No pathologies

Dosing and mixing technologies ensure high performance and durability of the asphaltic mass used for road paving

Due to its features of economy, comfort and resistance, the asphalt is being –for more than a century—the main component of paving works. Known as Petroleum Asphalt Cement (PAC), the material is a flexible, adherent, waterproofing and durable plastic substance. In addition, it resists to most acids, alkalis and salts. To be transformed in pavement, however, the PAC has to be mixed to the so-called “aggregates” that are not more than crushed stones—eventually mixed to additives such as hydrated lime and fibers of cellulose—surrounded by the binder. To ensure this mixing, they have to be classified according to the graduations specified in the design, composing a mix of different sizes. In addition, they have to be previously heated to remove moisture.

To ensure a good performance and high durability to the pavement, it is also necessary to obtain a homogeneous mixture. To match these requirements, a correct dosing of each component has to be carried out. In other words, it is necessary to select properly materials and proportions of the mixture, to attend the using conditions of the pavement. Of course, appropriate techniques of production, distribution and pouring of asphalt layers are also important.

PRINCIPLES

Plants are composed by a set of interconnected mechanic and electronic equipment. Its basic function is to perform the mixing of aggregates in an appropriate way, heat it and join it to the binder, producing a pavement mixture according to the specified features. This equipment varies in capacity of production and dosing principles, which may be gravimetric or continuous.

The first type is an older technique that started to work in the 70s. Since then, plants were producing asphalt pavements of low quality because they did not weigh or control appropriately the components. Currently, gravimetric plants produce in cycles or batch processing. “After a pre-dosing and drying, aggregates are transported vertically through a bucket conveyor to be sieved at the top of a tower”, explains Marcelo Zubaran, product expert of Ciber Equipamentos Rodoviários, that manufacturers both types of solutions. “After being sieved they are separated in hot bins according to their size and weighed according to the designed proportions.”

PAC is then transported from the storage and heating tank to a compartment where weighing occurs. After being weighed, the materials are transported by gravity to the mixer, where the production is carried out in cycles. “Therefore, a determined quantity of asphalt pavement is produced in a determined period of time”, says Zubaran. “Then it is unloaded or stored, starting a new cycle.”

In continuous plants, the entire cycle—from the introduction of materials to the end of mixing—proceeds continuously. The process is started with the introduction of the aggregates in dosing bins. These aggregates are then directed by gravity to dosing conveyors, which weigh them dynamically during their transport. There is an instantaneous correction of motor speeds according to the material that is being weighed, to maintain the proportion of the components.

After the dosing/ weighing, aggregates are transported to the dryer, to remove moisture. “Dry aggregates are continuously transported to a mixer while the PAC is transferred to the storage and heating tank”, explains Zubaran. “Therefore, materials are introduced by one side and extracted by the other, in a process carried out along the length of the mixer.”

After this process, the asphaltic mass is transported to a storage bin, and then is discharged in trucks.