How to choose a concrete mixer?

　When selecting a mixer, it should be chosen based on its purpose, classification, and model. Uses, classifications, and models of mixers The purpose of cement concrete mixers is to mechanically mix cement concrete; there are many types of them, and their classification methods and characteristics are as follows. 　　(1) Classified by operation mode, there are cyclic operation and continuous operation types. 　　The three processes of feeding, mixing, and discharging in a cyclic operation are carried out at regular time intervals, that is, mixing in batches. Since all the materials used in mixing are accurately weighed, the mixing quality is good. This type of working method is currently used in most cases. 　　The aforementioned three sequential processes are carried out continuously within a long cylinder. Although its productivity is higher than that of the cyclic operation type, the mixing quality is poor due to the difficulty in controlling the mix ratios of various materials and the mixing time. It is used less frequently nowadays. 　　(2) Classified by mixing method, there are two types: gravity mixing and forced mixing. 　　A gravity-fed mixer places the mixture inside a rotating mixing drum; as the drum rotates, the blades inside lift the mixture to a certain height, after which it falls back down due to its own weight. This process is repeated until everything is well mixed. This type of mixer is generally used to mix plastic and semi-plastic concrete. 　　In a forced-action mixer, the mixing drum does not move; instead, mixing is forced by blades mounted evenly on the rotating shaft inside the drum. This mixer provides good mixing quality and high production efficiency ； But it has high power consumption and the blades wear out quickly. It is generally suitable for mixing dry and hard concrete. 　　(3) Classified by installation method, there are fixed and mobile types. A fixed-type mixer is installed on a pre-prepared base, and the entire unit cannot be moved. It has a large volume and high production efficiency. It is mostly used in mixing towers or mixing stations. 　　The mobile mixer is equipped with its own driving wheels, and due to its small size and light weight, it has good mobility. Applied to small and medium-sized temporary projects. 　　(4) The discharge methods are divided into tipping type and non-tipping type. 　　Unloading is achieved by tilting the mixing drum, rather than through the reversal of the mixing drum. 　　(5) Depending on the shape of the mixing drum, there are five types: pear-shaped, drum-shaped, biconical, disk with vertical shaft, and circular groove with horizontal shaft. The first three types are self-falling type mixers ； The latter two are types of forced mixing, which are less commonly used in China at present. 　　(6) Classified by mixing capacity, they include large-sized (discharge capacity of 1000–3000 L), medium-sized (discharge capacity of 300–500 L), and small-sized (discharge capacity of 50–250 L). 　　 This post was last edited by Tjuleo on 2009-4-5 11:03]

Selection of concrete mixers (summary) Concrete preparation involves mixing various components together to create a concrete mixture that has a uniform texture, consistent color, and sufficient fluidity. The concrete mix ratio is designed on the principle that the fine aggregates just fill the gaps between the coarse aggregates, with the cement paste being evenly distributed over the surfaces of both the fine and coarse aggregates. If concrete is not prepared evenly, a dense concrete cannot be obtained, which affects its quality; therefore, preparation is a very important step in the concrete construction process. For the preparation of concrete, mechanical mixing should be used in all cases, except in those where the volume of work is very small and scattered, when manual mixing is employed. Concrete mixers are classified into two types based on their mixing principle: gravity-type and forced-action type (Figure 3-35). The inner wall of the mixing drum of the free-falling mixer is fitted with arc-shaped blades; as the mixing drum rotates around a horizontal axis, these arc-shaped blades continuously lift the material to a certain height, after which it falls freely and mixes with itself. Therefore, self-falling mixers are primarily designed based on gravitational principles. In this type of mixer, the path of movement of the material is as follows: the material that is not within the range influenced by the