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 and 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 contain 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 shoveled 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 of it, 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 measuring the water in pails. This insures a more uniform mixture than by spraying the mass with a hose.