Screed over fresh concrete

[Tonythecat]

I need to raise the level of freshly laid concrete. By about 30mm I Planned on laying 4.1 screed over it but I'm hearing that it needs to fully dry out before I can add the screed. If it is possible would I need to apply a cement slurry first

 

[JohnD]

but I'm hearing

Who from?

 

[Tonythecat]

Who from?

Sorry meant reading not hearing

 

[JohnD]

So where did you read it?

Was it about damp?

 

[Tonythecat]

So where did you read it?

Was it about damp?

Just general search on net saying it needed 1 day per mm before I could screed over it

 

[robinbanks]

Doesn't seem logical to me; no one lets foundations dry before they build a wall on them with cementitious mortar, so why dry concrete out before pouring cementitious screed?

I'd use self levelling screed,not trowelled sand and cement btw

What's your floor finish? Have you considered coming up in rigid insulation?

 

[Tonythecat]

Doesn't seem logical to me; no one lets foundations dry before they build a wall on them with cementitious mortar, so why dry concrete out before pouring cementitious screed?

I'd use self levelling screed,not trowelled sand and cement btw

What's your floor finish? Have you considered coming up in rigid insulation?

I was thinking exactly the same regarding footings. Floor is suspended timber and I want the screed to finish at floor level. The current level of concrete is about 30mm below floor so I need to raise it by that

 

[noseall]

I was thinking exactly the same regarding footings. Floor is suspended timber and I want the screed to finish at floor level. The current level of concrete is about 30mm below floor so I need to raise it by that

Laying cement based products on fresh concrete will only stick better, whilst it is green. The more it dries out, the more the need for a bonding agent. You may have got away with 30mm of screed whilst it was green. Not good once it has had time to really dry out. It will likely de-bond.

 

[Tonythecat]

Laying cement based products on fresh concrete will only stick better, whilst it is green. The more it dries out, the more the need for a bonding agent. You may have got away with 30mm of screed whilst it was green. Not good once it has had time to really dry out. It will likely de-bond.

Ok I see so really I need to chop it all out and start again. Just a thought but could I try and add a mix similar to render. No idea what mix is used for this but aren't thicknesses similar.

 

[robinbanks]

Not good once it has had time to really dry out. It will likely de-bond.

(Which might be important if it's 5 mm of self leveller but I wouldn't really care a hoot if 27 tons of screed wasn't stuck to its concrete substrate on every square millimetre..)

Also, I gotta point out things like concrete blocks are well cured by the time they're built into a wall and I don't remember the last time I worried about the cement mortar between them de-bonding to the extent that I thought "ooh the mortar might not stick to those blocks unless I SBR em first"..

I think the risk is low and the negative consequences non-existent in this case

The current level of concrete is about 30mm below floor so I need to raise it by that

Might as well take the gap out with insulation as something thermally conductive in my humble opinion

 

[Tonythecat]

(Which might be important if it's 5 mm of self leveller but I wouldn't really care a hoot if 27 tons of anhydrous screed wasn't stuck to its concrete substrate on every square millimetre..)

Is there anything I can do to remedy this. I'm at my wits end with it all. I'm a diyer so just need advice. Surely when people chop out old hearths they sometimes have to patch them up or raise the level

 

[stuart45]

If that's the screed in your hearth as previous photos I wouldn't worry. Just damp down the concrete. For bonding concrete the old saying was saturated, surface dry. Remember when slabs were done after the brickwork was up to DPC, and then the floor was screeded near the end of the job straight into the concrete with no bonding agents.

 

[^woody^]

Damp concrete will actually help screed to stick to it by controlling the initial drying and then setting.

However 30mm thick is a bit thin. You'll need to ensure it's given extra help by way of SBR or a proprietary bonding agent, and maybe cover it to prevent rapid drying.

 

[Tonythecat]

If that's the screed in your hearth as previous photos I wouldn't worry. Just damp down the concrete. For bonding concrete the old saying was saturated, surface dry. Remember when slabs were done after the brickwork was up to DPC, and then the floor was screeded near the end of the job straight into the concrete with no bonding agents.

I've attached a pic of what it's currently like. You can see I need about 30mm. So would a dryer screed be best or a mix more similar to mortar

 

[jj4091]

Yes a nice wet mix