Concreting over slab

[duke666]

I've had a 'builder' pour a slab for a shed and honestly it's the worst job I've ever seen. Usually I would have done it myself but didn't have time.

The top is too uneven to simply screed over, so I'm plannig to a pour about 30 to 50mm depth over the top.

Obviously never had to do this before.

Do I need to prepare the top of the slab with anything for the new concrete and is 30 to 50mm ok?

Thank you.

 

[SpaceWorm]

Usually when pouring a concrete topping, you normally need to scrabble or roughen the concrete prior to placing the new slab, but if it's is very rough, it might not be required - it should always be nice and clean and free of dust and detritus though. You should also use a smaller maximum aggregate size than normal, say 10mm instead of 20mm. If you were more like 75mm in thickness, I would also consider using some A142 mesh, but you shouldn't do this if you are going to stick at the thickness you mentioned - it's too thin for mesh.

Is it possible you could increase you topping thickness to something like 75mm? Doing this and using 10mm aggregate and the mesh will make it a proper job.

To me, 30-50mm would be fine for a wet screed?

 

[tell80]

As above: clean and loose debris free.
Prime the slab surface with say a couple of coats of SBR. Second coat tacky.
If you can get it, Granite "fines to dust" are good for a shallow sand & cement mix.
Be aware of raising the FFL & causing yourself further problems, esp at thresholds.

Did the builder use a membrane (a DPM)?

 

[noseall]

Be aware of raising the FFL & causing yourself further problems, esp at thresholds.

I've had a 'builder' pour a slab for a shed