I'm redoing my bathroom and had to replace a toilet flange. The old one was rusted cast iron whose flanges had all but disintegrated, and I ground down what was left of it with an angle grinder. It was fastened directly to the pipe, not the subfloor--which is concrete (house is on a slab). Here's what the result looks like:
Then I bought a new flange that fits inside the existing pipe. Unfortunately I haven't found a good way to secure it to the subfloor, which is old (very hard) concrete. I tried Tapcons with little success. The problem is the angle of the concrete I need to drill into:
The walls of the existing concrete are so steep that it's almost impossible to drill pilot holes.
So what should I do? I've contemplated pouring some new concrete to fill the void and embed threaded rods in it to which I can fasten nuts though the flange, but I worry that it won't adhere well to the old concrete and tightening the nuts will just pull it loose.
What do you guys think?