Drum mixers are one of batch mixers having a container with a cross section, which can be divided into non-tilting drum mixers, reversing drum concrete mixers and tilting drum concrete mixers.
Features of drum mixers
The blades are attached to the inside of the movable drum. Their main purpose is to lift the materials as the drum rotates. In each rotation, the lifted material drops back into the mixer at the bottom of the drum and the cycle starts again. Parameters that can be controlled are the rotation speed of the drum and, in certain mixers, the angle of inclination of the rotation axis. There are three main types of drum mixers:
non-tilting drum concrete mixer;
reversing drum mixer;
tilting drum mixer
Non-tilting drum mixers
The non-tilting drum mixer implies that the orientation of the drum is fixed. The materials are added at one end and discharged at the other (Fig. 2). The reversing drum (Fig. 2) is similar to the non-tilting mixer except that the same opening is used to add the constituents and to discharge concrete. The drum rotates in one direction for mixing and in the opposite direction for discharging the concrete. 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 second 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 discharge and mixing. Reversing drum mixers are usually used for batches up to 1 m 3.
Concrete mixer truck mixer
The truck mixers belong to the reversing category of drum mixers. The driver of the truck can control the speed of rotation 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 premixed concrete uses only 0.2 rad/s (2 rpm) to 0.6 rad/s (6 rpm) . In the United States, most ready-mixed concrete is mixed in trucks  and not premixed in a plant.
In a tilting drum mixer (Fig. 3), the inclination can be varied. When the drum is almost horizontal (inclination ≈ 0°), more energy is provided to the concrete because more concrete is lifted to the full diameter of the drum before dropping. It is during the drop that the concrete is knitted and mixed. Therefore, the higher the drop, the higher the energy imparted to the concrete. If the axis of rotation is almost vertical the blades cannot lift the concrete and the concrete is not well mixed. The drum axis usually stays at an angle of about 15from horizontal during mixing. To discharge the concrete the drum is tilted downwards (Fig. 3) below the horizontal plane.
The tilting drum is the most common type of drum mixer for small batches (less than 0.5 m 3 ) both in the laboratory and in the field.
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