New Concrete Slab - Advice

[Mr Punter]

1 hour ago, saveasteading said:

If the ground might heave (clay)  the perhaps concrete is needed.

 

I don't think the concrete will help unless it has a void under.  May be less good than MOT, which at least will not crack.

[saveasteading]

4 minutes ago, Mr Punter said:

which at least will not crack.

I am proposing concrete with fibre additive so it wont crack either.

[Fallingditch]

On 30/04/2022 at 15:00, saveasteading said:

That is 2 of us think that then.

I cant see what the bottom slab is meant to be doing other than provide a prepared and clean surface.

So why 150mm?

I am in the middle of a 'discussion' on this with our Engineer (I am not on the Scottish register). He is insisting on A193 mesh s well, and resisting my proposal to use fibre mesh for crack control (which is still over the top but not costing a lot)

 

So can I ask @saveasteading how you got in in those discussions?

 

Did he accept fibre reinforced rather than A193?

 

And had he specified 150mm or 100mm thickness ?  (I suspect it was 100mm, since you asked @Chanmenie "why 150" )

 

I have (almost) decided to bit the bullet, take out the old slab and replace it (along with other works) but the SE has specified 150mm slab with A252 mesh and UFH in screed, so I want to push back, but wondering what the best approach is ...

 

 

[Chanmenie]

4 minutes ago, Fallingditch said:

 

So can I ask @saveasteading how you got in in those discussions?

 

Did he accept fibre reinforced rather than A193?

 

And had he specified 150mm or 100mm thickness ?  (I suspect it was 100mm, since you asked @Chanmenie "why 150" )

 

I have (almost) decided to bit the bullet, take out the old slab and replace it (along with other works) but the SE has specified 150mm slab with A252 mesh and UFH in screed, so I want to push back, but wondering what the best approach is ...

 

 

I ended up using 100mm fibre reinforced EcFlow Concrete, structural engineer was happy with that. It actually has higher strength than the rebar originally specified, works out cheaper and much easier to do compared to rebar mesh 

[saveasteading]

10 hours ago, Fallingditch said:

So can I ask @saveasteading how you got in in those discussions?

 

Did he accept fibre reinforced rather than A193?

Yes. We used plastic fibres and the slab was 100mm  I think. It was laid by the family who had no preconceived bias. So the fibres added about £10/m3 to the concrete, and saved all the steel. Labourwise the concrete is a bit stiffer and added 30 minutes, but was countered by much easier access without mesh to cross. And no mesh labour.

Normally we would slice crack control next day but for somd reason ig wasnt done. But there are no cracks anyway.

The junior SE was reluctant but the senior one stepped in to say it was ok. This was not a normal discussion though, as we are as qualified as their senior guy.

 

To summarise, we laid sub-base, dpm, 100 concrete, 125 pir, more dpm, 60 poured screed. The latter was by professionals.

 

When concreting do not add water to the mix.

[Gus Potter]

For all..

 

Recommendation.. pick @saveasteading brains.. you could save a fortune.. but you will get true professional  CE advice for free so what about chucking £50 quid to BH as a donation?

 

 

 

 

 

 

 

 

[Fallingditch]

7 hours ago, Gus Potter said:

Recommendation.. pick @saveasteading brains.. you could save a fortune...

 

back on April 30, in response to "Why a slab and a screed ? That’s like doing it twice", @saveasteading wrote 

"That is 2 of us think that then. I cant see what the bottom slab is meant to be doing other than provide a prepared and clean surface."

 

and then yesterday, wrote "To summarise, we laid sub-base, dpm, 100 concrete, 125 pir, more dpm, 60 poured screed. The latter was by professionals."

 

I've probably misunderstood something here, but that seems to be suggesting one thing but doing another - so there are clearly other factors in play here ! 🙂

 

We are taking the old slab out in order to lay an insulated new slab with UFH.  (It probably does not make sense financially, but thats not the point). We are also constrained as to how deep we will be able to excavate without requiring underpinning.  Our trial pit shows we should be OK with removing 300mm, so by limiting my slab to 100mm, pouring the concrete on top of the insulation and setting the UFH pipes in the slab, I can squeeze more insulation in, and avoid an additional screed.  I'm sure there are some downisdes/risks with this approach - I'm just not sure what they are 😟

 

[saveasteading]

Apologies for the confusion. I don't write this stuff with the care I would if doing it professionaly.

If your ground is really bad there may be a need for the industrial loading slab you show below the pir.  It can carry forklifts.

But think of the load on your floor ....feet and furniture. This then sits on pir with very low strength. Therefore you don't need even more strength beneath that.

All you need is a flat, clean surface for the pir.

In theory that could be just sand blinding, but in practice that is difficult.

So my suggestion for you is  75 or 100 stone, then 25 max blinding, then 100 concrete with fibres, then pir then screed. Plus dpm twice.

The concrete could be thinner but is trickier to lay accurately....go for 75 if you want.

 

But this has to go past your professionals. I don't know your project circumstances. If they say no, please ask why. Remind them of the cost...it is a good idea to have worked that out to tell them.

