concrete mixer
A concrete mixer is a machine for mixing of concrete . There are small mixers that can be filled manually as well as machines for industrial processes in different designs which, together with the upstream and downstream system technology, enable high and quality-assured performance.
Concrete production process
Concrete is made from cement , aggregates ( sand , gravel or chippings ) and water . It can also contain small amounts of concrete admixtures and additives (e.g. fibers , paints). Cement is the highest quality component of the recipe, it serves as a binding agent after contact with water. Its even distribution and complete wetting are of decisive importance for the quality of the concrete. The recipe components must therefore be reliably mixed to form a homogeneous suspension . The result is fresh concrete , which can have a more or less fluid consistency , depending on the requirements . After the fresh concrete has been placed in the desired formwork , it sets over time to become hardened concrete as a building material.
In order to make the concrete production process quality-assured and economical, the ready-mixed concrete industry developed in the western industrialized countries from the 1950s and has now established itself worldwide. Instead of a manufacture on the construction site, the concrete is in the central -mixed concrete plant manufactured and in truck truck mixers delivered. In ready-mixed concrete plants, the concrete components are weighed according to the recipe for each batch and then mechanically mixed in a concrete mixer to form fresh concrete. This process is documented on a delivery note . The fresh concrete is transported to the construction site and poured into the formwork there. The truck's rotating mixer drum prevents the concrete from setting prematurely and from segregating. Truck mixers are also referred to as “concrete mixers”, but this is incorrect.
A second form of concrete production is the production of precast concrete parts . In this case, the sales product is not the pasty fresh concrete, but an end product made of concrete (e.g. beams, pillars, slabs, roof tiles, concrete blocks, pipes, garages, etc.) manufactured according to customer requirements. Similar to the ready-mixed concrete plant, the fresh concrete is first produced in a precast concrete plant. However, the fresh concrete remains on site and is poured directly into the final precast element in further work steps. In this case, too, a mechanical mixing process is required for reliable concrete production.
Requirements for a concrete mixer
It is the job of a concrete mixer to mix the recipe components of the concrete reliably, quickly and with a high degree of homogeneity into the fresh concrete. Most of this happens in a batch process. Such a batch mixer must meet the following requirements:
- Achieving the desired degree of homogeneity in a very short time
- Reliable repeatability of the mixing result
- Documentability of the process
- Options for a quick recipe change
- Universal applicability for all conventional concrete formulations
- Low operating costs, especially in terms of wear and tear and energy
- High availability and ease of maintenance
In most countries, concrete production is regulated by an extensive set of standards in order to establish generally binding quality standards. Requirements for the mixing process are also specified here.
Common designs of a concrete mixer
Truck mixer mix (dry dosage)
With dry dosing in the ready-mixed concrete plant, the mineral concrete recipe components are only weighed and then filled into the truck mixer together with the mixing water. The truck mixer drum is then rotated for a few minutes at an increased speed to mix the whole thing. The disadvantages of this process are an insufficiently documentable homogeneity of the concrete, dust generation when filling with cement and high wear of the truck mixer drum. This production method used to be widespread in many countries and can still be found in North, Central and South America today.
Drum mixer or free-fall mixer
This mixer type is available in two sizes. As a small device with a volume of a few liters, they are widespread in the DIY and sometimes also in the craft sector for use on construction sites. In industrial sizes with a capacity of 0.5 and more cubic meters, they are used stationary in ready-mixed concrete and precast plants. The basic principle is the same. The mixing process takes place in an inclined, rotatable drum. In this inclined position of the drum, the loading takes place through the drum opening. Inside the drum there is a spiral that picks up and lifts part of the mix with each rotation. When the upper third of the drum rotation is reached, the material falls down. This is where the synonymously used term free-fall mixer comes from . The mixing process takes place when the material to be mixed falls and hits, and when the partial quantity is selected for the lifting process.
There are two options for emptying. With drum mixers without cardanic suspension , the direction of rotation must be changed so that the spiral blades convey the mix to the opening. With a cardanic suspension of the mixing drum, the drum opening can be swiveled with the help of a tilting device while maintaining the direction of rotation and thus the material can be emptied. Small drum mixers or free-fall mixers are still widespread among do-it-yourselfers and craft businesses for the production of small amounts of concrete on the construction site. In stationary concrete plants in North, Central and South America, free-fall mixers are still regarded as the standard technology today.
Ring trough mixer, planetary mixer, plate mixer
What these three types of mixer have in common is that they consist of a cylindrical mixing container that is filled from above and is equipped with mixing tools on the inside. Ring trough mixers and planetary mixers have a fixed container, while the container in pan mixers is rotatably mounted.
