Ten years ago I built a barbeque pit out of concrete blocks. I lined the inside of the fire box – the sides and bottom – with fire brick. I used stone tiles, 1/4″ x 12″x12″ for the outside face and on top for the work area, and regular grout and adhesive for outside applications. Then, about six years ago, some of the top tile cracked. I replaced them. Two years ago they cracked again, in the same place. There are no cracks in the sides anywhere so don't think settling is the issue. Maybe it's due to the heat of the firebox? I cook with wood. I am planning on replacing the cracked tile again but would like to find out why it keeps happening. Any thoughts?
The cracking could have to do with the expansion and contraction of the structure, since it gets so hot in that particular area. If this was a tile floor, the elastic underlaying beneath it would stretch when heated, but there's no give in this scenario. You might want to think about replacing the tile with a different material. Consider stone pieces instead of tile - or a slab. If it's cracking every couple of years, I'd just keep maintaining it. But if you want something that lasts longer, you probably need to consider a different assembly, executed by a professional mason. Good luck!