You'll need to scrape off any old thinset residue to make an almost smooth and level surface. If there are any crumbled spots in the concrete, patch and level. Other than that, you should be good to lay your new tile directly on top of the concrete.
It's a very common but much older method of making a masonry subfloor designed to hold tile. Floor framing is dropped several inches in the area to be tiled and a heavy wooden subfloor is installed. Usually this is a double layer of diagonal 1x planking installed in opposite directions. The 'slab' is then poured over a lathe wire support. When the slab is leveled out, it's top surface is on the same plane as the remainder of the (non-tiled) floor.
They either use a very high density concrete or maybe it's so hard because all of these have been curing since the 30's or 40's but every one of these I've come across are very difficult to remove. The lathe wire adds to that but the end result is that the usual 1-1/2" to 2" thickness is more than enough.
In many cases, the framing is doubled up or solid blocking is installed under the entire area to make it even stronger.This message has been edited. Last edited by: Jaybee,