So I called the local machine rental shop and got a quote for $48 for four hours on the electric jackhammer.
But I drove down to the shop and found that $48 rents only the puny 35-pound machine. The big boy costs $60. Plus, I was annoyed that I had to rent the machine for a full four hours, when I only needed it for half an hour (plus commute, so still under two hours).
I figured, hell, for $48 I can do it myself with a sledge hammer. So I did.
Was it worth it? Well, per swing I didn't save too much money. (It took a lot of swings.) The middle was a lot thicker than I thought judging from the edges. But it's not like running a jackhammer is easy work. Plus, I saved an extra forty minute commute back to the rental shop, plus gas.
And, of course, I can say I broke up a concrete pad with nothing but a sledge hammer.