Concrete mixer

A concrete mixer is a mixer for concrete. He will distinguish between stationary designs and truck mixers. Concretes are now mixed primarily at the factory and delivered in defined batches ( batch mixing).

Concrete production process

Concrete is made ​​of cement as a binder, various aggregates (sand, gravel or crushed stone ) and water. It may also include concrete admixtures and cement additives ( eg fibers, colors) in small amounts. The most expensive component of the recipe is the cement. Due to its property as a binder is its share and its uniform distribution in the mass for the concrete quality is crucial. During mixing of the formulation components to a suspension containing the fresh concrete having a more or less liquid consistency. Fresh concrete is then introduced into the desired mold and bonds over time from the hardened concrete as a building material.

To make the process of concrete production quality assurance and economical, ready-mix concrete production has developed in Western industrialized countries and become the worldwide from about 1960. In the ready-mix concrete plant, the various concrete components are exactly prescription weighed and then mixed in a concrete mixer machine to fresh concrete for each batch. This process is documented and recorded on the delivery note. The fresh concrete is then placed in a truck mixer truck that transports the concrete to the construction site where it is directly cast or distributed by concrete bucket or truck-mounted concrete pump. The rotation of the truck mixer drum serves to prevent segregation. In common parlance, truck driving mixers, respectively. Concrete mixer truck referred to as " concrete mixer ", but this - as shown above - is not correct.

A second form is the production of precast concrete. Selling product in this case is not the fresh concrete but a manufactured end product on the market requirements of the concrete (eg carrier, pillars, slabs, roof tiles, concrete blocks, garages, etc.). In precast concrete plants, the processing of the fresh concrete in the work itself takes place.

Requirements of a concrete mixer

Task of a concrete mixer is reliable, fast and with the highest possible homogeneity to mix the ingredients of the formulation of the concrete and the fresh concrete. This occurs predominantly in the batch process. The following requirements must be met by a batch mixers:

• In a very short time - batch after batch - reliably achieve a desired degree of homogeneity
• Universal application for all kinds of concrete mix
• Low wear
• Low energy consumption
• High operational reliability and ease of maintenance

Common types of a concrete mixer

Dry Dosage and truck mixer mixture

In the ready-mix concrete plant, the concrete formulation constituents are merely weighed and then placed together with the water in the truck - mixer truck. The truck mixer drum is then rotated for several minutes at an increased rate. The disadvantages of this process are a manufacturer part not sufficiently verifiable homogeneity and quality of the concrete dust during filling with cement and high wear of the truck mixer drum. This lack of preparation is still very prevalent in the United States. The procedural shortcomings are often compensated by an increased addition of cement.

Drum mixer or tumbler

This type of mixer is used in small sizes on small construction sites and DIY. In a rotary drum of the mixing process takes place. Inside the drum wall diagonally positioned blades are attached, which lift the material in the mixing position, and subsequently falls by gravity back down. In drum mixers without the gimbal rotation must be changed so that the blades convey ingredients for opening and thus may cause the emptying of the drum. In gimbals of the mixing drum whose outlet can be also rotated by means of a tilting device with maintained direction and at the same time the material is mixed and emptied. The disadvantage of both types are mainly a lack and also to strongly fluctuating mixture homogeneity even after prolonged mixing time. Drum mixer or tumbler at an industrial scale were formerly very widespread. Today, they are still viewed in North, Central and South America as the standard technology.

Pan mixer, ring pan mixer, planetary mixer

These three types of mixers have in common is that they consist of a circular, cup- like vessel that is filled with the produced concrete.

When ring pan mixer and planetary mixer, the mixing tools rotate in a stationary mixing container on a ring road or on planetary orbits. You take over both the task of the material transport within the mixing container and the actual mixing process. Partially located next to the mixing arms still high rotating agitators in the mixing chamber to improve the mixing effect. The discharge takes place via one or two at the periphery of the vessel is embedded in the bottom slide.

