I had gotten this thread confused with another. But fortunately some info that may be useful to you developed when HJ suggested a closet flange spacer.
4" Styrofoam Closet Flange Spacer, eliminates needless chipping of concrete, styrofoam breaks away or melts with flame. Harvey makes one, as well as Sioux Chief. A search will turn up such spacers. The point is that the spacer keeps the concrete from the space that your flange bottom will have to go through.
To help with your mounting problem, you could fill the area where the mounting screws would go with mortar mix. Then your new flange could screw into the concrete with your favorite concrete fastener. I like Tapcons.
To help with your mounting problem, you could fill the area where the mounting screws would go with mortar mix. Then your new flange could screw into the concrete with your favorite concrete fastener. I like Tapcons.
