Why is concrete squeezed before a bridge beam carries traffic?

Concrete handles compression, or squeezing, much better than tension, or pulling apart. When a simply supported beam bends downward under a load, its upper region tends to shorten while its lower region tends to stretch. Tensile stress can therefore develop along the bottom.

Prestressing gives the concrete a useful starting condition. Strong steel strands are tensioned so that they put the concrete into compression. In pretensioning, stretched strands transfer their force to hardened concrete through bonding after release. In post-tensioning, tensioned tendons transfer force through anchors. The concrete begins its working life already squeezed.

Later bending must first offset that compression before producing net tensile stress in the precompressed region. The diagram uses a simplified centered preload of 4 megapascals, where a megapascal is a unit of stress. At the bottom, a bending contribution of 4 megapascals in tension combines with 4 megapascals in compression to give zero net stress. At the top, the same preload adds to bending compression.

This changes the beam’s stress distribution rather than magically changing concrete’s material strength. With a larger load, tension can still return. Actual designs account for tendon position, creep, shrinkage, relaxation and other losses, as well as several possible failure modes. Prestressing helps engineers control cracking and deflection and achieve useful spans within those limits.