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Concrete structure
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Introduction.- Reinforced concrete is an excellent building material, adaptable to many uses. It is strong, fore-resistant and durable when well made. On the other hand, it is a heavy material, and its use generally results in rather bulky members so that its greatest field of usefulness is in relatively low building sand structures where its mass, rigidity, and strength are advantageous. Tall building may be made of reinforced concrete but, when they are more than six or eight stores high, it is desirable to question the economy and advisability of such construction for industrial purposes.***
Structures built of concrete should be planned upon the basis of the characteristics of the material itself, and upon the essential nature of the construction processes. Concrete is not a substitute for structural steel in terms of member for member. Architects and engineers, figuratively, should throw away many of their ideas derived from experience with steel-framed structures, then tackle the project at hand on the basis of utilizing the concrete to the best advantage(*). Many have done this and are now producing plans for concrete is the most desirable material for a structure and should give careful consideration to the general proportions of the structure and to the uses for which it is intended. Because of the nature of concrete construction, careful planning is needed in the first p-lace because extensive alterations and radical changes of future use are likely to be both difficult and expensive.
Some General Principles.- The planning and the detail designing of concrete structures are so influenced and circumscribed by practical procedures and considerations that the engineer should attack such problems entirely upon the basis of the best use of this particular material. He should remember constantly that, except for possible precast members, he is creating a structure to be made of “artificial stone”, of material placed in position in a plastic state so that it must be supported temporarily by something other than itself, and of a material that will and should conform to every detail of the surfaces with which it is in contact when the plastic concrete is deposited.
Not only does concrete improve with the use of good materials, but its quality depends largely upon the excellence of the workmanship used in its manufacture, the adequacy and thoroughness of its placement, and the care with which it is cured. The attainment of the intended high quality is almost completely in the hands of the artisans, who, in the field, convert heaps of aggregates, barrels of cement, and gallons of water into a structure for the use of man. Not only does its strength depend upon highly skilled labor, but the quality of its surface and the beauty of its appearance do likewise. It is foolish to forget these obvious truths. Yet occasionally such important operations are delegated to unskilled, inefficient workmen.* Fortunately, this is the exception. Expert workmen produce surprisingly fine results.
When planning concrete work, an engineer should consider the following matters, along with others.
Concrete should be placed in the forms within⅓ to ¾ hr, after it is mixed; thereafter it should remain undistributed for a few days or weeks, depending upon the rapidity of its
1. setting, in order to allow the chemical action to proceed without any fracturing of the partially set material.
2. Since every detail of formed surface must be provided in advance in the forms themselves, the cost of the forms and their actual construction should be considered when the shape and details of the members are planned. Portions of concrete projecting out from the main surfaces, therefore, cause difficult form work, whereas recesses of the main forms.
3. The character and capacity of the mixing equipment inevitably affect the quantity of concrete that can be placed in a given time. It is obvious, of course, that the structure and its parts must be planned so that pours that should be monolithic can be made so.
4. It is desirable to plan structures and equipment so that the concrete can be lowered vertically into place. Dropping concrete through long distances, chuting it at flat angles, moving it horizontally inside the forms by means of shovels or vibrators -such procedures tend to cause segregation and so should be avoided deliberately. Pumping may be advantageous when concrete is to placed in large, monolithic pours at reasonable distances from the source or mixer, or at great altitudes above it. Generous proportions should be used for parts to be built under water by the use of a tremie or a a tight buckets in order to allow for uncertainties of placement as well as for dilution of the cement paste.
5. Forms should be left in place long enough for the concrete to attain sufficient strength to support itself and any loads to be applied to it immediately and in the near future. With ordinary Portland cement, this may require 2 or 3 days for vertical members such as walls and piers, or 2 weeks, for heavy slabs and beams.
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*** it is desirable to question the economy and advisability of such construction for industrial purposes желательно исследовать вопросы экономичности и целесообразности такой конструкции для промышленных целей; to question здесь используется в значении исследовать, изучать
(*)then tackle the project at hand on the basis of utilizing the concrete to the best advantage затем взяться за имеющийся проект для рассмотрения его с точки зрения наиболее целесообразного использования бетона(или: рассмореть его и т.д.); at hand(буквальное значение- под рукой) здесь используется в качестве определения к существительному project, поэтому at hand переводится имеющийся, находящийся в распоряжении; to the best advantage с наибольшей выгодой, наиболее целесообразно.
such important operations are delegated to unskilled, inefficient workmen такие важные работы поручаются неопытным рабочим. В значении поручать используется глагол to delegate в данном тексте.
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