MIXING CONCRETE 79. Preparation of Materials.—In making concrete, the materials should be properly prepared and conveniently placed for use, as the labor cost of concrete work is largely a matter of arrangements for handling materials. The work should be systematized so that it goes forward smoothly, without loss of time in any of its parts.
Broken stone can usually be obtained within such range of sizes as may be desired. In preparing crushed stone, the crusher is set to the maximum size allowed, and the product varies from this size to dust. This product is then passed tluough rotary screens inclined at a small angle to the horizontal, which are made in sections of different sizes of openings, and admit of screening the stone into several sizes at one operation. When considerable quantities of materials are being used, the cost of handling the stone may not be materially increased by using several sizes and grading the aggregate. In any case where the aggregates available are badly graded, the advantage to be gained by grading them should be carefully con sidered.
The screened stone usually drops from the screens into bins, which are arranged so that the contents may be drawn off through chutes into cars or wagons for transportation to the work. The sizes and arrangement of the bins depend upon the need for storage and the kind of transportation. It is usually desirable that the bins he of sufficient size to equalize variations in the rate of use, or short delays in the crushing plant, so that work may proceed continuously.
Gravel and sand nearly always need to be screened. When con siderable quantities are to be handled. and power for operating the screen is available, rotary screens are desirable, giving the most economical handling of the material, and admitting of division into required sizes.
In small work it is usual to employ hand screens, which are set up in an inclined position, and the material thrown against them with a shovel, the finer material passing through and the coarser sliding down to the foot of the screens. Sometimes two or more
inclined screens are placed so that the material which passes one falls upon the one below, each being hinged so that its inclination to the horizontal may be adjusted.
Sand and gravel frequently require washing to remove dirt and fine material, which is often accomplished by supplying water in the chutes leading to the screens, the dirt being washed through a fine screen which retains the aggregate. Sometimes the material is washed crown a sloping trough, with a fine screen set in its bottom to permit the dirt to pass through. Portable plants for screening and washing are available in a number of forms, and often provide the most economical means of handling work of this kind. Wetting the material while in a pile, for the purpose of cleaning it, is useless.
Some storage of materials where the work is to be done is usually necessary, in order to have a supply which permits work to proceed continuously. The location of the materials with reference to the mixer, or mixing platform, should be carefully considered, as their convenience to the work affects the cost. of mixing the concrete. The amount of storage should be as small as is consistent with assur ing a continuous supply to the mixers.
80. Hand Mixing.—Concrete may be mixed by any method which will produce a homogeneous mass of uniform consistency. The arrangement of the work and methods of manipulating the materials in hand mixing vary greatly with the character of the con struction and the ideas of the men in charge. The costs vary as widely as the methods.
Measuring the a laterials.—Bottomless boxes are sometimes used for measuring the aggregates, the box being placed on the mixing platform, filled, and then removed, leaving the material on the plat form—an accurate means of measuring, and desirable when it can be employed without materially increasing the cost of handling the aggregates.