I have to screed a concrete floor slab. Screed is going over concrete, not over insulation. Screed depth was supposed to be 60 to 70mm, but the slab ended up a bit thicker than intended, so in some areas, the screed will be down to about 40mm thick. Will a normal dry mix sharp sand / cement be OK at that thickness, or should I add anything (like SBR etc.) Floor will be tiled. I was going to use sand / cement in case I need to slightly slope levels to match floors in two adjoining rooms. Or should I consider any other type of screed ? Cheers, Simon.
I would have though 40mm was OK, but it's getting near the limit.
Perhaps prime with a cement slurry as you go to achieve a decent bond with the concrete?
SBR will be bombproof at that thickness (and less).
The main downside with SBR (apart from the expense) is it makes for messier handling - you'll get covered in stickiness, your tools will and it's a bugger to get off if it's not washed immediatedly (ie white spirit and/or metal scourer).
On the upside, your screed will not fall off, ever, if you follow the 3 step method:
Wash down concrete with dilute SBR
Prime as you go with cement/SBR slurry
Use SBR modified screed.
The handling is also a little weirder - it plasticises the screed immensely, so despite a very dry mix, watch out for minor (2mm) humps forming behind the tamper as it backflows (I had this happen). If you catch then the next day, they can be "planed" off if small by hand with a metal paint hook scraper and a certain amount of swearing. Wait a week and it's angle grinder time.
I wouldn't say it's much harder than normal screed if you are forwarned
- mine was a learning curve!
Google for Cememtone SBR datasheet (and/or other brands) - they give the mixing and application regimes in some detail. If you can't find them, email me (posting addy valid) and I'll email you my collection of PDFs.
I've recently had 65^2 of uneven concrete screeded and I know it must be thin in places as I had 1m^3 left over. even where it tapers of by a door it hasn't given a problem yet though I fully expect to have to make good with some epoxy.One chap and a laser level laid the lot in a day without expansion joints which surprised me a bit.
Nah - normal mixer is fine. However, they do mention that air inorporation may be a problem. So I mixed the ingredients dry, then added the liquid (premixed SBR/water) and mixed for the minimum time to get it right.
Fill the mixer with water as soon as you dump the load though, and leave it turning, or you'll never get the SBR off.
Slurry I mixed with a spoon in a bowl and applied with a large paintbrush as I went.
What consistency should you aim for - dry as possible as for plain sand/cement screed, or a little bit wetter ?
The other thing they mention is cleaning the concrete base first (they say shot-blasting etc !!!). One area of base was poured last week, the other is 2 years old. Did you prepare the base first ? I could wash the lot with brick acid first I guess.
Also (sigh) I need to clean out my cement mixer. Its a bit encrusted at the moment, and I'm sure you want it smooth and clean so the stuff slides out happily !
I had a labourer helping who'd done dry screeding before. We mixed according to the data sheet, which to me seemed very dry and to him seemed normal (so he said). However, it acted like it was wetter. Now, I'd gone with a plan of screwing level battens to the wall and using a
2x4" taper bar with "ears" screwed on the ends at the top so that when the ears rested on the wall battens, then the tamper would tamp to the correct level. This turned out to be a good idea (though a bit alien to my employee) as the screed acted wetter than it was (there are warnings to this effect, if not in your data sheet, then one of the other manufacturer's).
So if you are planning to screed the old fashioned way by running screed round the edges of the room, then infilling, you might want to make it a notch dryer than you think. OTOH, if you are going to splat it on and screpe it level, then float it off (the sort of thing I'd do 'cos I can't do screeding the proper way) then I think you'll find the mix OK if done by the sheet. It trowells well with a steel float, and it behaves quite well with tamping and scraping off (rather like a *very* viscous plastic mass).
I didn't bother. Onto old concrete, I hoovered with a VAX to remove excess dust and crap, then poured SBR/water in a 1:4 (that's on the sheet) and brushed it in to saturation. That will bind up anything loose in the surface. This can be left to dry as long as you like, or done the day before. The cement/SBR slurry will need to be applied as you go so it maintains a wet interface to the screed and that will bond the screed to the concrete in a highly reliable way.
The only thing I would to is ensure you don't have large areas of bitumen on the surface, should you have just peels tiles or wood parquet off - even SBR doesn't like bitumen.
Mine's not that clean. It was second hand, and I did take an SDS to it to take of the thicker crap - but it is most certainly not gleaming steel. Don't worry about that - the SBR screed will plop out OK. It's leaving SBR screed in it that's the problem - you will not shift that half as easily.
Beware of the ground you dump the mixer cleaning water on too - I managed to make my lawn go like plastic afterwards. Raking out a small ditch next to the mixer to contain the washings might be an idea if you have a fine garden. Don't dump the washings on your block drive or it's liable to go shiney. SBR is amazing stuff at not coming off.
Sounds like you'll need a fair quantity. You can get it in 25l cans for about 70-80 quid all in. I got mine from here by internet:
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were fast when I last ordered (3 days IIRC).
Have white spirit and wire wool handy to clean anything metal like the float - if it sets on your tools, white spirit works.
Can't remember who put me onto SBR, but it was here. I've been using it for all manner of floor work since and in particular patching loose screed.
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that, the light grey areas are SBR screed patching to infill deep damage. The brown is the original very weak screed - wasn't weak by the time I'l poured some 4-5 gallons of SBR/water over it - twice!. The patching screed was to save on the rather expensive StopGap 900 that was going on top, and also to keep the Stopgap 900 within its limitation of depth. The SBR screed was typically 10-20mm thick and feathered at the edges, but testing with a chisle showed that it had bonded perfectly to the crap underneath. Even teh concrete wasn't that strong. The floor is now tiled with slate and is showing no signs of being unhappy.
I did the daughter's bedroom and the bathroom as a full rescreeding job down to concrete - using SBR as the thickness was about 30mm and also as an aid to holding back damp (we have a patchy DPM in this house - it's old). In that respect it seems to work.
Thanks, excellent information. I think the SBR screed might be good at doing the floor in stages / since it will all stick so well. Perfect for my needs. I'll let you know how it goes - probably not doing it for a few weeks. Simon.
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