A Mixed Bag of Options

TxDOT Districts Sampling Hot Mixes for Road Rehabilitation

By Angelle Bergeron

Gone are the days, it seems, of a single generic hot mix that is applied in varying viscosities and layer thickness.

Recent research underscores the infinite combinations that have become available, depending upon the formula of asphalt, modifiers, aggregates and binders.

"Hot mix can be engineered to meet the application necessary for traffic conditions," said Harold Mullen, executive vice president of the Texas Asphalt Pavement Association. "A lot of that was developed when Superpave came about. That gave us a lot of stone-on-stone contact."

Many of today's hot mixes are highly advanced, surpassing the conventional A, B, C and D mixes of yesteryear for boutique varieties of advanced mixes. For example, stone matrix utilizes bigger aggregates that must pass flat and elongated tests to ensure a closely woven "skeleton" within the mix, Mullen said.

"The liquid asphalt has also changed over the years, moving to a performance-graded binder system that allows you to use a liquid asphalt binder that accommodates for specific temperatures and traffic conditions," he added.

The mix that is predominantly used in Texas is a 64 minus 22, Mullen said. That means it will handle temperatures in that range of degrees centigrade. For higher traffic areas, a 76 minus 22 is used. "In that mix, to make the liquid binders more strength-oriented, they add modifiers," Mullen added.

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He called performance-friction course, the newest golden boy of hot mix, one of the best riding surfaces available. "It reduces water spray, is extremely quiet and, because it has more asphalt, is a darker color so motorists can easily see road stripes," he said. "PFC mixes are turning in ride numbers that are some of the lowest we've ever had," Mullen added, referring to the numbers on the International Roughness Index.

Although access to the technology for hot mixes has been available since the mid-'90s, and some TxDOT districts have been using new mixes since 2000, it is still experimental for many districts, Mullen said.

As rehabilitation needs arise, the individual district searches for ways to accommodate its unique circumstances.

In the Beaumont District work was recently completed in Jefferson County on 6.78 mi. of State Highway 69 South from Beaumont to south of Interstate 10 to Port Arthur.
Koch Pavement Solutions of Wichita, Kan., supplied Nova Chip, a brand-name PFC, for the $1.68 million project. The asphalt-over-concrete rehabilitation included 228,000 yds. of the product and 170,000 lbs. of fiber-reinforced polymer patching, said Kal Kincaid, vice president of estimating and marketing for Apac Texas Inc., the Houston-based general contractor.

"We were working on existing concrete pavement that was not in pristine condition," and some surface patching was needed, Kincaid said. "The subcontractor installed epoxy material into all spalled areas, sealed large cracks and did surface protection on the concrete so that when we overlaid it, it was as good as it could be."
The application process required special equipment that is uncommon for contractors to own. Fortunately for Apac, its sister company in Dallas owns such equipment.

Unlike a regular asphalt paver that lays the material at grade, the Nova Chip application requires a spray-bar configuration that applies a liquid-asphalt membrane immediately prior to placement of the asphalt hot mix, Kincaid said. "It is combining the two steps in one, and the piece of equipment that does that is pretty unique."
The particular PFC was chosen in an effort to reduce hydroplaning. The membrane promotes adhesion, and the application process is designed to minimize reflective cracking from the underlying concrete pavement, Kincaid said.

While TxDOT doesn't have a program in place to test materials, as rehabilitation needs arise, districts sample mixes in different areas to determine if performance matches needs. "We call them test sites or review sites, but not necessarily what we call a test program," said Charles Gaskin, district construction director for the Houston District. "We are constantly testing and evaluating, and then area engineers are asked to find a location, a job where we can use these materials."

New mixes are generally tested in lower-volume areas rather than in heavy-traffic areas such as Houston.

In May the Waller/West Harris area >> office of TxDOT completed the rehabilitation on a section of U.S. Highway 90 from FM 359 to IH-10. Angel Brothers of Houston fulfilled the $388,000 contract by applying a crum-rubber seal coat, topped with 1.5-in. asphalt rubber permeable friction course on the 5-mi. stretch. The project is the first of its kind in the Houston area, said Tony Yrigoyen, design engineer with the district.

"I've been evaluating new types of mixes to check how well they perform under various conditions," Yrigoyen said. "The job we just finished on U.S. 90 looks extremely good."
Yrigoyen is still conducting tests to determine the ideal specifications for the soon-to-be-let $7 million rehabilitation of a stretch of State Highway 6/FM 1960 from IH-10 to State Highway 249. "If the testing meets my expectations, I'll probably use the same mix for that job," Yrigoyen said.

The crum-rubber seal coat application with the PFC was chosen primarily to reduce splash but, since completion of the project, Yrigoyen also noted some noise reduction with the new surface.

He plans to conduct tests before and after the rehabilitation on Highway 6 to determine just how much the PFC does affect sound levels.

A handful of rehabilitation projects are scheduled to be let for summer 2005 because the application requires temperatures of 70 degrees or more, Yrigoyen said.
After Highway 6, the next project scheduled to be let (in late spring or early summer of next year) is an 8.9-mi. section of U.S. 90 from FM 359 to Fort Bend County Road. That will be let in the late spring or early summer of 2005.

In TxDOT's Fort Bend area office, a thin-bonded PFC is being used for the first time on the $5.6 million rehabilitation of State Highway 6 from the Fort Bend Harris County line to U.S. 90A. Richmond-based Silva Contracting Company began work in June 2003 on the project, the largest of seven such projects being done in the district this year.

State Highway 6 is a fairly busy, seven-lane throughway, including three lanes in each direction with a continuous turn lane. "We had received reports of hydroplaning during rainstorms from the general public," said Mike Hobbs, design engineer at the Fort Bend area office.