Stress to make concrete stronger In reinforced concrete steel rods are embedded in wet cement and held in tension while the mixture dries. Once fully dry the tension is removed, and the concrete should be stronger as a result of this process. However, I am not 100% sure as to why this works? I thought it is because by applying tension the elastic modulus (which is given by (tensile stress/(tensile strain)) becomes higher and thus the material has stronger resistance to deformation. It sounds overly simplistic though.
Can anyone confirm/clarify this?
 A: For simplicity let's make that a concrete beam with a single pre-tensioned rod.
After the concrete has cured the external tension is removed, so that the contracting force exerted by the reinforcing rod is now exerted on the concrete.
Let the beam be in a horizontal position, spanning a width, so it has to carry its own weight.
If that beam would not have any reinforcing then having to carry its own weight would mean that there is tension along the bottom part of the (horizontal) beam.
The effect of a pre-tensioned reinforcing rod is that it adds the same compression load to the top and bottom part of the beam.
When that beam is carrying its own weight:
The total compression load on the top part is then the  carrying-its-own-weight load plus the pre-load.
The bottom part: if the pre-load exceeds the tension load that the beam would be subject to if not reinforced, then the result is that the bottom of the beam does not come under tension load, but that it remains in compression.
And of course, with a sufficient amount of pre-tension the beam can carry more than its own weight.
