Concrete plant
A concrete mixing plant is a plant for the industrial production of concrete . Modern systems with electronic control take over the storage, dosing, conveying and mixing of the raw materials as well as the production and loading of the concrete.
Primarily concrete mixing plants are in stationary concrete factories to use, ready-mixed concrete to produce according to the standard. For better utilization, mortar and screed are often also made. The mixing plants also supply the fresh concrete for the production of precast concrete parts and other concrete products. Concrete mixing plants are used much less frequently on the construction site. Only when large quantities of concrete are required, such as when building concrete highways or airports, is this the more economical option.
Concrete mixing plants are listed in the BGL / EUROLISTE under device sub-group B.4.5 - Concrete mixing plants, compact design . The characteristic value is the nominal volume in m³, i.e. the volume of the fresh concrete that can be produced in one work cycle in a compacted state.
Plant components
Modern plants for the production of quality concrete consist of the following components:
- Storage capacities for the individual recipe components (aggregate, cement, water and concrete aggregates)
- The individual types of cement (or types of binding agent) are stored next to the system in silos that are filled pneumatically. The silos have a closure at the bottom and from there the cement is transported to the cement scales via screw conveyors .
- Transport, dosing and weighing equipment for every recipe component
- The sand dosage is mostly done with double-segment closures. For larger aggregates from 4 mm, a single-segment closure is sufficient for dosing. The subsequent conveyance of the dosed aggregate takes place with a conveyor belt. The cement, on the other hand, enters the concrete mixer with the help of screw conveyors. The individual recipe components are weighed with high accuracy with the help of calibrated container scales equipped with electronic load cells. The size of the container is based on the maximum possible batches of the mixing plant. As an alternative to container scales, weighing belts or elevator buckets with load cells can also be used.
- Sand moisture meter
- A capacitive working probe measures the electrical resistance during the dosing process of sand and thus determines its moisture. Using the measured value obtained, the proportion of sand or the proportion of water in the batch can be adjusted.
- Consistency knife
- Several probes built into the concrete mixer measure the conductivity of the concrete. Depending on the amount of water added, the conductivity and thus the consistency of the concrete changes.
- concrete mixer
- The mixer is the actual heart of the system. It has to mix the batch very homogeneously in a very short time.
- Transfer device for the fresh concrete
- Dosing and process control technology
- dust filter
- Dust filters on the cement scales and the concrete mixer ensure that dust emissions are kept as low as possible
- Residual concrete recycling plant
- Many ready-mixed concrete plants are still equipped with a residual concrete recycling plant. After cleaning the truck mixer, this additional system separates the concrete suspension into solid and liquid components. This prevents the concrete residue from setting. The extracted components are then often added back to the concrete manufacturing process.
Flow diagram
The following flow diagram shows the main components and the material flow in a concrete mixing plant (stationary and mobile) for quality concrete.
| Dosing of aggregate |
Dosing cement |
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| Sand moisture measurement | Cement screw | ||||||||||||||||||||
| Scales aggregate |
Scales cement |
Scales or counter water |
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| concrete mixer | Dosage of concrete additives |
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| Handover of fresh concrete | |||||||||||||||||||||
Basic types
In the case of concrete mixing plants, a basic distinction is made between vertical plants and horizontal plants.
Vertical system
The different aggregates are stored in star-shaped chamber segments which, together with the cement silo, form a tower-like structure above the mixer and weighing platform. The tower is fed via a long conveyor belt, a bucket elevator or, more rarely, an elevator. The aggregate is fed to the weigh feeder and mixer by gravity. The advantage of vertical systems is a high level of efficiency and protection of the aggregates from the effects of the weather. In these systems, up to 130 m³ of fresh concrete can be produced per hour. However, this construction method is also associated with higher investments . It is therefore used almost exclusively in ready-mixed concrete and precast plants.
