Mike-
The pad was most likely broken up by the heat of the fire.
Concrete absorbs moisture and the heat and resulting steam cracked the slab. Doesn't make sense they would put any physical effort into busting up the concrete. Creosote soaked ties would burn pretty well also.
Lazy, stupid teenagers or maybe transients?
IIRC - our circle centers at Tulsa Glue Dobbers are septic tank lids (precast concrete).
this is more what it sounds like to me as well,, perhaps it was a "senior" barbeque,, it is about graduation time, and heck, how responsible,, lighting their bonfire on the concrete so they dont torch the whold area,,
( hey its just an idea,, it sounds plausable to me,, and the fire would definitely shatter the concrete) sorry to those who fly there, thats a real let down either way