I deal with ice all winter long. My favorite ice buster is ordinary boiling water. And a pick ax. But wear safety goggles if you go to hammering away at it.
The secret is to attack it late in the morning or afternoon on a sunny day. The sun melts the ice underneath, creating a sheet of water. This loosens it. Take something pointy that you can hit with a hammer, like a very skinny-head screw driver. This focuses the energy of your hammer blows and the ice should easily shatter. Treat it as if it was a window pane of glass and be very careful.