Floor Screeding

[sheba48]

I am about to screed a floor in our new extension which is 50m squared but broken up into three bays by internal walls. The screed thickness is 75mm and is going onto a new concrete sub base. Can I unibond the concrete base before the screed is laid and do I let the unibond dry before laying the screed. What mix screed is best to use 3.1 or 4.1 and do I need any additives or fibres in the screed. Many thanks

 

[roughcaster]

The first thing to say is this,, "never" lay a floor screed onto "any" dry surface,( uni-bonded or not),, always screed onto a wet base. When i lay a floor screed, i wet the the surface of the concrete sub base with clean water,, just poured on and brushed evenly (using a sweeping brush), and then mix up a runny cement grout, (stirring it regularly cos it settles) and pour this over an area of the wet floor to be screeded, again, brushing it evenly across the floor. Just do a few sq yds at a time, because you dont want it to dry in, before you screed onto it.
If you go down the uni-bond route,, the same applies, wet/brush the floor evenly, (don't overwet), and then brush the pva mix evenly over a small area as you go,, then screed straight onto it while it's wet, (again,, not overwet), or it will make the screed mix too wet, by soaking up into it.
Make it a 3 to 1 "semi dry", sharp sand and cement mix,, and yes, fibres in a floor screed nowadays is common,, but you don't really need any other additives.

Roughcaster.

 

[Micilin]

Just to chip in , there is no need for PVA if you slurry as described by the RC. What also helps the key, is if you wet the slab the day before.

 

[noseall]

F**k to pva is what i say.

Cement based products love water and will readily bond to water or damped down surfaces.

The problem with any screed is 'curling' whereby the edges of the screed try and separate from the sub base. This is more to do with the thermal aspect and the problems associated with thin screeds.

This is why a screed should be no thinner than 75mm.

 

[sheba48]

thanks for your reply are you saying to use fibres in the screed and what mix ratio do i apply for the fibres also where would i purchase them thanks

 

[roughcaster]

F**k to pva is what i say.

Absolutely!!, floors have been screeded for many decades using cement grout/slurry as a bond,,, as regards to the fibre ratio etc,,,( a product i have never used in a floor screed),,, anyway,, for you're info, have a look at this link:

http://www.tdpltd.com/products-construction/concrete&screed-fibres.html

Roughcaster.

 

[Micilin]

IIRC, a 75 mm screed is an independent slab, and I would only mesh/fibre if on top of insulation. You can screed down to 3 mm onto solid concrete if prepped properly.

 

[roy c]

IIRC, a 75 mm screed is an independent slab, and I would only mesh/fibre if on top of insulation. You can screed down to 3 mm onto solid concrete if prepped properly.

I would like to see that done!!! I have never screeded down less than an inch and half, and only done that on recomendation. We have always screeded at 2" . But nowadays you get retarders in the screed (12 to 36 hrs,) and fibres, we have always used 4-1 mix no additives and of course the good old cement slurry. No Probs. now when we do floors it is delivered to us and all the retarders and fibres are optional. We will soon be laying slabs and we are getting a 10-1 bedding mix for them delivered at least now its a gauranteed mix and a lot of mixing saved

 

[Micilin]

Roy, I should have added that if you were going that tight with the screed, you'd add sbr to mix and slurry in your prep.

Have a look at Ronafix website if you ever need info on mixes.

It is a great way of bringing different floors in together (such as when removing a wall, or marrying an extension floor into an existing floor) It's also ideal for putting falls into rooms for sluices , showers etc. on top of existing floors. another good application is where one section of a floor (with normal screed elsewhere) has to go really tight because a fall is needed, or perhaps the concrete subfloor has been put in at the wrong level, not allowing enough room for normal screed.

Long shot, but if you were anywhere near Wendover, Bucks I could show you one in a club that was screeded six months ago, not been tiled or carpeted yet and still coping with traffic. Thickness varying from 30mm running into nothing.

 

[roy c]

Cheers for that info Micilin .I am not too clued up with all the new tecnniques, but I understand what you are saying now, will take a note of the ronafix products. If i ever had problems with bringing in different floor levels or falls I have always used "Quick-set floor tile adhesive" and found that to work well. But I'm the old dog you can teach new tricks.. Cheers....