A newer technology for measuring asphalt density is hoping to get to the core issue (pun intended) of getting the most bang for the buck for both pavement owners and contractors.
While the idea of utilizing Ground Penetrating Radar (GPR) for measuring pavement density has been around since the late 1990s, the Dielectric Profiling System (DPS) method of measuring air voids in asphalt with GPR is considered “newer technology.” This DPS technology has state DOTs and other agencies seeing an opportunity to provide better and longer-lasting asphalt pavements.
DPS is a way to continuously measure asphalt compaction by utilizing GPR. A positive of DPS technology is that asphalt material density – where the higher the density results in a better opportunity for longer-lasting pavement – can be measured “more holistically.” In other words, DPS can be used to measure the whole kit and caboodle of pavement in real-time – which is an improvement on current measuring methods.
“What DPS allows us to do is measure essentially everything,” said Ohio Department of Transportation (ODOT) State Pavement Engineer Craig Landefeld, who added that the use of GPR can easily provide over 50,000 measurements a day through DPS recordings compared to the current use of 10 core (there’s the pun) samples. “When we do that, we have a much better handle as an engineer of what I have in place out there,” Landefeld added.
According to Asphalt magazine (“Dielectric Profiling Systems Prepare to Hit the Road,” Spring 2023), “By measuring and mapping the density of the entire pavement during construction, the contractor can spot deficiencies in their operation and make improvements in real-time.”
Interest in the use of DPS is growing among pavement owners, who believe the technology provides a better overall look at the quality of the asphalt compared to current testing practices. “Many agencies,” according to the Asphalt article, “rely on taking cores to determine the density of in-place pavement. Depending on the pavement owner’s acceptance specifications, as few as five randomly selected cores may be chosen to represent the density of 5,000 or more tons of (asphalt).”
Landefeld, a 14-year veteran of ODOT, said that while DOTs have utilized core samples to test pavement density since the late 1980s, the small sample size can be detrimental. “This is where (DPS) technology, in my mind, shines,” he said. “What we really want from our pavements is not only high-average density, we want that density to be uniform. Uniformity is the key because we want the pavement to perform the same … I think the technology is a very good one, and it is unique in that aspect because it allows us to start to measure things holistically.”
“What DPS allow us to do is measure essentially everything.”
Craig Landefeld, ODOT
Prior to core sampling, asphalt pavement was measured through “methodology standards,” such as a roller going over an area a certain number of times. ODOT also utilizes gauge density specifications that are compared to traditional averages, as well as statistical percent-within-limit guidelines for core sampling.
Top of mind for ODOT is longer-lasting pavements. “If we calculate all the dollars that ODOT spent on doing pavement repairs in 2022, that’s $112.8 million,” Landefeld said of the DOT’s $471-million resurfacing program for the year. “That’s a big percentage spent on repairing things. That’s why I’m so interested in this (DPS) technology. We’re not going to stop all that (repair work), let’s not kid ourselves … But what if we can prevent 50% of those? That’s – let’s say – $50 million annually; and that doesn’t account for service life savings from monitoring our projects better. It’s very feasible that we could get another year or two of (service) life from them.
“It could literally be $60 million to $100 million potential savings if we just do it better … Anything we can do better, and save,” Landefeld added, “allows us to do more someplace else on our transportation system.”
DPS technology evolved from late-1990s studies at the Texas Transportation Institute (TTI), when researchers saw “the relationship between the void content and the dielectric calculated by ground penetrating radar (GPR),” according to Asphalt magazine. In 2013, through a Federal Highway Administration (FHWA) request, the forerunner of today’s DPS equipment was created. In 2019, FHWA formed a Transportation Pooled Fund study of DPS, with the Minnesota Department of Transportation accepting the lead agency role.
Testing of DPS technology has grown in the past 10 years, as Ohio has joined Minnesota and other leading states, such as New York and Alaska, in determining its future use.
ODOT, which has two DPS units, received its first in 2017 through an FHWA procurement. Of the initial DPS unit, Landefeld said ODOT wanted to see for itself the ease of use and to study its testing reliability. The DPS unit is comparable in size to a residential lawnmower or child’s stroller. “You push it along like a lawnmower, basically,” he said of the approximately 50-pound piece of equipment, which during operation has an extendable horizontal bar that holds the DPS sensors.
In 2019, ODOT made use of a grant from FHWA’s State Transportation Innovation Council (STIC) to purchase a second DPS unit – this one featuring three sensors and lab setup. “It has all the bells and whistles,” Landefeld said of the $60,000 DPS unit. “If we’re testing this out, we’re going to purchase the whole setup.”
