Have my builders screwed up ???

Question

My builders are working on a new bathroom with underfloor heating -

100mm concrete slab on top of a damp-proof membrane, followd by 100mm of celotex and a 50-75mm screed containing the heating pipes.

They've removed the old (very cold) concrete floor, dug down to get the floor levels right and have laid the 100mm slab on top of a damp-proof membrane. All OK so far... However they've immediately put the celotex right on top of the sopping wet concrete. When I asked how it was supposed to dry out, they said that that all the water in the concrete mix would be absorbed as the concrete set.

Is this right? It sounds a load of rubbish to me. At the moment just a few hours after they finished laying the concrete, if I walk on the celotex it goes 'glug, glug, glug' underneath as all the water sloshes abouut and bubbles up around the various holes in the celotex around the underground drains, etc.

Should I insist that they leave it to dry out properly before they go any further? Surely there's no way all that water will be used up in the concrete reaction as it sets. As the celotex is foil covered on both sides, any excess water will be trapped between the celotex and the damp-proof membrane until the next ice age, won't it?

Have thay also knackered the celotex by doing this? It's effectively floating on a load of water, so if it absorbs any presumably it will have to be replaced - or can you safely immerse celotex in water without degrading it?

Advice pleaee from someone who knows what they're doing (because I'm beginning to have doubts that these builders fall into that category).

If I'm talking a load of rubbish and worrying about nothing, then please let me know. I just need to know where I stand before they show up tomorrow morning.

Thanks

Ian Stirling · Accepted Answer

If the mix is correct, and idiots don't go walking on it, then it'll befine.Concrete sets by a reaction which absorbs water.

Andrew Mawson · Answer

100mmdamp-proofslosheseffectivelycategory).The setting process of concrete is actually a re-hydration of thechemicals involved, so the water is combined into the cement ratherthan drying out. If the concrete mix is correct then there should beno problem other than that of a worried owner walking on it as it setsand ceases to be nice and flat !! Frequently concrete is allowed todry out too quickly (warm or windy weather) and the upper surface ispowdery due to the lack of water at the time of curing.AWEM

Lobster · Answer

Indeed, and the way to prevent too-rapid drying is to cover it with an impermeable polythene sheet or similar... Celotex, with its covering of silver foil, will do the same job nicely.David

Guy King · Answer

The message from                  contains these words:Yes. Don't walk on it.Even if it weren't completely dry after setting (which it should be ifit's been mixed properly) the water would eventually escape round theexpansion gap at the edge.

mheden · Answer

Thanks to all of you for your reassurance.  The concrete looked like areasonably dry mix as it was laid, so I'll relax and let them get onwith it.Thanks againGuy K

Weatherlawyer · Answer

You can pour concrete under water -sea water even. What kills concreteis too much water in the mix. There was thread about that happening onhere a few weeks back.

mheden · Answer

"...and idiots don't go walking on it..."

Point taken. This particular idiot isn't walking on it anymore!

However, in my defence m'lud, the builders spant ages walking on it while they laid the celotex, and I only took a few careful steps on it . I guess any foot traffic over the celotex will have an effect that's a bit like 'over tamping' the slap - it will draw the water to the surface.

Ian Stirling · Answer

Sorry, it did rather sound like you were trampolining up and down on itto hear it make noises :)Concrete is funny stuff that is fairly subtle and easy to get wrong.

mheden · Answer

"Sorry, it did rather sound like you were trampolining up and down onit..."No problem at all, and absolutely *no* need to apologise!I'm just so grateful to you all for putting me right so quickly.

Broadback · Answer

Ah, happy memories, I made my own mooring by making a large slab of concrete, very quickly, at low tide. The tide came in and covered it long before it had hardened, used it for years with no problems!

The Natural Philosopher · Answer

And it doesn't matter how wet it is *under* a damp proof membrane.

Have my builders screwed up ???

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