RB wrote:
We have a massive concrete step that I'm going to replace. The
question is whether or not I can bust it up with a sledge, in place.

The piece is 3'W x 4'L x 8"Thick.

Will whacking on it with a sledge finally break through that 8"
thickness and start shattering pieces off?

The reason I'm asking before trying is because, if I can bust it up,
it will have to stay in place and maintain function and appearance
for a short interval until I can figure out how to rig it out there
in one piece.

I'm hoping sledge blows will put fault lines though the mass and
start it breaking up.

Drill hole.
Pound wedge.

Alternate:

Drill hole(s).
Insert wood wedges.
Fill with water.