This is also called as free fall mixer or gratify mixer. In this type of mixer, concrete is mixed by lifting the ingredients with the help of fixed blades inside a rotating drum and then dropping the material by overcoming the friction between the mixture and the blades. The drum of the mixer can be filled and emptied by changing its direction of rotation, opening it, or tipping it up. Rotation speed must carefully follow machine-specific instructions and should not be too fast so that the free fall of the mixture is not interrupted by the centrifugal force.
The drum mixer suits concretes that are not too stiff; usually with a slump up to 50 mm is commonly used on construction sites. Truck mixers also use free fall mixing. Since the ingredients in a free fall mixer fall from height, there is some attrition of aggregates.
This type of mixer is used for producing large volume of concrete. These mixers are noted for having high production speeds, low maintenance, and ideal for slump concrete.
This drum mixer is available in the following three forms:-
Non – tilting Mixers
1. Tilting Mixers
Tilting mixers are usually trailer-mounted or otherwise portable, small to mid-size mixers, used either as main concrete mixing equipment on small sites or as ancillary equipment on sites served by concrete plant. The drum has two axes: one around which the drum rotates and another that serves to change from loading and mixing position (drum opening up) to discharging position (drum opening down). This position change is done manually by a dump wheel (or handle, in the smaller mixers), while drum rotation is electric, gasoline, or diesel powered. Drums are traditionally made of steel, but polyethylene drums for easier cleaning are now offered. Most mixers are of the side-dump type, but end-dump mixers are also manufactured. Material is generally loaded manually, directly into the drum. There are larger-size self-loading units equipped with a tilting hopper. After being filled manually at ground level, the hopper is tilted up mechanically and dumps the material into the drum. A built-in mechanical drag shovel to facilitate aggregate loading into the hopper is optional in some models.
The drum is conical or bowl shaped with internal vanes and the discharge is rapid and unsegregated so that these mixers are suitable for mixes of low workability and for those containing larger size aggregate. The drum axis usually stays at an angle of 15 degree from horizontal during mixing.
Tilt mixer offers consistent mixing with lower operating and maintenance cost. Tilting drum is the most common type of drum mixers for small batches.
Tilting mixers can handle larger size aggregates more easily and will discharge mixed concrete rapidly than a non tilting mixer.
2. Reversible Mixers
The reversible drum mixer is similar to the non tilting mixer except that the same opening is used to add the constituents and to discharge the aggregate. The drum on a reversible mixer has one horizontal axis around which it rotates. In mixing position, the drum rotates in one direction; while for discharging, the rotation is reversed.
Commonly self-loading, reversible mixers are mid-size to large-size units mounted on a two-or four-wheeled trailer for transportation between sites. The mixer is equipped with a tilting hopper similar to that occasionally found on a tilting mixer, or with a hoist-like hopper that moves up and down on a short inclined set of rails and that dumps the ingredients through a bottom opening into the drum. A built-in mechanical skip to facilitate aggregate loading into the hopper is optional on many models.
Most models of this mixer type are equipped with a water tank and meter, and optionally (the larger models) with a built-in aggregates batcher.
3. Non-Tilting Mixers
A non tilting drum mixer is one in which the axis of the mixer is always horizontal, and discharge takes place by inserting a chute into the drum or by reversing the direction of rotation of drum.
Because of slow rate of discharge, some segregation may occur.
There are two openings, one at each end of the drum: one for feeding the ingredients, the other for discharging the mixture.