cementing

[Gazz]

i need to level a concrete slab, putting a shed on it, where the slab will be the floor you stand on in the shed,

slab is about 9 foot by 8 foot, half is level, half sloping... origionaly for drainage as was a floor for a dog kenel.

max depth of new cement will be 2 inches i rekon, tapering to nothing as i get to the flat portion.

What do i need to do to get the new cement to bond to the old? removing the existing slab and pouring new is not an option, but i can break the sloping part of the slab up a bit to give it a key.

What ratio of sand, cement and aggregate should i go for?

never done cementing before, will hire a mixer, and buy normal portland cement in 25 kilo bags, sharp sand and small stone aggregate. any ideas how much i need to get? worst case, 4 foot by 8 foot, 2 inches deep,

i know i wont get a great finish where i get to the level portion of the slab due to the aggregate, but it dosent have to be perfect, and maybe pour of thinset after the main leveling work is done will make it perfect?

[Tim Watts]

I would probably use just sand and cement (about 1:4 or 1:5 but that's a bit of a guess, 1:4 is quite a strong mix, 1:3 is about the strongest possible).

I can say though with some surity that the way to get it to bond is:

1) Clean the old slab totally - pressure wash or hose and scrub with a wire brush. 2) As you are laying the top coat, brush down a cement slurry made of pure cement with enough water to make it roughly the thickness of thin honey. Brush into the old slab well but thinly and apply new layer to that. Do not attempt to go to nothing but top the whole lot off going down to an inch, maybe 1/2" at thinnest. There remains the risk that the thinner part will crack off at some point. If you wanted bomb proof, use an SBR modfied screed (1/2 water, 1/2 SBR for the sand cement mix and 1/2-1/2 SBR to water in the cement slurry). That is rated to 10mm thick and should not detatch.

But to be honest, it's a low stress situtation - you'll probably be OK without the SBR as long as you accept some risk of damage in the future.

[Gazz]

Cheers, i ended up using 3 sand, 1 cement and 2 'white marble chippings' as i had 3 bags of them laying about and was never going to use them, this was to do the deepest areas near the edges, used 2 bags of cement for this bit, got the thinner bits to do tomorrow, where i'll use no aggregate as suggested.

was pressure washed 2 days ago to get all the stains off from the dog that lived on it before we moved here.

Excelent, i'll do that, thanks for the tip.

okay, i'll do a half inch top layer over the lot tomorrow, the shed is going to be used one half for storage of the mower, bikes etc, the other side is the entry to the aviary, thats where the deepest concrete has been laid so should be the least likely to crack,

main thing is the get the slab level, so i can put the shed directly on the slab without fannying about with packing the parts up that float in the air, i'll be putting some damp proofing membrane stuff under the timbers that will sit on the concrete.