We only use reinforced concrete. The type of work where simple concrete is enough are very rare.
As for decay, IIUC, it loses pH gradually, from the time you mix it, and that pH is the most important protecting factor that stops the steel from rusting.
Obviously though, a lot of factors have a huge factor on lifetime, including composition, construction, environmental conditions ...
It’s rarely physics, almost always economics.
Physics, economics, and bureaucratics is a good summary of structural engineering. The last one can’t be ignored.