Concrete over existing slab. Minimum thickness?

[VxDuncan]

Hi, I'm currently building a new garage - 4.2 x 5.7m roughly. There's an existing slab from the old garage - 2/3rds of it is solid, around 3.5 inches thick. The other third drops away, and is thinner - around 2 inches. It's also lower. The plan is to put a thin bed of sand over the top, DPM and then pour more concrete over the top. I'll either put metal fabric (reinforcing bar shreddies) down or use fibre reinforced concrete for strength (opinions?).

The old base slopes, so the new base will be thinner one end. What I'm wondering is how thin the cap over the concrete can be? It's convenient to have the base 2.5 inches one end, increasing to 5 inches the other end due to the brick courses.*

• This was all supposed to be sorted with the brick courses, but the setting of the level for the foundations was done in the rain...

[caziques]

The usual "rule" AFAIK is 4 inches. However I had to lay a small slope of concrete when we had a garage extended and ended up with a 2 inch kerb. Hairline cracks where it gets driven over every day after six years is all.

[VxDuncan]

That's promising. Brick courses to the proposed slab level were completed earlier, so hopefully tomorrow I'll be able to get a better idea of how thin the concrete would be (when I've shifted the pallet of blocks and bulk bag of sand that's in the way).

Would welcome other input

[MOTORVATOR]

Dependant on the underlying surface the minimum you should use is 2 1/2 times the aggregate size. So for 20mm aggregate then 50mm is the thinnest you should use, 10mm agg and you can drop to 25mm.

This won't constitute a construction in itself though and would only be used as a structural topping to say a beam and block garage floor or similar.

Fibremesh is only really any good for stability over very small deflections, if the underlying strata needs to be bridged (areas of differing load capacities)then use reinforcing mesh.

A typical garage floor specification as a ground bearing solution would be 150mm of 20-30n concrete reinforced with A193 mesh top and bottom. That would be placed over a subbase material to give a CBR of 20% ish so 150 of crushed concrete or similar.

Without seeing your existing I would guess from your description that what you have there at the moment is not much more than the equivalent of a subbase material so I wouldn't cut the spec down too far.

[VxDuncan]

Maybe I do need to increase the thickness... Issue is the current base is roughly the right ground level, so adding too much to this will mean a climb into it. The added issue is it's a sloping site - I'm already looking at 6 courses of engineering bricks below the DPC, which is very visible from the house, and need to keep the whole thing under 2.5m for planning.

Knackers. Might have to get rid of the old base