/diy/ - Do It Yourself

>>1449376
Something likely changed the alkalinity of that concrete. It looks like it holds up a roadway so it could've been salted in winter.

>Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection. At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1 µm per year. Without the passive film, the steel would corrode at rates at least 1,000 times higher (ACI222 2001).

>Because of concrete’s inherent protection, reinforcing steel does not corrode in the majority of concrete elements and structures. However, corrosion can occur when the passive layer is destroyed. The destruction of the passive layer occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level.

http://www.cement.org/learn/concrete-technology/durability/corrosion-of-embedded-materials

Also here's the first re-enforced concrete bridge from 1875.

>>1449379
>Cost. It will continue to rust until it's gone.
And this may happen slower than I thought.