Mixing of Concrete

MIXING OF CONCRETE The methods of mixing concrete vary greatly in detail, but may be broadly divided under two heads—mixing by hand and mixing by ma chinery. A further distinction is made among mixing processes, which may be either batch mixing or continuous mixing. Batch mixing, as the name implies, is an intermittent process, the ingredients being measured out, and the mixing done for only a "batch" or limited, definite quantity at one time. Continuous mixing is always done by machinery, the materials being fed to the machine in an uninterrupted stream, continuously during the entire mixing process. Batch mixing may be done either by hand or by machine. In machine mixing, both continuous and batch, the proportioning of ingredients is sometimes done automatically by the machine itself.

Hand Mixing vs. Machine Mixing.

A choice as between hand mixing and machine mixing will depend chiefly on considerations of cost. Both methods will produce equally good con crete if sufficient care is taken to insure a thor ough and uniform mixture. This is harder to make sure of with hand than with machine mix ing, for, with the increasing fatigue of a day's work, and possibly that carelessness of work manship which is bred of familiarity and mo notony, a workman will naturally and inevitably show some falling-off in sharpness of attention and thoroughness of work toward the close of the day. Therefore, even with the closest su pervision kept up by a good foreman, hand mixed concrete is likely to be not quite so strong or so uniform in quality as the machine-mixed product—and this may be the case without any intentional neglect on the part of workmen.

Under any method of mixing—hand or ma chine—a careful and constant watch should be kept upon every detail of the process, to see that the proper relative quantities of the dry in gredients are being used, that the mixing is be ing thoroughly done, and that the concrete is of the proper consistency or degree of wetness. The same careful supervision should be kept up over every detail of the subsequent depositing, ramming, spading, drying, etc., of the concrete.

On small job.

even those requiring as much as several hundred cubic yards of concrete—it is ordinarily much cheaper and more expedient to mix by hand. This is, of course, especially true

where only a small crew—say two to four men— are available for the mixing, and where the work 'is often interrupted or frequent moving is necessary.

On large jobs it is more economical to mix by machinery, and this is the method generally adopted. The economy of machine mixing, on large jobs, depends to some extent on the use of labor-saving appliances in handling the raw materials and the finished mixture. Wheelbar row work in delivering materials to the mixer and carrying away the concrete to where it is deposited, will make the cost relatively high in any case, which will be greatly reduced if the mixer can be fed by gravity from bins, and the concrete dumped into hauling cars. On small jobs, however, the saving in hand labor is usually more than offset by the cost of the mixing plant and its maintenance and operation. The rela tive cost of the two methods usually depends on local circumstances, and these must be taken into consideration in each particular case.

Tests made by United States government en gineers at Duluth, Minn., to determine the rela tive strength of concrete mixed by hand and by a batch mixer, showed that at 7 days hand mixed concrete possessed only 53 per cent of the strength of machine-mixed concrete; at 28 days, 77 per cent; at 6 months, 84 per cent; and at one :Tear, 88 per cent. There was also less uniformity of strength in the hand-mixed concrete.

Mixing Concrete by Hand Details of the mixing process differ consid erably among different workers, one preferring one method, another another. Usually the sand and cement are mixed first, dry, and afterward mixed with the stone before the water is added. Sometimes the cement and sand are first mixed into a mortar, before being mixed with the stone. The stone may be added to the dry cement and sand, or to the mortar, or vice versa. Sometimes the stone, sand, and cement are spread in suc cessive layers, slightly mixed, and shoveled up into the form of a crater, into which water is poured, and the mass turned with shovels.