Concreting over an existing concrete slab?

[Specialized29er]

I have a small concrete slab 1600W x 2000L x 300H (don't know why so thick) and because of the thickness I'd like to leave the slab where is and concrete over the slab.

The slab sits level with the ground and I'd like to cover it with reinforcing mesh and another 100mm or more of concrete.

The new slab needs to be 3200W x 4300L X what ever the recommended depth of the new concrete is.

Has anyone done this job before and would this be successful and not have the concrete crack.

The slab is to have a 2 post MaxJax vehicle hoist installed on it to the side of a shed.

[rwbuild]

Since you are installing a vehicle hoist on it I strongly suggest you have structural engineer design the slab for you, its your life and liability at stake.

[joe greiner]

I'm a structural engineer (bridges mostly), and this sounds like a terrible idea because the new slab is larger than the existing. Mesh alone won't provide the needed reinforcing for bending across the edge of the underslab, and the new thickness would have to be substantial.

You would be better served with separate foundations for the posts, designed by a local structural engineer, with enlarged flooring isolated from those foundations.

Cheers,
Joe

[ian]

I have a small concrete slab 1600W x 2000L x 300H (don't know why so thick) and because of the thickness I'd like to leave the slab where is and concrete over the slab.

The slab sits level with the ground and I'd like to cover it with reinforcing mesh and another 100mm or more of concrete.

The new slab needs to be 3200W x 4300L X what ever the recommended depth of the new concrete is.

Has anyone done this job before and would this be successful and not have the concrete crack.

The slab is to have a 2 post MaxJax vehicle hoist installed on it to the side of a shed.

look it's doable, but the cost in engineering design, reinforcing bar and additional concrete will be more than the cost of removing the existing slab.

[Skew ChiDAMN!!]

Will the existing slab be in a corner of the new slab?

If so, you could box it in and bring it up to height. Then pour the rest of the slab, with expansion joints along the edges of the old. It wouldn't be pretty and there's no guarantee the new slab won't shift in relation to the old, but at least you won't have cracking.

If one of the hoist poles would be positioned over the old slab you'd need to cut the slab back, making clearance for a suitable footing(s) for the hoist.

[cava]

How can you be sure that the thickness of the existing slab is a uniform 300 mm across the entire slab?

[doug3030]

How can you be sure that the thickness of the existing slab is a uniform 300 mm across the entire slab?

It is a common practice to make the edges of slabs thicker than the centre. I agree with Cava that if you have measured the thickness of the slab on the edge it is no guarantee that it is that thick in the middle.

[ian]

Will the existing slab be in a corner of the new slab?

If so, you could box it in and bring it up to height. Then pour the rest of the slab, with expansion joints along the edges of the old. It wouldn't be pretty and there's no guarantee the new slab won't shift in relation to the old, but at least you won't have cracking.

if you did this the new slab will crack from the inside corner of the boxing. Stopping the cracking might require 16 mm bars at 300 mm centers.

the new slab at 3.2 x 4.3 is close to the maximum size before planned contraction joints need to be incorporated.

dredging the memory,
to work, the new slab would need to structurally incorporate the existing slab and then cantilever beyond the edges of the old slab.
A tricky design involving a concrete depth of upwards of 250 mm with far more steel than is found in "reinforcing mesh" PLUS knowledge of the existing slab's concrete strength and reinforcing.

and like the best laid plans would likely not work if the existing slab is edge thickened rather than a uniform 300 mm.

[Skew ChiDAMN!!]

if you did this the new slab will crack from the inside corner of the boxing. Stopping the cracking might require 16 mm bars at 300 mm centers.

I was thinking of a single box foundation for the hoist posts then pouring floor slabs around that. Full width expansion joints, similar to a tic-tac-toe board with the box in the middle. As I said: not pretty and no guarantee the floor'd stay level. But serviceable.

To make it as one pour... yeah, the old slab'd have to go.

Edit: forget the above suggestion. I'm too used to thinking of pier support and forgot about unequal loading. Sorry... having a D'Oh moment.

[Specialized29er]

Thanks guys and yes you're right the cost of the engineer and the additional reinforcing would be greater than removing the slab.
The slab was drilled in several positions and yes she's a 30cm solid slab and may not even have any reinforcing mesh.
I'll hire a jack hammer tomorrow and see if it'll break up into small pieces for removal.

[doug3030]

... and yes she's a 30cm solid slab and may not even have any reinforcing mesh...

If there's no reo it won't take much to break it up, to look on the bright side.

[rwbuild]

Just a question, the hoist, is it for your own personal use or business use? If it is for business, get a structural engineer to design the slab because if something fails, guess what, your wont own anything anymore, no insurance cover and litigation will bankrupt you. Even if it is for personal use and you do a favor for a friend and something happens, same scenario. Its simply not worth saving the dollars for a proper certified design.

[Specialized29er]

Private use on a property, way out western Queensland and the concretes arriving in 1/2 hour, 2 full days to dig the foundations, box up and setup the mesh and internal steel structure.
The fastening down tubes are welded to structural steel frames in the concrete and where the posts sit we've dug 60 deep x 60 x 60 holes for extra support as well a 40 deep trench all the way around the slab.
Had a builder/concreter mate take a look and is extremely happy with the foundations and even said a tad over engineered but hey safety first.