The selection of aggregates has a great importance to achieve a good quality of the final products. The objective of the manufacturer of concrete precast 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 therefore also the performance of the production machine, both in the filling of the mould and in the compression of the part.

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 of the aggregate moisture content, can be an inconvenience because the unit weight varies so we must prevent this from happening.

The classification of aggregates by granulometry is as follows:

Fine Aggregates: Grain dimensions less than 0,1 mm.
Sands: Grain dimensions between 0,1 and 5 mm.
Gravel: Grain dimensions of the grains 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 the dust were very large 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.