In the making of high-quality projected concrete or shotcrete, it is key to have the right ‘recipe’ setting out the correct proportions in which to dose the different ingredients required.
But having the right ‘recipe’ on its own is not enough, as both the way in which the ingredients are mixed and the sequence in which it is done will determine the quality of the final product. This post will look at the basic requirements of a concrete plant, and its role in the final quality of the resulting shotcrete.
Main types of plants
Concrete plants can be categorized according to
- The type of concrete:
-
Dosing plants:
These plants simply weigh and combine the materials in the required amounts as set out by the ‘recipe’, with the actual mixing done in the mixer truck.
-
Mixing plants:
These plants include a mixer that is in charge of processing all the ingredients in one go, before discharging into the mixer.
-
- According to their mobility:
-
Fixed
-
Mobile
-
Shotcrete, in-transit mixers & central mixers
As we mentioned earlier, the actual mixing of concrete in a dosing plant happens in the in-transit mixer truck. On the other hand, mixing plants feature a central mixer that is responsable for the homogeneity and final quality of the shotcrete, rendering it an indispensable item.
The central mixer’s capacity will determine the amount of cycles required to fill up the in-transit mixer truck.
- Example: if the plant’s central mixer has a capacity of 1 m³ , it will need to do three batches of 1 m³ to fill up a mixer truck with 4 m³ capacity
To determine a mixer’s capacity, one of the key aspects to bear in mind is the homogeneity and quality of the resulting shotcrete. In order to ensure the best possible finishing quality, as is the case with shotcrete, a vertical mixer is normally recommended since they can produce a very homogeneous mix in the least amount of time.
Vertical mixer pans consist of a cylindrical pan with a set of rotating blades and scrapers set as specific angles to create a uniform mix.
In order to avoid excessive wear, the arms typically boast shock-absorbing springs, and both the tank and the arms are covered in wear-proof steel.
How batching plants work
For the perfect concrete, you need to combine the right proportion of aggregates, cement (binder) and water, as well as admixtures such as chemical additives and fibers.
Find out more about shotcrete’s key ingredients, and the shotcrete grading curve.
In a central plant, the amounts used for the mix are carefully monitored by a station control panel that ensures the correct delivery of each ingredient in the right proportion, making the job of the plant supervisor simpler.
Usually, aggregates are delivered from the hoppers to the mixer via extractor belts, and weighed by its integrated weighing system, while water is dosed with a programmable water meter. Given the importance of weighing accuracy, some plants include hydro-probe moisture sensors to monitor humidity levels and correct them should it be necessary.
For the cement, there are different feeding systems including bag splitters, big bag hoppers or silos, depending on the necessities of each project.
To check out a central mixing plant with a vertical mixing set, click here
Sources:
- Robert FJ Goodfellow, Concrete for Underground Structures: Guidelines for Design and Construction, Society for Mining, Metallurgy & Exploration, 2011
- http://concrete.nist.gov/CREMEpapers/Concrete/Overview/j62fer.pdf
