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:

 

[flanagaj]

3 hours ago, Nickfromwales said:

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:

 

Thanks @Nickfromwales the manufacturers state that no further finishing is required, eg power floating.  Looks like they are being slightly disingenuous with that claim.

[Nickfromwales]

2 hours ago, flanagaj said:

Thanks @Nickfromwales the manufacturers state that no further finishing is required, eg power floating.  Looks like they are being slightly disingenuous with that claim.

Nope, just you’re misinterpreting what they’re saying.

 

It gets “finished” by the dapple bar, and that is not ‘perfect’, but is as near to ‘shit hot’ as you’ll ever be getting.

 

FYI, power floating also goes seriously wrong if the person using it is a numpty.

 

You’re not going to get anything like you’re expecting, so realign yourself with whats to be expected in reality, as it’ll need ‘refinishing’ no matter what you put down; unless Jesus is on the bullfloat.

[flanagaj]

2 hours ago, Nickfromwales said:

Nope, just you’re misinterpreting what they’re saying.

 

It gets “finished” by the dapple bar, and that is not ‘perfect’, but is as near to ‘shit hot’ as you’ll ever be getting.

 

FYI, power floating also goes seriously wrong if the person using it is a numpty.

 

You’re not going to get anything like you’re expecting, so realign yourself with whats to be expected in reality, as it’ll need ‘refinishing’ no matter what you put down; unless Jesus is on the bullfloat.

Ok.  Thanks for clarifying.  I've realigned my expectations and accepted that more money will need to be spent and more trades hired 😞than I wanted too

[Nickfromwales]

51 minutes ago, flanagaj said:

Ok.  Thanks for clarifying.  I've realigned my expectations and accepted that more money will need to be spent and more trades hired 😞than I wanted too

I’ll be brutal, apologies in advance, but you need to accept that you can’t cut out necessary steps or avoid processes / associated costs. 
 

What you ‘wanted’ is perhaps unachievable, unless you become the master of all and DIY this single-handedly, a-la @Russell griffiths , but the side effect is then how long it then takes you to do these things. Saves money, but costs time.

 

A guy I worked for many moons ago told me “short cuts take 3 times longer and always cost you more in the long run”. He was correct.

[marmic]

assuming 'slab' not structural have a look at liquid screeds. Finish that good we just needed primer and tackifier adhesive + suitable LVT straight on top.  No 'leveller' (which should be called 'smoother' in my book anyway - that's another topic and daily conversation for me!).

[Nickfromwales]

1 hour ago, marmic said:

assuming 'slab' not structural have a look at liquid screeds.

 

20 hours ago, flanagaj said:

am in the process of getting the hardcore in for the ground bearing slabs

Structural  

 

1 hour ago, marmic said:

Finish that good we just needed primer and tackifier adhesive + suitable LVT straight on top.  No 'leveller' (which should be called 'smoother' in my book anyway - that's another topic and daily conversation for me!).

If you’ve LVT going down without a feathering (smoothing) compound, then you will likely see some ‘tide’ marks when the sun shines across the floors.

 

Did you rub the floor over with a carborundum block first, at least? That’s the minimum I’d ask for. 
 

Agree there are some very good screeders out there, but not seen a floor good enough to omit any feathering out. 

[marmic]

25 minutes ago, Nickfromwales said:

 

Structural  

 

If you’ve LVT going down without a feathering (smoothing) compound, then you will likely see some ‘tide’ marks when the sun shines across the floors.

 

Did you rub the floor over with a carborundum block first, at least? That’s the minimum I’d ask for. 
 

Agree there are some very good screeders out there, but not seen a floor good enough to omit any feathering out. 

believe me it was good. we had planned to use smoother (leveller) but was pointless!   Not your normal LVT with trowelled adhesive -  more like a carpet tile without any fluff.  (Interface) 

[Nickfromwales]

3 minutes ago, marmic said:

believe me it was good. we had planned to use smoother (leveller) but was pointless!   Not your normal LVT with trowelled adhesive -  more like a carpet tile without any fluff.  (Interface) 

Ah! Ok, different beast and a but more forgiving

 

Clockwork screed guys were shit hot, and I’d recommend anyone in their catchment areas to use them. Cost was very reasonable too.

[marmic]

but if you asked me about groundworker whole different story.  all fine in end but the grief!

 

[flanagaj]

The reason for attempting it myself is mainly due to an inability to find anyone who can do the job OR who I trust to do a good job.  Having already had a poor experience of trades, my confidence is at rock bottom.

 

[Russell griffiths]

I think you should go the standard route. 
hardcore, concrete ,insulation , screed, finished floor. 
this method gives you the ability to juggle finished floor height. 
without wanting to sound like a wa##er you have had a few problems so far, if you get the concrete height wrong by 15mm when going for the final finish height this is going to totally bugger you up with door cill heights and all manner of other things. 
especially as you have finished brickwork outside. 
 

stick to putting a screed on top of the insulation so that level can be worked out at a later date when doors are in and you have a better idea on floor finishes. 
 

building 15mm up with self leveling compound over 100m will be a major ball ache if you change from tiles to LVT, but it’s easy to get the screeders to add another 15mm when they do their bit. 

