Texas Goes Perpetual on I-35

How Gilbert Texas Construction met compaction specifications for a 16-inch, four-lift pavement and an 8-inch stabilized base.

The Texas Department of Transportation has wasted no time implementing perpetual pavement materials and specifications for major reconstruction projects in its federal aid highways. One of the first such projects undertaken in 2002 is an 8-mile, two-lane section of Interstate 35 in Cotulla, about 60 miles north of Laredo on the Mexican border.

In 2001, average daily traffic for this section was 12,400 vehicles, with trucks comprising about 36% of the traffic. However, traffic volume is picking up every year, partly because of the NAFTA-inspired cross-border trade, and DOT projections call for average daily traffic counts of 23,100 vehicles by 2021.

This volume, plus the hot, dry climate of south Texas, convinced the state DOT to replace the existing asphalt pavement with a roadway designed along the guidelines the asphalt industry has created for long lasting, or perpetual pavement. The I-35 specifications call for a 16-inch-thick, four-layer, five-lift asphalt pavement over an 8-inch stabilized subbase.

Gilbert Texas Construction of Fort Worth started the job by removing 81,347 square yards of old asphalt pavement, and hauling it away for recycling. Following that, the company stabilized the 8-inch subbase with lime, compacting it with a 30-ton Dynapac CP271 pneumatic roller, followed by a 37,000-pound, 84-inch-wide Dynapac CC722 tandem roller.

The first asphalt lift in a perpetual pavement is designed to provide fatigue resistance to the pavement. For the Cotulla project, the 2-inch base layer was a Superpave mix using a nominal maximum aggregate of 0.5 inches and Performance Grade 64-22 binder. The lift was compacted to 93.5% average density using one vibratory and two static passes with the CC722, and a final static pass with a 26,100-pound, 77-inch-wide CC522 double-drum roller.

The intermediate layer is designed to lend rut resistance to the pavement. The Cotulla project’s intermediate layer measured 8-inches thick and was done in two 4-inch lifts of stone-filled asphalt using a 1-inch maximum aggregate and a stiffer PG 70-22 binder. The lifts were compacted to 95.5% average density with a rolling pattern that started with the CP271 pneumatic roller, two passes with the CC722 double-steel-drum roller operating at 2,700-vpm, and four passes using two CC522 double-drum rollers.

Between the intermediate layer and the wear course was a 3-inch-thick stone-filled asphalt lift using 0.75-inch aggregate and PG 76-22 binder, which doubled the stiffness of the intermediate layer. To achieve an average 93.7% compaction density with this mix, the contractor used the pneumatic roller for breakdown, then four 2,700-vpm passes with the CC722, followed by a total of four static passes with the CC522 rollers.

For a wear course, TxDOT specified a 3-inch stone matrix asphalt design for both rut and wear resistance. The mix used 0.75-inch aggregate and a PG76-22S binder, along with cellulose fiber and mineral filler to add durability. Gilbert Texas Construction achieved 95% average density — and qualified for a TxDOT bonus — by using six passes with the 84-inch CC722 at 2,700 vibrations per minute and an amplitude of 0.028 inches in the mix’s tender zone. After the mat temperature dropped to 200 degrees F, one of the CC522 rollers made a vibratory pass followed by the second CC522 in static mode. Rideability requirements for the pavement were exceeded.

Scott Roe, Gilbert project manager, explained the company’s selection of compactors, saying, “We normally would only have two double drums on hand, but we felt three would give us the edge on the mix design and allow us to stay up with the paver.” Roe felt the 84-inch CC722 was especially well suited to the project, due to the combination of its heavy weight, drum width, and large 60-inch-diameter drum. “A roller that combines heavy operating weight, a wide drum, and a large drum diameter helps you manipulate the mat without pushing it,” said Roe. “That’s a must on this job.” 

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