I have in my barn (concrete floor) a pit, of about 36" x 60" x 10" deep which was originally used to install some machinery by the previous owner.
I have no use for it, and would rather have the floor space than a safety hazard. I don't know how deep the rest of the slab is, nor how much is left under this "pit". It is smooth sided and bottomed.
I'm not sure if I should fill it full with concrete or use filler?
Also, i am not sure if I need to prep the sides at all for adhesion or what kind of concrete to use.