While removing ceramic tile floor from our basement bath, that was attached directly to the concrete floor, I found a small patch of concrete between the toilet and the tub that is crumbling and sounds hollow. I'm guessing when putting the bathroom in many years ago they removed some concrete to put in the toilet drain, and replaced it poorly???? I have no experience with concrete. Anyone have any ideas on what caused this? Any solutions? Do I need to tear out all of the crumbling concrete? Thanks in advance for your help!