Repairing a concrete floor
Clean out the small depressions and cracks to be filled with a club hammer and chisel. Beware of flying chippings. Use a trowel to fill in the depressions with mortar and to level off to provide a suitable surface for the tiles.
Filling cracks and hollows
If you have a concrete floor which is flat, dry and level you can go ahead and lay tiles without further preparation. Often, however, the floor is not level or there are cracks and small hollows on the surface. Indentations should be filled with mortar (a 3:1 sand:cement mix is suitable) mixed to a creamy but not too runny consistency. For mortar with a good bond add some PVA bonding solution to the mix. Cut back the holes to a clean shape and brush out any loose material so it doesn’t mix in with the mortar making it difficult to get a smooth surface. You can also coat the holes with a PVA bonding solution to help the mortar adhere.