Concrete is alkaline, which prevents corrosion. There are galvanized, epoxy coated, stainless, fiberglass, and probably other types of rebar which can be used in highly corrosive environments. Paint wouldn't work, cause by the time the rebar is wrangled I'm place, most of it would be gone.

Steel doesn't corrode in concrete unless either salt concentration reaches a critical concentration or the pH is reduced. This is because concrete had a very high initial pH and steel is passive and doesn't corrode at that pH. Either salt reacting with the steel, or carbon dioxide from the atmosphere can over time change the pH and cause corrosion to occur. The most likely cause of corrosion is salt concentration.

When you coat reinforcement with epoxy you still get small coating holidays, it's unavoidable. During those holidays, corrosion can rapidly occur under the coating and even sever a bar. Where if that same installation was uncoated the corrosion would be more diffuse across the bar and the concrete would crack before the bar would be too damaged.

This behavior means that inspectors can be sure if there are no cracks in the concrete, the bars are intact.

Even worse, if corrosion on coated reinforcing steel is found, the most common repair techniques like cathodic protection don't work because corrosion can occur under the coating and no electricity will pass through the coating.

So coating epoxy means the failure mode becomes more severe with less obvious signs of decay and limits repair options.

It is far better to just use more expensive concrete mixes, more concrete cover or even stainless steel rebar.

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