Horizontal system
In horizontal systems, the aggregates are stored horizontally next to the concrete mixer, dosed, weighed and fed into the concrete mixer by means of a conveyor belt or elevator bucket. In some cases, horizontal systems can be implemented and installed quickly and inexpensively. The efficiency of the horizontal systems is somewhat lower than that of the vertical systems and is around 110 m³ of fresh concrete per hour.
There are four different types of storage:
- Star bearing
- The different aggregates are piled open in star-shaped chambers. In the center there is a rope-guided scraper (also called scraper), with the help of which the aggregates can be pulled into the distribution star. From there, the surcharges fall onto a scale, which usually also serves as a conveying container for charging the mixer.
- High silo
- With this design, the various aggregates are stored in a tower silo next to the concrete mixer. They fall in the required quantity onto a weighing belt below and then get into the concrete mixer with the help of an elevator bucket.
- Pocket silo
- The pocket silo stands on a hill and consists of four chambers. Ramps are built on both sides so that the chambers can be loaded with the wheel loader.
- In-line silo
- The various aggregates are stored in row silos that are fed by a wheel loader. A dosing device is located under the silos, which delivers the individual aggregates to a conveyor element according to the recipe and then feeds the mixer.
Areas of application
Mixing plant in the ready-mixed concrete plant
From the beginning of the 1960s, stationary ready-mixed concrete plants with truck mixer operation replaced the manually operated construction site mixing and weighing systems that had been common up to that point. Modern ready-mixed concrete plants are almost exclusively equipped with stationary concrete mixing plants. The concrete is produced according to the concrete type directory using valid standards. The entire manufacturing process is documented on a delivery note. Information on the delivery note includes the type of concrete, the exposure, strength and consistency class as well as the type and quantity of the components used.
Mixing plant on the construction site
Special concrete mixing plants have been developed for large construction sites with a high demand for concrete, such as the construction of airports, concrete highways or dams. As a rule, these are mobile large-scale mixing plants with an output of up to 300 m³ / h hardened concrete with the same recipe. They consist of transportable individual components that can be quickly moved and assembled quickly. In the case of construction site systems, the concrete is also transferred to construction site dump trucks, crane pegs or conveyor belts.
Mixing plant in the precast concrete plant
In the precast concrete plant, the production of fresh concrete is only an intermediate stage. Solid concrete elements are then made from it and transported to the construction site. The concrete output of a precast plant is usually lower than that of a ready-mixed concrete plant, since the concrete molding is usually the capacity-determining factor. The production process for fresh concrete is basically the same as for ready-mixed concrete, but other recipes with more fines are often used in the aggregates. The fresh concrete is usually handed over using internal transport buckets, from which the prefabricated formwork is then filled.
Winter operation
If the concrete mixing plant is operated in wintry weather, certain measures must be taken in order to obtain the required fresh concrete quality. For example, the most important system components (concrete mixer and transport, dosing and weighing equipment) should be enclosed and heated. In addition, so-called warm concrete can be produced. This is done by heating the aggregates in the silo with warm air. For this purpose, warm air is blown into the lower part of the silo pockets and in this way the aggregate is heated upwards. There is also the option of using warmed mixing water for the production of concrete and thus obtaining sufficiently preheated fresh concrete.
Manufacturer (selection)
Major manufacturers of concrete mixing plants are:
swell
- Horst König: Machines in construction: Basics and application . Vieweg + Teubner Verlag, 2008, ISBN 978-3-8351-0250-7 , page 9 ff.
- Manfred Hoffmann: Number boards for the construction company . Teubner Verlag, 2006, ISBN 3-519-65220-X , page 588 ff.
- Gerhard Drees, Siri Krauss: Construction machinery and construction methods . Expert Verlag, 2002, ISBN 3-8169-2060-8 , page 1 ff.
- Konrad Zilch, Claus Jürgen Diederichs, Rolf Katzenbach (Hrsg.): Handbook for civil engineers. Springer Verlag, 2002, ISBN 978-3-662-07714-6 , pages 2–284 ff.
