Self compacting concrete floor slab. Save money and omit screed?

[flanagaj]

I am in the process of getting the hardcore in for the ground bearing slabs and have been looking at SCC with interest.  Cemex and others state that when finished with a dapple bar it provides smooth flat surface that does not require further finishing.  So this has got me thinking.  If SCC can be laid and finished with ease, why bother going down the traditional route of insulation under screed and on top of slab, when I can omit the screed layer and go with insulation under slab.

This removes another stage in the process and ££ too.  Our original plan was to go with a power floated floor finish, but we scrapped that due to the ridiculous quotes and also having read negative comments.

I know @Oz07 has used it before, but has anyone used it with UFH attached to the rebar mesh and then simply tiled or used engineered flooring directly on top of the SCC

[sgt_woulds]

Replace the hardcore with a sufficient depth of foamed glass aggregate (e.g. Geocell) and you remove the need for additional insulation under the slab and also for a DPM.  (Unless it is a radon area, all you need is a geotextile membrane seperating the concrete from the foamed glass).

 

I'd be interested to know more about RRP of SCC.  I used floated conrete over Geocell, but unfortunately it still required a leveling screed since my builders were a bunch of amateur monkeys.

[Spinny]

For our extension we have the heating pipes in the concrete slab. So the makeup is hardcore with sand blinding layer over, DPM (in our case also a radon barrier), 150mm PIR, Vapour Barrier, about 120mm readymix poured (pumped) concrete slab with reinforcing mesh and heating pipes attached to mesh.

 

This was the preferred method of our architect who advised this was more efficient for heating given the higher thermal mass of the concrete slab.

 

I repeatedly asked our builder how level the slab would be and was told 'within 6mm'. I always found this hard to believe. Yes it was false and the slab had a 30mm variance from highest point to lowest point when actually done. I measured it and queried it when it had dried but was told it was not too high and would be fine and a refusal to come to site and measure it with me. Of course when they opened the knock through to the existing house 2 months later it was clear the slab was too high. We had to overcome this by raising the floor level in the existing house using a pipe in board u/floor heating system.

 

There are some challenges associated with putting the pipes into the slab, particularly arising from the fact the slab is constructed at an early stage as the build is coming out of the ground...

1/ You will need to know your pipe loops and layout and have it installed before the slab is poured. Including connecting the pipes to a manifold and ensuring they are pressurised and the pressure in monitored/checked.

2/ The slab needs to be protected from water ingress. That is stop water from being able to get onto and under the completed slab and into the insulation layer underneath. It might soak through the PIR edge insulation, or enter through apertures for conduit and services etc. The walls and roof structure are not in place. So you need to keep rain off the slab whilst you build the walls and the roof structure. A seperate temporary roof over the site is much preferable at this point - but our builder did not quote or provide for one. Consequently we spent money and time protecting the slab as best we could as the build went up. The risk is the insulation layer turns into a small underslab lake necessitating cutting out pieces of the slab and pumping it out etc - mucho cost and delay if this were to happen.

3/ Is a slab laid outdoors by ground workers ever going to be properly flat and at the right level ? A slab that is too high is potentially a major problem. You need good professionals with the right equipment, experience, and verifyable skill levels to get a good result. Do you know your FFL ? Are all your door openings at the right level ?

4/ You need to get your conduits and services/apertures in place before the concrete pour.

 

My experience is minimal (1 extension by troublesome builders) but I would find it very hard not to plan to use a floor leveller over the top of the slab once the build is dry.

 

I wouldn't say pipe in slab is a no brainer cost saver. Best to have pros that have done it before providing some assurance of qualtiy/levels etc.

[flanagaj]

Interesting.  The problems you face seem to be common.  Too many 'builders' will happily say "Yes, we can do that", when in reality, they can't and should be honest and upfront that they cannot

 

If I read your post correctly, you did not use SCC for your slab?

[Nickfromwales]

2 hours ago, flanagaj said:

I know @Oz07 has used it before, but has anyone used it with UFH attached to the rebar mesh and then simply tiled or used engineered flooring directly on top of the SCC

MBC do it for every client in their passive rafts, unless someone says they are going to rub their arms to keep warm……

 

You’ll need some smoothing, so tile adhesive if tiling, and self levelling / smoothing compound if anything else. 
 

Nothing will go directly on to a newly laid slab, that’s just wishful thinking.
 

Current MBC one, great guys, SCC: