About ten years ago I built a bbq pit ut of Concrete blocks.I lined the inside of the fire box, sides and bottom, with fire brick using a little clay in the mortar because I was told you should bdo this with firebrick. I used stone tiles, 1/4" x 12"x12" for the outside face and on top for work area. The tile on top ties the concrete block and fire brick together. I used regular grout and adhesive for outside applications. Problem is about 6 yease ago some of the top tile cracked. I replaced them and about two years ago they cracked again in the same place. There are no crackes in the sides anywhere soI don't think settling is the issue. I thought it might be due to the heat of the firebox. I cook with wood. I am planning on redoing it again but thought I might see if I could find out why it keeps cracking. Any thoughts???