Utah Thick Lift Stands Up to Truck Traffic
In the middle of the Bonneville Salt Flats at the state line between Nevada and Utah lies the Interstate 80 port of entry.
Subject to intense summer sun and temperatures regularly hitting more than 100 degrees Fahrenheit, as well as brutally cold winters where temperatures regularly reach the single digits, the port of entry faces a set of circumstances that would present a challenge to any pavement.
Pair that with heavy truck traffic slowing, stopping, and accelerating through the port of entry day in and day out on its way inland from the California ports, and you’ve got a recipe for pavement distress.
“With all the trucks stopping in the same places, it’s a very difficult place for any pavement to survive out there,” said Howard Anderson, state asphalt engineer with the Utah Department of Transportation (UDOT) for more than 30 years.
“A number of solutions have been tried out there and have shown issues within days,” said Luke Peterson, asphalt quality control manager for the South Wasatch Front for Staker Parson Materials and Construction, a CRH Company, in Draper, Utah.
In early June 2021, Staker Parson executed an innovative test strip at the port of entry, paving a single 6-inch-thick lift of highly polymer-modified asphalt with 1 percent air voids.
It’s a mix design and concept Anderson has been investigating for the past five years, one that seemed well suited to such strenuous circumstances. As UDOT Region 2 Materials Engineer Lonnie Marchant said, “If it works here, it’ll work anywhere.”
I-80 along the port of entry has an average annual daily traffic count of 7,600 per day, with 51 percent trucks.
Lots of Polymer, Less Air Voids
“Normal mix designs aim for 4 percent air voids,” Anderson said. “In Utah, we moved to 3.5 percent (75 gyrations), but on this job we moved to 1 percent with only 50 gyrations. That’s like heresy in the asphalt world.”
Anderson, who studied under Jon A. Epps when he was at University of Nevada – Reno, was aware of the rutting risks associated with air voids below 2 percent. However, he thought aiming for 4 percent air voids was too conservative and based on the binders of the past.
“Back then, they didn’t have polymers to help hold everything together,” Anderson said, adding that UDOT uses polymer-modified binders on all state roads as well as 100 percent crushed aggregates. “Now that we have a stronger ‘glue’ we don’t have to stay at 4 percent air voids.”
In early 2016, Anderson asked UDOT Central Lab Technician Clark Allen to run a series of Hamburg rut tests using 76-34 polymer-modified binder instead of UDOT’s standard 64-34 polymer-modified binder. The 76-34 is two grades higher than UDOT’s standard binder, with twice as much polymer.
Allen ran the tests at binder contents of 4.8 percent (the Superpave mix design content), as well as 5.8 percent and 6.8 percent. “Even with over-asphalting, the mix using a 76-34 binder wouldn’t rut,” Anderson said, even when extra weight was added and another test was run on the same slab. “That just blew my mind because it goes against everything I learned in school.”
What Anderson came to understand was as the binder approaches 7 percent polymer, it begins to perform differently. “It takes on more of the properties of the polymer than the regular asphalt,” Anderson said. “The mix didn’t become sensitive to over-asphalting, like we’d see if we had used a normal binder or even a normal polymer-modified binder. When I saw this, I knew we could be doing things differently.”
This higher binder content, without the elevated risk of rutting, could not only reduce air voids and help mitigate cracking, Anderson said, but it could also enable compaction of thicker lifts of asphalt than what has been possible in the past.
“Our industry usually doesn’t pave thick lifts because it’s hard to get density,” Anderson said, “so we instead pave two lifts with a tack coat in between.” However, Anderson continued, increasing binder lubricates compaction throughout the thick lift. Anderson first presented the idea of paving a rich thick lift using highly modified binder at the WASHTO Conference in Salt Lake City in April 2016. “It took some time and further presentations to get such a radical idea on a project,” he said. Eventually, an opportunity arose in Utah to put Anderson’s ideas to the test: the I-80 port of entry.
The Test Strip Before the Test Strip
The 330-ton thick lift test strip at the port of entry was part of a much larger and more conventional project, milling 2 inches off 20 miles of I-80 and replacing it with 2 inches of stone matrix asphalt (SMA).
“We were going after the large 40,000-ton SMA project,” said Staker Parson’s South Wasatch Front Construction Manager Jonas Staker. “When we saw the spec in there for a 6-inch lift on the port of entry, we were willing to give it a shot.”
The mix was produced at Staker Parson’s Beck Street Plant in Salt Lake City, a 1.5-hour haul from the port of entry. “We felt we needed to give ourselves any advantage we could, which is why we decided to use Evotherm,” Peterson said, adding that WMA is very common in Utah. “I think that helped with the long haul and also with compaction.”
The final mix design was a 12.5 mm 50 gyration Superpave mix with 15 percent RAP, PG76-34 binder with 0.5 percent Evotherm and a total asphalt content of 6 percent, with 1 percent air voids. UDOT’s usual mix is a 12.5 mm 75 gyration mix with about 5 percent binder and an air void target of 3.5 percent.
When Mike Stevens, Staker Parson’s superintendent on the project, began talking to Peterson about the thick lift test strip well in advance of the job, Stevens knew the project was “something special.”
