When is a thick concrete slab ready for screeding and a finished floor?

[cmdrawesome]

I have a 250mm thick RC35 fibre-reinforced, waterproof-admixed concrete slab that has a ~30mm dip in the middle so it will need a screed before a finished floor can go in. It has the underfloor heating already installed at the bottom of the slab, with a further 225mm of insulation underneath that.

 

At some point soon I'll have a roof on and will be able to start drying the place out with a couple of rented dehumidifiers, and once the glazing is in and the plumbing is connected up I'll be able to fire up the UFH too.

 

My question is - How can I know when slab will be dry enough to screed?

I can get a cheap surface tested from screwfix, but that's not going to tell me the moisture content of the core of the slab. One screeding specialist was recommending simply waiting 1 day per mm, but I have 250mm!

[Nickfromwales]

What m2 of area do you have the ‘dip’?

[JohnMo]

3 hours ago, cmdrawesome said:

30mm dip in the middle

I would look how this is reflected in room layout, some rooms may only need straightening out with self levelling compound, others may need way more, it also depends on the area of the floor how much needs to be corrected.

 

We did all the correction that was needed a short time before floor covering went on.

[saveasteading]

13 hours ago, cmdrawesome said:

waiting 1 day per mm, but I have 250mm!

They always say that. There is no logic to it so I've no idea where the nonsense originates.

 

Is this indoors and under cover (not getting rained on?)

How long has it been down?

 

Unless the contactor added water, which is against the rules and the supplier would have made them sign on the ticket, there is very little free water in the slab after the chemistry.

 

30mm is a lot. Was it by a specialist? get them to fill it in.

 

 

What is the finish at present and was it intended to be the end result?

 

We have 30mm in the barn, as it was agricultural quality.

The sole plates are packed to level. The remaining variations become less severe once the walls are in, and we are putting an extra 25mm insulation in just one area to reduce the screed depth there.

Are you intending the concrete as the finish or affixing finishes to that? 

 

Any dampness on the bottom doesn't matter. the top may i suppose, if the screed is not compatible with damp surfaces.

 

it's likely you can screed any time now but please supply the info as above.

 

[Mr Punter]

You will want the screed to cure and probably bond to the concrete, so a dry slab may not help.  The concrete may need some prep.

[Nickfromwales]

5 minutes ago, Mr Punter said:

You will want the screed to cure and probably bond to the concrete, so a dry slab may not help.  The concrete may need some prep.

Yup. 
 

When we feather dry sand and cement screeds from 10mm to 30mm, there needs to be a very particular primer used, one iirc that stays tacky unlike PVA or SBR, to stop the layer from separating from the slab. 
 

From 10mm to 0mm you then use SLC.