Corrosion of steel in reinforced concrete is a serious problem that can lead to the deterioration of concrete structures. When steel corrodes, it produces rust, which is a much larger volume than the original steel. This expansion of the rust can cause the concrete to crack and spall, leading to structural failure.
Corrosion of Steel in Reinforced Concrete
There are two main types of corrosion that can affect steel in reinforced concrete: carbonation corrosion and chloride-induced corrosion.
Carbonation corrosion occurs when carbon dioxide in the air reacts with the calcium hydroxide in the concrete. This reaction produces carbonic acid, which lowers the pH of the concrete. When the pH of the concrete drops below 9, the protective layer on the surface of the steel is destroyed, and the steel begins to corrode.
Chloride-induced corrosion occurs when chloride ions from seawater or deicing salts penetrate the concrete and reach the steel. Chloride ions can also be introduced to concrete during the mixing process or through the use of contaminated aggregates. When chloride ions reach the steel, they react with the iron in the steel, forming rust.
Both carbonation corrosion and chloride-induced corrosion can lead to the deterioration of concrete structures. However, chloride-induced corrosion is generally considered to be more serious because it can occur more quickly.
There are a number of things that can be done to prevent or mitigate the effects of corrosion on steel in reinforced concrete. These include:
- Using high-quality concrete with a low water-to-cement ratio.
- Using pozzolans or other admixtures that can improve the durability of the concrete.
- Using epoxy-coated or stainless steel reinforcement.
- Avoiding the use of chloride-containing materials in contact with the concrete.
- Providing adequate drainage to prevent the accumulation of water on the concrete surface.
- Monitoring the concrete for signs of deterioration and taking corrective action as needed.
By taking these steps, it is possible to extend the lifespan of concrete structures and prevent the costly and disruptive repairs that can be caused by corrosion.
Here are some additional details about the two main types of corrosion that can affect steel in reinforced concrete:
Carbonation corrosion
Carbonation corrosion is a slow process that can take many years to develop. It is most likely to occur in concrete that is exposed to air or water that contains carbon dioxide. The concrete must also be porous enough to allow the carbon dioxide to penetrate.
The first sign of carbonation corrosion is usually a white powder on the surface of the concrete. This powder is calcium carbonate, which is produced when the carbon dioxide in the air reacts with the calcium hydroxide in the concrete. As the carbonation process continues, the pH of the concrete drops and the protective layer on the surface of the steel is destroyed. The steel then begins to corrode, producing rust.
Rust is a much larger volume than the original steel, so it can cause the concrete to crack and spall. This can lead to structural failure, especially if the concrete is in a load-bearing element.
Chloride-induced corrosion
Chloride-induced corrosion is a more aggressive form of corrosion than carbonation corrosion. It can occur more quickly and can cause more damage to the concrete.
Chloride ions can enter the concrete through a number of ways, including:
- Seawater: Concrete that is exposed to seawater is at risk of chloride-induced corrosion.
- Deicing salts: Concrete that is exposed to deicing salts in winter is also at risk of chloride-induced corrosion.
- Contaminated aggregates: Concrete that is made with contaminated aggregates can also be at risk of chloride-induced corrosion.
Once chloride ions reach the steel, they react with the iron in the steel, forming rust. Rust is a much larger volume than the original steel, so it can cause the concrete to crack and spall. This can lead to structural failure, especially if the concrete is in a load-bearing element.
Chloride-induced corrosion can be a very serious problem, and it is important to take steps to prevent it. These steps include:
- Using low-chloride concrete.
- Avoiding the use of deicing salts in winter.
- Using clean aggregates.
- Monitoring the concrete for signs of chloride-induced corrosion and taking corrective action as needed.
By taking these steps, it is possible to extend the lifespan of concrete structures and prevent the costly and disruptive repairs that can be caused by chloride-induced corrosion.