“There were so many unknowns about the material,” Peterson said. That’s why Staker Parson decided to pave its own 100-ton test strip in its pit the day before paving UDOT’s test strip at the port of entry. “We were very committed to making sure everything was going to succeed in the end.”
The test strip provided a number of insights, including some changes to the rolling pattern and an opportunity to calibrate all of Staker Parson’s equipment. “We were able to shoot density on the test strip and then core it to make sure our gauges were reading correctly,” Peterson said. “That made us feel very comfortable using a nuclear gauge on the job.”
“It also helped us realize how stiff the mix was and how fast we’d have to pave it down,” Peterson said.
They also learned how long such a thick lift would retain heat. They planned to core the test strip at their pit that afternoon, but it was still too hot. “We cored it that night and ran tests through the night to make sure we had the information we needed for the next day,” Peterson said.
When the pavement was still 150 degrees Fahrenheit the following morning, there were discussions about cancelling the project altogether. However, Anderson was confident that once the mat had cooled, it would set up strong.
The original plan was to open the port of entry to traffic the same day of paving, but Staker Parson’s test strip in its pit prompted UDOT to wait to open the port of entry to traffic. The job was paved on a Thursday and didn’t open to traffic until Sunday afternoon.
“Normally we won’t have to wait that long,” Anderson said, “but we were unusually nervous since this was our first job with it.” He suggests waiting until the pavement reaches a temperature of 100 to 120 degrees Fahrenheit before reintroducing traffic, which the test strip in the pit reached two days after it had been paved.
“The test strip told us that, as much as we needed to pave it rapidly, we needed to be patient after it was compacted to give the mat time to let that heat out,” Stevens said.
A Sticky Success Story
Before Staker Parson’s crew could pave the port of entry on June 3, 2021, subcontractor Construction Material Recycling (CMR), Coalville, Utah, first milled out 3 inches of asphalt and 3 inches of concrete 300 feet ahead of the scale and 100 feet after the scale at a width of 14 feet. In addition to milling, they had to chip off the concrete stuck to the metal housing of the scale and sweep before paving could begin.
Staker Parson exhibited the same commitment to quality on the job as it did at its plant.
“Anything we thought we might need during the job, we brought out there – plus spares – just in case so we wouldn’t find ourselves waiting on anything,” Stevens said. They even had a spare paver, material transfer vehicle and roller out there. “We lined up everything for our success.”
This included devoting nine belly-dump trucks to the job. “Normally, we’d have four trucks running between the job and the plant, but we didn’t want to do that with this job,” Stevens said. The company wanted to make sure they had the tonnage on site to complete the job so they could pave continuously. “Once you stop to wait for a truck with this type of mix, you’re going to run into problems.”
Staker Parson staggered its trucks from the plant 15 to 20 minutes apart. The only issue they hit was when a truck blew a tire on the 1.5-hour haul from the plant and paving had to briefly pause. That mix unfortunately had to go to the recycle pile.
“When you’re putting down such a thick lift, you’re running through the mix fast,” Anderson said. The lift was 7.5 inches thick, pre-compaction. “You have to have more trucks and maintain a steady stream of material. It’s a lesson learned, but definitely something we can deal with.”
Despite how sticky the mix was, pickup wasn’t a problem for Staker Parson’s Weiler E2850 material transfer vehicle, nor its Cat AP1055E paver. And, although the mix was stiffer and stickier than Stevens thought it would be, the company’s experience with SMA meant its crews were used to mixes that require excellence off the screed.
Staker Parson paved 100,000 tons of SMA in 2021.
“It’s not like a normal mix that you can take a shovel to and re-rake it,” Stevens said. “Thankfully, we had planned ahead, which is what made this job a success.”
The rolling pattern included seven passes with a Hamm HD140VV double drum roller and a Cat CCS9 finish roller to take the lines out of the mat. According to Peterson, the area was so small so they didn’t have enough room to warm up the tires before compaction. With a pneumatic-tired roller, he added, the mix would have stuck to the cold tires. They did however use vibratory mode on the first pass. Although the crew had an extra double drum roller on site just in case, Stevens said they were able to achieve compaction throughout the entire lift without the backup roller.
“We were constantly monitoring the mat with Troxler 4640 nuclear gauges to see where we were on compaction,” Stevens said. Although they weren’t able to core the pavement until the following day for fear of parking a truck on the mat before it had cooled, the pavement had an average density of 97 percent.
“We didn’t compact at 1 percent voids and I didn’t think we could,” Anderson said. “When you do a 4 percent air void mix design, you’re shooting for 6 to 7 percent in the field. When we designed for 1 percent, we hoped for 3 to 4 in the field.”
Originally, UDOT was only going to test compaction on the top 3 inches of the 6-inch lift. However, based on the cores UDOT pulled, Staker Parson was able to achieve compaction throughout the 6-inch lift.
