If it's just cast iron a 16-22 oz framing hammer and a strong arm will bust it up fairly quickly. Make sure it's not a vent for your sewer or the sewer itself first. If you can legally take it out once the floor area is clear keep busting concrete around the hole about an 1" bigger than the hole itself. Go down about two inches and keep busting the cast iron down to that level if possible. Take a big plug of insulation and stuff it at least 6" down the hole, mix and 10# bag of sackrete towards the dry side and smooth it out with the top of concrete.
If it is steel you will need a sawzall with a metal cutting blade. (A Lenox 10 tooth x 8-10" bimetal blade should do it) Lay the blade down flat on the concrete and try to cut the STEEL pipe off perfectly flush with the top of concrete. Then jamb a tight plug of insulation about 6" down the pipe and mix your concrete and smooth off level withtop of concrete.
If it is steel you will need a sawzall with a metal cutting blade. (A Lenox 10 tooth x 8-10" bimetal blade should do it) Lay the blade down flat on the concrete and try to cut the STEEL pipe off perfectly flush with the top of concrete. Then jamb a tight plug of insulation about 6" down the pipe and mix your concrete and smooth off level withtop of concrete.
