Dr. Doom wrote:

(a). any kind of plastic will burn (given sufficient conditions -
the circumstances which create those conditions are highly
unpredictable). why chance it ? use metal ducting.

Metal ducting under concrete means cast iron pipe! Certainly not aluminum
sheet metal.

Very many plastics are fireproof. One of the earliest, Bakelite, for
example.

PVC is flame-resistant and self-extinguishing. It will burn only if a flame
is continuously applied and even then very reluctantly.