I'm no engineer, but I hope our tunnel operators have learned from
those tunnel fires in the Alps.
One documentary I saw said that the concrete roof had some melting due
to a semi-truck fire in one of the mountain tunnel.
I'm no engineer either, but I guess about anything will melt if it gets hot enough.
Concrete of course is re-enforced with steel for most any structure. If the steel gets got enough the structure will fail. It's easy to bend a piece of steal once it's red hot.
Even if the concrete isn't hot enough to turn to liquid rock, if the steel gives, it's all over. The concrete can't support it's own weight without it.