Built to Last

More Texas Pavements Qualifying for "Perpetual" Designation

By Mary Lou Jay

Two perpetual-pavement projects under way on different stretches of Intestate 35 prove that the asphalt approach to roadbuilding in Texas aims to stick around.

After completing its first perpetual pavement hot-mix asphalt project on a 2.5-mi. section of IH-35 north of Waco in 2003, the Texas Department of Transportation continues to use the extra-depth mixes on several other major highway projects.

The perpetual pavements that TxDOT is constructing today may someday receive honors such as the Perpetual Pavement award recently given for State Highway 173 in Bandera County, which opened in 1963. Shown, crews at work in May on the first-ever pervetual-pavement project for TxDOT's San Antonio District, a section of IH-35 in New Braunfels.

The department hopes to prolong the life or these roads while reducing the need for maintenance.

But one much older highway in the state has already made its mark for longevity. The Asphalt Pavement Alliance has designated a section of State Highway 173 in Bandera County the winner of a 2004 Perpetual Pavement award. Although they use the same "perpetual" terminology, the APA and TxDOT programs are not related.

To qualify for the APA's Perpetual Pavement award, a road must be at least 35 years old, have undergone no major rehabilitation since its construction and have at least 13-year average intervals between resurfacing. The award-winning portion of SH 173, which runs from State Highway 16 to the Kerr County Line, opened to traffic in 1963 and has required only one seal coat, which was in 1987.

The highway was built with a 5-in. flexible base and a .5-in.-thick, two-course surface treatment. It was one of only nine highways in the country to win the Perpetual Pavement designation this year.

"When building this highway, we were able to take advantage of some very good flexible-pavement material sources in the area, including limestone rock," said David Kopp, director of construction for TxDOT. "In addition, the road was laid over a high-quality subgrade. This has helped it last."

"Well-built roadways are the result of good planning, design and construction," said Mike Behrens, executive director for TxDOT. "The public doesn't always see it, but the planning and design work that goes into a project before construction is absolutely critical to a successful project. We couldn't be more pleased with the recognition this award brings."

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Building the next award winners?

The perpetual pavements that TxDOT is constructing today may someday receive similar honors. "We're building thick asphalt pavements that are designed not to fail from the bottom up," said Dale Rand, flexible pavements branch director for TxDOT.

"We normally use a lot of granular base plus 4 to 6 in. of hot mix for our roads. With perpetual pavement we don't put in a granular base. We stabilize the subgrade, then we put in thick, multiple layers of hot mix." TxDOT may have to replace the road surface every 12 to 15 years but should not have to do any underlying work at that time."

Perpetual pavement is not a specific mix, but rather a pavement design concept, he said. It allows roads to be built quicker than with concrete while lasting just as long. "We only place it in areas designed for more than 30 million ESALS - mainly our heavy truck routes."

In the Waco District, Young Contractors Inc. of Waco is general contractor for a $42 million perpetual-pavement project that involves a 4.4-mi. widening of Interstate 35 north of Hillsboro. The road will increase from four lanes to six lanes, with two additional transition lanes for merges.

The total depth of hot mix for the road will be 22.5 in., according to Duane Schwarz, director of construction for TxDOT's Waco district. That doesn't include the 12 in. of lime-stabilized subgrade, followed by a 6-in. base layer of crushed stone.

The perpetual pavement starts with a 4-in. subgrade layer of dense graded hot mix, a .75- in. nominal-size B mix with a PG 64-22 binder. "This is the fatigue resistant layer, made with a 98 percent laboratory density to give it good flexibility and to help waterproof it," Schwarz said.

Above this is TxDOT's version of Superpave, a 12-in. layer of 1-in. nominal-size, stone-fill asphalt concrete pavement with a PG 70-22 binder. "This is our main load-bearing layer," he said. This is followed by a 3-in. layer of .75-in. stone-filled asphalt concrete with the same binder; the smaller aggregate should provide a better ride.

Next is a 2-in. layer of stone-matrix asphalt with .5-in. nominal size and using a PG 76-22 binder with tire rubber modifier. "This is the rut-resistant layer, with 6 percent or more asphalt as opposed to 5 percent or less on the other layers," he said. "We also add cellulose or mineral fibers to help stiffen the binder so that it won't drain down."

The top layer is a 1.5-in. porous friction course with 18 to 22 percent air voids. "The purpose of this is to allow water to drain off the roadway. This drastically reduces the amount of truck spray and improves the visibility of road markings," Schwarz added. "This layer also gives the road a pleasing appearance, since it is uniform in color and texture - two of the big factors that the public notices."

In TxDOT's San Antonio District, the work on a 3-mi. section of IH-35 from the Guadalupe River to FM 306 involves adding lanes in both northbound and southbound directions.

It is the first perpetual-pavement project ever for the San Antonio District. Hunter Industries Ltd. of San Marcos is the general contractor for the approximately $48.5 million contract, the last stretch between Austin and San Antonio to expand IH-35 to three or four lanes on northbound and southbound lanes.

This perpetual-pavement design calls for 21.5 total inches of HMA: 4 in. of Superpave, 12 in. of 25 mm stone fill, 2 in. of 12.5 mm stone fill, 2 in. of high-density hot mix and finally 1.5in. of PFC.

TxDOT and Hunter Industries have been modifying the project's original specifications as they work through it, according to Michelle Kopp, TxDOT assistant area engineer. "Since this was one of the first perpetual pavements done in the state, we had to change our design as we went through to meet current acceptable practices," she said. "For example, we started out with 2 in. of Superpave on the bottom course, but we found that wasn't adequate."

Both IH-35 projects should be completed by late 2007 or early 2008.