Q. Can I rebuild my garage on my existing concrete pad? There was a major fire that completely destroyed the structure. How can I tell if the floor is ok?
A. Most of the time the heat from the fire will weaken the surface of the concrete floor and cause pop outs and/or surface scaling.
If you don't have any of this happening to your concrete floor, try tapping the surface with a hammer, if it seems solid, not brittle, then you are probably all right.
I would hire a concrete testing company to test the concrete to be sure. They can core drill some small holes and test them for strength.
Better to know now then have to replace it after your new garage is built.