The best concrete mixer is the one that turns out the maximum of thoroughly mixed concrete at the minimum of cost for power, interest, and maintenance. The type of mixer with a complicated' motion gives better and quicker results than one with a simpler motion. There are two general classes of concrete mixers - continuous mixers and batch mixers. A continuous mixer is one into which the materials are fed constantly, and from which the concrete is discharged constantly. Batch mixers are constructed to receive the cement with its proportionate amount of sand and stone, all at one charge; and, when mixed, it is discharged in a mass. No very distinct line can be drawn between these two classes, for many of these mixers are adapted to either continuous or batch mixing. Generally, batch mixers are preferred, as it is a very difficult matter to feed the mixers uniformly unless the materials are mechanically measured.

Continuous mixers usually consist of a long screw or pug mill, that pushes the materials along a drum until they are discharged in a continuous stream of concrete. Where the mixers are fed with automatic measuring devices, the concrete is not regular, as there is no reciprocating motion of the materials. In a paper recently read before the Association of American Portland Cement Manufacturers, S. B. Newberry says:

"For the preparation of concrete for blocks in which thorough mixing and use of an exact and uniform proportion of water are necessary, continuous mixing machines are unsuitable; and batch mixers, in which a measured batch of the material is mixed the required time, and then discharged, are the only type which will be found effective."

There are three general types of concrete mixers: gravity mixers, rotary mixers, and paddle mixers.

Gravity mixers are the oldest type of concrete mixers- They require no power, the materials being mixed by striking obstructions which throw them together in their descent through the machine.

Fig. 122. Portable Gravity Mixer.

Fig. 122. Portable Gravity Mixer.

Their construction is very simple. Fig. 122 illustrates a portable gravity mixer. This mixer, as will be seen from the figure, is a steel trough or chute in which are contained mixing members consisting of pins or blades. The mixer is portable, and requires no skilled labor to operate it. There is nothing to get out of order or cause delays. It is adapted for both large and small jobs. In the former case, it is usually fed by measure, and * by this method will produce concrete as fast as the materials can be fed to their respective bins and the mixed concrete can be taken from the discharge end of the mixer. On very small jobs, the best way to operate is to measure the batch in layers of stone, sand, and cement respectively, and feed to the mixer by men with shovels.

Fig. 123. Operation of portable Gravity Mixer

Fig. 123. Operation of portable Gravity Mixer.

Fig. 124. Rotary Mixer with Cubical Box.

Fig. 124. Rotary Mixer with Cubical Box.

Fig. 125. Rotary Mixer Mounted on Frame.

Fig. 125. Rotary Mixer Mounted on Frame.

Fig. 126. Cross Section of Drum of Rotary Mixer (front half cut away), Showing Blades and Lining.

Fig. 126. Cross-Section of Drum of Rotary Mixer (front half cut away), Showing Blades and Lining.

There are two spray pipes placed on the mixer: for feeding by hand, one spray only would be used; the other spray is intended for use only when operating with the measure and feeder, and a large amount of water is required. These sprays are operated by handles which control two gate-valves and regulate the quantity of water flowing from the spray pipes.

These mixers are made in two styles, sectional and non-sectional. The sectional can be made either 4, 6, or 8 feet long. The non-sectional are in one length of 6, 8, or 10 feet. Both are constructed of 1/8-inch steel. To operate this mixer, the materials must be raised to a platform, as shown in Fig. 123.

Rotary mixers, Fig. 124, generally consist of a cubical box made of steel and mounted on a wooden frame. This steel box is supported by a hollow shaft through two diagonally opposite corners, and the water is supplied through openings in the hollow shaft. Materials are dropped in at the side of the mixer, through a hinged door. The machine is then revolved several times, usually about 15 times; the door is opened; and the concrete is dumped out into carts or cars. There are no paddles or blades of any kind inside the box to assist in the mixing. This mixer is not expensive itself, but the erection of the frame and the hoisting of the stone and sand often render it less economical than some of the more expensive devices.

Rotating mixers which contain reflectors or blades, Fig. 125, are usually mounted on a suitable frame by the manufacturers. The rotating of the drum tumbles the material, and it is thrown against the mixing blades, which cut it and throw it from side to side. Many of these machines can be filled and dumped while running, either by tilting or by their chutes. Fig. 125 illustrates the Smith mixer, and Fig. 126 gives a sectional view of the drum, and shows the arrangement of the blades. This mixer is furnished on skids with driving pulley. The concrete is discharged by tilting the drum, which is done by power.

Fig. 127. Ransome Batch Mixer.

Fig. 127. Ransome Batch Mixer.

Fig. 123. McKelvey Batch Mixer.

Fig. 123. McKelvey Batch Mixer.

Fig. 127 represents a Ransome mixer, which is a batch mixer. The concrete is discharged after it is mixed, without tilting the body of the mixer. It revolves continuously even while the concrete is being discharged. Riveted to the inside of the drum are a number of steel scoops or blades. These scoops pick up the material in the bottom of the mixer, and, as the mixer revolves, carry the material upward until it slides out of the scoops, which therefore assist in mixing the materials.

Fig. 128 represents a McKelvey batch mixer. In this mixer, the lever on the drum operates the discharge. The drum is fed and discharged while in motion, and does not change its direction or its position in either feeding or discharging. The inside of the drum is provided with blades to assist in the mixing of the concrete.

Paddle mixers may be either continuous or of the batch type. Mixing paddles, on two shafts, revolve in opposite directions, and the concrete falls through a trap door in the bottom of the machine. In the continuous type the materials should be put in at the upper end so as to be partially mixed dry. The water is supplied near the middle of the mixer. Fig. 129 represents a type of the paddle mixer.