I had to break up a 16x8x6 slab some years ago.

Started out by hand and as some of the slab had already cracked and settled
it was easy enough to get a wedge under pieces as was previously suggested.
As the demo continued I discovered that the slab covered a set of patio
steps. These things laughed at my sledgehammer. Whoever poured them must
of wanted to finish up the concrete they had because these things were
massively overbuilt.

Headed to the rental place and rented an electric jackhammer. I forget how
much that was for 4hrs but it was in the neighborhood of $30. It went
through the steps in no time and made me realize how stupid I'd been to do
the slab by hand.

Spend the money and rent a jackhammer. You won't regret it.

ml