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As has been discussed a time or two on this very blog, and throughout the concrete industry, corrosion of steel rebar in concrete is a silent killer of concrete structures. But exactly how does this happen?

A Venn diagram shows the connection between concrete corrosion and iron, water, and oxygen.

Reinforced Concrete Corrosion Occurs When Three Elements Combine 

There are three essential components necessary for corrosion in reinforced concrete: steel, water, and oxygen. Eliminating any one of the above will prevent the oncoming chemical reaction and damages incurred due to corrosion. This is why there is no corrosion in dry concrete. This is also why concrete fully submerged in water has limited corrosion — except in instances where the water can entrain air.

A textured gray background shows off the coloring of concrete.

But Generally, Concrete Offers Great Protection Against Corrosion

Overall, concrete is a great host for rebar. Due to the high alkalinity of concrete, the steel reinforcing bars are passivated by an iron oxide film (Fe2O3) that provides a protective layer to the steel. In this state, concrete normally provides reinforcing steel with excellent corrosion protection.

Some weathered concrete can be seen corroding past a meshed gate.

That’s Not Always the Case Of Course

While hardening, concrete develops minute pores that can become a source for the ingress of corrosive agents into the concrete. These corrosive agents, entering into the concrete through those voids, lead to a breakdown of the passive protection layer around the concrete. Without the passive iron oxide film protecting the steel, corrosion is then able to commence at a much higher rate.

The passive layer can also break down over time due to atmospheric carbon dioxide, which, through a process called carbonation, lowers the pH of the concrete until the passive layer becomes unstable. The passive layer can further be rapidly broken down by aggressive chemicals, such as chloride, which are present in coastal environments and also used in deicing chemicals.

Once the passive layer is compromised, steel reinforcement will corrode if there is moisture and oxygen present at the surface of the steel.

This is just another reason why mitigating the risk of water penetration by using the most reliable concrete waterproofing product is vital to the sustainability of your project.

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