Warm Mix Asphalt Technologies and Research
European countries are using technologies that appear to 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. With the decreased production temperature comes the additional benefit of reduced emissions from burning fuels, fumes, and odors generated at the plant and the paving site.
There are three technologies that have been developed and used in European countries to produce WMA:
- 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 binder 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.
The Aspha-Min and Sasobit products have been used in the United States. Additional technologies have been developed and used in the United States to produce WMA:
- Plant production with an asphalt emulsion product called Evotherm™, which uses a chemical additive technology and a "dispersed asphalt technology" delivery system.
- The addition of a synthetic zeolite called Advera® WMA during mixing at the plant to create a foaming effect in the binder.
All five 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.
This technology could have a significant impact on transportation construction projects in and around non-attainment areas such as large metropolitan areas that have air quality restrictions. The reduction in fuel usage to produce the mix would also have a significant impact on the cost of transportation construction projects.
The benefits of these technologies to the United States in terms of energy savings and air quality improvements are promising but these technologies need further investigation and research in order to validate their expected performance and added value. It is important to note that producing HMA at lower temperatures is the desired product to achieve these benefits, not the particular technology that is used to produce the WMA mix.
(Note: These products are listed for information only. The FHWA does not endorse any particular proprietary product or technology. These applications should be considered as experimental.)
Aspha-Min is a product of Eurovia Services GmbH, Bottrop, Germany. It is available in a fine white powdered form in 25 or 50 kg bags or in bulk for silos. It is a manufactured synthetic zeolite (Sodium Aluminum Silicate), which has been hydro thermally crystallized. The percentage of water held internally by the zeolite is 21 percent by mass and is released in the temperature range of 185° - 360° F. By adding Aspha-Min to the mix at the same time as the binder, a very fine water spray is created. This release of water creates a volume expansion of the binder that results in asphalt foam and allows increased workability and aggregate coating at lower temperatures.
Eurovia recommends adding Aspha-Min at a rate of 0.3 percent by mass of the mix, which can result in a potential 54° F reduction in typical HMA production temperatures. This reduction in temperature is reported to lead to a 30 percent reduction in fuel energy consumption. Eurovia states that all commonly known asphalt and polymer-modified binders can be used as well as the addition of recycled asphalt.
The zeolites are framework silicates that have large vacant spaces in their structures that allow space for large cations such as sodium, potassium, barium and calcium and even relatively large molecules and cation groups such as water. In the more useful zeolites, the spaces are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow the easy movement of the resident ions and molecules into and out of the zeolite structure. The most well known use for zeolites is in water softeners. Zeolites are characterized by their ability to lose and absorb water without damage to their crystal structures. They can have the water in their structures driven off by heat and other solutions pushed through the structure. They can then act as a delivery system for the new fluid.
WAM-Foam is a product of a joint venture between Shell International Petroleum Company Ltd., London, UK and Kolo-Veidekke, Oslo, Norway. In WAM-Foam the binder is formed using two separate binder components in the mixing stage. By dividing the binder into two separate components, a soft binder and a hard binder in foam form, lower asphalt mixture production temperatures can be achieved. The soft binder component is mixed with the aggregate in the first stage at approximately 230° F to achieve full aggregate coverage. The hard binder component is mixed in a second stage into the pre-coated aggregates in the form of foam. Rapid evaporation of water by injecting cold water into the heated hard binder as it is added to the mix produces a large volume of foam. The hard binder foam combines with the soft binder to achieve the required final composition and properties of the asphalt product.
Shell states that WAM-Foam's success depends on careful selection of the soft and hard components. In some cases it is recommended to use an adhesion improver in the first mixing stage. Shell also states that initial coating of the aggregate in the first mixing stage is vital to prevent water from reaching the binder and aggregate interface and entering the aggregate and that water must be removed from the asphalt mix to ensure a high quality end product. Shell reports that the decreased production temperatures of the WAM-Foam process can lead to plant fuel savings of 30 percent, which results in a 30 percent reduction in CO2 emissions.
