Asphalt innovation

New technology makes asphalt more competitive, longer lasting

By Angelle Bergeron

In Louisiana, researchers relentlessly concoct asphalt mixes suited to the state's unique environmental and usage demands. Technology, materials and new application procedures and equipment allow asphalt to be tailor-made for virtually every sort of application.

"I've been studying asphalt since 1987, and over the years technology has advanced so much that it allows us to apply tools for the characterization of asphalt mixtures," said Dr. Louay Mohammad, director of the Engineering, Materials, Characterization Research Facility at the Louisiana Transportation Research Center.

"That will allow us to understand the load and viscosity over a range of temperatures and densification properties of a sample while it is compacting. If we can engineer the right material, we can prevent failure."

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This month, LTRC kicked off a new study funded by the National Cooperative Highway Research Program. The study, called Optimization of Tack Coat for HMA Placement, will determine the optimum application methods, equipment type, calibration procedures, application rates and asphalt-binder materials for the various uses of tack coats.

"We want to come up with a best material that will allow the layers (of asphalt applications) to act as one monolithic system," Mohammad said. "The idea is to improve pavement performance."

Density is usually difficult to achieve with one layer, but finding the appropriate glue to hold the layers together is also a challenge. Cracking and slippage of whole layers are typical problems that LTRC is hoping to remedy through this study.

Huey Stockstill, a Picayune, Miss., contractor, recently applied a test section of trackless tack coat on La. Hwy. 3235.

"It's a polymer modified tack coat that sets up in about 10 minutes and doesn't track up," said Sam Cooper, senior asphalt research engineer at LTRC. The test site marked the first time LTRC used the material, and evaluations were scheduled to be complete by the beginning of this month.

The cure time varies according to the application levels and temperature, but the section actually cured in about nine minutes, said Leo LeBourgeois, project manager for Huey Stockstill.

"They claim you can shoot it days ahead of time and the weather won't hurt it," LeBourgeois said. "We didn't have any rain, so we didn't have a chance to test that, but we did leave it overnight."

The tack test section represented only a small area of the 5-mi. overlay project that ran from Golden Meadow to La. Hwy. 3162 in Lafourche Parish. The $4.1 million contract required application of a very fine mix of Level 1 Superpave, LeBourgeois said.

"They were concerned about the subgrade breaking down, not being able to take the compactive effort, and this mix doesn't require too much compactive effort to get the proper density on the asphalt," he added.

Because of the fineness of the aggregate, the contractor had difficulty obtaining the desired mix.

"We had to redo mix designs >> several times because we weren't able to get the amount of voids in the aggregate that we needed," LeBourgeois said. "We did do some test patterns on the existing asphalt with the rollers to decide what compactive efforts could go into the subgrade, and LTRC came out and tested before we even started laying down the asphalt."

Gilchrist Construction Co. of Alexandria is working on an $18.7 million project on U.S. Highway 171 from Noble to Converse that will also feature Level 1 Superpave.

"We have a good mix design so it shouldn't be a problem," said >> Raymond Boniol, company engineer for Gilchrist. "We will be doing the hot mix ourselves and will probably start laying asphalt in late July or early August. We've done Superpave before, and our equipment is in good shape for it."

L&A Contracting of Hattiesburg, Miss., has a $15 million four-laning contract for a section of U.S. Highway 165 from Pollock to LA 524 that is primarily Level 1 Superpave.

"The town section (through Pollock) is concrete paving, but the rural areas are all asphalt," said Lawrence Salard, project superintendent. The town portion represents only .25-mi. of the 3-mi. job, which also includes construction of five new bridges and removal of three existing bridges.

"U.S. 165 has been a pinpoint area for DOTD and there is construction south and north of us," Salard added.

D&J Construction of West Monroe recently completed a repair and overlay project for DOTD on a section of Interstate 20 using an open-graded friction course that is another test project for LTRC. D&J bid on the Britton Road to Vancil overpass job in October and won the A+B (base bid plus calendar days), $1.8 million, 60-day contract.

The contractor began work in mid-April cleaning the section, sealing cracks and applying a 1-in. overlay of open-graded friction course.

"We were overlaying existing asphalt because it's wheel-rutted and the aggregates have become polished, causing a hazard when motorists apply brakes," said Jerry White, vice president of operations for D&J. "They were having problems with hydroplaning in this particular section."

In 1984, DOTD placed a moratorium on open-graded friction course because of moisture-related problems that resulted in raveling, LTRC's Cooper said.

"The mix really hasn't changed that much, but one of the aggregates gave us a problem in the '80s," he added. "Now we've got the asphalt contents back up to where they were in the late '70s, and with the addition of polymers and fibers we don't have the problems they did back then." Although chosen for durability and reduction of hydroplaning, the open-graded friction course has the added benefit of providing noise reduction.

"It's at least a 50 percent reduction in noise already," White said. "We have asphalt on one end of the site and concrete on the other, and you can really tell the difference when those cars come off the concrete on the other end. And there is virtually no overspray with those big 18-wheelers because the water runs through the OGFC and out the side."

Reduction of splash and spray are the primary benefits of OGFC, but the asphalt mix is limited because it is thin, Mohammad said.

"It can only work if it is put on a good layer to start with," he added..

D&J's White said OGFC is a good product, but it requires a special self-priming layer for application.

"It carries its own tack coat on the paver that is applied before the asphalt," he added. "I had to hire a four-man crew from E.J. Breneman out of Pennsylvania to come in and lay it."

White, who oversaw an ultrathin job last year using another tack coat, said D&J would buy the special paver if the state would let enough work to justify the purchase.

Barriere Construction Co. of New Orleans had to make some adjustments to its Boutte plant to produce a stone-matrix asphalt mix it is applying on Interstate 10 in Metairie.

The SMA placement is the first under the new DOTD specifications but will probably become the norm for interstate jobs because the mix is durable and long-lasting, said Don Weathers, executive director of the Louisiana Asphalt and Pavement Association.

"We've done SMAs in the past, but we didn't have all the goodies we do now, the polymers and fibers," Weathers added. "It's a lot more expensive, but holds up better under heavy loading and will last. That's why they're putting it down on I-10."

Barriere is placing a 2-in. single-lift overlay of the SMA, which is a stone-on-stone contact mix with high asphalt cement content and a great deal of fibers and dust to control the drain-down of the liquid in the mix.

"Ordinarily, the liquid would drain down to the bottom and the asphalt would not last as long. It would come apart," said John Victory, quality control manager for Barriere. "The fibers and dust make it like a chewing gum consistency where it won't drain down."

DOTD will now require the new mix for all projects with 35,000 average daily traffic or greater, said Chris Abadie, materials research administrator for LTRC.

"This is the premium mix, the mix with the longest durability and fatigue life, so you want this on the surface of all your high-traffic areas," Abadie said. "It's the best mix we have but it's also the most expensive, so we have to make a decision where we want to put it."

The 15-year life expectancy of SMA is still half of concrete, but it is less expensive and may be applied in a thin layer, he added.

The consistency of the mix and nighttime application require the contractor to apply it quickly. The contractor established rolling patterns to meet density requirements and closely monitored travel times from the plant to the site.

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For more about research initiatives at LTRC, go to: http://www.ltrc.lsu.edu/