 to improve coating and workability, adhesion promotes, and emulsification agents.
All these four technologies appear to allow the production of WMA by reducing the viscosity of the asphalt binder at a given temperature. This reduced viscosity allows the aggregate to be fully coated at a lower temperature than what is traditionally required in HMA production. However, some of these technologies require significant equipment modifications.
The Sasobit® additive, which is a product of Sasol Wax, is one of these technologies that allow the reduction of mixing and compaction temperature of hot mix asphalt. It was selected by the WMA task group of Graniterock for the trial section because of its minimal plant modification requirement and relatively low cost.
Sasobit® is a fine crystalline, long-chain aliphatic polymethylene hydrocarbon produced from coal gasification using the Fischer-Tropsch (FT) process.(2)
Sasobit® is described as an “asphalt flow improver”, both during the asphalt mixing process and during laydown operations, due to its ability to lower the viscosity of the asphalt binder2. This decrease in viscosity allows working temperatures to be decreased by 32–97°F (18–54°C). Sasobit® has a congealing temperature of about 216°F (102°C) and is completely soluble in asphalt binder at temperatures higher than 248°F (120°C). At temperatures below its melting point, Sasobit® reportedly forms a crystalline network structure in the binder that leads to the added stability2, 3. An example of how Sasobit will affect the mixing and compaction temperature is shown in Figure 24. Figure 2 demonstrates how Sasobit® can reduce viscosity in the mixing and compaction temperature range while producing approximately the same (or in some cases greater) viscosity at in-service pavement temperatures.
During the production of HMA, Sasol recommends that Sasobit® be added at a rate of 0.8 percent or more by mass of the binder, but not to exceed 3 percent.
In commercial applications in Europe, South Africa, and Asia, Sasobit® has been added directly onto the aggregate mix as solid prills or as molten liquid via a dosing meter. Marshall tests performed on mixes produced in this manner indicated no difference in stability or flow as compared to premixing with the binder5. In the United States, Sasobit® has been blended with the binder at the terminal and blown directly into the mixing chamber at the same point cellulose fibers were being added to an SMA. Commercial supplies of Sasobit® are available in 20 kg bags and 600 kg super-sacks.
GRANITEROCK WMA TRIAL
The Quarry Road WMA trial is the first application of the Sasobit® warm mix in California, and the project should provide quantifiable results for further evaluation and implementation of this new technology.
The Quarry Road project called for 330 tons of the warm mix containing the Fischer-Tropsch wax, Sasobit®. The WMA mix was paved in a one lane strip that has 540 meters (1800 feet) in length, 3.9 meters (13 feet) in width, and 64 mm (2.5 inches) in thickness. For easy evaluation of the long term performance, a control HMA section was built just aside from the WMA trial section. The only difference for the WMA mix comparing to the control HMA mix was the addition of 1.5% of Sasobit® wax by weight of the asphalt binder. The Sasobit® wax was pre-blended with the asphalt binder before it was pumped into the asphalt tank.
There were no changes required at the production plant when blending the warm mix, other than lowering the mixing temperature. Graniterock produced the warm mix at its Astec drum plant in Aromas, CA at a temperature of 270°F and 250°F (Control HMA mix was produced at the same plant at 305°F). The placement and compaction temperatures were lowered accordingly for WMA mix. As shown in Figure 3 and 4, there was significant difference between the emissions indicated by the smoking from the dumping of the truck load.
The lab and field testing results are encouraging for this new technology. The WMA mix achieved good compaction of 97.7 percent under the normal compaction pattern