Concrete and metal are challenging w/regard to corrosion.
The concrete is hygroscopic and cools the metal, then moisture in the concrete and air condenses on the metal cartridges with every little rise in dew point..
It really is best to keep ammo in the same pristine conditions that our guns should be kept in (right up to moment of use).
With this, I keep loose ammo in my front pants pockets in the field. I pull out/load one round at a time, when I'm ready to kill something.
As load developed the same way, it also mitigates ammo temperature variance, which affects our loads (summer or winter).
When I sit with my gun in the field, I drape a towel over it to keep the sun's radiant heat from changing my barrel temps.
Might seem excessive, but I am really into single shot cold bore kills.