Two years ago, Texas' first contracted perpetual pavement project was completed in the Waco District on Interstate 35 on the north side of Waco near Craven Avenue. According to the Texas Department of Transportation's (TxDOT) Waco District Director of Construction Duane Schwartz, that pavement has measured up to expectations, and the district has now begun a second perpetual pavement project on I-35 north of Hillsboro where I-35 East and I-35 West meet south of Dallas-Fort Worth.

Perpetual pavement is not a single design, but rather it is a concept that gives engineers the ability to design for specific modes of distress by incorporating rut and crack resistant characteristics into base layers that should have a lifespan of at least 50 years, with only the ride surface requiring periodic replacement.

During a Texas Transportation Commission meeting a couple of years ago, TxDOT Waco District Engineer Richard Skopik stated that "The intent of the district was to design a pavement structure that would be equivalent to concrete pavement, particularly in terms of embankments. It would match up against concrete and be a design that would be intended for a heavy-duty, heavy-volume surface much like what we have on I-35. The result of that endeavor plus development of stone-on-stone type asphalt pavement specifications at the state level provided for inclusion of the full-depth asphalt pavement design that we jointly developed with industry and academia into an I-35 reconstruction project, expansion project north of Waco in Lacy-Lakeview."

The perpetual pavement project on Waco's north side was constructed by Young Construction/Knife River Corp., Inc., of Waco, built on a 12-inch-thick lime-stabilized subgrade underneath a 6-inch flex base. The 22-1/2-inch-thick pavement structure started with a rich bottom layer consisting of 4 inches of Type B with a 98-percent lab density requirement to make it waterproof and fatigue resistant. The load bearing layer is 12 inches of 1-inch nominal gradation Superpave, with a 3-inch layer of 3/4-inch gradation Superpave above that. The rut resistant layer is 2 inches of Stone Matrix Asphalt (SMA), and the surface layer is a 1-1/2-inch-thick Permeable Friction Course (PFC) with high air voids.

Skopik says that the most noticeable aspect of the design is the PFC ride surface, because "this surface produces a very quiet ride, it's very noticed by the public and it minimizes tire spray in wet weather. The best illustration is to just drive it when we've got a wet period. These two things become an advantage not only to the safety but to the environment in terms of noise."

The second perpetual pavement project in the Waco District, currently under construction on I-35 north of Hillsboro, is also being built by Young Construction, Inc./Knife River Corp., and is expected to be completed in 2008.

Kirk Krause, TxDOT's Waco District engineer in the Hillsboro area, says, "This is the second project using a full-depth hot mix perpetual pavement section in the Waco District. As we construct this project, additional testing is being performed on these asphalt lifts by researchers from TTI as part of their study to determine improved mixed designs and better criteria for the pavement design for perpetual pavements."

On the current project, existing pavement is being milled to a depth of 8 inches. The existing hot mix pavement is being milled to produce material that will pass a 1-inch sieve, while existing continuously reinforced concrete pavement (CRCP) is being removed to produce material that will not be larger than 12 inches by 12 inches by 10 inches. Up to 30 percent of the recycled material being produced is allowed to be used in the Type B asphalt base and is also being used to backfill pavement edges. The remaining recycled asphalt and concrete will be salvaged for use by TxDOT maintenance crews.

The perpetual pavement project on I-35 in the Hillsboro area is a 30-year design which is being built on a 12-inch lime treated subgrade with a 6-inch flex base above it. The perpetual pavement layers begin with 4 inches of 98-percent density Type B HMAC (PG 64-22) as a fatigue layer to prevent bottom cracking and keep water out. Next is a 12-inch-thick layer of 1-inch stone-filled hot mix asphalt (HMA) (PG 70-22) in three 4-inch lifts. Above that is a 3-inch layer of 3/4-inch stone-filled HMA (either PG 70-22 S or TR). The top two layers are 2 inches of 1/2-inch heavy-duty SMA (PG 76-22 TR) and finally, a single 1-1/2-inch PFC ride surface consisting of Class A Aggregate and PG 76-22 TR asphalt. Lime is used as an additive at a rate of 1.0 percent by weight of the total aggregate.

This project also includes the removal of five existing bridges, two of which have already been removed by Stomper Demolition Co., Inc., of Euless, and the construction of two new bridges. Traffic has been rerouted off the mainlanes twice for bridge demolition and will be rerouted one more time for removal of the remaining bridges.

TxDOT's Kirk Krause explained a test involving three projects in this area, "This perpetual pavement section is being placed in the vicinity of a recently completed 14-inch CRCP section with similar traffic, and there is another project that recently started between these two projects that will have 14-inch CRCP as well as the implementation of a 2-mile research project using 9 inches of post-tensioned concrete pavement. These three different pavement sections constructed in a relatively close time frame with very similar traffic will provide an excellent comparison of the life span and costs for the different types of pavement sections over time."