I had no idea.
Where is the oxygen coming from to drive the oxidation (i.e. rust) process. From the concrete itself?
The ingredients in concrete are highly caustic. You can consider it a type of acid.
Some minute air bubbles are trapped in the concrete when it is mixed and poured. (air-entrained concrete)
Entrained air bubbles are very small - just a fraction of a millimeter.
Additionally, much of the water in the wet concrete mix dries out as the concrete sets. When the water molecule evaporates it leaves behind a void that is filled with air.
Furthermore, some of the remaining water is entrapped in the concrete as just minute water droplets. Water is composed of hydrogen and oxygen with most of the atomic mass being oxygen.
The amount of entrained air, entrapped air and entrapped water varies with the type of concrete mix and the way it is mixed and cured.
So in general, the necessary components to feed the deterioration of steel reinforced concrete are in the concrete to one degree or another from the moment it is poured.
Rust producing moisture and oxygen come from cracks in concrete. Even prestressed reinforced concrete is going to crack over time. Cracks are caused by the loads concrete structures and mats carry, by the tension and compression that can come when concrete members expand when hot and contract when cold, from bending due to wind forces, from vibrations due to traffic or small earthquakes or even noise from close jet engines, depending what and how close.
Moisture is the cause of rust and steel’s enemy. In the past we resorted to galvinizing the reinforcing steel or even coating it with a plasticized coating. Nothing prevents rust, only slows it down.
Since cracks allow moisture access to the steel, cracks are our enemy and if we could only eliminate cracking, steel woul never ruspt or would take a very long time at the least.
One of the most interesting developments in concrete is the possibility for “self-healing concrete” that could repair its own cracks.
http://www.iflscience.com/chemistry/self-healing-concrete-repairs-its-own-cracks/
It is not a complete answer as some concrete may keep repeatedly cracking faster than the concrete could repair, such as high constant winds. But think of bridge decks where various forces cause hairline cracks in the deck. Constant traffic will eventually widen these cracks to where moisture can infiltrate it and ripust the embedded steel, corroding it. Instead of spending millions on bridge maintenance to seal those cracks every few years by hand with a low viscosity Methacrylate seal, if the deck cravks rapaired themselves they would never get wide enough for watervto get in.
Exciting times for technology.