According to asphalt plant and equipment manufacturer Ciber Equipamentos Rodoviários (Ciber), the new inputs available for the production of hot milled asphalt concrete are providing opportunities to create special mixes with properties more relevant to the complexities of paving works.
Marcelo Zubaran, Product and Application Engineer at Ciber says one of the main highlights in this space is petroleum asphalt cement. It's a product that comes from refineries with properties related to petroleum, but is insufficient when it comes to dealing with road traffic and climate conditions.
He says additives such as polymers and rubber dust improve the performance of this type of binder, but may also affect its production in an asphalt plant.
Mr. Zubaran says asphalt cement is considered to be the residue of petroleum distillation, which comes from lighter gases and oils evaporating and being collected during the process in the refinery. This binder gives the asphalt mix flexibility and impermeability, which are necessary to deal with the dynamics of loading and climate conditions.
"However, certain rheological properties of this binder, such as viscosity, softening point, elastic recuperation and thermal susceptibility, may be insufficient to a certain traffic volume or climate of the place of application. In those cases, modified asphalts stand out as the solution," explains Mr. Zubaran.
He says that modified asphalt allows for the development of special projects such as a draining asphalt coating and mixes with high superficial adherence.
The modifying additive is mixed into the binder, normally through the provider, and goes through specific processes to fuse the additive and the asphalt.
The polymers present a higher fusion capacity than the rubber dust and it can be measured through a phase separation test.
When the phase separation is higher than 5 per cent, Mr. Zubaran says, it is recommended to introduce stirrers in the tank with rotation higher than 100 revolutions per minute to avoid decantation of the additive. If the phase separation is lower than 5 per cent, a recirculation system through a pump is enough to keep the product homogeneous.
Regarding the quality of the asphalt mix, he says it is fundamental that the binder is heated for the mix with other aggregates according to their viscosity.
According to Mr. Zubaran, the combination of modified asphalt and aggregates should happen in a mixer that gives the mix a high shearing rate.
Certain mixes also require a dry mixing stage so that the thickness of the binder pellicle is homogeneous on all faces of the aggregates. Mr. Zubaran says those technological capabilities may be found in asphalt plants with a pug mill mixer in continuous production flow or per batch.
Mr. Zubaran explains that another system that requires monitoring in those applications is the system for asphalt binder spreading. If the asphalt viscosity is too high, it's recommended to remove the injector nozzles to stop the nozzles from blocking.
"The asphalt plant should cater to the production of the most varied asphalt mixes. The tuning between the criteria of operation and maintenance and the inputs used then help add to a project's success.