What Is the Rate of Corrosion in a Concrete Structure?

Concrete

• Concrete is a compound containing sand, gravel, cement and water. The cement and water act as a paste that binds the sand and gravel together. Concrete hardens as it absorbs water, this is why concrete hardens even when under water and in other aqueous environments. Concrete becomes stronger as it ages because the bond between the binding paste and aggregate strengthens the longer they are in contact with one another.

Structural Support

• Concrete structures are held in place using a variety of structural support systems. Wires, mesh and bars are strategically positioned to counteract the stresses and tension found naturally within large, concrete blocks. Concrete's shear weight cannot, at times, support itself. Architects and engineers measure and predict where stress will be greatest and position iron or other metallic supports where they are needed most.

Corrosion

• Concrete may crumble or lose its integrity over time but it does not corrode. Corrosion occurs to the metallic supports positioned within concrete structures. These supports, when exposed to environmental conditions, like water and salt, will begin to rust and corrode. As these supports weaken, the surrounding concrete structure will begin to collapse. The concrete itself can, at times, cause structural corrosion because of its chemical make-up and pH balance. Concrete is a highly alkaline substance. As concrete ages, a chemical process called carbonation occurs. Carbonation changes the concrete's pH balance into a neutral balance similar to that of iron. When exposed to neutral pH levels the iron begins to undergo weakening corrosion. In other situations, such as a lack of oxygen within the concrete matrix, steel rebar corrosion is also a possibility.

Considerations

• Corrosion is prevented by carefully monitoring the concrete's pH levels and ingredients. High quality concrete is less likely to undergo carbonation or exhibit a lack of oxygen. Other preventive methods include positioning metallic supports in a manner that seals them entirely within concrete, limiting their exposure to weather conditions and deicer.