Warming Up to Cooler Mixes

Advances in warm-mix asphalt technology allow TxDOT to expand its inventory of paving materials while reducing emissions.

By Debra Wood

Nearly two years after first placing warm-mix asphalt paving on a small stretch of road in San Antonio, the Texas Department of Transportation continues monitoring the segment and has begun testing the lower-temperature products on longer, more heavily used highway.

Testing helps pave the way for new warm-mix asphalt applications. Image courtesy NAPA.

“The primary advantage, overall, is less emissions,” says Dale Rand, director of the flexible-pavements branch of the Texas Department of Transportation.

“There’s a huge benefit in the reduction in volatiles coming off the pavement,” adds Ralph Browne, North Tarrant Area engineer in the Fort Worth District, which has a $5 million, 5-mi-long warm-mix project under way.

In addition to decreasing smog-producing emissions at the jobsite, the asphalt plants burn less fuel because warm mix is not heated to temperatures as high as those of hot mixes. Even so, Rand says warm mix increases the cost $1 to $3 per ton, and the state does not want to pay a premium for warm mix.

“The big hurdle is who pays for it,” Rand says. “But the economics are getting more competitive.”

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Rand adds that because warm-mix asphalt is heated to lower temperatures, it does not age as much as traditional hot mixes. He also expects less long-term cracking because the asphalt is not overcooked or overoxidized at the mix plant.

Richard Izzo, a flexible pavements engineer in the materials and pavements section of TxDOT in Austin, says warm mix allows contractors to pave at lower ambient air temperatures, which should extend the paving season.

“We’re seeing performance improvement out of the mix--longer-haul distances and improved compaction,” says Harry Bush, manager of technical services at Vulcan Materials Co. in San Antonio, which provided warm mix with the Evotherm additive on TxDOT’s first warm-mix project in San Antonio. “Compaction is everything for long-lasting pavement.”

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Jonathan MacIver, business development manager for Asphalt Innovations, MWV Specialty Chemicals of Glen Allen, Va., maker of Evotherm, says the chemical additive helps the asphalt become more compactable at lower temperatures.

McIver says the company is working to bring cost down. The lower temperatures at the plant allow producers to cut the energy bill in half, saving about $2 per ton. But, he adds, contractors are not yet sharing that cost savings and they won’t until they have done enough jobs to prove the benefits to themselves.

“Volume will play a critical role,” McIver adds.

And that is picking up. In addition to a few TxDOT projects this year, Myles McKemie, vice president of Ergon Asphalt & Emulsions in Austin, a local supplier of Evotherm, says his company is providing the additive to contractors for county road projects and commercial work.

“In five years, it will all be warm mix.” McKemie says. “[Contractors] will find it to be less expensive and more comfortable for the workers. The benefits will save contractors more than it costs.”

Making warm mix Asphalt producers achieve the lower temperatures through a variety of means, including additives and foaming processes. Bush reports 11 technologies currently exist for producing warm mix. He says that with foaming technologies, the temperature can be reduced about 50 degrees from the 300 degrees to 320 degrees necessary for hot mix.

Pinto Construction Co. of Nacogdoches placed four types of warm mix on a 5-mi. stretch of roadway in Lufkin that the Texas Transportation Institute will monitor to compare to hot mixes. Photo courtesy of TTI.

MacIver says the Evotherm additive lets producers drop the temperature more than 100 degrees below a traditional hot mix.

And the Evotherm warm mix has been hauled for one hour with minimal temperature loss, adds John Morgan, a sales representative with Asphalt Innovations.

The other products being tested in Texas include: Sasobit, with a wax additive; Rediset WMX, with a solid additive; and Advera WMA, which employs a foaming method.

Advera WMA contains about 18% water in a synthetic zeolite that does not change the chemistry of the oil and releases chemically bonded water into the mix as long as it remains heated, says Ed Myszak, warm-mix project director for PQ of Malvern, Pa., the product’s manufacturer.

“The water releases a little at a time to keep the performance of the mix in a workable state,” Myszak adds.

John Shaw, business unit manager for asphalt additives for Sasol Wax Americas of Shelton, Conn., maker of Sasobit, says it allows producers to lower the temperature by 25 degrees to 75 degrees. It does not add any moisture to the mix.

Likewise, Rediset adds no water, says Jim Wright, the Rediset marketing person for Akzo Nobel Surface Chemistry in Texas. It also incorporates an antistripping additive and allows the plant to lower the mix temperature by about 40 degrees.

