What’s the trick?

[Mike Davis]

I don’t think there’s any rebar in it.
I haven’t hit any metal at all in the 20 or so holes so far.

 

[Hmerkle]

Drill some short holes pour in some water, get some dry ice and see if you can get them to "pop" a line...
To be honest with 8 inch thick concrete you will even have hard time with a sledge unless you can "prop-up" a portion of the concrete in some way to break it, if it is on the ground, you can beat it as much as you want to and it is unlikely it will crack, the ground absorbs the blows.

 

[Gotcha6]

Plan C: Rent an electric jackhammer......

 

[Hmerkle]

Plan C: Rent an electric jackhammer......

Good idea, but hire a 20 or 30 something along with it - that thing will beat you up!

 

[Gotcha6]

Good idea, but hire a 20 or 30 something along with it - that thing will beat you up!

Yep. BTDT