Precast and prestressed concrete

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Precast concrete.- The use of pre-cast concrete members and parts of members is a matter that warrants careful study. The possible savings in formwork are obvious, but the handling of heavy pieces in the field may require special equipment. If portions of a structure are to be precast, the original planning should be based upon this fact, be incorporated in an otherwise poured-in-place structure*, but provision should be made for the support of these heavy pieces during construction. Most poured-concrete structures gain much from the stiffness derived from the continuity secured at the junctions of parts, whereas a building made of heavy, loosely connected, precast parts by means of poured sections at the junctions if proper bonding of reinforcement is provided. Although such procedures have not been common in the past, their use should be investigated with an open mind **. Because of the possibility of appreciable economies, the use of precast parts will undoubtedly increase in the future.

Prestressed Concrete. - Prestressed-concrete members are useful for certain structures. Their basic purpose is to avoid harmful deformation and cracking when the intended loads are applied. Two different principles that may be used in prestressed concrete are illustrated in Fig.5. The following comments may help to clarify them:

1. In sketch (a), a smooth rod is coated with a substance that prevents bond of the concrete to the steel when the former is poured, or the rod may be inside a mastic-coated cardboard tube. After much of the shrinkage of the concrete has occurred and when the concrete has sufficient strength, the nuts at the ends of the rod are tightened until the steel has the desired tensile unit stress. Of course, the steel stretches inside the concrete, and the latter compresses slightly. Thereafter, it is intended that further shrinkage of the concrete will merely relieve part of the tension in the steel but that the rod will continually cause some compressive stress in the concrete so that cracks cannot open up. Furthermore, when external forces try to elongate or bend the member, they may be considered to substitute for the compression in the concrete, but they should not annual the compression entirely. Since the compression in the concrete generally is a low unit stress, little deformation occurs during its relief, and the increase of tension in the rod is not usually excessive. Only when the tensile stress in the steel is balanced by the effect of the external forces is the compression in the concrete relieved completely; thereafter, an increase of the tension in the rod will cause elongations that might crack the concrete. Since the initial tension in the rod is supposed to exceed any that will be applied by the action of external loads, and because the original tensile strain in the steel should exceed the shrinkage and compressive deformations of the concrete, the rods should be made of a steel which has a high elastic limit and which can be used at a high unit stress.

A second method of prestressing a member is illustrated in Fig.5.(b). Here the reinforcement is generally high-strength wires or small rods that are tightened until they have the desired tensile deformation and unit stress, using the form or some external anchorage to resist the pull. The reinforcement is then embedded while in this stressed condition, and the bond of the concrete to the steel grips the latter so that, when the member is later removed from the form and when the tightening force is released, the tension in the steel causes a compressive stress in the concrete, but without slipping of the reinforcement. This method is useful in manufacture of precast, prestressed beams and slabs.

an otherwise poured-in-place structure* сооружение, отлитое из бетона иным способом. В качестве определения к существительному structure использован целый оборот otherwise poured-in-place, в котором, Past participle (poured) имеет свое определение, выраженоое наречием(otherwise) и обстоятельством места(in-place).

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Across the valley gloaming,
Across the cattle-horns of early stars.

Feathers and people in the crotch of a treetop
Throw an evening waterfall of sleepy-songs.