Today concrete is probably the most widely used man made building material. Some concrete structures built during the Roman Empire still stand today. Concrete is strong and durable because it has a very high compression strength.
But we all know that concrete can crack and break apart. That’s because concrete has a very weak tensile strength. Tensile strength is the force needed to pull something apart until it breaks. When steel rebar is added to concrete we now have a building material that is both high in compression AND tensile strength.
Rebar is graded by its yield strength. A rebar’s yield strength is the force needed to permanently deform or bend it. Any force less than that and the rebar will spring back to its original shape.
Grade 60 rebar for example, has a minimum yield strength of 60 ksi (60,000 psi) and a minimum tensile strength of 90 ksi (90,000 psi). This difference between yield and tensile strength means that rebar has a certain amount of ductility. If a force bends Grade 60 rebar, it will take another 30,000 pounds of force per square inch to break it apart. Imagine how important that can be when designing a building in an earthquake zone.
Just think – the steel reinforced concrete you pour today might still be around 2000 years from now!