UDOT pulled four cores and ran them on the top 3 inches and the bottom 3 inches. The top 3 inches of the first core had a density of 97.9, the bottom, 98. The second core had a density of 97.8 on top and 94.4 on bottom. The third core had a density of 97.2 on top and 92.8 on bottom. And the fourth core had a density of 97.3 on top and 97.6 on bottom. “Four cores can’t tell you the whole story, but we were happy with the level of compaction we achieved,” Peterson said.
The only core that was low, at 92.8 percent, was located where paving briefly paused due to the haul truck that had blown a tire. “The other cores show we were able to get full depth compaction,” Anderson said.
One full 6-inch core Staker Parson took on its own test strip achieved 97 percent compaction. “They didn’t even cut it in half,” Anderson said.
Ultimately, Staker Parson was able to achieve 100 percent pay for gradation, binder content and density.
“One of the challenges with paving thicker lifts is the rideability spec,” Peterson said. “When you increase thickness, you’re often decreasing smoothness because of the way it compacts. So, to achieve a better rideability, paving two 2-inch lifts can help improve performance on that ride spec.”
“Each time you put down a layer of asphalt, you can improve rideability and smoothness,” Anderson said. “Only paving one lift means less opportunity to make it smooth.” However, he added, this project hasn’t experienced any issues relating to smoothness.
Staker attributes this rideability to the crew’s commitment to the continuous paving method. They did, however, have to grind down the end joints. But Anderson considers that a benefit of how dense this pavement is; it’s possible to grind the surface without opening the pavement to water.
Several months after the test strip was paved, Anderson returned to the port of entry and found that it continued to perform well. “Even with the brutal temperatures of this past summer, it still didn’t budge,” Anderson said. “I think it’s past the danger zone where we’d see it rut if it was going to rut. If it made it through this past summer, I think it’ll do just fine next summer.”
Industry Expresses Interest
The team at Staker Parson also attributed the success of the project to partnership and trust between contractors, the asphalt industry, and UDOT.
Peterson said the establishment of the Utah Asphalt Pavement Association (UAPA) in 2012 had a significant impact on the asphalt industry within the state. “Before UAPA, there were a bunch of single voices talking to UDOT and it was just a lot of noise,” he said. “UAPA talks to UDOT with one voice and it’s possible to have a conversation back and forth.” Now, he said, UAPA sits down with UDOT once a month to go over ideas, including Anderson’s thick lift idea.
“I think this project shows a high amount of collaboration between UDOT and industry,” said UAPA Executive Director Reed Ryan. “Anderson envisioned this type of trial project, put the parameters of the mix design through the proverbial ‘ringer’ and proved out a concept that was worth pursuing for the DOT. Marchant was willing to put this application down in a very rigorous and tough setting (the port of entry). Then came industry with the willingness to assume some of that risk and actually build and place the mix.”
“We learned together and communicated together long before the mix ever went down and I feel that is vital to the success of this trial project,” Ryan said. “I commend UDOT because we (industry) never felt left out of this process.”
Ryan was also present on the day the port of entry was paved, along with a number of UDOT regional directors, high-up UDOT and Staker Parson staff, Dave Johnson from the Asphalt Institute, and other industry leaders. Word of the project has even made it to the Utah state legislature.
“There is so much interest in this,” Anderson said, adding that thick lifts offer a number of benefits. They could speed up contractors’ production rates and reduce disruption to the traveling public. Paving thick lifts could also result in cost savings from eliminating the tack coat, reducing traffic control to what’s required for one lift, and not having to perform quality control on two separate lifts. “If you can save money on all of those things, you can instead put that money into the pavement by spending a bit more on a high polymer mix,” Anderson said.
Furthermore, this could expand Utah’s paving season. “The contractors are excited that with this idea, we could put out a project in November or April,” Anderson said. “We could actually use the cooler weather to speed up how fast the mat cools down after compaction.”
Already, other DOTs are seeing the potential. Several have reached out to him for the specs. Within Utah’s state lines, UDOT higher-ups have expressed their support. “I think you will see more thick lifts happening in Utah, more projects with 1 percent air voids, and more use of 76-34 binder,” Anderson said.
His own personal dream is to see this concept on the interstate. “If I could get a 10-mile section of road, remove the pavement, and put in up to 8 inches of a new high polymer, low void mix, I think it would be a perpetual pavement,” he said.
Although the lift at the port of entry was 6 inches thick, Anderson said he thinks they might be able to go as thick as 8 inches and still achieve compaction.
“If we can pave at a similar depth to concrete, we could put this where we might normally put concrete,” Anderson said. “If we can place this faster than concrete and open pavements up sooner, with similar or even better performance because there won’t be any joints, this could be a very exciting opportunity for the asphalt industry.”
“For the asphalt industry as a whole, we owe it to ourselves to start thinking out of the box when it comes to the mix design and construction process,” Ryan said. “I recognize we must be careful and responsible when it comes to the hard-earned dollars of taxpayers in the country, but we currently stand on the shoulders of giants – people in the past that gave us Marshall Mix Design and Superpave Mix Design – and we have to take on that legacy and do something with it. What was true 70 years ago with Marshall and 30 years ago with Superpave still works today, but we are in a time and place where we can and should do more to build on that legacy.”