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Proportioning Concrete

voids, materials, proportions, fine, volume, cement and coarse

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PROPORTIONING CONCRETE Arbitrary Proportions.—The common method of propor tioning concrete is by assuming ratios between the volumes of cement, sand, and coarse aggregate. These proportions are varied accord ing to the character of the work, and sometimes are adjusted to the qualities of the materials. A formula. of definite proportions does not always lead to the same result unless the method of measuring the materials is the same, as cement measured loose may vary con siderably in weight for the same volume. A barrel of cement may measure from 3.5 to 5 feet, according to its degree of compactness. It is desirable to follow the recommendation of the Joint Committee on Concrete and take one sack (94 pounds) of cement as a cubic foot, or a barrel as 4 cubic feet in measuring the materials.

Specific fixed proportions have to a certain extent become stand ard in ordinary practice for various kinds of work. For reinforced concrete in building construction and where it is necessary to develop considerable strength, the porportions of 1 part cement, 2 parts sand, and 4 parts broken stone are commonly employed. For positions where strength is of special importance, as in column con struction, or work in light superstructures of buildings, the propor tions 1 : 11 : 3, or sometimes 1 : 1 : 2, are used. In more massive work and where only compressions are to be carried with ample sections, the proportions 1 : 3 : 6 and sometimes 1 : 22 : 5 are employed.

The common proportions are based upon the requirement that the volume of fine aggregates shall be one-half that of the coarse aggregate. For materials commonly used, this gives a quantity of mortar sufficient to fill compactly the interstices in the coarse aggregate. The quality of the mortar is varied by changing the ratio of cement to fine aggregate, and the strength of the concrete varies accordingly. The ratios between fine and coarse aggregates are often varied when the coarse aggregates contain more or less voids than is usual, and 1 : 2 : 3, 1 : 3 : 5, 1 : 2 : 5 or 1 : 3 : 7 con crete is frequently used.

Good results have been obtained in practice by this method of proportioning, when proper attention has been given to the quality of the aggregates. More careful methods of adjusting proportions would often be more economical, and equally good results might sometimes he obtained with less cost for materials. Many users

of concrete employ ordinary proportions for all concrete irrespective of the character of the materials, and a wide variation in the quality of the concrete is frequently the result.

74. Proportioning by method of proportioning some times followed is to determine the voids in the aggregates, and use enough cement to fill the voids in the fine aggregate and enough mortar to fill the voids in the coarse aggregate. A small excess of fine materials is used in each case on account of inequalities of mix ing. If the fine materials would all lie in the voids of the larger materials, this method would always give the desired result, and produce the concrete of maximum density and greatest strength. In practice, however, the voids cannot be completely filled, the volumes of the larger materials are increased by the smaller par ticles lying between them, and the distribution of fine material through the mass is not uniform.

Usually a volume of mortar 5 to 10 per cent in excess of the voids most nearly fills the voids without leaving appreciable excess of mortar. More mortar than this swells the volume of the concrete without increasing density, and has the effect of weakening the con crete. If, for instance, sand containing 50 per cent voids is used with stone containing 40 per cent voids, and just fills the voids in the stone without increasing the volume, the resulting mixture will have 20 per cent voids. If an excess of sand be used, this excess will give an increase in volume having 50 per cent voids.

This method of proportioning is an improvement over that of arbitrary selection of ratios, and usually gives approximately the most desirable proportions. Variations in the relative sizes of the materials, however, may change considerably the proportions neces sary to give the most dense concrete. A certain sand may easily work into the voids of a given broken stone without materially increasing its volume, while with another stone containing the same percentage of voids but of different sizes, the same sand may produce quite different results, and to secure greatest density would need to be differently proportioned. The object should be to get the greatest density in the final mixture of fine and coarse aggregates.

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