The greatest weakness of concrete is its low tensile strength which has been remedied by using steel rebar on the tension side of the concrete. However, a major and unavoidable disadvantage of steel rebar is its susceptibility to corrosion.
Having a concrete cover that’s too thin allows water to penetrate and react with steel rebar causing the concrete to crack. Occasionally concrete aggregates react with the steel causing the concrete to spall and rust.
If steel rebar rusts it will reduce the strength of the reinforced concrete. When rust forms around steel rebar, the internal pressure will significantly increase around the surrounding concrete, leading to crack propagation.
When steel bars corrode, deterioration of the structure will be quite rapid and dramatic.
Other disadvantages of steel rebar are its high weight and conductivity.