Concrete over concrete

[Sir Stinkalot]

Hi,
We are looking at concreting a new entertaining area .... thats fine I have no problem there .... but we wish to tint the concrete to a charcoal colour. There is a path that leads around the house and will meet up with this new entertaining area that is already concrete. What I would like to do is put a new layer of concrete over the existing concrete so the colours will match. I assume I would have to rough up the existing concrete to ensure good adhesion. Are there any problems with putting concrete over concrete that I should be aware of? What is the minimum depth of concrete over that I can get away with?

Thanks

Stinky.

[DaveInOz]

when I was contemplating similar (didn't go though with it) I was told to use Bondcrete to ensure the new concrete sticks to the old.

[Sir Stinkalot]

Ahhh ... Thanks Dave .... at least it can be done. I just need to keep its thickness to a minimum to save on costs. I hope your information wasn't from one of the helpful chaps at Bunnies

[q9]

For thin concrete you might also want to think about using something to slow down the "cure" and help prevent cracking.

[journeyman Mick]

50mm is generally considered minimum thickness of a topping, otherwise it will drum. Bondcrete is no good in permanently wet or weather exposed situations, it's basically PVA and we all know how water proof it is(n't). If the concrete is clean and mould free and unpainted the "suction" of the wet concrete on the dry concrete should stick them together nicely. You may need to high pressure clean the existing concrete and chemically etch it if it is very mouldy and or painted. To reduce shrinkage place the concrete as dry as possible and don't work it too late or early. Steel trowel it after floating, when it is hard enough that it rings when you work the steel trowel over it. This will compact and "densify" the surface giving you a hopefully crack free surface. When it has set enough that you can walk on it with thongs wet it and cover with plastic - a slow cure is a good cure. Keep it covered for up to a week. Then again others may have better information, I've usually avoided concrete like the plague, its messy and heavy and back breaking to work with, I even got rid of my mixer two years ago and I've been pretty successful in staying away from it since then.

Mick