I used this product ... you cut a piece off and knead it together until its mixed and then press it into the crack. I roughed up the surrounding area with a sandpaper sponge block. Works well but needs re-doing periodically dependent on people stepping on it etc. Look for it at the hardware store or CTC if you're in Canada.

http://www.henkelna.com/cps/rde/xchg...ram1=technical