By Mark Rea

The Texas Department of Transportation - always exploring more cost-efficient ways to get motorists to and from their destinations - has completed its first project using perpetual asphalt pavement.

On a 2.56-mile stretch of Interstate 35 just north of Waco, TxDOT in mid-June completed a roadway that has a 20½-in. hot mix asphalt concrete pavement, believed to be among the thickest ever recorded on a hot mix project in Texas.

Costing nearly $21 million, the project will serve as a pilot program for TxDOT as it searches for alternatives to the concrete pavement used throughout much of the state's high volume roadway system

"A big part of why we did this project was the interest of the Texas Asphalt Paving Association," said Duane Schwarz, TxDOT's Waco district director of construction. "Along with other parties in the hot mix industry, the state association wanted to develop a bituminous design that would compete against concrete on roadways like Interstate 35.

"The association knows about all of the future work we have scheduled for IH 35 and other major roadways throughout the state, so it got together with highly qualified industry experts, and designed the pavement structure for this particular project."
To develop the unique blend of design mixtures for this project, TxDOT worked with a group that included Dr. Jon Epps, professor emeritus of the University of Nevada at Reno and past president of the executive committee of the International Society for Asphalt Pavements, the Asphalt Paving Institute and Charles Smoot, director of technology for TxAPA.

After removing the center grass median for additional northbound and southbound lanes, crews placed 10 in. of lime-treated subgrade atop an additional 6 in. of base material for the new pavement. The result will provide IH 35 with three lanes of traffic in both directions divided by a concrete median.

The pavement itself consists of six courses of hot mix material, each featuring its own unique mixture.

Atop the subgrade and base material is 4 in. of Superpave hot mix asphalt concrete pavement designed to contain high asphalt content and low air void. The mix carries a 98 percent laboratory density and a performance graded binder rate of 70 degrees to minus-22 degrees Celsius. (That computes to a temperature range the mixture can withstand of 158 to minus-7.6 degrees Fahrenheit.) It is virtually impermeable to ground water and is designed to resist any fatigue cracking as well as withstand nearly every adverse weather condition.

"This particular mixture has additional asphalt to make it more flexible and resistant to fatigue," Schwarz said. "The higher density is something TxDOT doesn't normally specify. Normal specs call for 96 percent lab density and this has a 98."
Two separate layers of 1-in.-sized stone-filled hot mix asphalt concrete pavement goes atop the Superpave. These layers - 5 in. each - have the same PG binder grade used in the first layer and are 4.6 percent liquid asphalt.

Next comes 3 in. of three-quarter-inch-sized stone-filled hot mix asphalt concrete pavement with 4.8 percent liquid asphalt followed by 2 in. of half-inch heavy-duty stone matrix asphalt, or SMA, concrete pavement with a higher asphalt rating of 6 percent. The latter mixture includes cellulose fibers and a mineral filler of very fine dust to help fill voids between rocks to keep moisture out. The mixture features only 4 to 5 percent air voids.

Finally, 1½ in. of permeable friction course, widely known as PFC, made with cellulose fibers, completes the perpetual pavement. All of the rocks in the final mixture are one size, which helps the layer to have stone-on-stone contact, but about 20 percent air voids. This layer is so porous that surface water can trickle through. But once moisture reaches the layer of SMA, it does not seep through and flows to the edges and out of the pavement, leaving a water-free surface on top to reduce hydroplaning.

Break In Schedule

Work crews from Waco-based Young Contractors Inc. began completing the final phase of the project in early May by placing the top PFC layer.
The project began in January 2001 and was originally designed to be completed in late 2002.

"We completed the SMA portion of the project last year right around Thanksgiving," said Young Contractors project superintendent Homer Pricer. "But our engineer recommended not trying to lay the final PFC in the wintertime because that mix is so porous."

