How to build up concrete slurry under a new bath

Question

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?

Answer 1

I have only done this once before and I have no idea if it is the correct way but we just poured the concrete onto the floor (shaped so it was cented where the tub bottom is) and then squished the tub into it before it dried.