METHODS OF MIXING 133. The two methods used in mixing concrete are by hand and by machinery. Good concrete may he made by either method Concrete mixed by either method should he carefully watched by a good foreman. If a large quantity of concrete is required, it is cheaper to mix it by machinery. On small jobs where the cost of erecting the plant, together with the interest and depreciation, divided by the number of cubic yards to be made, constitute a large item, or if frequent moving is required, it is very often cheaper to mix the concrete by hand. The relative cost of the two methods usually depends upon circumstances, and must be worked out in each in dividual case.
134. Hand Mixing. The placing and handling of materials and arranging the plant are varied by different engineers and contractors. In general the mixing of concrete is a simple operation, but should be carefully watched by an inspector. He should see (1) That the exact amount of stone and sand are measured out; (2) That the cement and sand are thoroughly mixed; (3) That the mass is thoroughly mixed; (4) That the proper amount of water is used; (5) That care is taken in dumping the concrete in place; (6) That it is thoroughly rammed.
The mixing platform, which is usually 10 to 20 feet square, is made of 1-inch or 2-inch plank planed on one side and well nailed to stringers, and should be placed as near the work as possible, but so situated that the stone can be dumped on one side of it and the sand on the opposite side. A very convenient way to measure the stone arid sand is by the means of bottomless boxes. These boxes are of such a size that they hold the proper proportions of stone or sand to mix a batch of a certain amount. Cement is usually measured by the package, that is by the barrel or bag, as they con tain a definite amount of cement.
The method used for mixing the concrete has little effect upon the strength of the concrete, if the mass has been:turned a sufficient number of times to thoroughly mix them. One of the following methods is generally used. (Taylor and Thompson's Concrete.) (a) Cement and sand mixed dry and shcveled on the stone or gravel, leveled off, and wet as the mass is turned.
(b) Cement and sand mixed dry, the stone measured and dumped on top ()fit, leveled off, and wet, as turned with shovels.
(c) Cement and sand mixed into a mortar, the stone placed on top of it and the mass turned.
(d) Cement and sand mixed with water into a mortar which is shoveled on the gravel or stone and the mass turned with shovels.
(e) Stone or gravel, sand, and cement spread in successive layers, mixed slightly and shoveled into a mound, water poured into the center, and the mass turned with shovels.
The quantity of water is regulated by the appearance of the concrete. The best method of wetting the concrete is by measur ing the water in pails. This insures a more uniform mixture than by spraying the mass with a hose.
135. Mixing by Machinery. On large contracts the concrete is generally mixed by machinery. The economy is not only in the mixing itself but in the appliances introduced in handling the raw materials and the mixed concrete. If all materials are delivered to the mixer in wheel-barrows, and if the concrete is conveyed away in wheel-barrows, the cost of making concrete is high, even if machine mixers are used. If the materials are fed from bins by gravity into the mixer, and if the-concrete is dumped from the mixer into cars and hauled away, the cost of making the concrete should be very low. On small jobs the cost of maintaining and operating the mixer will usually exceed the saving in hand labor and will render the expense with the machine greater than without it.
136. Machine vs. Hand Mixing. It has already been stated that good concrete may be produced by either machine or hand mix ing, if it is thoroughly mixed.
Tests made by the U. S. Government engineers at Duluth, Minn., to determine the relative strength of concrete mixed by hand and mixed by machine (a cube mixer), showed that at 7 days, hand mixed concrete possessed only 53 per cent of the strength of the machine-mixed concrete; at 2S days, 77 per cent; at 6 months, S4 per cent; and at one year, SS per cent. Details of these tests are given in Table X.
It should be noted in this connection, that the variations in strength from highest to lowest were greatest in the hand-mixed samples, and that the strength was more uniform in the machine mixed.
The mixture tested was composed of 1 part cement and 10.1S parts aggregate.