The physical-chemical characteristics of bitumen basically depend on petroleum and its refinement conditions, which result in known, foreseeable characteristics.
“This means you know exactly what to expect from the asphalt binder,” says Marcelo Zubaran, Engineer, Trainer and Technical Application at Brazil-based asphalt plant manufacturer, Ciber.
“For their part, aggregates present characteristics intrinsic to the processes of their formation, according to the climate and other phenomena, and this leads to materials with unique qualities.
“Aggregates have the characteristics of the minerals that make them up, so rock type can help you make some broad predictions about its physical-chemical traits, but the margin of error can be high.”
With experience producing high-end asphalt plants, Ciber provides insight on technical aspects of the product it specialises in. Mr. Zubaran looks at how aggregates, moisture control and process adaptations affect asphalt production.
One of the premises for producing a HMA is to remove all the moisture from the aggregates, either on the surface and/or absorbed moist.
According to Mr. Zubaran, the asphalt plant itself is mainly a thermal system in which all the moisture from the aggregates is removed to allow bitumen adherence in the aggregate’s surface.
“The aggregates are naturally more attracted to water than to bitumen. This asphalt binder does not adhere to the aggregate’s surfaces if there is water, and because of that the asphalt mix will lose quality and will not fulfill the objective established in the project,” says Mr. Zubaran.
In regards to aggregate moisture, he says that removing water trapped on the surface of the rocks isn’t a tough job.
“Adsorption is the phenomenon that keeps water adhered to the edges of the soil/aggregates,” he says.
“For its part, absorption is the quantity of water an aggregate can absorb in its pores when immersed in water. Removing absorbed water is much more complex and time-consuming.”
After removing the water absorbed by the aggregates, the bitumen partially enters the water-free pores.
The greater an aggregate’s absorption, the greater its absorption of bitumen during mixing. This means the mixture will consume more bitumen in the HMA, making the mixture more expensive but with no added benefits.
The right material
“One of the characteristics of rocks that makes drying difficult, especially absorbed water, is the presence of clayish minerals, especially if the minerals belong to the smectite group,” says Mr. Zubaran.
These materials retain water in their structure, which becomes more viscous and makes flow more difficult.
Mr. Zubaran says that aggregate pore geometry needs to be taken into account, particularly as the smaller the pores where the water penetrates means there is a longer exit path for water, which makes the drying process harder.
Likewise, the type of rock used as aggregate influences the plant drying process.
“Rocks like granite and gneiss tend to present more clayish minerals, while rocks like basalt can also feature clayish minerals, although usually in smaller quantities,” says Mr. Zubaran.
“In general, basalt is more porous than granite and gneiss.”
Uncrushed aggregates from rivers, such as sand and rolled stones, have high capacity for absorbing water and roundish shapes, which Mr. Zubaran says is prejudicial to the asphalt mixture because of their reduced shearing resistance and tendency to deform permanently.
The bonding between aggregates and the bitumen also completely depends on the aggregates drying and their surface polarity.Mr. Zubaran says an alternative to correct granite and gneiss acidity is to add hydrated lime originating from calcite.
He says this reverses the surface polarity of acidic aggregates and imrpoves its ability to adhere to the bitumen, since it is slightly acidic.
Utilising the equipment
Mr. Zubaran says the asphalt plant itself should be set to work according to the characteristics of the materials used to produce the HMA.
“High porous aggregates should spend more time in the thermal system, while acid aggregates should be heated more than alkaline ones. The variations of these characteristics must be understood by the asphalt plant, which must compile these variations to produce proper and quality asphalt mixes,” he says.
During the heat exchange process, between the flame of the burner and the aggregates, Mr. Zubaran says three phenomena of heat exchange take place.
“The inflow of aggregates to the drum is assisted by a conveyor belt that projects them into the dryer system. At this stage the cascading process begins, and its main heat transfer mechanism is the convection,” he says.
Removal of moisture from the aggregates occurs in the convection zone. The larger this zone is, the greater its capacity to dry aggregates.
“As a result, high porous aggregates should stay in the dryer’s convection zone longer,” adds Mr. Zubaran.
The cascading flights are configurable, in order to vary convection times. It is also possible to change the quantity of retention blades, deflectors that are located between two zones of cascading flights and reduce the flow speed of aggregates in the dryer.
After the cascading zone there are mixed flights that provide heat exchange through convection, radiation and even conduction, since the aggregates are closer to the heat source and stay longer on the blades.
At the end of the dryer the cascading stops and the aggregates run through the lower part of the drum, receiving heat mainly by radiation, heating up the aggregates to the temperature stipulated by the project.
The horizontal angle of the dryer and its spinning speed also determine the time aggregates remain in the thermal system. The higher the angle and speed, the greater the drying time and capacity.
There are drier technologies that can control the drying time through a variation of its turning speed, adapting to different aggregates.
“After going through the entire thermal system of the plant, the aggregates should be dry and reach the temperature set on the project,” says Mr. Zubaran. “In practice, it accepts up to 0.3 per cent residual moisture of the aggregates, but the objective is to reduce the maximum residual moisture.”
Depending on the geometry of the pores and minerals in the cavities, the aggregates might take a long time to dry completely.
“At times the stone is dry on the surface, with bitumen adhesion occurring in the mixer even with the water temporarily confined in the pores,” he says.
“In these cases, at a certain point the water begins to boil inside the pores and breaks the adhesive bond with the bitumen.”
Dripping water is often seen in the truck right after mixing and/or during application.
Mr. Zubaran says this loss of adherence not only harms mixture cohesiveness, it also changes some volumetric properties of the mixture.
“Because the portion of bitumen that would be absorbed in the pores of the aggregates isn’t absorbed, it leaves more effective bitumen in the mix, and this reduces the mixture air volume.”
In these cases, he suggests that the aggregates should stay longer in the initial zone of the dryer, where heat exchange by convection occurs. If this adaptation is not enough, he further suggests that changing aggregates and avoiding both plants with parallel flow dryers and those with internal mixing, even the counterflow types.
“These technologies are less efficient at removing moisture from the aggregates.
“This may occur when we work with river sand or aggregates with a high level of absorption and large contents of smectite clay minerals.”
Mr. Zubaran says it is common to use anti-stripping agent when working with acidic aggregates with high absorption and when no lime originating from calcite is available.
“This additive reduces bitumen surface tension and thereby improves bonding even with slightly moist aggregates,” he says.
“The type of bitumen can also help in adherence.”
He asserts that more viscous asphalt cement increases the thickness of the binding film around the edges of the aggregates, which improves mixture cohesion.
Recent techniques such as warm mix asphalt or warm mixtures also make adhesion possible with slightly moist aggregates.
Mr. Zubaran says that, regardless of the material of choice, the aggregates on site should always be stored in a covered location, especially the finer ones.
“When aggregates are porous and contain clayish materials, coverage becomes even more essential. The productivity of an asphalt plant is inversely proportional to the humidity of the aggregates,” he says.
“Therefore, the lower the moisture, the lower the plant’s fuel consumption will be, in addition to the benefits related to mixture quality.”
He says that these sorts of measures will easily pay off their own investment in a few months and lead to financial returns.