Concrete slab drying time before insulation, ufh and screed?

[mattwaddy]

So I'm having a 12cm slab poured next week ready for me to continue building up the next layers of PIR insulation, installing and pressure testing ufh and then have screed pumped in. How long will I need to leave the slab until I can start on the next steps do you think? I.e. dropping 150mm pir over it, slip layer and stapling down ufh pipes?

 

[Mr Chibs]

I'm sure I read that to dry properly it's a mm per day. It should be walkable 24 hours after.
In reality you probably won't be able to leave it 120 days, so just leave it as long as you can, a week or three.
I think it's more important not to rush the screed drying time, especially if tiling.

 

[mattwaddy]

I'm sure I read that to dry properly it's a mm per day. It should be walkable 24 hours after.
In reality you probably won't be able to leave it 120 days, so just leave it as long as you can, a week or three.
I think it's more important not to rush the screed drying time, especially if tiling.

Yeah we can leave the screed layer as long as it needs, but obviously need to crack on with the next layers after the slab asap so we can get moved in and stop paying rent+mortgage. Will likely get at least two weeks based on current plans and other work we need to do

 

[JohnD]

If you mean "dry" so damp does not rise into the house, put a DPM on it.

If you mean "harden" this does not result from drying out. It results from the fresh concrete staying wet as a slow chemical reaction takes place between water and cement. It will be reasonably strong after being wet for two weeks. As soon as it dries, it stops gaining strength.

 

[jeds]

There’s no definitive answer to this but most sites will lay insulation and screed over a 100mm slab after three weeks or so. The aim is to get the concrete dry enough, rather than dry, and dry enough means that free water will not accumulate under the insulation. You could test that by laying down a sheet of insulation, weighed down, and check it after a week or two. I would say three or four weeks will be safe.

PS. It’s not the same thing as dryness for laying flooring over new screeds, which is a completely different thing. In that case it needs to be really very dry – screed is something like a day per mm and then should really be tested. Most flooring manufacturer’s specify a certain moisture content, so that’s what you should go by.