It’s not very strong, but it is resilient and can stand up to stretching out (tensile force). Concrete is anchored against tension forces with reinforcing bars. In order to maximize the strength of the rebar it needs to be stretched making what’s known as prestressed concrete.
When a large weight is placed on concrete, it flexes downward. The top of the concrete is stronger than enough to withstand the compressive pressure, while it is stretched at the bottom through tension. The result is cracks that form which can eventually cause an unavoidable breakdown.
Concrete could have rebar in it, but it isn’t able to prevent cracks when concrete isn’t pre-stressed. It is acceptable for certain buildings, but for bridges and buildings that require prestressed concrete, it’s best to prevent harm to people using them. Concrete is laid over rebar. A machine can take apart the rebar whilst it is forming. It expands the rebar which makes it more resistant to the force of tensile.
Prestressed concrete works great on bridges, as mentioned above. Bridges can be both long and slim, and therefore concrete with a thinner thickness will more easily fail due to tension. Precast concrete is strong and provides thickness without degrading the strength of the concrete. To learn more take a look at the video on the right.