Screed info required

[Donwill]

I intend laying a 50mm screed and ceramic floor tiles over a concrete floor which was laid approx 6weeks ago and floated to give a smooth finish.

I was told that I should use 3.5 to 1 sharp sand/ cement mix , this seems a little strong to me and could it incur some cracking due to shrinkage? Should I use a bonding agent on the concrete ? would PVA be OK?

So questions:

What strength mix should I use?

Any additives to the mix? i.e Fibres or plasticiser?

Do I need to bond it to the smooth existing concrete floor?

Any tips appreciated

Regards Donwill

[keith_765]

When i was helping the floor sceedereds at work the mix was 20 washed sand 1 cement 1/2 bucket of clean water

No plasticiser

You can mix cement powder with water and just brush the sub base to form a bond. Now days PVA is used. Follow the instuction on the container.

Dont make the mix to wet or it wont roll. Start to lay the screed round the outer edges and level as you proceed round until there is only a gap for access. then using the outer edges as a level fill in the centre from the furist point away from the access. fill in about 600mm or 2 ft by the width of the room at a time, wall to wall enough so you can reach. When you are happy with the level just lightly using a curclar motion using a platic or wood float skim the surface. T his will show up any small depression. To fill theses in just add a small amount of screed on top of the hollow and repeat with float.. Finaly glide over with a steel float to smooth and polish. If the screed wont roll when laying then its to wet and you wont be able to trowel, all you will get is water floating on the top.

[Donwill]

Many thanks for the info, however I came across this fairly comprehensive reference approx half hour ago,

seems to indicate that the screed should be 3.5 to1? I'm now even more puzzled :-). However, apart from the S/C ratio which needs to be clarified, your practical advice is most gratefully received. Cheers Donwill

[keith_765]

The screed mix should be 20 washed sand 5 cement or 4 to 1. My mistake not

20 to 1. Sorry for the type error.

KS Nottm