Rednadnerb wrote:
Steel needs oxygen as well as water to rust. Reinforcing bars will rust
and the concrete spall if there isn't enough cover of concrete over
them or if the concrete isn't properly compacted around them.

I agree. Also concrete starts off being highly alkaline which prevents
oxidisation. However carbon dioxide in the atmosphere penetrates the
concrete at about 1mm per annum which turns it acid. When this
'carbonation' reaches the rebar it will rust and in time spall the
concrete.

There are many ways to prevent this happening. (by the way, wire
brushing rust on MS just polishes it)