“All products claim to do the same thing,” says David Kopp, director of construction for the San Antonio District of TxDOT. “It will boil down to ease and cost.”

In collaboration with the Texas Transportation Institute of College Station, TxDOT is researching which products last longer and will perform best overall.

“We want to look at performance to make sure it holds up under traffic-rutting and cracking-and compare that to mix placed at the normal hot-mix temperature at the same job,” says Cindy Estakhri, a TTI researcher.

Fort Worth sees largest Texas warm-mix job yet On U.S. 287 near Saginaw in the Fort Worth District, Sunmount Corp. of Roanoke started placing warm mix with Evotherm on a 5-mi section in early April. The contract calls for milling 2 in. off the top before overlaying in some sections and paving atop the existing surface in other locations. The contractor will use 20% recycled asphalt, not necessarily from this road, in the mix. MacIver says the warm mix will cost $3 more per ton.

Richard Williammee, Fort Worth District materials engineer, says it looks great.

It is Sunmount’s first warm-mix job. Soon after starting, crews realized they could obtain the necessary densities with fewer roller passes, says Jason McLear, general manager for Sunmount.

In addition to the generic benefits of warm mix, the project presented a special need. TxDOT had used crack sealant on the road. Typically, if hot mix is placed atop the sealant, the material tends to expand and create bumps. Estakhri says the cause is unknown, but moisture in the sealant may contribute. She says this project will help the team learn if lower mix temperatures can decrease these problems.

“Rollers have trouble getting density on top of the crack sealant, because it liquefies and is like gum,” says Williammee, adding that the Missouri Department of Transportation has had success with placing warm mix atop crack sealant.’

“When you decrease the mixing temperature, you avoid activating the crack filler,” MacIver says.

North Tarrant Area engineer Browne expects the warm mix will cool more quickly, allowing traffic to start using the road sooner.

Lufkin tries multiple warm mixes In Lufkin, TTI and TxDOT are trying four different warm mixes on FM 324. The approximately $2.5 million portion of a $9.9-million, 5-mi-long project began in summer 2007 and was completed in May.

TxDOT first tested warm mix on Loop 368 in San Antonio. Shown, Dean Ward Co. of New Braunfels lays the pavement, which the Texas Transportation Institute continues to monitor. Photo courtesy of TTI.

Pinto Construction Co. of Nacogdoches placed Advera WMA, Evotherm, Sasobit and Rediset. TTI will monitor the road through August 2009.

“The goal is not to compare one to another to show one is better, but to compare all the products to hot mix,” Estakhri says.

Estakhri says all four products went down well. TTI sampled the mixes and is running lab tests on them.

The Austin District also plans to test warm mix. Asphalt Paving Co. of Austin will apply it to two of four lanes on a $2.3-million, 4.5-mi-long repaving project on State Highway 71 in front of the Austin-Bergstrom International Airport.

San Antonio’s demonstration project TxDOT started testing warm mix in 2006 for the first time on a 2-mi-long project on Loop 368 in San Antonio. Vulcan Materials supplied about 1,200 tons of warm mix and a hot-mix control. Kopp says the warm mix was $6 per ton more expensive on the job than hot mix. Dean Ward Co. of New Braunfels laid the pavement.

San Antonio District director of construction Kopp says the agency began looking at warm mix in San Antonio to help the region meet U.S. Environmental Protection Agency air-quality standards. He adds that when an area falls out of attainment, it makes it harder to complete construction projects because of the additional modeling and testing that’s required. “This was driven by the environmental benefits,” Kopp says. “The results are good, in regard to how the pavement looks and appears to be performing.”

TTI regularly takes core samples from Loop 368 and will monitor it through August 2009.

“It’s performing well,” Estakhri says. “There is no difference between the warm mix and hot mix.”

Later this year, TxDOT will use more than 25,000 tons of warm mix on a $30 million paving project on IH-37, from Loop 13 to South Cross, in San Antonio. The project calls for a test section with Evotherm, a section with Sasobit, a section with one or the other and a fourth section with hot mix. Williams Bros. Construction Co. of Houston is the contractor.

“Warm mix seems promising,” Estakhri says. “Some of these big jobs in Fort Worth and San Antonio will be on high-volume roads and will be a good test to see if it holds up under heavy truck traffic.”