My SE is recommending I do NOT embed the UFH in the insulated slab...

[zzPaulzz]

8 hours ago, Gus Potter said:

 

@zzPaulzz Go back and have another chat with your SE. It seems like you are working at cross purposes.

Good advice Gus, thanks.  I see no reason why I can’t layout the UFH loops to minimise crossing the SE’s expansion joints, assuming these are even necessary. This local SE firm has only completed two insulated slab foundation designs and never come across UFH in the slab. If we can’t find a way forward then I’ll switch SE. 

 

 

 

[zzPaulzz]

8 hours ago, Gus Potter said:

For all .. Nick mentions a " passive raft" this tends to be a structrual concrete slab and by my previous post you would expect that to have no movement joints.

But you can also do passive house design with a direct ground bearing slab on insulation and this would often have movement joints. We make these thin as we can with minimal anticrack reinforcement or use concrete with fibres to control the cracking.. @saveasteading has implemented this approach.

I believe the SE has designed a direct ground bearing slab on top of a crushable layer (to handle ground heave) hence he wants to include movement joints cut into the slab. 

[zzPaulzz]

Update after chat with SE.  The slab is a 'raft' not ground bearing, so it will be 175mm thick, with additional edge thickness for the walls.  He's planning on two meshes set 40mm from the top and bottom. The top mesh will be sectioned around the joint lines. 50mm formers will be used to make the joints in the slab - no sawn cuts.  Each joint line will have an intermediate mesh at 58mm (1/3rd depth) from the top to maintain the structural strength.  We've agreed to extend this intermediate mesh so that I can use it to lay the UFH pipes.  He stressed again that fixing a burst pipe will be more complicated than just digging out the concrete and backfilling with a bit of mesh, but admits he knows nothing about UFH pipe strength/resilience.  Any views on this?

[saveasteading]

I posted this in error in another discussion on the same subject. and have mixed the 2 up I think. No time to review so here it is again.

 

My thoughts.

questions first.

Where is the floor insulation?

Is the ground levelish and good bearing capacity?

 

Assuming the ground is good, I would lay harcore and 150mm slab. 175 maybe .

Crack control mesh is all that is needed.

This suffices in warehouses and commercial vehicle garages so is ok for your sofa.

Lay PIR on that with ufh on top, then screed in 60mm poured or hand laid.

 

If you need more capacity for internal loadbearing walls then it can either have a thickening in the slab, or a trench footing, as the ground dictates.

 

I think rafts are specified too often. I really don't like structural slabs on top of insulation.

 

Now some other points.

 

85m3 isn't much and I will stab at it being about 8m x 10m.

I'd have to check but this is close to not needing any contraction joints for a finished surface.

But with a screed over it that is irrelevant because the slab will  crack well before the rest of the work, and will not reflect through the screed.

 

Even as a finished surface this cracking would be trivial.  With careful mix control (absolutely don't allow extra water added on site) and fibre additive (£30 or so) any cracking will be microscopic.

Cracks would be tiny and many. The UFH pipes will likely debond at cracks but are also well capable of stretching the 1/10th mm required.

 

I did put UFH into a 175mm slab once, as the  client's specific requirement. (It was a factory and this was the area where people stood to do craftmanship stuff.)

It had anti-crack mesh in the floor, and the UFH pipes were fixed on top of it.  Shrinkage joints were either the old fashioned method of pouring alternate long strips, or crack inducers built in....I can't remember. So there was no cutting into the slab afterwards. More mesh reduces crack width if necessary, which it wasn't in this case.

I guess the shrinkage cracks were about 1mm.

It went well and worked welI. I heard no more about it so it must  have been ok.  I had a further job for them so it's not as if we lost touch either.

It was an uninsulated for various reasons I won't divert into.

 

btw I've done 200,000 m2 of slabs and never more than 175mm  when ground bearing. 

 

So there are choices.  

Even the best SEs don't know all the ins and outs of costings (it isn't just the  slab but the walls, the insulation , UFH , access to site)......That needs a close collaboration between contractors and consultants.

The biggest and best contractors will know, if they have an SE inhouse, or have developed a standard method.  But that will cost you in either overheads or conservative design.

Small contractors may well know, but that is rare, and with some, the less they know, the more they think they know. BUT perhaps they and your SE together can optimise this.

 

If there is a reason against my suggested method, then please say.

[zzPaulzz]

@saveasteading In my case the slab is roughly 15x9m, passive design so 400mm EPS on layer of sand, then on ground, 175mm structural raft on top with a ring beam around the perimeter. Clay with trees ground condition so heave has to be mitigated.

[saveasteading]

15 minutes ago, zzPaulzz said:

heave has to be mitigated.

OK. I have seen heave once. I think clay heave is assumed too often but I don't know your ground conditions.

Clay heaves when it was dry and gets wet. If the whole area is rising and falling then it doesn't matter. Under a 15x9 slab will it be much drier or wetter than the surroundings?

If you had conventional footings at 1m deep then they won't budge, and the slab will be on fairly standard ground.

 

OR refer to another discussion on here about beam and block. No heave.