Too much stress can hardly be laid upon the necessity of thoroughly mixing the concrete if the best results are to be attained. It has already been shown that thoroughness in mixing mortar is repaid by greatly increased strength, and the result is even more marked in the case of concrete. Every grain of sand should be coated with cement, and every piece of stone should be covered with mortar. In general, the Cost of mixing is from one-tenth to one-fifth of the total Cost of the concrete in place. If by doubling the Cost of mixing we can increase its strength more than one-tenth or one-fifth in these respective cases, or permit a corresponding decrease in the amount of cement necessary for a given result, the additional labor in mixing is justified.
318. Concrete may be mixed by hand or by machine. Opinions vary as to the comparative merits of the two systems, but as a machine properly installed usually furnishes much the cheaper method of mixing, it is usually employed. The saving by this method, however, will evidently depend upon the Cost of labor, the total amount of work to be done, and the degree of concentration of the work, or facilities for distributing the concrete. In certain sections where cheap labor is abundant, the Cost of hand mixing may be as low as machine mixing.
With proper supervision, hand mixing may be thorough, and the chief argument against it, aside from its Cost, is that such hard work is likely to be slighted. The best forms of mixers now on the market, however, give results quite equal to the best hand work.
We will assume that the materials have been brought within easy reach of the mixing place. If the concrete is to be mixed near the point where it is to be deposited, the mixing platform must be made portable. Three platforms, each 8 by 14 feet, built of two-inch plank or of two layers of one-inch boards, nailed to four 2x6 inch longitudinal scantlings laid flat, will be suitable for such a case. The platforms should be made without vertical sides, though if desired a narrow piece of one-inch board may be laid flat around the edges and nailed. A short piece of rope attached to each corner of the platforms, or to the ends of the longitudinal scantlings, will be found convenient in moving them. These mixing boards are placed side by side.
The sand, which may be delivered to the mixing platform in wheelbarrows, is first dumped on the board and spread evenly over the surface. If the sand is measured, the barrows should be so arranged as to hold the required amount after "striking" with a straight edge. This will make the measurement independent of the judgment of the shoveler. If the sand is delivered in cars, bottomless boxes of two or three barrels capacity, according to the proportions used, will be found more convenient for measuring than barrels. If the sand is determined by weight, which as has been shown is the more accurate method, the scales should be set at a weight which is a factor of the total weight, and but little time will be required to bring the scales to a balance for each barrow.
If it is possible, the batch should be of such a size as to take either one or two full barrels, or a certain number of full sacks of cement. This will obviate the necessity of measuring or weighing the cement. The sand having been spread over the surface of the mixing board, the cement is dumped upon it and spread evenly over the sand. These ingredients are then mixed dry, the required amount of water is added at one time in the center of a ring formed of the dry materials, and the whole is thoroughly mixed as described under the head of mortar-making.
320. The mortar having been spread evenly over the board, the broken stone is dumped upon it and evenly distributed over the surface. Four shovelers then mix the concrete. Each shoveler starts at a corner of the board and turns each shovelful completely over, casting toward the end and spreading the mortar a little as he draws the shovel toward him. The two shovelers at each end work toward each other, and meeting at the axis of the platform, return to the side and repeat. When the four shovelers meet at the center of the board, they turn the mass again by casting toward the center in a similar manner. If in mixing the concrete it is found that sufficient water has not been used, more may be added from a rose nozzle, or sprinkling pot, previous to the last turning of the mass. The shovel should always be used at right angles to either the side or the end of the board, never diagonally; and it should always scrape the mass clean from the board, never cut it at mid-depth. From three to five turnings are required to thoroughly mix the concrete.
The mode of mixing has been thus minutely described, because if a gang of men are started properly they will soon become expert, working in unison; whereas if each man is allowed to mix according to his notion, confusion is sure to result. It is sometimes preferred to spread the stone on a separate board and cast the mortar upon it, but this necessitates one handling of the mortar which does not appear to contribute much to the incorporation of the ingredients.
While the shovelers are engaged in mixing the concrete on one platform, the mortar mixers have proceeded to the next platform to mix another batch of mortar, and the cement and sand are being placed upon the third platform. Thus the work proceeds in regular progression without delays. The shoveling of concrete is hard work, and it will be found necessary not only to pick good men for this duty, but to cull them until the evolution results in the proper men for the work. An extra compensation for men who perform satisfactory service in the mixing of concrete will usually be repaid in the character and quantity of the output.
321. With the method described above, a working gang would consist of the following men under ordinary conditions: —
Measuring and supplying cement and sand......... 1
mixing mortar..................... 2
Delivering stone from bin, one man with horse and cart, or two men with barrows.................. 2
Shovellers to mix concrete and cast or wheel to place .... 4
water boy....................... 1
Spreading and tamping concrete.............. 1
Total men required.............. 11
If it is found impracticable to mix the concrete near the place of deposition, it may be necessary to put on two or more extra men to wheel the concrete to place. This gang of eleven men may be doubled and still work on the same three platforms when so desired.
With a moderate length of wheel for the materials and the finished concrete, a gang of eleven picked men, working according to system, will be able to make from twenty-five to thirty cubic yards per day of ten hours, or about two and a half yards per man. The double gang of twenty-two men may not work to quite as good advantage, and will probably not put in more than from forty to fifty cubic yards per day. It would therefore be somewhat more economical to work two gangs of eleven men each on separate sets of platforms, especially as in this way a rivalry is created. Lack of room, however, will frequently preclude this arrangement.
The amount of concrete stated above, two and a half yards per man, may be taken as a maximum. With wages at $1.75 per day this would correspond to a Cost of about seventy cents per yard, exclusive of the wages of a foreman. Numerous examples might be cited where the mixing costs more. Colonel Mendell, in writing of the fortifications at Fort Point, California,1 states that a foreman (at $4 per day) and twenty laborers (at $2 per day) made forty-five cubic yards per day of eight hours, the cost of mixing being thus about $1 per cubic yard. It is stated that "the circumstances were exceptionally favorable".
As an instance where hand mixing was done at a very low Cost, the Lonesome Valley Viaduct2 may be mentioned. At this point colored labor was used at a cost of $1 for eleven hours' work. A gang of men, distributed as follows, would mix and lay forty cubic yards of concrete per day: —
1 Jour. Assn. of Engr. Soc, March, 1895.
2 Construction of Substructure for Lonesome Valley Viaduct, Gustave R. Tuska, Trans. A. S. C. E., Vol. xxxiv, p. 247.
Filling sand barrows and handling water...... 1
Filling rock barrows............... 2
mixing sand and cement............. 4
mixing stone and mortar............. 4
Wheeling concrete................ 2
Spreading concrete in the molds.......... 1
Tamping concrete in the molds . . ........ 1
Fifteen men at $1 per day, and foreman at $2.50 per day, makes a Cost of $17.50 for forty yards of concrete, or at the rate of forty-four cents a yard for mixing. Had the laborers received $1.75 per day, however, the Cost would have been 72 cents per yard.
323. In the construction of the Forbes Hill Reservoir and standpipe at Quincy, Mass.,1 all concrete was mixed and placed by hand. "The ordinary concrete gang was made up of a sub-foreman, two men gaging materials, two men mixing mortar, three men turning the concrete, three men wheeling concrete, one man placing, and two men ramming. Two gangs were ordinarily employed, placing about twenty cubic yards per day each, or about 1.43 cubic yards per man. The concrete was turned at least three times before placing." With labor at $1.75 per day, this would give the Cost of mixing and placing $1.22 per cubic yard. The actual Cost of mixing and placing varied from $0.97 to $1.53, according to the character of the work.