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 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 with an average of 50°F temperature reduction. The lab testing results indicate that the WMA mix will perform similar or even better than the HMA control mix in terms of air voids, stability and moisture damage sensitivity.
The added cost of the Sasobit material was almost fully compensated by the fuel saving (up to 37% with 50°F temperature reduction) due to the lower mixing temperature. The benefit of low emission and less odor, better pavement durability due to less aging, and the potential to promote low temperature or night paving capability are just extras.
Emission is one of the biggest issues with the rubberized asphalt mix. The production rate is limited to approximately 1000 tons per day because of the restriction for maximum allowed emission. The WMA technology appears to be able to significantly reduce the emission of asphalt plant therefore allows a significant improvement in the plant production rate for rubberized asphalt mix.
A message from NCAPA asphalt bulletin6 indicates that HMA industry needs to be cognizant of the current environmental regulations that HMA industry will face in the coming years. Several limits including NOx and CO2 limits have been developed and implemented. This is evident in Environmental Protection Agency (EPA)’s implementation of the National Ambient Air Quality Standard (NAAQS) and the recent adoption of the San Joaquin Valley Air Pollution Control District (SJVAPCD) Rule 4309. International Agency for Research on Cancer (IARC) will conduct a major study of asphalt fume research and make a determination as to whether asphalt fume is carcinogenic. All these major efforts will have significant effects on how the industry produces HMA.
How we address these challenges will determine the future of HMA as a viable and environmentally friendly road building material. With all the benefits on energy consumption saving and a commensurate reduction in CO2 emissions and a reported up to 90% reduction in fumes7, the WMA technologies appear promising though there is still a lot to learn about this new technology.
- U. S. Department of Transportation, Federal Highway Administration. “Warm Mix Asphalt Technologies and Research”, www.fhwa.dot.gov/pavement/asphalt/wma.cfm, accessed July 2005.
- Damm, K-W, J. Abraham, T. Butz, G. Hildebrand, and G. Riebeschl. “Asphalt Flow Improvers as ‘Intelligent Fillers’ for Hot Asphalts—A New Chapter in Asphalt Technology.” In Journal of Applied Asphalt Binder Technology, April 2002, Pp 36–69.
- Butz, T., I. Rahimian, and G. Hildebrand. “Modifications of Road Bitumens with The Fischer-Tropsch Paraffin Sasobit®.” In Journal of Applied Asphalt Binder Technology, October 2001, Pp 70–86.
- Hurley, G.C., and B.D. Prowell. “Evaluation of Sasobit® for Use in Warm Mix Asphalt” NCAT Report 05–06, Auburn, AL 2005.
- Brits, C.H. “Sasobit Inverstigation” Report No. 100035/S9/2004/11/05/CHB/av/1, Geostrada Engineering Materials Laboratory, South Africa, 2004.
- NCAPA, “Northern California Asphalt Pavement Association’s Asphalt Bulletin”, Volume 10, Issue 1, Feb, 2006.
- Newcomb, D., “Warm Mix: the Wave of the Future”, HMAT, July, 2005.