Reinforced concrete is almost a perfect marriage between the low cost and versatile concrete and the high strength and flexible carbon steel. Concrete needs the steel for its strength and the steel needs the concrete for its corrosion protection.
When the concrete loses its high alkalinity that protects the carbon steel from corrosion is a one example where alternative solution is required. This process is known as carbonation and it can take more than 100 years but ultimately is inevitable. Stainless steel does not rely on the concrete for its corrosion protection and is a straight forward replacement and solution for long life design buildings such as temples or monuments.
The second is where the concrete is subject to the ingress of chlorides from either marine environments or de icing salts. In this process the chlorides build up to such a concentration level at the surface of the rebar that they break down the corrosion resistance mechanism and initiate corrosion. Stainless rebar can resist the initiation of corrosion with chloride concentration levels more than 10 times that of carbon steel and again is a straight forward replacement and solution to the problem.
The third is where equipment is particularly sensitive to magnetic fields and needs non magnetic reinforcement. Carbon steel rebar is magnetic as too are most stainless steels but austenitic stainless rebar is not. Once again it is a straight forward replacement and solution to the problem