A mixer serves mainly for the creation of aggregates and pieces of batching plant. Depending on the pieces we manufacture, the number of aggregates must be taken into account.
The quality of the pieces will largely depend on the proportion and blending of aggregates.
The hoppers that store the aggregates have to be covered so that if it rains it will not filter the water and it cannot vary the degree of humidity of the mixture.
The selection of aggregates is of great importance for achieving a good quality of the final products. The objective of the manufacturer of concrete prefabricated products is to achieve a piece of the mechanical properties required by the international standards with the least amount of cement.
The mixture has to be as homogeneous as possible, to achieve it we have to make a good dosage of aggregates and cement.
The most important thing is the quantity of water, as it completely modifies the aggregate, and thus the performance of the production machine, both in the filling of the mould and in the compression of the piece.
The main characteristics of aggregates are: • Hardness • Weight • Finishing • Dimensions
These properties will be achieved if we choose good aggregates, and a good dosage between them. The weight of the piece should normally be the minimum, to facilitate road transport and installation on site.
The dosage of aggregates can be in two ways: • By weight: we weigh the desired amount of aggregates. • By volume: a much simpler system, we regulate the feeding time of each aggregate.
For a good dosage we will need three hoppers with aggregates of different granulometry although the minimum advisable are two hoppers.
The variation in the moisture content of aggregates can be an inconvenience because the unit weight varies so we must avoid that this happens.
The classification of aggregates by granulometry is: Fine aggregates: Grain dimensions less than 0,1 mm. Sands: Grain dimensions between 0,1 and 5 mm Gravel : Grain dimensiones between 5 mm and 8 mm
The aggregates cannot be larger than a third of the thickness of the walls.
A recommended mixture to achieve a balance among all properties would be:
• 45 % of medium-sized aggregate. • 40 % of thick aggregate. • 15 % of fine aggregate.
The powder is necessary in the dosage of the aggregates to help the cement form a paste that perfectly mix all the grains. If there is no dust at all, we will have to increase the quantity of cement and water, but on the contrary, if there were a large amount of dust we would not be able to make a resistant piece. The quantity of powder should not exceed a weight of 5 % regarding the total quantity of the aggregates.
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