[flanagaj]

2 minutes ago, Russell griffiths said:

I think you should go the standard route. 
hardcore, concrete ,insulation , screed, finished floor. 
this method gives you the ability to juggle finished floor height. 
without wanting to sound like a wa##er you have had a few problems so far, if you get the concrete height wrong by 15mm when going for the final finish height this is going to totally bugger you up with door cill heights and all manner of other things. 
especially as you have finished brickwork outside. 
 

stick to putting a screed on top of the insulation so that level can be worked out at a later date when doors are in and you have a better idea on floor finishes. 
 

building 15mm up with self leveling compound over 100m will be a major ball ache if you change from tiles to LVT, but it’s easy to get the screeners to add another 15mm when they do their bit. 

That does seem like the logical approach.  Do you know whether the SCC can be easily worked so it provides a suitable surface for Radon membrane / insulation.  My concern is that the concrete slab is laid and the surface resembles the Alps and then needs remedial works to get it flat enough for PIR / Radon.
 

My plan is to put the the Radon membrane on top of the slab and a sacrificial DPM below the slab.  The utilities are quite high and if Radon membrane goes below the slab, it's going to be a hatchet job sealing around the pipes, due to the fact that at that level I am either on the start of the curve of the rest bend, or sat directly on the rest bend collar.  Going above the slab puts me on the vertical and will mean I can use the top hats for sealing.

 

[marmic]

1 hour ago, flanagaj said:

My plan is to put the the Radon membrane on top of the slab and a sacrificial DPM below the slab.

has your detail changed?

 

[Russell griffiths]

Stop trying to reinvent this job. 
 

sealing around the service pipes is easy. 
 

you do not try and cut a hole in the membrane and seal that to the pipe. 
cut a 300mm hole in the membrane, poke it over the pipe, then cut a 600 mm square of extra membrane, cut a 120 mm hole in the centre, poke it over the pipe and tape it up, then tape your patch down to the main sheet. 
easy, each pipe penetration will take 15 mins max. 
 

why buy two membranes to do one job. 
 

if your finish concrete is rough the pir insulation won’t give a monkeys it will just deform to the shape of the slab when it has 15 tonne of screed on top of it. 

[flanagaj]

I am obviously overly complicating these things.  It's not my intention, to but I spent nearly 2k on Visqueen membrane, pre formed corners and top hats as I thought it needs to be done with precision.  I get the impression that it's a case of just do a Heath Robinson job on it.  The PIR tip is useful and I naively thought it needed to be flat and smooth.

Having been a furniture maker in a previous life, this hackery is not something I am used to.

[Russell griffiths]

2 hours ago, flanagaj said:

I am obviously overly complicating these things.  It's not my intention, to but I spent nearly 2k on Visqueen membrane, pre formed corners and top hats as I thought it needs to be done with precision.  I get the impression that it's a case of just do a Heath Robinson job on it.  The PIR tip is useful and I naively thought it needed to be flat and smooth.

Having been a furniture maker in a previous life, this hackery is not something I am used to.

If you have the top hats then use them they work. 
the problem is these things get designed on a computer by a bloke who wouldn’t know what end of a trowel to hold. 
on site you very rarely get perfect conditions as you have found out with the pipe collar being exactly where you need to seal it. 
so you will probably find you chuck a couple of top hats away and just use more tape.   Hardly heath robinson this one, it’s just done, finished move on get the concrete on top and it will become a distant memory. 

Edited 9 hours ago by Russell griffiths

[flanagaj]

3 hours ago, Russell griffiths said:

If you have the top hats then use them they work. 
the problem is these things get designed on a computer by a bloke who wouldn’t know what end of a trowel to hold. 
on site you very rarely get perfect conditions as you have found out with the pipe collar being exactly where you need to seal it. 
so you will probably find you chuck a couple of top hats away and just use more tape.   Hardly heath robinson this one, it’s just done, finished move on get the concrete on top and it will become a distant memory. 

I have lots to learn and need to understand that things are not always perfect and accept improvisation will be required.

[Stratman]

We recently had self compacting concrete insulated raft with UFH fixed to the steel mesh. We were told it is as good as a power floated finish. It is not. Dapple bar ridges prominent, high areas needed grinding back. Not happy - should have ignored the hype and stuck with power floated.

[flanagaj]

1 hour ago, Stratman said:

We recently had self compacting concrete insulated raft with UFH fixed to the steel mesh. We were told it is as good as a power floated finish. It is not. Dapple bar ridges prominent, high areas needed grinding back. Not happy - should have ignored the hype and stuck with power floated.

Sorry to hear that.  Thanks for sharing your experience though.