Concrete is often reinforced with steel rods, wires and bars to increase its strength for civil structures.

You may wish to read the section about composites on concrete.

When the reinforced concrete is constructed properly, the steel is not exposed to a corrosive environment and therefore will not corrode.

However, if the concrete is not prepared correctly, and the steel is not protected from water, then a problem known as concrete cancer occurs.

If there are cracks or pores in the concrete then water may seep in and react with the steel reinforcing to form rust. As was mentioned previously, oxides can sometimes have a higher volume than the metal from which they have formed and this is the case for rust. Concrete cancer is therefore a problem because the corrosion product is bigger than the metal from which it formed and also because the portion of the material that provides the strength is corroding.

Corrosion that produces rust will force open bigger cracks, degrading the concrete further by progressively letting more water in.

Concrete cancer is becoming less of a problem in new buildings, as designers and makers of concrete are aware of the problem and greater efforts are made to protect the concrete from initially forming cracks or pores. Coating or painting the steel is also used as a precaution if water does seep into the concrete.

Concrete cancer is still a major concern in older buildings.