Sasobit is a product of Sasol Wax (formerly Schümann Sasol), South Africa. Sasobit is described as a modifier or "asphalt flow improver". It is available in 2, 5, 20 and 600 kg bags. On request it can be supplied in flakes or powdered form. Sasobit is a fine crystalline, long chain aliphatic hydrocarbon produced from coal gasification using the Fischer-Tropsch (FT) process and is otherwise know as an FT paraffin wax.
In the FT process, carbon monoxide is converted into a mixture of hydrocarbons having molecular chain lengths of 1 to 100 carbon atoms and greater. The starting point for the process is synthesis gas, a mixture of carbon monoxide and hydrogen (CO + H2) produced by gasification of coal, a process involving the treating of white-hot hard coal or coke with a blast of steam. The gas is manufactured in vast quantities for commercial use. It is important in the preparation of hydrogen and as a fuel in the making of steel and in other industrial processes. The synthesis gas is reacted in the presence of an iron or cobalt catalyst; heat is created and products such as methane, synthetic gasoline, waxes, and alcohols are made. The liquid products are separated and the FT waxes are contained.
The makers of Sasobit emphasize the difference between naturally occurring bituminous waxes and FT waxes in terms of their structure and physical properties. The difference reportedly arises from their much longer chain lengths and the fine crystalline structure of the FT waxes. The predominant chain length of the hydrocarbons in Sasobit is in a range of 40 to 115 carbon atoms. The chain lengths of bituminous paraffin waxes naturally found in asphalt range from 22 to 45 carbon atoms, resulting in a lower melting point than FT waxes.
Sasol Wax states that the melting point of Sasobit is approximately 210° F and is completely soluble in asphalt binder at temperatures in excess of 240° F. It produces a reduction in the binder viscosity. This enables production temperatures to be reduced by 18° - 54° F. At temperatures below its melting point, Sasobit forms a lattice structure in the asphalt binder that is the basis for the reported stability of asphalts that contain Sasobit. At service temperatures, Sasobit modified asphalts are reported to display an increased resistance to rutting. In addition Sasol Wax reports improved "compactibility" with an increase in the degree of compaction for the same roller loading as unmodified asphalt.
Sasol Wax recommends adding Sasobit at 3 percent by weight of the mix to gain the desired reduction in viscosity and should not exceed 4 percent due to the possible impact on the binder's low temperature properties. Sasobit can be blended into hot binder at the blending plant using a simple stirrer. A high shear mixer is not required. It is anticipated that in-line blending of melted Sasobit with the asphalt binder stream at the plant will be finalized in the near future. Direct blending of solid Sasobit at the plant is not recommended because it will not give a homogeneous distribution of Sasobit in the asphalt.
Evotherm is a product developed by MeadWestvaco Asphalt Innovations, Charleston, South Carolina. Evotherm uses a chemical additive technology and a "Dispersed Asphalt Technology" delivery system. MeadWestvaco states that by using this technology a unique chemistry customized for aggregate compatibility is delivered into a dispersed asphalt phase (emulsion). During production, the asphalt emulsion with Evotherm chemical package is used in place of the traditional asphalt binder. The emulsion is then mixed with the aggregate in the HMA plant. MeadWestvaco reports that this chemistry provides aggregate coating, workability, adhesion, and improved compaction with no change in materials or job mix formula required.
MeadWestvaco reports that field testing has demonstrated a 100° F reduction in production temperatures. MeadWestvaco also reports that the decreased production temperatures of the Evotherm process can lead to plant energy savings of 55 percent; which results in a 45 percent reduction in CO2 and SO2 emissions, a 60% reduction in NOX, a 41% reduction in total organic material, and benzene soluble fractions below detectable limits.
MeadWestvaco has introduced its third generation of the technology co-developed by Paragon Technical Services, Inc & Mathy Technology and Engineering Services. The technology called Evotherm 3G and also branded as REVIX™ Reduced Temperature Asphalt is water free and does not rely on the principles of asphalt binder foaming or other methods of viscosity reduction. Mathy states that the technology is based on work that shows the additives provide a reduction in the internal friction between aggregate particles and the thin films of binders used to produce bituminous mixtures when subjected to high shear rates during mixing and high shear stresses during compaction; and rheological testing is used to determine the mix production and compaction temperatures.
