Our house has a flower bed immediately in front of the foundation.
There's a patch in the flower bed that has no significant plants growing
in it, and we decided to add some. When we tried to dig a hole to plant
them, we discovered why there aren't any plants the

It seems that someone had some leftover concrete, perhaps from pouring a
stairway nearby that goes from driveway level up to front lawn level,
and they simply dumped the excess concrete into the area that would
eventually be the flower bed. There is a chunk of concrete about 6
inches thick, 6 feet long, and 2.5 feet wide in there, with a few inches
of dirt over it. The concrete is not attached to the foundation or the
sidewalk, it's just lying there. But it's too heavy to move as a single
piece.

So I've been breaking it up into smaller pieces, using a single-point
concrete chisel and a 3 pound club hammer. This just doesn't work very
well for breaking 6 inch thick concrete. I end up holding the chisel in
one hand and the hammer in the other until I get the chisel embedded far
enough to stand up on its own, then I switch to two hands on the hammer.
Sometimes this works in a half-dozen strikes, sometimes it never works
and I try moving the chisel somewhere else. I've probably spent a
couple of hours on this already, and it's down to half the original
size, but progress is discouragingly slow.

The two ways to improve the situation seem to be: get a bigger hammer
(e.g. a long-handled sledgehammer), or some sort of power hammer. What
would be suitable for 6 inch concrete?

Dave