“This technology correlates very well with density…”
Craig Landefeld, ODOT
ODOT has tested the DPS units on more than a dozen projects throughout Ohio, including this year in Hancock County on U.S. Route 30. “Right now, we’ve scattered the units on projects around the state, looking at how they measure materials from different sources and how they handle various issues,” Landefeld said.
Ohio’s DOT has been spotlighted by the FHWA for its DPS initiatives:
“ODOT found that the DPS unit could assess more territory, more quickly, than by obtaining cores. The DPS unit has three antennas that can cover up to 6 feet transversely. They take continuous measurements and report reading every 6 inches longitudinally. On a day’s production on I-77 in Guernsey (County), Ohio, for example, 10 random cores represented nearly 3 miles of pavement, while the DPS collected over 45,000 density readings in the same area …”
(Source: “Spotlight on Pavement Density: Ohio Department of Transportation: Starting Out with Dielectric Profiling Systems”).
ODOT is currently utilizing the DPS units on projects solely to improve its own methods and knowledge of using the system and is not holding contractors accountable to the data being collected. “But, if we notice something onsite, like, ‘Hey, I think we can make this (portion of pavement) better,’ we’ll certainly bring it up and show the paving crews (on the DPS readings),” Landefeld said.
Contractors seem to be looking at DPS technology for those real-time density readings.
“We would be getting information in real-time,” said The Shelly Company Vice President of Quality Control Larry Shively of the advantages of DPS feedback. “So, if we had to make a change, if we saw that our density was erratic or decreasing, we could alert the paving crew, the roller-train crew, and make changes on the go.”
Wanting to stay atop a potential move toward DPS technology by project owners, like ODOT, contractors are purchasing and testing their own DPS units.
“The reason we got it is because it seemed like it is technology that ODOT was leaning toward in determining density profiling; and with that we thought we needed to be one step ahead or even with ODOT, so that we really understood it,” said Shively of The Shelly Co.’s purchase of a two-monitor DPS unit. “So, eventually if there is a specification for it, we can provide some intelligent input; that we would know and be familiar with some of the limitations, the pitfalls, with the device.
“… Our exposure to date is limited,” Shively said of The Shelly Co.’s use of its DPS unit. “But my plans are to get the equipment out in the field, as soon as we get some extra staff, and run it in conjunction with our normal required QC tests and start analyzing it and see what kind of data we’re generating.”
Shively, who has been with The Shelly Co. for 48 years, foresees ODOT’s asphalt acceptances on projects “being more profile-based rather than individual test locations. And with that, you must have a method to do that, and it seems like the DPS would accomplish that,” he said.
Not Too Fast
Ironically, while DPS units can provide quick feedback when it comes to measuring air voids in asphalt pavement, ODOT is taking its time when it comes to DPS’s future.
“Where we are today is that it works,” Landefeld said. “This technology correlates very well with density, so we shouldn’t be questioning that at this time. Where we are now is, how does this work? How do we implement it? And every state does things differently.”
Contractors are appreciative of ODOT’s methodical testing and vetting of the technology.
“One of the concerns is: ‘What can influence it?’” Shively commented. “… What happens if we get into an area that has a soft base, or had an intermediate, or an existing pavement that we are having some issues? We can only compact as well as what we’re compacting on. Is it going to be reading that and giving us some false information?”
Shively said pilot projects utilizing DPS technology would go a long way in having buy-in from the industry. “… That would prompt the industry to take note and probably buy some equipment, and then you would start seeing some usage of data-driven procedures.”
Those data-driven procedures, according to Shively, would be regarding things such as: How do you run it? How fast do you run it? Currently, it’s a push device, but DPS devices are coming out where it can be vehicle mounted. “So, the bottom line for the industry is to see that ODOT is very interested and letting contracts that require it as information and not as a specification, but to gather information. Then you will get a lot of interest, I think, from the industry,” Shively said. “There are a lot of questions about it, and it’s not a cheap tool. Our biggest concern is that we don’t want to jump into it – either ODOT or the industry – and not have it give us the benefits that we think that we’re going to get from it.”
Landefeld is aware of contractors’ concerns. “Before ODOT really dives into this, we want to make sure we have support networks for ourselves and contractors,” he said of the availability of DPS equipment retailers, parts manufacturers, as well as repair technicians. “What we’re trying to do is flesh out and answer these questions that we need before we implement this … We are getting down to the nitty gritty of ‘Hey, if I’m going to use this every day, how would I do this?’” Landefeld added. “We’ve answered the big questions, now it’s down to the details. And the devil is in the details.”
“(DPS) does have some validity,” Shively said, “and I think we just need to take some baby steps to see what it can do for both of us – not just ODOT but for the industry – before it justifies that expense.”
