If, therefore, we consider a beam made of plain concrete, and spanning a certain distance, it will at once be realized that the beam’s own weight will cause the beam to “sag” or bend. This sagging at once puts the lower edge of the beam of tension, and if the cross-sectional area is small, causes it to break, especially if the span is relatively large.
If, on the other hand, we use a beam of similar cross-section, but incorporate steel bars in the lower portion, the steel will resist the tensile stress derived from the sag of the beam, and thus assist in preventing it from breaking.
In prestressed concrete steel is not used as reinforcement, but as a means of producing a suitable compressive stress in the concrete. Therefore any beam (or member) made of prestressed concrete is permanently under compression, and is consequently devoid of crack under normal loading, or so long as the “elastic limit” is not exceeded.
Prestressed concrete is not only used for beams but is now employed extensively for columns, pipes, and cylindrical water towers, storage tanks, etc.