Concrete holds up well when exposed to fire
It holds up well enough that most, perhaps, all insurers assume that a concrete slab foundation is not damaged when a house burns to the ground. For all practical purposes, they assume that a slab foundation can be reused when there is a fire.
Concrete is used as a fire protectorate
It is common to apply concrete to steel framing to protect the steel from a fire. For that reason, insurers assume that the heat has no adverse effect on the slab. The reality is that the rebar can be damaged. The heat can cause it to expand and push against the concrete cracking it. When everything cools, you may find that some or all of the rebar is no longer bonded to the rebar. Proving this without damaging the slab and spending a lot of money is not easy.
The problem gets worse
The City of Houston will not issue a building permit for rebuilding a fire damaged house unless a Texas Professional Engineer certifies that the slab foundation is structurally adequate. Since any damage is not likely to be visible because it is buried in the concrete, most engineers are very reluctant to make such a certification.
And worse again
It is highly likely that the foundation would not pass muster under today’s codes. The stiffening beams are probably too shallow and spaced too far apart. A home built in the seventies is likely to have beams around 16 inches deep spaced as far apart as 20 feet. The beams also may not all be continuous. rendering the slab less likely to be successfully underpinned should that become necessary.