Hi, I am replacing several exterior doors, and a couple of them had concrete sills rather than wooden/aluminum ones set over the framing. The first one was relatively easy to break up and remove with a hammer and chisel, but the second one entering the basement is proving to be much more difficult to remove, as the concrete they used is very strong. I'm making little progress with hammer and chisel, and have also tried an air chisel with almost no progress at all. Any suggestions as to what tool may make this easier. I've seen some rentals for electric breakers (one from Bosch). I suspect this is designed for larger jobs, but any advice would be helpful.