 

The readymix company rep may not be used to fibres, but they are standard and from Sika. They simply pour them into the wagon from a premeasured bag 1/m3.

[Fallingditch]

I might have confused matters further here ...

 

Back on April 30th, I thought you were agreeing with @Chanmenies post namely "put the slab on top of the insulation".

 

Then you later described your floor makeup, which was concrete slab below insulation.  My point was "did you have a strong reason for putting your slab below the insulation when you did your floor?".

 

(I think some people do it one way - others the other way.  Completely take on board your suggestion to use 100 concrete with fibres,  And your suggestion to estimate costs and use that to support the revised proposal is also a very good one.  If this were simply my decision, i woouldn't have a problem - but I will have to convince my SE of the merit of using a thinner slab, and possibly even pouring the slab on top of the insulation).

[saveasteading]

Yes if the stone could be laid level, that should really mean that you don't need the lower concrete. After all, commercial sheds don't need insulation so there is one slab, straght onto dpm on stone.

It is really down to conditions and expectations.

Doing away with the slab would have been an argument too far with our SE...we settled for this as there were bigger arguments on other stuff. We saved about £15k but it could have been more. If we hadnt been professionals ourselves i dont know if we could have prevailed.

In reality, the bottom slab gives control and cleanliness and it is easier to get the pir tidy.

Did you say earlier that there is existing slab? Can it stay? 

 

[Chanmenie]

27 minutes ago, saveasteading said:

Yes if the stone could be laid level, that should really mean that you don't need the lower concrete. After all, commercial sheds don't need insulation so there is one slab, straght onto dpm on stone.

It is really down to conditions and expectations.

Doing away with the slab would have been an argument too far with our SE...we settled for this as there were bigger arguments on other stuff. We saved about £15k but it could have been more. If we hadnt been professionals ourselves i dont know if we could have prevailed.

In reality, the bottom slab gives control and cleanliness and it is easier to get the pir tidy.

Did you say earlier that there is existing slab? Can it stay? 

 

It’s not too difficult to get the hardcore base flat and level to within a few mm, any undulations are fixed with the blinding sand which is not too difficult to get within 3-5 mm if using a lazer level. So with the blinding sand nice and level lay the DPM then the PIR with UFH pipes then the concrete. 
The fibre reinforced c28/35 concrete I used was more like a screed as the max aggregate size was 10mm but mostly 6-8 so it poured flat and is self compacting.

saved me th3 additional £3000 for a separate screed just for the ufh pipes that wasn’t needed. 
 

 

[Fallingditch]

3 hours ago, saveasteading said:

Did you say earlier that there is existing slab? Can it stay? 

This is a refurb of an 1850 building. We want to install wet UFH (and impove damp control), which means that we have to get (say) 150mm PIR minimum under the UFH pipes. So the 1970's uninsulated concrete slab (which replaced rotten wooden flooring) has to come out, and then the dirt underneath it scraped away to provide sufficient depth, after which we reconstruct the floor with membranes, PIR, slab.  

 

The approach @Chanmenie took appears to require the least digging out(and clearly because we are working in an existing building, the less digging out the better 😊

 

(Really useful discussion btw).

[Roger440]

27 minutes ago, Fallingditch said:

This is a refurb of an 1850 building. We want to install wet UFH (and impove damp control), which means that we have to get (say) 150mm PIR minimum under the UFH pipes. So the 1970's uninsulated concrete slab (which replaced rotten wooden flooring) has to come out, and then the dirt underneath it scraped away to provide sufficient depth, after which we reconstruct the floor with membranes, PIR, slab.  

 

The approach @Chanmenie took appears to require the least digging out(and clearly because we are working in an existing building, the less digging out the better 😊

 

(Really useful discussion btw).

 

Or you could do this, especially as you are doing an old house, one presumes without DPC? https://www.lime.org.uk/products/sylfaenr-foundation-system.html

 

This is what i did. Total build up depth, 225mm, so minmal excavation. Which is good as it had minimal foundations. Admitedly, insulation level not comparable to PIR, but i was more concerned with solving damp issues in the walls. You proposed route may make it worse.

 

On my previous garage, i did as suggested above, Stone (MOT1) sand blinding, then 4 inch insulation (XPS), then concrete, 6 inch with thicker sides. So a raft That was plenty enough to drive cars over.

 

Like you, i still dont understand why people lay a slab, then insulation, then a slab. Makes absolutely no sense at all.

 

The XPS manufacturer wanted +/- 5mm over the length of the board (2400mm) which is tight, but doable with care and a laser.. 

[saveasteading]

4 hours ago, Fallingditch said:

the least digging out

Ok all makes sense. Do you know the depth snd construction of your footings?

Edited December 20, 2022 by saveasteading

[Fallingditch]

Its 1851 built, in E.Cornwall. Slate (rubble) stone construction.  Walls are about 530mm thick.  We've dug down to about 500 and not reached bottom of the footings, so should be no need for underpinning.  However there are two internal stone walls - don't know how far down they reach.

[saveasteading]

8 hours ago, Fallingditch said:

down to about 500

So that is unusually deep, and good news. You can excavate your floor depth without undermining.