Warming To Warm Mix Asphalt

By Liz Moucka, Photos courtesy of Astec Inc. and the Louisiana Asphalt Paving Association | September 28, 2010

State DOTs are warming up to warm mix asphalt, a process developed in Europe and introduced into the United States in 2002. In Shreveport, LA, Madden Contracting, Astec Inc., Astec Mobile Screens, the Louisiana Department of Transportation & Development (DOTD), and the Louisiana Asphalt Paving Association recently conducted a demonstration/workshop introducing the warm mix asphalt (WMA) process into Louisiana. Over 120 attendees, including contractors from all over Louisiana, southern Arkansas, as well as DOTD engineers, got their first look at this process in late August.

With the price of materials skyrocketing and with government agencies striving to become more environmentally conscious, the timeliness of this showcase could not have been better to raise awareness, according to Chris Abadie, materials research administrator for the LTRC (Louisiana Transportation Research Center).

Benefits of Warm Mix

Warm mix asphalt uses less fuel to reach its lay down temperature of 250 degrees to 270 degrees Fahrenheit, as compared to between 300 degrees and 320 degrees Fahrenheit for hot mix. A decrease in both greenhouse gases and in that distinctive aroma can be attributed to the lower temperature, a factor that will help nonattainment cities meet air quality standards. The lower temperature and reduced odor also create a more comfortable environment for the workers.

Another benefit of WMA is that a higher percentage of recycled asphalt pavement (RAP) can be incorporated into the mix, reducing the amount of expensive oil needed. In this time of environmental consciousness, recycling materials and reducing the consumption of oil are attractive benefits for state and local contracting agencies.

Two factors of this process allow the incorporation of a higher percentage of recycled asphalt pavement (RAP) into the mix: first, fractionating the RAP into smaller sized aggregate and, second, foaming the asphalt, which decreases the viscosity by expanding the asphalt binder and enables better mixing of the RAP.

The Process

There are several approaches to warm mix asphalt; most incorporate a foaming action using water or additives. Madden Contracting decided to go the route of foaming using high-pressure water injectors, which involved plant modification. This is a one-time expense that over the long term will save them the cost of various per-batch additives such as Aspha-Min®, Sasobit® or Evotherm™. Sasobit requires a less expensive plant modification of installing a hopper to inject the wax pellets into the mix.

Madden had the Astec Double Barrel® Green System high-pressure water injector retrofitted onto his Astec Double Barrel dryer/mixer at his Shreveport plant just prior to the late August seminar and demonstration. A second unit has since been installed at Madden's Natchitoches facility.

“We chose foaming because we were getting excellent test results from the foaming and it's a lot cheaper than the chemicals,” said James Madden, co-owner of Madden Contracting. “It takes about a pound of water to a ton of mix.”

Astec and Terex both have retrofit water injectors added as plant modifications to the drum mixer/dryer to inject water into the mix along with the liquid asphalt cement. The injection of water causes the liquid asphalt to foam and expand in volume, and the foaming action helps the liquid asphalt coat the aggregate at a lower temperature.

Foaming reduces the viscosity of the asphalt binder by expanding it, allowing more recycled asphalt pavement (RAP) to be incorporated into the mix, according to Abadie. “It also gives us more time to compact the mixture which allows for improved density, a big plus.”

Partly because of the foaming action, but also because of the better splitting and fractionation using the newer breed of crusher/screeners that further reduce the size of RAP, more of the old road can be incorporated into the mix. A crusher/screener like the Astec ProSizer will run the RAP through its grinder in successive passes until it reaches a smaller size, most optimally 1/2-inch by 1/4-inch and a 1/4-inch-minus product.

“We sent some mix components to the National Center for Asphalt Technology (NCAT) for mixing with the Wirtgen laboratory foaming device just to compare standard mixing with the foamed mixture. LTRC tested the mixture in the Loaded Wheel Tester and found that the foaming action improves the mixing of high-RAP mixtures,” commented Abadie.

Regionally Speaking

Nationally, there have been about 200 WMA projects/test sections of pavement. All the states are researching and experimenting with WMA to some degree. Texas and California have already begun actively adopting warm mix into their asphalt specification provisions.

“MDOT (Mississippi Department of Transportation) is similar to many states in that we are trying the technology but have not fully implemented it for widespread use,” said James A. Williams, III, P.E., state materials engineer for MDOT. “MDOT has done one WMA pilot project in District 1 with one of the several technologies available.”

Tennessee DOTs have begun to let projects that allow WMA within the past year, prompting local asphalt mix producers to research the most cost-effective method of production, and some are beginning to modify their plants. Engineers from the Oklahoma DOT are paying attention to the WMA trend and have attended demonstration sessions. They are in the process of identifying a project that would lend itself to a trial run. The Arkansas DOT is likewise in the early learning stage, but has already allowed two WMA projects thus far at the request of the contractor.

“Texas has already had trials in several areas using the different types of warm mix, so they're way ahead of us in that regard,” said Abadie, “which is fine because we can learn from them.” Warm mix can be adapted to almost any mix specification.

“What I am learning from them is what I believe to be true anyway – the way we should approach adopting this new idea is to ask for mixture performance criteria in the lab and on the roadway and not really worry about the 'recipe.'”

The Future Trend

As long as the contractor's design meets the laboratory rut test and can verify the ability of the mix to resist moisture damage to the DOTD's standards, the DOTD should be satisfied. “LTRC certainly wants to continue testing all warm mix processes,” stated Abadie.

The only problem Abadie sees with incorporating more recycled material in the mix is the availability of recycled material. Not all general contractors have the material available, and not all projects are conducive to removal of great quantities of RAP. If use of warm mix with high RAP content was left as an option for the contractors, it could provide a competitive advantage for those with RAP stockpiles.

LA DOTD plans on featuring warm mix in their Transportation Conference in Baton Rouge Feb. 9-11, 2009. The department hopes to include some enabling contracts to allow contractors to use WMA as early as spring 2009.

Madden Contracting will be ready for the change. “It makes economic sense,” Madden added. “We've been in business 50-plus years, primarily in the highway asphalt overlay business, and this is one of the better things that has happened to us in a long time. It's a win-win. It's not only good for the contractors, it's good for the environment and for the economics of our highway departments. Anybody who wants to stay in business, I don't think they can do anything but adopt this new process.“