Concrete can be strengthened before the concrete is put in place.
Prestressed concrete employs rebars in a certain method to protect against damage caused by Tensile force. Concrete can stand up to compressive forces effectively. An example of compressive force can be when you step onto an empty soda can and then crush it to flat. Tensile force (also known as tension) occurs when you pull something apart.
For concrete structures, you will have both types of force that are acting on concrete. Take a bridge for instance. The bridge’s top bends when an enormous load is transferred over it. The bridge is able to stand up to compressive forces and hold its own. Its bottom portion of the bridge pulls apart. Without rebars, the top of the bridge could crack and fail.
Concrete is created by using pre-stressed methods that inject rebar into concrete. The rebar is stretched out with the help of special machines. Once the concrete is set in place, the bar puts the right quantity of pressure on the concrete in order to keep it together against tensile force.
You can find more details in the video.