I have had to do the same thing. I rented a drill with a six inch hole saw. I made a series of holes almost touching each other for the path of the pipe I was adding. Then I hit them with a sledge and pried out the round concrete hole pieces. That left a pretty much open slot that a hammer and chisel cleaned up enough to lay the pipe down in.

After plumbing was complete, I poured a bucket of bag concrete over the slot and finished with a trowel.