On 28/05/10 12:53, sm_jamieson wrote:
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.

--
Tim Watts

Hung parliament? Rather have a hanged parliament.