94587

STRUCTURAL PRECAST CONCRETE

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Производство и промышленные технологии

In evaluating any method of construction the economic advantages which it can lead to must weigh heavily in the estimation of the designer. Under certain conditions, precasting has advantages over in situ concrete, the main points in its favour being the economy of factory production, the saving in formwork and scaffolding...

Английский

2015-09-13

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STRUCTURAL PRECAST CONCRETE

In evaluating any method of construction the economic advantages which it can lead to must weigh heavily in the estimation of the designer. Under certain conditions, precasting has advantages over in situ concrete, the main points in its favour being the economy of factory production, the saving in formwork and scaffolding, and the increased speed of construction made possible by it. The development of prestressing in recent years has greatly emphasized these advantages. There are many structures where the use of precasting, either wholly or in part, can result in a saving of time and cost, and it may be worth while investigating new projects which we would normally design and build with concrete in situ from this point of view to determine if precasting will not be more suitably applied to them.

The main advantages of using precast units for the construction of concrete structures may be considered, firstly, from the point of view of casting the concrete member itself and, secondly, from the 'point of view of the erection of the structure as a whole. Those falling in the first category are:

  1.  Technical control is better applied in the workshop casting yard than on the site, resulting in a higher quality of concrete produced, and a greater precision of shape and regularity of finish.
    1.  Mass production and standardization facilitate design and make possible the employment of permanent labour which has a better chance of becoming skilled and organized.

3. The large number of identical units to be produced permits a large re-use of moulds.

Let us examine these advantages in detail. Because of the careful selection of materials and control of production in the factory or casting yard, higher concrete stresses may be permitted in precast than in monolithic construction. The cross-sectional dimensions and concrete cover can be smaller, and consequently the structural members can be lighter. The fullest advantage of very high strengths of concrete can, however, be obtained only with the employment of prestressing. Prestressing also makes it possible to divide a structural member into small segments which are precast and then united by prestressing. Incidentally, it may be mentioned that very high strengths of concrete — of the order of 15,000 to 20,000 psi — can only be obtained by precasting small elements.

Again, mass production .of precast units leads to a continuous process independent of weather conditions. Repetition work trains skilled labour and supervision becomes a matter of daily routine, resulting in more accurate work and higher quality of products. As the units are standardized, only a few designs are required, and greater care can therefore be taken with them than is possible with the wide range of members usual in a structure cast in situ.

Moulds for precast units are usually made of steel with very great accuracy in all dimensions. They are, of course, expensive but they can be re-used scores of times so that their cost has little bearing upon the price of the precast product.

Of the advantages of precasting from the point of view of erection of the structure as a whole, the following may be mentioned:

  1.  The amount of formwork and scaffolding is considerably reduced.
    1.  The wet trades are practically eliminated from the site; so also is the transport of bulk materials and their storage.
      1.  The speed of construction is greatly increased and a smaller force of labour is required at the site.
      2.  The effeсts of shrinkage and temperature are no longer of importance and1 joints may be therefore be omitted.
      3.  Precast structures may be easily dismantled and re-erected elsewhere.

Perhaps the most simple and evident advantage of using precast units for a structure is the avoidance of formwork. This advantage is more marked for structures of considerable height and spans which require a disproportionately large amount of timber for scaffolding and formwork. The absence of a maze of props and shuttering, as well as the elimination of stockpiles of bulk materials and of the paraphernalia of concrete making leads to a clearer, drier, and cleaner site, and permits immediate starting of interior finishing work once the precast members are fixed in position.


 

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