infidel that lacks faith, and more than wicked, that has not gratitude enough to acknowledge his obligations.” —George Washington
I've never heard of sand over the foam in a radiant slab. I can see Skwimm from here and I would install a rebar mat over the foam, on 1" dobies, run the tubing over the rebar and tie it with tie wires, then pour 4" of concrete over that. No sand. You may be thinking of the sand as a "blotter" layer to absorb bleed water out of the concrete, but I would not do that, ESPECIALLY with tubing involved. All you need is the concrete.
Some guys would staple the tubing to the foam and pour the slab. I would rather get the tubing up to mid-slab than have it sitting on the bottom. That has been discussed here. I have never heard of anyone putting the tubing in sand and then pouring concrete over that with the tubing entirely under the slab. It sounds like a major mistake to me.
There's pros and cons all three ways
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What's the pro to putting sand over the tubing and then pouring??
when ever you place concrete on sand without vapor barrier, the concrete will crack.BOB is always right, ALL HAIL BOB
maybe that is a phenom for Florida sand?
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brownie, Why is that ?
Not disputing the statement, just wanting to understand your rational.
They can't get your Goat if you don't tell them where it is hidden.
sand is a porus material. it will wick moisture from other material. By placing concrete on top of sand, no matter how wet the concrete is, it will wick the moisture out of the bottom layer causing the bottom to set quicker than the top. this different is setting is what causes the cracking. you want the slab to set slowly from side to side not bottom to top.dont believe me, ask Gabe..BOB is always right, ALL HAIL BOB
but when the sand is damp, only 2" thick, and there is VB right under it, that does not happen.
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>>By placing concrete on top of sand, no matter how wet the concrete is, it will wick the moisture out of the bottom layer causing the bottom to set quicker than the top.
Seems to me that most concrete people are saying the opposite--the sand retains moisture under the slab, causing the bottom to cure more slowly than the top, which is exposed to air. The result can be slab curl.
You're saying "set", they're saying "cure". My experience is the above, that sand leads to curl, even if the top is kept wet. I've had much better luck pouring over foam directly, no sand.
most concrete people are saying the opposite--you have to take into account the soil condition of the area. If you are in a clay area , then this would occur. But rememebr my world is sand, all sand, everywhere sand, no clay for hunderd miles.most people in clay area, isolate a slab from water by using a permable clay.we work to keep the mositure where it at and build on top of it. Here water is our friend..BOB is always right, ALL HAIL BOB
exactly what I meant about you being on Florida sand
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You know how much I hate to defend Brown Bagg but he's exactly right here. The plastic does a much better job of holding the water in the concrete as it cures. It needs to stay wet for 28 days to develop full strength so you need to protect the top from drying out too quickly a well. I pour concrete in NC where it is all clay and we would never put anything to allow moisture to fall out of the bottom of the slab. And we cover it with plastic the day after the pour to keep it from drying out too quickly.a re-bar grid and tie the pipe to the top of the re-bar is my regular drill. What you call dobies we call chairs but the drill is the same get the tubing and the re-bar in the middle of the slab or staple the tubing to the foam and stick the re-bar in the middle of the slab. I don't buy that there is much diffenerce in having the tubing in the middle or the bottom. I've done both ways and not noticed any difference. But definately not sand under the slab wet or dry it can still absorb a lot of water...------------------
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There's no such thing as dry sand here. Even if I buy it in August, the first scoop the loader takes at the quarry will be nice and wet an inch from the surface. Then it'll get rained on after we spread it out. I have to worry like the OP that my slab prep will be a swimming pool on the day of the pour.
You of all people know about the 2 kinds of concrete.
Cracked & gonna crack ;-)
“The Hand of providence has been so conspicuous in all this, that he must be worse than an infidel that lacks faith, and more than wicked, that has not gratitude enough to acknowledge his obligations.” —George Washington
protecting the PEX from careless concrete placement and tooling while increasing the thermal mass heat sink
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I don't think I've ever heard of this, and I would definitely not recommend pouring a slab over sand.
Lex,
In fact, many builders have reported extraordinarily expensive problems arising from exactly the situation you are describing. Read this:
http://www.buildingscience.com/bsc/resources/foundations/sand_layer_under_slab.htm
Wet sand between poly and a slab is a big no-no. It can take forever for the slab to dry out. Flooring failures due to this condition have cost many contractors tens of thousands of dollars.
Anyone using poly under a floor slab should ensure the poly is placed directly under the slab, with no intervening sand. My advice to you: Punch numerous holes in your poly. Then install a new poly layer right before the slab is poured.
I'd probably puch a couple holes where the worst puddling is too7
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