Cracks on floor screed

[Saycheese5]

Hi,

I have an original solid floor which is made up of concrete, mortar screed, Damp Proof Membrane (DPM) and vinyl tiles. Wanting to level some areas of the floor, I noticed some parts were cracked while others sounded hollow, so I took away with a cold chisel what needed to be replaced. I then wet the scabbled concrete base and added a coat of PVA, then poured some pre-mixed mortar (for bricks and general purpose) onto it to a thickness of about 30mm to make the floor levelled with the rest. I waited one month for it to be completely dry but it started again, it felt hollow when I tapped onto it and some areas had very minor hairline cracks. I’m thinking of re-doing the work but this time using sharp sand and a stronger cement base 3:1. Could that help? Could I also use a thin layer of about 3mm of pre-mixed mortar which contains finer sand, where I’ve taken out broken vinyl tiles only? I’ve used self levelling compound on two previous occasions and it expanded and cracked for some reason. Eventually I want to have a laminate wooden floor.

Finally, could I build in one area a second screed of 10mm to make the base for a hearth? The tiles for the hearth will be 15mm thick.

Thank you for your answers.

 

[noseall]

Sounds to me like you need a completely new screed!

Building sand is useless for thin trowelled applications, it will shrink, curl and crack.

30mm is very tight but you should get away with screed sand (sharp sand, grit sand) and cement mixed at 3:1.

Adding fibres controls shrinkage.

Making up 3mm or so can only be done with floor levelling compound. Either that or put back some vinyl tiles. 3mm aint no hill o beans so a decent laminate type underlay may deal with small discrepancies.

 

[Saycheese5]

What fibres could I use to control shrinkage? Would a plasticiser help?

Could I also build above the floor a small 10mm mortar screed on top of a coat of DPM as a base for a hearth? The tiles for the hearth will be 15mm thick.

I will have to make a wooden frame to allow the edges to be straight and I'm trying to avoid using a self levelling compound because from experience the heat could eventually affect it.

Thank you.

 

[noseall]

Building sand is useless for thin trowelled applications, it will shrink, curl and crack.

Could I also build above the floor a small 10mm mortar screed on top of a coat of DPM as a base for a hearth? The tiles for the hearth will be 15mm thick.

 

[Saycheese5]

Thanks for your answer Noseall, but I'm not completely sure what you mean. Am I to understand that a 10mm screed between the DPM and thick quarry tiles on top to form the hearth is not strong enough?