Moisture from rain penetrates the surface layer of concrete. This is not, in itself, a problem. The damage comes when the concrete is attacked by either chloride – that is, the salts in the air combined with moisture (typical on buildings near salt water) – or “carbonation” – that is, the rain that is naturally acidic. One or both of these culprits break down the protective film surrounding the reinforcing bar. Typically, this happens when the reinforcing bar is too close to the surface of the concrete. Moisture will penetrate and affect the rebar if it does not have a sufficient layer of concrete to protect the steel. Optimally, the rebar should have a minimum of 1.5” concrete cover to protect it.