The basic quality standard of a hot asphalt mix is adhesiveness, which is the adhesion capacity among the materials.
Often the choice of aggregates is limited and the material’s adhesiveness can become an issue. Adhesiveness is a quality an aggregate must have to be covered by bitumen, which has to resist damaging factors, such as water, without breaking.
The adhesive capacity of the asphalt mix can be measured in the laboratory through qualitative or quantitative testing. The results of this analysis should be used for improving the mix.
Qualitative adhesiveness tests, by displacement of the bitumen film, are commonly used given their simplicity. However, these tests only determine if there is binder displacement of the aggregate’s surfaces.
Marcelo Zubaran, Application Engineer and Product Specialist at Ciber Equipamentos, says the concept of adhesiveness goes beyond the aggregate being “painted” or “black” as set in a qualitative test. For this reason, modern designs consider quantitative tests, such as the induced moisture damage, or Lottman, test.
“This test indirectly measures the adhesion capacity between the aggregates and bitumen from one per cent to 100 per cent,” he says.
In order to improve natural adhesiveness between aggregates and the asphalt binder, granular or liquid additives can be used in the design phase.
A typical example of a granular additive is hydrated lime of calcific origin, capable of reversing the surface polarity of acidic aggregates. This makes the surface of the aggregates basic and improves attraction with the bitumen, which is slightly acidic.
“Liquid additives, called anti-stripping agents, are surfactant compounds that lower the superficial tension of the bitumen. Such additives are added directly into the bitumen tank, dosing normally between 0.1 and 0.3 per cent in relation to the binder weight,” Mr. Zubaran says.
The best additive to use depends on the cost, particular reaction of each mix to each additive, as well as the particle size, as lime increases the amount of fine that can pass through the 200 sieve.
Once the project is defined, the characteristics of the plant mixer and the mixing time affect the adhesiveness of the mix that is to be applied to the pavement.
External pug mill mixers, in continuous and batch plants, transfer higher shear rates to the mix, tending to aid the coating of the bitumen on the aggregate’s surfaces.
“It is also possible to make a dry mix, using coarse and fine aggregates, lime and other additives, prior to the injection of bitumen. This pre-homogenisation of the dry granular materials enables an equal thickness of the binder film on the aggregates surfaces,” Mr. Zubaran says.
This allows for a reversal of the superficial polarity of acidic aggregates when they are mixed with lime, and makes additives, such as cellulose fibers, homogeneous to prevent segregation of the additive in the pavement.
“Another relevant factor is the mixing time. In batch plants, this is freely assisted by the operator, according to the need of the application, but it has a direct impact on the plant’s production rate,” he says.
Modern continuous plants with a pug mill mixer are able to adjust the mixing time, which is controlled by plant automation, with no need for mechanical intervention in the mixer.
“In continuous plants, the mixing time is an indirect measure of the amount of material inside the mixer and it depends on the plant’s production rate. By keeping the amount of material constant in the mixer, the lower the production rate, the longer the mixing time,” Mr. Zubaran says.
The correct measurement of the adhesiveness of an asphalt mix, mainly by quantitative tests, guides the insertion of additives, allowing for a mixing project of higher quality to be implemented.
Mr. Zubaran says in the application, the asphalt plant technology can affect the homogenisation of the mix produced.
“The most modern continuous plants present high flexibility regarding the mixing time, something that would only have been possible before in batch plants. Therefore, the continuous Ciber plants of the new generation have exceeded expectations when it comes to the mixing process.”