Warm Mix Asphalt Studied

By Aram Kalousdian, Editor | September 28, 2010

Warm mix asphalt technology has the potential to significantly reduce mixing temperatures, improve compaction, extend the paving season, reduce fuel consumption, and reduce emissions. Wayne Jones, regional engineer with the Asphalt Institute in Lexington, Ky., was a member of a Federal Highway Administration (FHWA) and American Association of State Highway and Transportation Officials (AASHTO)-sponsored 2007 Scan Team that traveled to Europe to observe warm mix asphalt construction projects. Jones reported on the group's findings at the Asphalt Pavement Association of Michigan's (APAM) 52nd Annual Asphalt Paving Conference in Sterling Heights in February.

The group spent 16 days and traveled to four countries. "The suppliers and contractors in Europe are the ones developing warm mix asphalt and are the driving force behind it. We're anticipating that same relationship to work here in the United States. We don't expect the departments of transportation to mandate warm mix asphalt. We think that once the contractors understand it and get comfortable with it and situations come up where they want to use it, then they will approach the departments of transportation and ask permission to use it. At that point, we hope the states will have an approval process in order to give the contractors approval," Jones said.

The group went to Oslo, Norway; Frankfurt, Germany; Cologne, Germany; Paris, France; and Brussels, Belgium.

"The purpose of the trip was to learn about the technologies with an emphasis on long-term field performance," Jones said. "We identified certain areas we wanted to concentrate on. We wanted to look at the processes, materials and construction practices that are used in Europe, their mix design and construction procedures to see how they differ, and performance.

"What we found in Europe is that the driving force is environmental concerns. They want to be able to maintain their development at a pace that they can live with. Everyone's talking about green construction these days. Certainly, the warm mix asphalt fits right into that category.

"The main driving force in Europe in terms of reduction of CO2 exposure is the Kyoto Protocol. It is mandated as part of the European Union's ratification of the Protocol. However, what we found in Europe is that Kyoto has no direct impact on the hot mix asphalt industry. But because the industry recognizes that they need to be environmentally sensitive, they take a proactive approach. They want to participate in CO2 reduction and reduce energy consumption.

"Another driving force in Europe is something called Registration Evaluation and Authorization of Chemicals (REACH). This is a cradle-to-grave chemical tracking system that took effect in the summer of 2007." The system looks at the minimum amount of chemical exposure that affects the environment. Every chemical must have this number and asphalt is no exception.

"The ongoing research has shown that there is a correlation between temperature and fumes that are generated, so they anticipated negative effect levels for liquid asphalt and hot mix asphalt. Application levels less than 400 degrees Fahrenheit are safe. That's well below the mix temperature of any mix that we put down in this country and the majority of the mixes that they put down in Europe," Jones said.

"So, what's driving this? It's a little different in each country. In Norway, it's contractor/supplier driven; in Germany it's contractor driven, mainly because of a product called goose asphalt; and in France it's contractor driven but it's agency supported. In France, the contractor must get its process approved by the national transportation authority. Once they do that, then they can go out and market it. These processes are only approved if they can prove to be sustainable technologies.

"Goose asphalt is also known as mastic asphalt in Europe. It has high binder content with zero voids. They use it for sidewalks and, believe it or not, buildings, with the logic that once they put this down and it cools down, then they can start building walls for the building and they don't have to wait 28 days for the concrete to cure. They've modified concrete finishing machines to place thin layers of this material on their autobahn. In certain locations such as sidewalks, they sprinkle a chip on it and roll it in. But typically, it's placed by hand at temperatures up to 450 degrees."

Jones explained that the typical hot mix course is placed at above 275 degrees Fahrenheit, and warm mix asphalt is anything between 212 degrees and 275 degrees. Half-warm asphalt is placed at approximately 210 degrees.

"There are three major classes of warm mix technology. There are those that use some form of additive, water or some combination of the two. Generally, the additives are some type of organic wax additive, foaming processes where foam is used to expand the asphalt, and an emulsion-based process. We saw some vegetable-based synthetic binders that can make colored asphalt," Jones said.

"The processes that use wax decrease the viscosity by melting high temperature wax. Having said that, wax has always been a negative point in liquid asphalt. The refineries do everything they can to get the bad paraffin waxes out of their liquid binder before they sell it. But these high temperature waxes act a little differently. If you choose them properly, they melt at a temperature higher than the in-service temperature of the asphalt, which lowers the viscosity and makes it more workable at lower temperatures. The only thing that you must be careful of is that if you choose the wrong wax or load it up too high, you can get thermal cracking in cold weather."

John C. Friend, P.E., Bureau of Highway Delivery director for the Michigan Department of Transportation (MDOT), reported on the department's upcoming construction season.

"As we look at 2008, I really think that we're going to have a pretty solid year at MDOT. Our program is going to be approximately $710 million, but the economic stimulus is going to be approximately $205 million in addition to our program, which is going to include a couple of different categories. There will be approximately $150 million primarily funded through bonds, which will start to show up around the June letting. The intention is to have work take place either this summer or this fall. The remaining approximately $55 million will be used for a concept called design-build-finance. This will include an approximately $50-million road project in Southeastern Michigan and a $5-million bridge project. I think the time frame for that is about June," Friend said.

"I would be remiss if I didn't say that beyond 2008 the department is looking at some long-term funding challenges."

Title 23 of the Code of Federal Regulations, part 637 says that all states must develop a procedure for testing personnel and laboratories used in acceptance decisions for federal aid on projects for the National Highway System (NHS). Curtis Bleech, construction paving engineer for MDOT, explained the regulation.

The basic components of MDOT's hot mix asphalt laboratory qualification program include:

  • Quality assurance laboratory requirements, which include:
    • An American Association of State Highway and Transportation Officials Materials Reference Laboratory (AMRL) assessment and proficiency sampling
    • A completed and available laboratory quality manual
    • An annual evaluation by MDOT's Construction and Technology Division staff for compliance to the quality manual
    • Participation in a Construction and Technology Division-administered hot mix asphalt round robin
    • Employing qualified technicians
  • Quality control laboratory requirements, which include:
    • Completion of a laboratory quality manual
    • An annual evaluation by an internal or external source for compliance to the quality manual
    • Participation in MDOT's Construction and Technology Division-administered hot mix asphalt round robin
    • Employing qualified technicians
  • Hot mix asphalt quality assurance technicians must:
    • Maintain certification through MDOT-approved training at Ferris State University
    • Successfully complete an annual independent assurance test (IAT) for hot mix asphalt for appropriate mixture types (Superpave or Marshall)
  • Hot mix asphalt quality control technicians must:
    • Maintain certification through MDOT-approved training at Ferris State University
    • Successfully complete an annual internal or external evaluation for appropriate mixture types (Superpave or Marshall)

All of the laboratory qualification and technician qualification requirements will be kept in a database at MDOT's Construction and Technology Division. The federal regulation also requires a disqualification process. Failure to correct laboratory deficiencies within 20 days will result in notification to the engineer of MDOT's Construction and Technology Division. A decision will be made on what the next course of action will be. Technicians can be disqualified by falsifying data, failing to meet IAT program requirements, and failing to obtain recertification through Ferris State University.