There are mainly are 3 types of drum mixers in construction industry: non-tilting drum mixer.Drum mixers have a rotary mixing chamber that moves about a horizontal or inclined axis with a cross section similar to that shown in Fig. 1. The blades are attached to the inside of the movable drum and their usages are to lift the materials as the drum rotates. In each rotation, the lifted materials drop back into the mixer at the bottom of the drum and the cycle repeats again.
The mixtures are discharged by tilting or reversing the direction of rotation. The mixing effect is created by the free drop of the materials inside the rotating chamber and is intensified by mixing tools attached within the chamber. Parameters that can be controlled are the rotation speed of the drum and, in certain mixers, the angle of inclination of the rotation axis.
1. Non-tilting drum mixer
The Fig.2 vividly shows us the operating principle of non-tilting drum mixer. Its drum orientation is fixed and there is one inlet and one outlet places at the two ends respectively.
2. Reversing drum mixer
It is also commonly called a non-tilting mixer but there is some difference between them. In Fig.2 the materials are added at one end and discharged at the other, but in reversing drum mixer the same opening is used to add the ingredients and to discharge the concrete.
The mixer drum will rotate in a direction to mix the materials and in the opposite direction to discharge the concrete mixtures. There are two types of blades attached to the inner walls of the drum. One set drags the concrete upwards and toward the center of the mixer when the drum rotates in one direction; the other set of blades pushes the concrete toward the opening when the drum rotates in the other direction. The blades have a spiral arrangement to obtain the desired effect for discharging and mixing materials. Reversing drum mixers are usually used for batches up to 1m³. In addition, the truck mixer is one type of reversing drum mixer. The driver of the truck can control the rotation speed with a clutch in the cabin. The speed depends on whether the concrete has been well mixed prior to being placed in the truck or whether the truck has to do most of the mixing. Typically the speed for mixing is 1.57 rad/s (15 rpm), while the transport of pre-mixed concrete uses only 0.2 rad/s (2 rpm) to 0.6 rad/s (6 rpm)
3. Tilting drum mixer
In Fig.3 the inclination of tilting drum mixer cab be various. Two primary types exist: horizontal (one end has and opening for charging and a different end for discharging) and single drum (materials are charged and discharged through a single opening). At the condition that the drum inclination≈0°(that is almost horizontal), the concrete can get more energy because there are more concrete lifted to the full diameter of the drum before dropping. During the drop the concrete is knitted and mixed. Therefore, the higher the drop, the higher the energy imparted to the concrete mixtures. If the axis of rotation is almost vertical the blades cannot lift the concrete and the concrete may be not well mixed. The drum axis usually stays at an angle of about 15°from horizontal during mixing. To discharge the concrete the drum is tilted downwards (Fig. 3) below the horizontal plane. The tilting drum mixer is most suitable for concrete with large sized aggregate and, since they have a rapid discharge rate, are suitable for low workability concrete for the small batches (less than 0.5 m³) both in the laboratory and in the construction field.