- Advera® WMA
Advera WMA is a product of PQ Corporation, Malvern, PA. It is a manufactured synthetic zeolite (Sodium Aluminum Silicate) which has 18-21 % of its mass as water entrapped in its crystalline structure. This water is released at temperatures above 210° F. The water is released when the zeolite contacts the heated asphalt binder which creates a foaming of the binder in the mix. This amount of water, > 0.05% on the mix, allows improvement in workability of the asphalt mix, with minor binder volume increase. Production and placement temperatures are typically 50° F – 70° F lower than conventional hot mix asphalt.
PQ Corporation recommends the addition of 0.25% by weight of the mix, or 5 pounds of Advera WMA per ton of asphalt mix. PQ Corporation states that their technology works with dense, gap and open graded mixes and workability improvements can occur with polymer modified mixes and mixes containing higher recycled asphalt pavement (RAP) contents. Since Advera WMA is an inorganic material it does not change the performance grade of the asphalt binder. Advera WMA is manufactures in plants located in Jeffersonville, Indiana, USA and Augusta, Georgia, USA. It is available in bags, bulk bags (supersacks), and bulk delivery by truck and rail.
- Asphaltan B® (not used in the United States)
Asphaltan B is a product of Romonta GmbH, Amsdorf, Germany. It is available in granular form in 25 kg bags. Created specifically for "rolled asphalt", Asphaltan B is a mixture of substances based on Montan wax constituents and higher molecular weight hydrocarbons.
Crude Montan Wax is found in Germany, Eastern Europe and areas of the USA in certain types of lignite or brown coal deposits that have formed over millions of years by the transformation of fossilized sub-tropical vegetation that flourished in the Tertiary Period. Wax which once protected the plant leaves from extremes of climate did not decompose, but instead enriched the coal. Due to its high stability and insolubility in water, the wax has survived over long geological time periods. After mining, the Montan Wax is extracted from the coal by means of a toluene solvent that is distilled from the wax solution and removed with super heated steam. Romonta GmbH has a global market share of 80 per cent in the crude mined wax products sector.
Romonta recommends adding Asphaltan B at 2 to 4 percent by weight. It can be added to the asphalt mixing plant or directly at the binder producer and can also be added to polymer-modified binders. The melting point of Asphaltan B is approximately 210° F. Similar to FT waxes, it acts as an "asphalt flow improver" with associated reduced production temperatures. Romonta does not specify how much the production temperature can be lowered. Like FT waxes, Romonta also reports increased "compactibility" and resistance to rutting.
Current Status of Activities
The National Asphalt Pavement Association (NAPA) in cooperation with the Federal Highway Administration (FHWA) proposed a research program to investigate the performance of WMA products. Initial research was conducted on the feasibility of using these technologies in the United States through a cooperative agreement between the National Center for Asphalt Technology (NCAT) and FHWA. This study also included additional monetary support from NAPA and the individual technology providers. This initial research has been completed and the findings have been published by NCAT. The reports can be found on their website published as:
- Report 06-02 "Evaluation of Evotherm for Use in Warm Mix Asphalt"
- Report 05-06 "Evaluation of Sasobit for Use in Warm Mix Asphalt"
- Report 05-04 "Evaluation of Aspha-Min Zeolite for Use in Warm Mix Asphalt"
Although the initial research focused on the Aspha-Min®, Sasobit®, and Evotherm™ products, future research in this area is rapidly expanding to include the other WMA technologies.
