Warm Mix Asphalt
WHAT IS WMA?
European countries are using technologies that allow a reduction in the temperatures at which asphalt mixes are produced and placed. These technologies have been labeled Warm Mix Asphalt (WMA). The immediate benefit to producing WMA is the reduction in energy consumption required by burning fuels to heat traditional hot mix asphalt (HMA) to temperatures in excess of 300°F at the production plant. These high production temperatures are needed to allow the asphalt binder to become viscous enough to completely coat the aggregate in the HMA, have good workability during laying and compaction, and durability during traffic exposure. The additional benefits with the decreased production temperature include reduced emissions from burning fuels, fumes, and odors generated at the plant and the paving site.
Warm asphalt mixes will also allow longer haul distances and a longer construction season if the mixes are produced at more normal operating temperatures. There is another potential added advantage in that oxidative hardening of the asphalt will be minimized with the lower operating temperatures and this may result in changes in pavement performance such as reduced thermal cracking, fatigue cracking, and preventing the mix to be tender when placed.
TYPES OF WMA TECHNOLOGIES
There are three technologies that have been observed in the European countries to produce WMA1:
The addition of a synthetic zeolite called Aspha-Min® during mixing at the plant to create a foaming effect in the binder.
A two-component binder system called WAM-Foam® (Warm Asphalt Mix Foam), which introduces a soft and hard foamed binder at different stages during plant production.
The use of organic additives such as Sasobit®, a Fischer-Tropsch paraffin wax and Asphaltan B®, a low molecular weight esterified wax.
Another WMA technology was recently developed in the United States, and has been used for WMA application. This technology is chemical based, called Evotherm®, which is a chemistry package that includes additives 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 bee