Prestressed concrete

Текст C

Prestressed concrete is concrete with stresses applied to it before use so as to counteract stresses that will be produced by loads. Prestress is most effective with concrete, which is weak in tension, when the stresses which are applied are compressive. In order to produce compressive prestress a concrete member can be placed between two abutments, with jacks between its ends and the abutments, and pressure is applied with the jacks. The most common way is to stretch steel bars or wires, that are called tendons, and to anchor them to the concrete; when they try to gain their initial length, the concrete resists and is prestressed. The tendons may be stretched with jacks or by electrical heating.

Prestressed concrete is particularly useful for beams. It permits the use of steel at stresses several times larger than those permitted for reinforcing bars. Furthermore, it permits economical use of high-strength concrete, for when an engineer designs a member with reinforced concrete, he considers that all concrete below the neutral axis is in tension and cracked, and therefore ineffective, whereas the full cross section of a prestressed concrete beam is effective in bending.

A particularly desirable characteristic of prestressed concrete is that as long as the material is maintained in compression it cannot crack. If cracks appear under overload, they usually will close when the load is removed. Sometimes concrete is prestressed mostly in order to prevent cracking.

Basic principles. When you take a pile of books and apply pressure to the end pair not one of the books will fall out if the pressure is enough. The effect of compressive prestresses is much the same.

But if this concept were applied to a concrete beam in practice, steel tendons would be tensioned and placed along the centroidal axis of the beam. The prestress that would be achieved would result in a uniform compression at every section. Loads would produce both tensile and compressive stresses at the middle of the span. The prestress would combine with these. As the result the compression would be increased and the tension avoided. The whole concrete section would be resisting bending effectively, and there would be no cracks.

In practice, however, tendons are rarely placed along the centroidal axis. A smaller prestressing force is required, and therefore less steel for the tendons, if the steel is placed below the centroidal axis of the beam. With the eccentric prestress, stresses at each section of the unloaded beam may vary from tension at the top to compression at the bottom. When loads are applied to the beam, they produce both tensile and compressive stresses at the middle of the span. At the top of the beam they cause compressive stresses, which are reduced by the tensile prestress there. In other sections the tensile stresses produced by the loads are counteracted by the compressive prestress.

Notes:

centroidal axis центральная ось

concept n. концепция, понятие

eccentric prestress внецентренное предварительное напряжение

overload n. перегрузка

pile n. зд. стопка

tendon n. арматурный пучок

unloaded adj. ненагруженный

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