Thread: increasing thickness of slab

I reckon you'd probably want to put some starter bars in and tie them to your steel in the new slab. Drill holes in the old slab, chemset in some short lenths of a good sized bar.

You probably should talk to an engineer about it though, as it's going to form part of your foundations.

My first thought would be to place plastic over the original as a bond breaker and not tie it to the original. So now you have two opposite traines of thought so as Silent said speak to an engineer.
As far as the steel goes, IMO if you have such a good base no I wouldn't bother but you will need a contol joint though the centre.

In the past I've done like bleeding thumb suggested. WIth the old slabs you can never be sure they laid plastic underneath. I'd use some SL72 mesh in it and foam where it meets the walls. A 150mm slab should support itself, whereas if it was a thin topping <50mm I'd bond it to the old slab.

Cheers
Pulse

Actually you're probably right. If the existing slab is sound and you're pouring another 6" slab over the top, it's probably not necessary to tie them together.

However a raft slab would normally have a reinforced edge where the walls sit on it, so I suppose it depends on whether the old patio slab is inside the footprint of the new one, ie. are you going to dig footings around the perimeter of the new slab? Or are you intending for the old slab to act as footings too?

I can think of at least 3 engineering issues: Anchorage to the rest of the house slab, foundation support for your new walls, and edge turn-down for frost protection. As silent said, you should talk to an engineer.

I'm an engineer, and I'm not trying to drum up business for my colleagues. But I'm not licensed in Oz, so I can't give you much more advice than this.

Joe

Thanks for everybody's advice, there will be no loadbearing walls on this, the existing sidewalls will stay unaltered, only across the front there will be a window and a door in a 90mm studwall, we are basically shifting the existing door/window arrangement out by 1,5m. And frost is thankfully not an issue here in Bundaberg. As the existing building/slab is very well built (steel frame), I reckon I'll just put down some reo and pour over it. I have to say that I much prefer to make sawdust than wrestle with this stuff...