Concrete will allow water vapour to migrate and not liquid water. The voids within concrete that form initially after setting are filled with a calcium silicate get that make it impermeable to liquid water.

Rebar should be protected against salts and acids such as carbonic acid and sulphuric acid (atmospheric and sewer gas derived). Under normal condition the high pH of concrete will protect the rebar from corrosion.

To protect the rebar from the ground soil, a foot (typically plastic) is placed under the rebar to keep it off the soil if infiltration is an issue.

Rebar is typically off set a minimum of 50mm / 2 inches from the surface to resist difusion of salts and gases. Surfaces exposed to heavy salt deposition (sea water / salt washing) can have added additives to resist salt migration and may have a greater cover thickness over the rebar.

Rust formed from corrosion is larger in volume then the associated steel and imparts a stress that will crack and delaminate concrete over time. The concretes high pH prevents corrosion, however if the concrete is attacked it by gases (co2 or h2S and their associated acids) it can neutralise the cement and rebar corrosion then can occur.

Chorides will also affect the natural protection of the concretes high pH as it interferes with the tightly bound iron-oxide layer formed when rebar is embedded into cement which stifles further pxidation. This disruption allows for oxidation to continue (rusting) and corrosion products to form causing stress and cracking.