How to deal with these stones

[chrisb]

We're extending rearwards to create a large open plan kitchen / dining / living area. The existing kitchen / diner at the back of the house used to be two rooms (kitchen and back half of the lounge. I built a stud wall to divide the lounge and then took down the wall between the new room and the existing kitchen. The floor is very much less than flat, particularly in the middle where the wall used to be, but also around the edges of the lounge side of the room.

 

The intention was to dig up the existing slab and relay with insulation and UFH as per the new slab in the extension. I have taken up some of the screed to expose the slab. The screed varies between 10mm and 55mm thick. The slab is concrete approx 50mm thick, poured directly onto approx 200mm 20-30mm stones/pebbles. First thought was to take all of this up and replace. This will probably be OK in the original kitchen as there is almost certainly a footing under the doorway, and there is a footing where the old dividing wall used to be. However, in the other side, (what used to be the back of the lounge), the slab runs the whole length of the house, so there is unlikely to be a footing underneath where I have put the stud wall.

 

Any ideas how to prevent the stones under the front of the slab from being disturbed whilst I remove the stones from the back? I was thinking of some kind of shuttering, but not really sure how to go about installing it with 200mm of stones trying to slide around.

 

 

[Crofter]

I might not be understanding correctly, but why not just remove all the stones? The only useful thing I can see them doing is providing drainage, and if that's strictly necessary then you have a bit of a problem, IMHO.

I would get rid of all that shingly stuff and lay a new layer of crushed stone with fines (type 1) that will pack down hard. Then that gives you space for insulation on top. In fact you could make the whole 200mm insulation and that would be no bad thing, providing it was suitably, uniformly, supported underneath.

[Nickfromwales]

You could drill a series of core holes down to the 'chippings' and then make a very wet, high cement content slurry / grout and wash it down through to get them to bind. The only issue that may cause is introducing damp bridges up to the slab underside. Any idea what's there for DPM?

[chrisb]

@Crofter the issue is that I only want to replace half of one of the slabs (the part at the back of the house), so the stones will naturally slide down into the hole as I dig, probably destabilising the section of slab that I want to keep.

@Nickfromwales I hadn't thought of that. DPM is bitumen paint on top of the slab, under the screed.

 

Along the stud wall will be kitchen units (600mm deep). I wasn't planning on installing UFH under them, and don't think I need to take the new slab all the way back to the stud wall. So long as the join is under the units, so it isn't uneven to walk on, I can't see a problem with this?

 

Given I have about 500mm to play with under the units, I suppose I could break out the concrete slab, leaving the stones underneath. Then pour the wet mix, let it dry, and then remove the stones where I want to replace with insulation. The 'old' slab could then be reinstated with concrete over the stones.

 

I thought a picture may be helpful. The yellow and pink parts are all one slab. No idea if there any footings underneath, and if so, where they may be.

 

[Nickfromwales]

No current issues with damp? 

I would try and avoid breaking the slab up midway as it's reliant on being a complete raft. 

I think you may be better off just breaking out what you need to, let the chippings fall down, build your wall and then line it with a DPM. Then backfill with a very wet, but strong concrete mix with 10mm aggregate. Make a T-bar up to paddle the concrete up and down so it 'washes' under the slab and backfills the void, all the way up to the original slab ( top ) level, but short of the screed. Reapply the tanking solution and finish with leveller or screed. 

[chrisb]

Just to close this thread, after removing all the screed and deciding I'd had enough of breaking concrete, I handed this over to my builder who has done exactly what @Nickfromwales said above. New slab gets poured tomorrow.

[Nickfromwales]

It's a tough job, but someone's gotta do it . 

Thanks very much for the update .