blades rolls down automatically along the inclined surface of the mixture under the effect of gravity ； The material within the range driven by the blade, after being lifted to a certain height, first falls freely before rolling down along the inclined surface. Due to differences in falling time, landing point, and rolling distance, the material particles interpenetrate, mix, and blend with each other to achieve uniformity. Self-falling mixers are suitable for mixing plastic concrete. http://www.scutde.net/t14courses/1412-efmdljiaif/hunningtu/3.3/IMAGE/3-35.GIF Figure 3-35 Principle of concrete mixing a) Free-fall mixing ； b) Forced mixing 1—Concrete mixture ； 2—Stirring cylinder ； 3—Leaves ； 4—Shaft: The double-cone reverse-discharge mixer (Figure 3-36) is a good type of self-falling mixer, suitable for mixing plastic concrete. The mixing barrel of the double-cone reverse-discharge mixer is composed of two frustum cones. With each rotation of the barrel, the material circulates multiple times within it, resulting in high efficiency; moreover, the arrangement of the blades ensures that the material is lifted and mixed through its own gravity, while at the same time forcing it to move back and forth axially, thereby achieving a strong mixing effect. It rotates forward for mixing and reverses to discharge the material; it has a simple structure and is easy to manufacture. The twin-cone tilting discharge mixer is suitable for mixing concrete with large volumes, large aggregates, and high slump; in China, it is widely used in hydropower projects, bridge construction, and road engineering. The forced mixer (Figure 3-37) is primarily designed based on the shear mechanism. Such mixers contain rotating blades; as these blades move at different angles and positions, they pass through the material, overcoming its inertia, friction, and viscosity to force it to move in circular, radial, and vertical directions. This mechanism, in which the blades force the material to undergo shear displacement to achieve uniform mixing, is known as the shear stirring mechanism. http://www.scutde.net/t14courses/1412-efmdljiaif/hunningtu/3.3/IMAGE/3-36.GIF Figure 3-36 Double-cone reverse-discharge mixer 1—Feeding rack ； 2—Chassis ； 3—Hopper ； 4—Material discharge port ； 5—Tapered mixing drum 　http://www.scutde.net/t14courses/1412-efmdljiaif/hunningtu/3.3/PIC/3.3.1.JPG 　Double-cone concrete mixer (photo). The mixing action of a forced-action mixer is stronger than that of a gravity-fed mixer, making it suitable for mixing hard concrete and concrete with light aggregates. However, the rotational speed of forced-action mixers is higher than that of gravity-fed mixers, resulting in greater power consumption as well as more wear on the blades, liners, and other components. http://www.scutde.net/t14courses/1412-efmdljiaif/hunningtu/3.3/IMAGE/3-37.GIF Figure 3-37 Forced mixer 1—Feed inlet ； 2—Tumble drum cover ； 3—Stirring cylinder ； 4—Discharge port. Forced mixers are divided into vertical-axis and horizontal-axis types; the horizontal-axis type is further categorized into single-axis and double-axis types, while the vertical-axis type is divided into vortex-pump type and planetary type (Table 3-9). A vertical shaft mixer discharges material through a discharge port at the bottom of the disk, allowing for rapid discharge. However, if the discharge port is not properly sealed, the cement slurry can easily leak, so vertical-axis mixers are not suitable for mixing concrete with high fluidity. Horizontal shaft mixers have advantages such as a wide range of applications, short mixing time, and high mixing quality, and they are a type of mixer that is being widely developed both domestically and internationally. When selecting a mixer, it depends on factors such as the volume of work, the slump of the concrete, and the size of the aggregates. It is necessary to meet technical requirements while also considering economic benefits and energy conservation. In our country, concrete mixers are specified by their discharge capacity in cubic meters multiplied by 1000; therefore, the series of concrete mixers in our country are: 50, 150, 250, 350, 500, 750, 1000, 1500, and 3000. Table 3-9 Types of concrete mixers: Double-cone, self-falling, forced-type, vertical-axis type, horizontal-axis type (single-axis, double-axis), discharge by reversal, discharge by tilting, vortex type, planetary type, fixed-disc type, rotating-disc type.
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