I live in an apartment and have just chiseled up the toilet floor in concrete for a new plumbing installation to reach the floor below (ceiling with something like Ecophon tiles). Now, I'm contemplating how best to pour concrete to fill in the holes, one 200 x 400 mm with rough edges and a diamond-drilled hole for the old toilet drain. The floor is between 150 - 200 mm thick.
I'm planning to fasten a bottom formwork with a thin wire against a cross brace of some angle iron. This way, the formwork bottom is pressed up against the underside of the ceiling/floor. Seal the formwork bottom with a closing strip so that the concrete doesn’t seep away against the somewhat uneven ceiling.
How do you achieve the best adhesion, especially on the clinically smooth diamond-drilled hole? Should I roughen the surface or chisel it to create a coarser finish? Mix a slurry of concrete mortar or use a special slurry compound? Or primer? It will be a bit difficult to pre-wet, especially since spilling water downwards is not advisable.
Reinforce? Do I need to drill and insert some rebar? Is it