"And TxDOT agreed with that assessment. We didn't want to risk that the mixture could flash-cool and create too many problems laying and compacting the mix. So with the cold weather and holidays coming, we decided to shut down the job until we got to warmer weather."

Schwarz added that the specifications "require warm temperatures - 60 degrees surface temperature - and the material can cool fairly quickly. Since we were doing the work at night, it was going to be hit or miss in terms of whether or not the temperature would be warm enough. We felt that we would alleviate a lot of headaches if we just shut the paving down and returned in warmer weather. It was a TxDOT decision that came down to wanting the project completed correctly rather than just getting it finished."

To further ensure perfect cooling conditions for the mixture once the project resumed, Young Contractors finished paving at night after completing most of the other portions of the roadway during daylight hours. Workers typically began to close lanes on IH 35 about 9 p.m. while the first hot mix-laden trucks began to appear at the project about 11 p.m.

"We usually got off the road by 6 a.m. before the morning commute began," Pricer said. Work on the northbound lanes of IH 35 was completed first and opened to traffic before crews turned their attention to the southbound lanes.

While installing the final PFC layer, crews used approximately 700 to 800 tons of hot mix asphalt from Young Contractors' own hot mix plant, located about eight miles from the project site. The entire project contains more than 167,000 tons of HMA.

The new, seamless 12-ft.-wide main lanes were placed over the same base course used when this section of IH 35 was built in 1967. The existing 10 in. asphalt courses were removed and some was recycled as part of the roadway shoulder while the rest was stockpiled for later use in other projects.

New Mixes, Challenges

Working with advanced mixes led to some unique challenges for work crews, especially in the area of machinery maintenance.

"Just learning how to put down the different kinds of mixes and prevent clogging in our machines was the one of the major differences as opposed to working with what we consider normal hot mix," Pricer said. "Some of this new asphalt is extra sticky, and it doesn't fall off the belt of the transfer machine like other mixes, so clean-up was sort of a trial-and-error process.

"For example, when we began one of the courses, we had to pull the transfer machine offline three to four times a day just to clean it. As the project continued, though, we didn't experience the same problems. We learned a lot as we went along. As far as the actual laying of the mixes was concerned, asphalt is asphalt. It all goes down on the course pretty much the same."

On the final PFC course, Young Contractors used several different pieces of equipment, including transfer and laydown machines and two steel rollers. A water truck was also employed as well as a pickup truck to pull the power plant for the portable lights. Tying in bridgework involved loaders, motor graders and brooms. Each piece of equipment had its own individual operator with an additional 14 workers on the laydown crew to help fill in the course and perform clean-up tasks.

When the SMA course was installed, Young Contractors utilized additional rollers.
"In addition to the breakdown roller with a steel wheel, we used a pneumatic roller and then a finish roller," Pricer said. "The pneumatic roller smoothes the course a little better and brings up some of the fine material to the surface to cover the other larger aggregate."

More Perpetual Projects?

Schwarz said that from his perspective, "The jury is still out on the perpetual pavement concept. I don't know whether TxDOT will continue to do a lot of this or not. I would suppose a lot will depend upon how this particular project is received."

Nevertheless, there is already another perpetual pavement project on the design table. TxDOT is scheduled to let a perpetual pavement project in August for a stretch of IH 35 near the point in Hill County north of Hillsboro where the roadway splits to go northwest toward Fort Worth and northeast toward Dallas

"It will be almost identical to this design," Schwarz said. "We have picked up a lot of excellent information from this project for other locations, such as using the SMAs and PFCs in other combinations. And while those projects would not be considered perpetual pavement roadways, there has been a lot of valuable data and experience gained from working with these particular mixes."

PROJECT TEAM

GENERAL CONTRACTOR/HOT MIX SUPPLIER: Young Contractors Inc., Waco
OWNER: Texas Department of Transportation
LOCATION: Interstate 35, Waco
AGGREGATE SUPPLIER: Hanson Aggregates, Davis, Okla.