Concret floor slab: how long to dry, pre-screed?

[Lobster]

In the interests of economy (ie of cement mixer hire fees!) is it OK to lay a 4" slab on one day, then lay the final screed on the following day?

Thanks David

[80/20]

Need more info - how big a slab - what is the final screed composition?

[Lobster]

It's 1.7m x 1.7m, there will be a DP membrane on top of that, followed by 1" Kingspan, followed by 3" screed mortar (3:1 sand:cement). (BTW, there's a reason for this structure, which the BCO has approved).

David

[Lobster]

Oh, quick related follow-up question:

If you want to join sheets of DP membrane using tape, will that grey waterproof duck tape do, or is there special BCO-approved stuff?

David

[Chris Bacon]

[80/20]

composition?

I trust you have a DPM below the slab. I am always dubious about having a DPM above the slab as you are locking in the moisture and talking to a few people who do slab work as a living you will not be pleased to hear that they wouldn't want to put the screed on for

4 months. Yes I'll repeat that 4 months. These guys tell me that it takes on average a month for each inch of concrete. They say that the concrete will remain "green" for a good few weeks and will be prone to wear until it has started to harden fully. Concrete doesn't harden by drying out, but by chemical reaction - therefore get the mix wrong and you will affect it's strength.

I'm not directly in the concrete industry, but in the carpet industry, for us the slab has to be at 75% relative humidity or below before we do anything on it, for timber it's probably the same but would need checking.

Sorry to land you with this news.

Steve