Exploding Concrete Tiles

Rather than have this years bonfire on one of the veg plots I thought I would use the small concrete area in front of the terrace that used to house a brick barbecue.
This would enable us to stand or sit more comfortably around the fire.

This seemed like a great idea...until one of the tiles exploded!

This is the aftermath after I have cleared up.

Fortunately we were not close to the fire at the time. Needless to say I will make the bonfire somewhere else next year.

With a pot over the hole no one will notice.

 There are 20 tiles there and all but the four on the left of the last photo are embedded in at least 4" of concrete which has so far prevented me from taking them up. How/why it was laid on that wonky alignment I will never know.