I'm replacing our bath. It is a standard built-in plastic bathtub, with a concrete slurry built up underneath it. I talked to a builder some time ago who explained that it was to support the weight of the bath, I guess it distributes the weight of the full bath over the floor underneath.
How to I build up the concrete underneath the bath? Do I pour the concrete first and get the bath into position before it goes off, or shovel it in with the bath already in place?