I’ve heard of the risk of concrete curling when pooring a slab directly on top of vapour barrier. A capillary break of sand or gravel can be used in between the slab and vapour barrier to help reduce the risk. What about when pooring a slab on top of rigid foam insulation? Are the same risks involved?
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I'm not a concrete guy, but can say that I've had slabs poured directly over foam insulation on a number of jobs and haven't had any problems. Sometimes the slab guys whine about having to work over the foam, but I've never seen the curling that you mention.
Don't put sand on top of your foam, just pour right over the foam. Wet cure the top carefully to avoid curling. Last one I did I kept the top wet for several days while we were framing and had no problems. You can't just keep it wet overnight and expect good results.
skory,
for my own info - what is curling?
"it aint the work I mind,
It's the feeling of falling further behind."
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When you pour directly on a vapor barrier and the top of the slab dries out too quickly, the edges of the slab will curl up just like a board sitting on wet grass in the sun.
The solution is to protect the surface of the concrete from drying out too quickly, which also makes for much stronger concrete. Putting sand under the slab and letting it dry quickly just makes for a weak slab.
It's like done w/ a broom and oversized puck ... eh? :)
Noticed this was already answered, but for further info check http://www.nrmca.org. Decent site - go to"research and engineering", and then "concrete in practice"(#19 talks about concrete curling).
Nice Link. Thanks for posting it.
They can't get your Goat if you don't tell them where it is hidden.
for my own info - what is curling?http://www.youtube.com/watch?v=dXeXNHRPMMISome weird sport that is played on ice and the players use a broom to try and control the "puck".Don't flood the slab in the middle of the winter and then people can use it for curling..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Seems like I hear a different opinion on this subject every day, but I think putting sand in between a vapor barrier and the concrete is a bad idea.
http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems/2008-08-19.0671157037/download
The problem is not caused by what is under the slab but by allowing too much heat and dry air on top to dry it too fast on top. Wet cure or cover with plastic
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An inch or two of sand under the slab on top of plastic sure doesn't help. The moisture sits in the sand and wet cures the bottom. If you pour directly on the foam the water can't sit there, it comes up thru the slab.
Concrete hydrates. Water is part of the chemical reaction, not just a medium for mixing and placing. To much water in a mix reduces the strength by increasing the inter spacial distance of the reactant of the process and delaying the reaction. Water loss though rapid evaporation means there is to little water left to complete the hydration process, agian causing a strenght reduction in the finished product, and IMO is one the major causes of most cracking in slabs.
As Piffen pointed out the watere trapped below the slab cause a slower cure rate, while the surface or bleed water loss cause a faster cure rate there. These unequal cure rates cause stresses in the hydrated matrix, thus reducing its' strenght and in most cases causeing the slab yo fracture or crack along these stress lines. Wha t we refer to as shrink cracking and edge culing is the result of different cure rate occurring within a pour.
As you pointed out, wet curring is a good thing. After the intial set and finish of a slab , the addition of more water to the slab surface serves the same function as covering it with plastic. It slows or reduces the evaporative water loss in the slab and equalizes the cure rate throughout the slab.
Putting a blotter layer of sand under your slab, on top of your foam or poly, is old school. I would not recommend it. It makes it harder to keep the top wet enough to avoid curling.