Check to see how much movement you have at the crack.
You may also have a large enough field of tile to require an expansion
joint.
Robert A. Cibiras wrote:
I laid ceramic tile over a concrete slab - new
construction. There was a crack in the slab before
any tile was laid. I was told by a local tile guy,
to use a rubber membrane brushon coating to cover
the crack.

I did this as instructed. The specific product I
used was called Pro-Red Waterproofing Membrane,
"elastomeric waterproofing & anti-fracture
membrane". Part of the product description even
states, "reduces crack transmission in ceramic
tile or stone..."

Everything has been good for about 3 years. A
couple of months ago I noticed a cracked tile, and
put it on the "to do" list. Last week, it had
spread to include 3 tiles total. So I decide that
today is the day I will chisel the tiles out, coat
the crack with the membrane, and lay new tiles in
a day or two.

Imagine my surprise when I begin removing the
tiles and discover that they are some of the tiles
lying over the already coated crack.

So now I am essentially back to the beginning -
sort of. If the membrane didn't stop the tiles
from cracking the first time, what would make me
think a reapplication will be any different? Based
on this, can I expect the tiles along the other
25' of the defect in the slab to likewise crack
over time?

What can I use to prevent future cracks in the
secton of slab that is now exposed? What, if
anything, can I do to prevent other tiles that are
currently laying on the slab, over the membrane
coated fault line, from cracking?

Thanks for any help.