New concrete slab over ground

Hi all,

I’m doing some renovations in my house and I would need some advise.
I have an atrium that used to have a small pond and a fountain. I decided to get rid of both and after removing all rocks/etc we found out that around 60% of the area is in concrete and the rest is ground.

Dimensions are around 250 x 180cm - the height of the existing concrete is around 8 cm.

I would like to have a concrete floor there so I can put a sauna instead. I’m not planning to put tiles, the sauna will rest on the concrete and I'll put little stones over the remaining area.

As you can see from the image below, the existing slab is not leveled (2.5cm "missing" in the middle).

There are a couple of holes and a drain (I'm not sure if it still works) -

Anyway I think my options are basically:
1/ lay a new concrete slab that covers the whole area
2/ lay a new concrete slab on the new area first then put an overlay on top of the old one

In case of option 1, I believe separation will be needed to avoid binding. I do not want the floor to be any higher than what it is currently so I think this option won't work for me.

For option 2, my questions are:
1/ Should I leave a gap between the concrete and the walls/existing slab? If so how much gap is needed?
2/ Should I use a 'plastic layer' to protect the slab from humidity or should I leave a hole as with the other one?
3/ If I make that new concrete slab leveled, any potential issue putting an overlay over the old one?

Any comments/help appreciated.

Cheers.
B

Hi Bubblebeat
1. I would remove the slab, chances are there is no rebar
2. 100mm reinforced slab with edge thickening
3. Factor in a drainage groove or similar around the slab perimeter
4. 10mm Mastic slip joint between brickwork and slab
my2c

Hi and thanks for posting.
I did not think about this option but I can see it work. I'd be able to lower the floor a little which will be handy as I'd like to put a mezzanine later.
Do you know how much it would roughly cost to get this done by a contractor?
Cheers.

Hi there.

I did remove the existing slab and I am now almost ready to get the concrete in.
One question I have remaining. What is the best way to get a flat slab?
I'm thinking of putting wooden guides on each walls and then replace those guides with the expansion joints. Is that the right way to proceed or is there a better one?

Cheers.