My garage floor has a large crack in it that runs the entire length of the garage (approx. 20'). The crack is about 1/3 the way into the center of the garage. and a slab has sunken. my brother in law suggests I just pour self leveling concrete onto the floor and raise it about 2-3". I'm afraid that if I did that, whatever cracked the floor might just cause whatever I put down to crack also. A website said that I can put something down over the old floor, like plastic sheeting, to physically separate the old floor from the newly poured concrete and that will somehow prevent the crack from going into the new floor. I've also read online that I may need to rip up the entire floor and start from scratch. I want to turn my garage into a second living room/mancave with polished concrete floors. Can you offer me any advice?