When pan mixer, a cylindrical mixing vessel rotates relatively slowly around its vertical axis. In the mixing chamber, one or more eccentrically arranged mixing elements are arranged according to size. In the center of the mixing vessel is a circular emptying. A stationary wall and arranged scraper freeing the container from adhering and produces a coarse mixing of the mixture. This coarse mixing can be enhanced through the inclined from the horizontal arrangement of the mixing container and the use of gravity. In an opposite direction of rotation of the mixing tool and mixing vessel arises when inclined mixing tank, a particularly well-developed three-dimensional moving image of the mixture.

Due to this special design, the material transport is decoupled by the rotating mixing pan from the actual mixing process by the mixing tool. Characterized the shape of the mixing tool and the speed up to 40 m / s can be set freely, without causing to the centrifugation of the mixture at the Mischbehälterwandung, as is the case of mixers with fixed mixing vessel. Due to the centrifugation of the mixture are mixers with fixed mixing container in the choice of the maximum circumferential speed to about 4 m / s is about 6 m / s limited in ring trough and planetary mixers and twin shaft mixers.

Disadvantages in the ring trough and planetary mixers are a low filling ratio and consequently a relatively expansive dimension of the machine. Above all, a predominantly two-dimensional and therefore are often inadequate turbulent motion of the mix. With the use of agitators, this problem is reduced, however. Another disadvantage is a relatively high degree of wear in both the mixing tools as well as the trough lining. This is a result of the high peripheral speed of the mixing tools and the low degree of filling, which has a large trough inner surface result. This design is therefore especially in smaller batch sizes spread (up to about 1.5 hardened concrete ), such as those encountered in smaller precast plants.

When pan mixer with rotating mixing tank, the wear is significantly reduced compared to mixers with fixed mixing containers such as ring trough, planetary, double-shaft or pipe mixer, since the mixing tool no wall or floor runs smoothly. For mixers with fixed mixing tank, the mixing tool must move the mix through the machine and scrape off the walls and floor. These tools need to work with a minimum distance to the tank, which due to the high relative speed between the tool and container leads to high wear. When pan mixer made ​​by the rotating mixing container transport all the mixed material to the mixing tool with no relative motion between mix and mixing tank, the wear on the mixing container is thus reduced to zero.

Furthermore, the direction of rotation of the mixing tool (direct current or countercurrent ) and the mixing tool speed s at which any particular design principle of the disc mixer is between 1 m / s to 40 m / adjusted. This allows optimum adaptation to the task. During high local shear and thus tool speeds are necessary, for example for the complete digestion of fibers in ultra high strength concrete or the digestion of color pigments in facing concrete to catch up the agglomerates, very low tool speeds are advantageous for gentle mixing of lightweight aggregates or foam. For the production of ultra high-strength concrete is also offers the use of vacuum mixers to achieve a minimum air void content.

Twin-shaft batch mixer

This type of mixer is characterized by two horizontal shafts, each equipped with mixing tools and rotate against each other. The bottom of the vessel is formed corresponding to the two mixing circles as double rounding. In the longitudinal middle of the bottom there is the discharge opening, which can be designed as a rotary slide or a folding door. The mixing tools are ideally designed so that there is a circular and at the same time along each mixing shaft helical motion of the mixture in the trough. This causes a targeted three-dimensional material exchange across the mixing chamber. Depending on the manufacturer, there are also other forms of movement. A change in the speed of the mixing tools is easily possible with appropriate control technology to achieve although there are special requirements (SCC, UHP etc. ) reliably repeatable mixing results.

Also in the center of the mixing trough is an overlap of the two mixer circuits takes place, to enhance the turbulence of the relative motion of the mix in addition. Homogeneity grade of 95 % in just 30 seconds mixing time can be reliably achieved by twin-shaft batch mixer leading manufacturer, even at high batch sizes. The mixing time, however, depends on recipe and can therefore also vary. The twin-shaft batch mixers have therefore generally accepted than it is today dominant machine type in high-performance concrete plants and asphalt mixing plants in Europe and Asia.

Mixing of small quantities

Small amounts of concrete, for example, repairs may be made with a whisk, is clamped in a drilling machine. This works just like the whorl used as a kitchen appliance.