Great strides are continuously being made to improve the longevity of asphalt roads. The advent of mix designs such as Superpave and stone-matrix asphalt enable these flexible pavements to withstand the punishment of ever-increasing traffic loads. Additionally, the relatively new warm-mix asphalt designs now allow producers to be more environmentally friendly while saving on fuel costs through reduced mixing temperatures.
Arguably one of the most exciting recent advancements for the industry is a different approach to designing and building asphalt pavements that result in a longer-lasting road. The core of this perpetual pavement philosophy involves creating an asphalt pavement built to last indefinitely—longer than 50 years—without requiring major structural reconstruction. Only the surface layer will require minimum periodic maintenance through milling and replacement with a new surface course.
Traditional empirical approaches to road design take into consideration the volume of traffic the road will encounter over its intended lifetime, typically 20 years. The asphalt pavement and mixes are designed to withstand a certain number of equivalent single-axle loads (ESALs)—10 million, 30 million, 50 million or possibly higher—before major reconstruction of the pavement is necessary.
The mechanized design
Mike Heitzman, state bituminous materials engineer for the Iowa Department of Transportation, offers this analogy for the empirical approach. “Think of the road as being a wire coat hanger and the traffic is the force that pushes against and bends the wire. The empirical approach designs the wire to withstand a specific number of times it will bend before it breaks.”
The perpetual pavement approach, on the other hand, uses a mechanized design to analyze the effect that tire loads will have on each lift of the pavement. This approach takes actual raw data of current and estimated future wheel loads to calculate the necessary road thickness and the right base, intermediate and surface course characteristics to deliver increased longevity.
Going back to the wire hanger, the perpetual pavement focuses on the forces that will bend the wire and designs the hanger to withstand these forces.
“The concept of perpetual pavement is to design the pavement so the amount of movement will never cause a failure,” explained Heitzman.
The paving contractor tackling Iowa’s first perpetual pavement project is Brower Construction Co., a division of Fred Carlson LLC. This new approach forced a minor adjustment in how the company typically constructs a road.
“We have completed many full-depth reconstruction projects before, but this is our first perpetual pavement design,” said Mike Collins, Brower Construction’s QMA director. “It challenged us to review the mix designs lift by lift to meet the specific requirements of each layer, and we ultimately created four different mixes for the perpetual pavement and 11 for the entire project.”
Perpetual Highway 60
Running north-south through the northwest corner of Iowa, Highway 60 has become an important artery for goods and commuter traffic between Sioux City, Iowa, and the Minnesota border. From a mile south of Ashton to the Sibley, Iowa, airport, eight miles of the highway are being transformed from an outdated two-lane road into a modern four-lane divided highway. More than 200,000 tons of asphalt were required for the project.
The existing highway traffic lanes are being widened to 38 ft and overlaid with fresh asphalt to serve as the two new southbound lanes of Highway 60. Prior to the asphalt overlay, several full- and partial-depth patches of the roadway were completed to reduce the occurrences of reflective cracking. The overlay consisted of a 3-in.-thick intermediate design with a 3?4-in. minus aggregate and a 1 1/2-in.-thick surface course with a 1/2-in. minus aggregate. Both courses were designed for 10 million ESALs and included a PG 64-28 binder.
The two new northbound lanes and all four Ashton bypass lanes were constructed using the perpetual pavement approach. These lanes were laid in six carefully designed lifts. The base course required two different aggregate sizes. Three inches of a 1/2-in. minus base mix were followed by 7 in. of a mix that included a 3/4-in. minus aggregate. Both base designs used a PG 58-28 binder. “The first 3 in. of base material included more sand and binder, allowing it to flex more to reduce the chance of cracks forming at this level,” mentioned Collins. The spec also called for this lift to include a slightly thicker film.
Two-and-a-half in. of a 3?4-in. minus mix with a PG 64-34 binder served as the intermediate lift. When construction concludes this June, 1 1/2 in. of a 1/2-in. minus mix will serve as the durable surface. “The top two lifts consist of more rock and less sand and binder for additional strength,” added Collins.
The 130-working-day, $11.5 million contract started in June of 2005. Before paving commenced, critical sub-base construction had to be completed. From the sub-base up, the road was constructed to withstand all the future loads it will carry.
In a perpetual pavement project, sub-base construction is just as important as designing the right asphalt lift. “We prepped the sub-base with 10 to 12 in. of a modified material trimmed and compacted to an 8-in. depth at grade,” said Collins. Depending on moisture conditions, the sub-base’s California bearing ratio averaged between 63 and 85.
Asphalt mix was supplied by a portable Cedarapids parallel-flow drum mix plant, positioned at the southern end of the jobsite so as to reduce transport distances and time. Since the water table was within 2 ft of the road base along portions of the new highway, all mixes included a 0.75% hydrated lime additive to prevent asphalt stripping.
Brower’s paving train consisted of a Blaw-Knox paver maneuvering a Lincoln pickup machine. The machine transferred asphalt windrows into the paver’s hopper and helped the contractor to establish a continuous paving process. “We used a track paver on the first two lifts for better traction and finished with a rubber-tire paver,” stated Collins.
Initial densities behind the screed averaged slightly higher than 80%. Vibratory breakdown, pneumatic-tire intermediate and static finish rollers were used to achieve final spec densities of 94% at the shoulders and 95% on the main driving lanes. “As part of our QC program, we graph the densities behind the paver and each roller to ensure the crew is meeting spec,” said Collins.
Nothing but zero
Not only did Brower’s crew face challenges posed by the first-ever perpetual pavement project and meeting the spec densities, they also had to contend with the state’s stringent smoothness standards. Upon project completion, the crew’s work will have to stand up to a new zero blanking band smoothness testing standard.
“With a zero blanking band, every bump registered by the profilograph counts against the smoothness spec,” explained Collins. This dramatically changes the final numbers when compared with the state’s previous smoothness standard, which included a 0.2-in. blanking band. “With the old standards, contractors could average readings of approximately 1 in. per mile or better,” said Heitzman. “The zero blanking band standards increase typical smoothness readings to the 15 in. per mile range.”
On Highway 60, Brower’s crew will achieve maximum incentive bonuses if they can keep the imperfections to 6.3 in. per kilometer or better. If final profilograph readings are higher than 24.8 in. per kilometer, Brower will have to grind the imperfections.
“After we paved the base courses, the profilograph registered numbers between 500 and 600 mm per kilometer,” recalled Collins. “After the intermediate course, the numbers dropped to approximately 190 mm per kilometer.” Collins and Brower’s crew are shooting for the max bonus once the final surface lift is finished.
Even with the challenges that Brower faced in constructing the new highway using a different methodology, Collins sees the advantages to the perpetual pavement approach. “By building the road lift by lift to different specifications, we feel like we are building the road of the future,” said Collins. “This type of design could be used anywhere—county roads, state roads or interstates.”
Once final construction is completed this summer, commuters in this northwest section of Iowa will have a smooth and long-lasting road for their travels. “Provided maximum truck axle loads remain the same, the pavement structure on this new section of Highway 60 should not require replacement,” said Heitzman. “The surface layer will last 15-20 years before it will need to be milled and replaced.”