By Dr. Mark Buncher, P.E
A number of new technologies have been developed that allow asphalt mixtures to be produced and placed at significantly lower temperatures. These technologies are called warm mix asphalt (WMA). With these technologies, temperatures can be reduced by as much as 30 percent while still allowing the asphalt binder to adequately coat the aggregate during mixing at the plant and achieve the desired workability at the paving site. This is accomplished by reducing the viscosity and increasing the workability of a given asphalt binder at a given temperature.
Potential Benefits of WMA
WMA’s most often mentioned benefits regarding reduced temperatures during mix production are:
- Decreasing the energy and fuel consumed to make hot mix asphalt (HMA)
- Reducing possible emissions and odors from plants
- Improving the working conditions at the plant and paving site.
Reducing emissions is especially critical in and around non-attainment areas such as large metropolitan areas and cities that have tight air quality restrictions.
Other potential benefits of WMA are less recognized. Some contractors and agencies are starting to explore the use of WMA in order to extend the paving season in cool weather. Depending on location and climate, paving projects are typically prohibited after a certain calendar date because colder temperatures lessen the time available for the rolling operation to compact the mix to the desired in-place density. With WMA, mixes can remain workable at cooler temperatures, increasing the time available for compaction. This may make WMA a feasible option for those end-of-season projects that must be completed before winter, without compromising quality due to Mother Nature.
Another potential benefit may be the in-service pavement performance. While the many performance questions relating to WMA have just started to be explored, one potential benefit relates to the mix not being exposed to the elevated production and placement temperatures typical of HMA. Less oxidative hardening of the binder takes place with WMA, possibly reducing a mixture’s susceptibility to aging and cracking. Of course, with less hardening comes the potential for greater susceptibility to early rutting until the pavement has oxidized somewhat in service. Strategies need to be developed for determining when it is appropriate to select a higher performance grade to address this issue.
One Missouri contactor found that using WMA eliminated a recurring problem that occurred when they placed a HMA overlay over cracks that had recently been sealed. When laid, the hot mix would generate steam from moisture that was trapped over the fresh sealant causing a “snaking” effect into the new surface. By using WMA, no steam was generated and no more “snakes” showed up in the finished overlay.
Various WMA Technologies
WMA was first developed and used in Europe through three distinct technologies, all of which are proprietary.
- Aspha-Min®(a.k.a. zeolite): A synthetic zeolite is added at the plant at the same time as the binder during mixing. The zeolite releases internal water to microscopically foam the binder, decreasing the viscosity of the binder and increasing the workability of the mix.
- WAM Foam®:A two component system that introduces a soft binder and hard foamed binder at different stages during plant production.
- Sasobit® or Asphaltan B®:Organic additive compounds such as a small crystalline paraffin wax or a low molecular weight esterified wax.
WMA first came to the United States in 2002. The Aspha-Min and Sasobit products have been used here, as well as a fourth technology called Evotherm™. MeadWestvaco Asphalt Innovations, of Charleston, South Carolina, developed Evotherm, a nonproprietary technology, especially for WMA. This chemical package includes a dispersed asphalt (emulsion) technology that improves coating, workability and adhesion. No plant modifications are required for using Evotherm.
WMA Technical Working Group
The Federal Highway Administration (FHWA) and the National Asphalt Pavement Association (NAPA) formed a national WMA Technical Working Group (TWG) in early 2006. Members of the WMA TWG include representatives from several state highway agencies, state asphalt paving associations, HMA contractors, and other industry groups such as the Asphalt Institute, National Center for Asphalt Technologies, American Association of State Highway and Transportation Officials, etc. Matthew Corrigan from FHWA and Dave Newcomb from NAPA serve as co-chairs. While there is formal membership to this group, all the meetings are open to the public and typically attended by many industry guests. There were three meetings in 2006.
The mission of the TWG is to implement proactive WMA guidance, policies and procedures to evaluate and implement WMA technologies that contribute to high quality and cost effective pavements. Specific goals include:
- Technology transfer and implementation
- Research needs
- Procedures for product and material approval
- Testing and performance measurement protocols
- Guidelines for mix design and construction
One significant document that the WMA TWG has developed is called Frame work for Warm Mix Asphalt Trials. In order to maximize the benefit from the many trial WMA sections being built across the United States, agencies and contractors are encouraged to collect certain information that can be later shared with others for comparisons and analyses. This document provides a framework for a desired level of uniform information and testing data collection. The pertinent data to be collected as part of a WMA trial covers the project summary, material properties, production, laydown and testing.
This document, as well as other information on the WMA TWG and WMA in general, can be found online at www.warmmixasphalt.com.
NCHRP Project 9-43
The National Cooperative Highway Research Program (NCHRP) has just recently awarded Project 9-43, Mix Design Practices for Warm-Mix Asphalt Technologies. The objective of this $500,000, 36-month research project is to develop a performance-based mix design procedure for WMA in the form of a draft AASHTO recommended practice. The method will be based on Superpave, include a suite of performance tests, and be applicable to any of the WMA technologies. The project also includes the development of a half-day workshop.
More information on this project can be found at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=1625.
WMA Technologies Panel – European Scan
Sponsored by FHWA, AASHTO and NCHRP, a panel of about a dozen representatives from U.S. state agencies, FHWA and industry will visit Norway, Germany and France in May 2007 to meet with their European WMA counterparts. The purpose of the scan is to evaluate WMA technologies, materials and practices in Europe, and to determine variations from normal mix design practices and compatibility with current U.S. practices. Specific topics of interest are:
- What processes, materials and construction practices are used with WMA
- How do they differ from the U.S. practices
- How are WMA pavements performing in terms of rutting, cracking, etc.
- What type of roads and conditions is WMA best suited for
- What does the future hold for WMA.
Wayne Jones, AI Field Engineer in Ohio, will be representing the Asphalt Institute on the tour.
While the various WMA technologies seem to offer promising benefits, many questions need to be answered regarding mix design, performance and cost before their use becomes more conventional in our industry. Because of the variety of products and processes involved, this is no small challenge. National initiatives such as the WMA Technical Working Group, NCHRP 9-43, and the European Scan Team will hopefully provide many of the answers.
|Mark Buncher is the Asphalt Institute’s Director of Engineering Services.