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It is recommended that any concrete poured on top of an existing slab needs to be either:
1) Separated (but still supported by the lower slab) by means of 2 layers of plastic DPM to act as a slip layer,
or:
2) Bonded to the existing slab by means of an epoxy tie-coat (Sikadur32 or similar) to prevent
TOPPINGS / SLABS
Notes on Concrete Mix and Placement
Concrete mix should be designed for minimum shrinkage and have low water/moisture content, and not be a pump mix as pump mixes have a higher paste content and are therefore more prone to shrinkage cracks. Often poly fibre is added to reduce plastic shrinkage cracking in fresh concrete.
We recommend placement by wheelbarrow, and compaction by use of a handheld motorized vibrating screed. The edges of the old slab/new topping interface should be cleaned up as much as possible to produce a clean straight line between the two areas of floor. Existing slab to be covered must be clean and free from any loose or deleterious material (water blasting or something similar).
Curing of the slab is essential. We recommend wet curing the slab with water and covering with polythene for at least 4 days.The topping / screed can suffer from curling and delaminating as drying shrinkage occurs.
Bonded Slabs
Concrete poured on top of an existing slab needs to be bonded to existing slab by means of an epoxy tie coat must be applied immediately prior to the fresh concrete screed being laid to prevent the topping from curling and delaminating as drying shrinkage occurs. An epoxy tie coat (Sikadur 32) must be applied immediately prior to the fresh concrete screed being laid In accordance with the manufacturer’s instructions.
Any cut, cracks or joints in the underlying slab will reflect through the topping, so care should be taken to transfer marks from existing joints for saw cutting in the topping.
Separated Slabs
Slip layers are generally only suitable for toppings of 80mm depth or greater, and the surface of the underlying slab must be smooth and free from humps or hollows that will stop the slab from sliding freely.
Floor coverings
Tests may be necessary to ascertain moisture content if other floor levelling compounds or vinyl etc. are to be laid on the topping at a later date.
This is by no means a complete list of the issues involved with topping slabs, but a brief outline to give you an idea of the process involved.