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Prestressed concrete
Prestressed concrete is not a new material. It successful use has been developed rapidly during the last two decades, chiefly because steel of more suitable character has been produced.
Concrete is strong in compression but week when used for tensile stresses.
If, therefore, we consider a beam made of plain concrete, and spanning a certain distance, it will at once be realised 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 in tension, if the cross-sectional area is small, causes it to break, especially if the span is relatively large.
If, on the other hand, we used 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 cracks - 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.
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