New technology makes it possible to produce conglomerates at less than 100°C with a saving in the energy input for the process in the order of 40%. These “Warm” conglomerates (WMA Warm Mix Asphalt) can be produced using different procedures, but that which is undoubtedly giving the best results is the system devised by the French company LEA-CO, one of the pioneers in research on road building technology.
The procedure known as LEA® (Low Energy Asphalt) or EBE (Enrobage à Basse Energie) is based on the use of wet sand. The logic behind the process is that the energy required to transform water into steam is five times higher than that needed to heat aggregate from 0°C to 100°C; furthermore, fine aggregates like sand tend to have a higher humidity content, so if drying the sand can be avoided the energy saving may be more than 40%.
The procedure is as follows:
- larger gauge aggregate is dried and heated to about 150-160°C;
- liquid state asphalt, before it is injected into the mixer, has a special additive put into it;
- the asphalt and the large aggregate are mixed at high temperature (150-160°C);
- wet sand at air-temperature is added to this mixture (if it is not sufficiently damp it can be sprayed with water);
- when the high-temperature mix (large aggregate and asphalt) comes into contact with the damp sand, the water content is transformed into steam, which expands to create the effect of “foaming” the asphalt which thus spreads and covers the granules of fine aggregate;
- the fine aggregate, in contact with the larger gauge, is heated up and takes the mixture to an equilibrium temperature of less than 100°C;
- the residual water content re-condenses inside the mixture in small droplets which help to keep a good working consistency in the conglomerate, even at low temperature.
The whole process, which includes the recycling of milling material (RAP), is possible thanks to a series of specifically dedicated technological components and is controlled using a specialist software.