some builder has done a concrete slab an its 25-30mm down below exsisting floor what can go a that distance to bring it up was going to sbr the concrete floor just to bond what do ya think cheers
This is what I would do.
1. Thoroughly clean and make dust free the existing slab.Damp down the exsiting slab, give it a good drink, then prime it with 3 to 1 SBR water.
2. Start mixing your screed with a 3/4 to 1 mix , then add to every batch using 25Kg of cement ,4 litres of SBR, you may find the water to sand ratio varies ( especially if you are using a tonny bag,as sand gets wetter towards the bottom of bag).Mix the SBR with a little water and throw in the mixer before you start. Always bucket gauge your sand and cement.
3. Mix up a cement/SBR slurry using 1 to1 water SBR and neat portland cement.Mix in a bucket with drill to a runny yogurt like consistency. Generously paste this onto the floor with a brush or broom, and whilst it is still wet lay your modified screed over the top.Perhaps tack bond the whole floor firstand sprinkle screed over to a depth of 10mm then start to lay as you would normally.
4. Finish screed laying, following morning lay sheet of polythene over screed, (no one on floor now for at least 3 to 4 days if your lucky) . Remove sheet after 7 days, and there you have one nicely bonded SBR modified screed.
Why would want to waste your money on fibres in 30mm floor bonded directly to a concrete slab. ?? Fibres normally for floating floors. Even then I dont rate them.( and always try to spec D49 fabric as well.)
Because its been proved they reduce cracking :RpS_thumbup:
Liquid screed like gyvlon can be layed direct to concrete 25 mm
I have seen plenty of so called fibre screeds with cracks in them. One scenario to consider over an insulation quilt.In a perfect world the quilt is of one thickess.In reality sometimes this is not the case.and due to poorly installed oversites I have been n jobs where the variation in thickness of board and depth variation and resulted in differential drying and curing times.These instances I can always predictcracks in the screed,even with fibre. D49 half way throught the screed is a much more robust inclusion into the screed.
On reflection ,with 25 years screeding I have never seen a cracked screed which has been placed over a good sound concrete over site with out fibre. ( expansion details considered and there in lies another conversation altogether).We have encountered curled screeds due to delamination from under substrate due to poor bonding and curing.
However I note your comments and agree that it would not hurt to put them in if you wanted too.