Have my builders screwed up ???

[mheden]

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]

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

[Andrew Mawson]

100mm

damp-proof

sloshes

effectively

category).

The setting process of concrete is actually a re-hydration of the chemicals involved, so the water is combined into the cement rather than drying out. If the concrete mix is correct then there should be no problem other than that of a worried owner walking on it as it sets and ceases to be nice and flat !! Frequently concrete is allowed to dry out too quickly (warm or windy weather) and the upper surface is powdery due to the lack of water at the time of curing.

AWEM

[Lobster]

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]

The message from [e-mail] contains these words:

Yes. Don't walk on it.

Even if it weren't completely dry after setting (which it should be if it's been mixed properly) the water would eventually escape round the expansion gap at the edge.

[mheden]

Thanks to all of you for your reassurance. The concrete looked like a reasonably dry mix as it was laid, so I'll relax and let them get on with it.

Thanks again

Guy K> The message

[Weatherlawyer]

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

[mheden]

"...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]

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

[mheden]

"Sorry, it did rather sound like you were trampolining up and down on it..."

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]

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]

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

[Ian Stirling]

To an extent - as long as you don't have pools of water, which allow the floor structure above to flex and wear out the dpm, or the concrete is diluted.