Screed over fresh concrete

[stuart45]

Have you got any SBR handy?

 

[Tonythecat]

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.

Ok so my head is full of dilemmas. So I used sbs bond to reface some crumbling concrete window sills and applied a thin skip of screed (about 3mm) before painting. This has held for years so I know it's a good product my issue is that because the hearth will go right back and the stove will sit on top then will the bond be afected by the heat in anyway

 

[stuart45]

The heat from a stove on legs shouldn't be a problem. Sand/cement was used for the back hearth for open fires, and the heat from those was directly on them.

 

[Tonythecat]

The heat from a stove on legs shouldn't be a problem. Sand/cement was used for the back hearth for open fires, and the heat from those was directly on them.

Thanks for the reply. Ok so the stove is a cassette type. Most of these heat the hearth to 100 degrees or above. (As opposed to stoves in legs which don't heat to above 100) So this is what is worrying me having said that the bond layer would be approx 50mm below the surface as the screed will be 30mm plus tile and adhesive on top. Also that is a good point about open fires. I'm prone to overthinking things so maybe bond and then a 4.1 screed will be absolutely fine

 

[Tonythecat]

Thanks for the reply. Ok so the stove is a cassette type. Most of these heat the hearth to 100 degrees or above. (As opposed to stoves in legs which don't heat to above 100) So this is what is worrying me having said that the bond layer would be approx 50mm below the surface as the screed will be 30mm plus tile and adhesive on top. Also that is a good point about open fires. I'm prone to overthinking things so maybe bond and then a 4.1 screed will be absolutely fine

Have you got any SBR handy?

Yes I have some

 

[^woody^]

Oh, a hearth.

Vaccuum the surface with a brush attachment remove all loose stuff, brush your SBR on, and I'd be inclined to brush on a cement slurry too to fill the uneven surface that screed would not get into. Mix your screed a bit wetter than normal with what, half a cup of SBR and just lay it. Compact the base layer to ensure its all even and in contact with the concrete.

When it's starting to set and the surface has hardened, throw a bit of polythene over it for a week or so, or a cotton sheet but keep it damp.

 

[Tonythecat]

Oh, a hearth.

Vaccuum the surface with a brush attachment remove all loose stuff, brush your SBR on, and I'd be inclined to brush on a cement slurry too to fill the uneven surface that screed would not get into. Mix your screed a bit wetter than normal with what, half a cup of SBR and just lay it. Compact the base layer to ensure its all even and in contact with the concrete.

When it's starting to set and the surface has hardened, throw a bit of polythene over it for a week or so, or a cotton sheet but keep it damp.

Thanks very much for the info it's much appreciated I'll use a straight edge to level it off but should I float the surface as it starts to stiffen of just leave it.would floating give a harder final surface plus is 4.1 the best ratio for this

 

[^woody^]

Thanks very much for the info it's much appreciated I'll use a straight edge to level it off but should I float the surface as it starts to stiffen of just leave it.would floating give a harder final surface plus is 4.1 the best ratio for this

It wont really matter for this little bit of floor. The hardness of the surface is not the issue, rather it's the bonding. Do whatever you feel comfortable with or able to do to get a flat surface - if you have a wooden or plastic screed float fine use that, if not use a metal float. 4:1 will be OK, just make sure its well mixed.