Mixer with fixed mixing trough
In the ring trough and planetary mixers, the shovel-shaped mixing tools are driven from above, they rotate on a ring or planetary track. They take on the task of moving the material to be mixed in a circle within the mixing container. Due to the engagement of the shovels, the material to be mixed is divided and subsequently brought together, this essentially causing the mixing process. With the planetary mixer, this process is much more intensive than with the ring trough mixer because of the more complex sequence of movements. In many cases, the mixer is also equipped with high-speed agitators to improve the mixing effect. Emptying takes place via one or two slides embedded in the edge of the container bottom. In the case of ring trough and planetary mixers, the degree of filling is limited because the material exchange is essentially two-dimensional. Compared to other designs, this results in relatively large machine dimensions with the same batch output. This is one of the reasons why these mixers are essentially only used for batch sizes of up to approx. 1.5 m³ of hardened concrete.
Mixer with rotating mixing trough
In a pan mixer, a cylindrical mixing container rotates relatively slowly around its vertical axis. Depending on the size, one or more eccentrically arranged mixing tools are arranged in the mixing chamber. In the center of the mixing container there is a circular emptying opening. A stationary wall and floor scraper removes buildup from the container wall and creates a coarse mixture of the mix. This coarse mixing can be intensified by the inclined arrangement of the mixing container with respect to the horizontal and the use of gravity. If the mixing tool and the mixing container rotate in opposite directions, the inclined mixing container produces a particularly distinctive three-dimensional movement pattern of the material to be mixed.
Due to this special design, the material transport through the rotating mixing container is decoupled from the actual mixing process by the mixing tool. As a result, the shape of the mixing tool and its speed can be freely adjusted up to 40 m / s without centrifugation of the material being mixed against the wall of the mixing container. With the pan mixer, the rotating mixing container transports all the mix to the mixing tool without any relative movement between the mix and the mixing container, so there is only little wear on the mixing container.
Twin-shaft batch mixer
An essential element of this mixer type are two parallel, horizontal shafts that are equipped with mixing tools and rotate against each other. The mixing vessel is designed as a double curve corresponding to the two mixing circles. In the middle of the mixing trough there is an overlap of the two mixing circles in order to additionally increase the turbulence in the mixing process. In the bottom there is an emptying opening in the longitudinal center, which can be designed as a rotary valve with a horizontal shaft or as a flap door. The speed is comparatively low at 20 to 30 / min; it can be changed flexibly with the appropriate control technology. The mixing tools are designed so that there is a helical movement of the material to be mixed in the trough along a mixing shaft. At the end of the shaft, the material to be mixed is shoveled to the other shaft, so that overall there is a spiral transport flow along a circular path. This sequence of movements causes a three-dimensional and highly random material exchange in the entire mixing room. It is the physical basis to achieve a high level of homogeneity in a short time and repeatably.
The twin-shaft batch mixer is used worldwide for the production of all types of concrete in ready-mixed concrete and in precast plants. Today it has established itself as the predominant machine type in new concrete plants and also in asphalt mixing plants in Europe and Asia.
history
One of the first drum mixers was developed by TL Smith in Milwaukee in 1900 . It already had the basic design, which is still common today, with a tiltable conical drum (at the time as a double cone) with paddle blades. In 1925, at least two mixers built 25 years earlier were still in use (serial numbers 37 and 82). The Smith Mascot essentially corresponded to the design still used today for small mixers. The TL Smith Company in Milwaukee built the largest concrete mixers in the world in the 1920s. The company's mixers came and a. used in the construction of the Wilson Dam (six 2- yard and two 4-yard mixers, at that time the largest single installation of the largest concrete mixers in the world), the first stage of Ohio State University and the Exchequer Dam .
Web links
Individual evidence
- ↑ History of Concrete . BetonMarketing Deutschland GmbH. Retrieved January 24, 2015.
- ↑ Selection of the concrete . BetonMarketing Deutschland GmbH. Retrieved January 24, 2015.
- ↑ Pemat mixed media . Pemat Mischtechnik GmbH. Retrieved January 25, 2015.
- ↑ Eirich mixer in batches . Maschinenfabrik Gustav Eirich GmbH & Co KG. Retrieved January 25, 2015.
- ↑ Twin-shaft batch mixer . BHS-Sonthofen GmbH. Retrieved January 25, 2015.
- ↑ Double-shaft batch mixer also for precast plants . BFT International. Retrieved January 25, 2015.
- ^ The TL Smith Company (ed.): Smith concrete mixers and pavers . Catalog no. 526. Milwaukee 1927 ( archive.org [accessed October 9, 2019]).