Warm Mix Asphalt Technical Working Group (WMA TWG)
A WMA TWG was initiated by NAPA and FHWA. The mission of the WMA TWG is to evaluate and validate WMA technologies and to implement proactive WMA policies, practices, and procedures that contribute to a high quality, cost effective transportation infrastructure. The WMA TWG will foster an environment where transportation officials from government and industry share information on new, innovative, or proven WMA technologies and validate those technologies which will safeguard the transportation infrastructure of the United States and ensure that transportation funding and programs are efficiently and effectively utilized. The purpose of the WMA TWG is to proactively provide national guidance in the investigation and implementation of the concept of WMA technologies in the United States; to identify, review, validate, and provide technical WMA guidance that will provide a product with the quality, cost effectiveness and performance at least equal to conventional HMA with the benefit of reduced production and placement emissions; to discuss WMA technology problems and develop solutions to the problems. The WMA TWG is a group of individuals organized to discuss and individually respond to the subject of Warm Mix Asphalt. It is made up of representatives from the Federal Highway Administration (FHWA), the National Asphalt Pavement Association (NAPA), State Highway Agencies (SHA), State Asphalt Pavement Associations (SAPA), American Association of State Highway and Transportation Officials (AASHTO), the National Center for Asphalt Technology (NCAT), the Hot Mix Asphalt Industry, Labor, and National Institute for Occupational Safety and Health (NIOSH). Further information on the WMA TWG is found at http://www.warmmixasphalt.com/.
The WMA TWG submitted three high priority research needs statements that were combined into two projects by the National Cooperative Highway Research Program. Additional research needs statements have been submitted for consideration in fiscal year 2009.
NCHRP Project 09-43
The National Cooperative Highway Research Program (NCHRP) has approved research Project 09-43 "Mix Design Practices for Warm Mix Asphalt Technologies" for fiscal year 2007. The objective of this research project is to develop a performance based mix design procedure for warm mix asphalt in the form of a manual of practice. Additional information can be found at NCHRP's webpage http://www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID=977.
NCHRP Project 09-47
The National Cooperative Highway Research Program (NCHRP) has approved research Project 09-47 "Engineering Properties, Emissions, and Field Performance of Warm Mix Asphalt Technologies" for fiscal year 2008. The objectives of this project are to (1) establish relationships among engineering properties of WMA binders and mixes and the field performance of pavements constructed with WMA technologies, (2) determine relative measures of performance between WMA and conventional HMA pavements, (3) compare production and laydown practices and costs between WMA and HMA pavements, and (4) provide relative emissions measurement of WMA technologies as compared to conventional HMA technologies. Additional information can be found at NCHRP's webpage www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID=1625.
Continued FHWA Involvement and Support
The FHWA Office of Pavement Technology is actively involved with all these projects and demonstrations and is also working in cooperation with FHWA Turner-Fairbank Highway Research Center's (TFHRC) Bituminous Mixtures Laboratory to develop and monitor WMA demonstration projects and research and also to advance the knowledge and state of practice of these materials and technologies. FHWA will actively support further WMA research and validation through material sampling and performance testing provided through the Office of Pavement Technology's Mobile Asphalt Mixture Testing Laboratory (MAMTL) in addition to continued WMA research activities at TFHRC.
The MAMTL was on site for material sampling and testing and mixture performance testing with the Asphalt Mixture Performance Tester (AMPT) during several WMA projects:
A Missouri Department of Transportation WMA demonstration project on Hall Street in St. Louis. This demonstration project included construction of asphalt pavements using the Aspha-Min®, Sasobit®, and Evotherm™ products by Pace Construction. A large amount of volumetric and performance specimens were immediately fabricated for testing in addition to loose mix sampled for later testing to investigate any effects due to reheating and residual moisture.
A Colorado Department of Transportation WMA project on Interstate 70 near Dillon, CO. This project included construction of pavements using the Sasobit®, Advera®, and Evotherm™ products. The project location was on the uphill, east bound lanes as the elevation climbs between 8,800 and 11,100 feet toward the Eisenhower Tunnel.
A Federal Highway Administration, Western Federal Lands Highway Division project in Yellowstone National Park. This project placed a total of 30,000 tons of asphalt mixture split between a traditional hot mix asphalt control section, Sasobit®, and Advera® products. The asphalt mixture haul distance was between 50 and 55 miles from the mixture production plant.