[logfromblammo]

It's not always the reinforcement. It can also be the accumulated result of decades of very slow reactions happening in the cement binder, or between the binder and the aggregate, generally also driven by moisture and gases penetrating the pores in the concrete, so cracks and spalling accelerate the process.

If it was just the rebar, we could replace the iron rebar with drawn basalt fiber rebar.

Perhaps in the future, structural concrete will be covered in an outer layer of ceramic that is subsequently vitrified at a certain stage in curing. You keep your building from collapsing by burning it in a towering inferno first.