Specifications usually require concrete to be turned at least four times, and frequently six.
Machine Mixers. There are at least twenty-five concrete mixing machines upon the market. According to the method of charging and discharging, concrete mixers may be divided into two classes: (1) continuous mixers, those into which the materials are fed continuously, usually with shovels, and from which the concrete is discharged in a steady stream; and (2) intermittent or batch mixers, those which receive one or more bags of cement with the proper proportion of sand and stone, and after mixing the charge is discharged in one mass.
There are two forms of continuous mixers,—the power mixer and the gravity mixer. In the latter the ingredients are mixed in falling through a vertical or inclined chute by striking against rods or in falling from inclined shelves. Of the continuous power mixers there are two types: one in which the concrete is forced along a horizontal trough by an endless screw or by revolving paddles; and another in which the concrete is mixed in its passage through either a long horizontal box, square in cross section, or a cylinder which revolves about a horizontal axis.
The batch mixers have a mixing chamber in the form of a cube, or a cylinder, or a double cone, which revolves about a horizontal axis. In some of the machines of this type the contents are dis charged by changing the position of inside deflectors, without stop ping the machine; and in others the mixing chamber is tilted on a horizontal axis to discharge a batch.
Each machine has some advantages to recommend it. Batch mixers are usually preferred unless the materials are measured and fed mechanically, owing to the difficulty of uniformly feeding the continuous mixer.* Some of the mixers are equipped with auto matic measuring devices.
A machine mixer should be used in preference to hand mixing when the cost of setting up, taking down, and transporting the mixer is less than the difference in the cost of mixing the required amount of concrete by hand and by machine. Under ordinary conditions, if more than 150 or 200 cu. yd. of concrete are required, it is cheaper to mix by machine than by hand.