Research advances implementation
By Mark Blow, P.E.
Using intelligent construction technology to enhance asphalt pavement durability through increased pavement density is one of the most economical methods to extend pavement life and increase the sustainability of our roadways. Dielectric Profiling Systems (DPS) can assess nearly 100 percent of a new pavement surface placed during construction in real-time.
It all began
In the late 1990s, researchers at the Texas Transportation Institute (TTI) discovered the relationship between the void content and the dielectric calculated by ground penetrating radar (GPR). In 2013, the Federal Highway Administration’s (FHWA) Strategic Highway Research Program (SHRP 2) enlisted TTI and Geophysical Survey Systems, Inc. (GSSI) to package components into a device that would provide real-time profiling of asphalt mixture uniformity. In 2019 the Minnesota Department of Transportation (MnDOT) accepted the role of the lead agency for the FHWA Transportation Pooled Fund study TPF-5(443) titled “Continuous Asphalt Mixture Compaction Assessment using Density Profiling System (DPS).” MnDOT engineers Kyle Hoegh, Ph.D. and Shongtao Dai are the program managers for this effort.
Doesn’t GPR measure sub-surface characteristics?
Dr. Hoegh explains GPR and dielectric properties in his 2018 article in the Transportation Research Record.
“GPR provides a nondestructive testing alternative that allows for walk-behind or vehicle-mounted measurements. Determination of dielectric properties of the asphalt layer with GPR has been traditionally done through measurement of either round trip travel time to reflection at the depth of the AC layer or surface reflection. The travel time approach covers a greater depth but relies on a known thickness. The asphalt thickness is often unknown and spatially variable. Moreover, if the asphalt layer is placed in several lifts or as an overlay over an existing asphalt pavement, it may be difficult to separate the travel time in the individual lifts from the overall travel time of the electromagnetic signal in the asphalt layer.”
DPS uses the AC surface reflection method. “The AC surface reflection method uses the ratio of the amplitude of the GPR signal reflection from air to the asphalt surface … to the incident amplitude … to determine the bulk dielectric constant of the asphalt … The advantage of this approach is that if the upper lift is sufficiently thick (thicker than 30 mm) then the measured AC surface reflection depends only on the properties of the upper layer.”
Transportation Pooled Fund study TPF-5(443)
To develop DPS into an effective and efficient technology that can be readily put into practice, each of the 14 pooled fund member states contributed toward completing the tasks identified by the group, which include:
• Improving DPS software and hardware by working with a variety of vendors, to advocate for hardware improvements that increase functionality, improve data collection and analysis, and make the technology easier to use.
• Developing an AASHTO specification for use in the adoption and advancement of DPS technology.
• Developing testing and calibration methods to assure the tools and technology are properly calibrated to perform as expected.
• Accrediting equipment operators to operate the various elements of DPS equipment with precision and interpret the data produced.
• Helping other states learn about DPS and share the benefits the technology offers to increase its acceptance as a viable alternative to traditional density testing measures.
• Providing training and assistance to help other states put DPS into practice.
• Promoting the technology. By speaking at conferences and other events, pooled fund members can disseminate the information that has been learned about DPS so far to increase interest in further study and encourage investment.
So why do we need this?
Many agencies 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 product. If all goes well, cores tested the next day will show whether the finished pavement is accepted or if incentives/disincentives will be applied. In a worst-case scenario, for the agency and contractor, a pavement may need to be removed and replaced.
With so much time and money on the line, the process can feel like a high-stakes game of chance. DPS can give contractors more control and confidence over this outcome. 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. It is also safer for field crews who are often drilling cores the next morning in low light conditions or taking longitudinal joint cores adjacent to oncoming traffic. DPS testing enables that work to be completed behind the compaction train as the paving day progresses.
Minnesota case study
To test the capabilities of DPS technology in the field, MnDOT collaborated with a team of contractors on a recent state highway repair and rehabilitation project. Working together, the groups used DPS to monitor compaction and map areas of high and low density in real-time, giving crews the opportunity to determine optimal rolling configurations.
Construction of a 12-mile stretch of a multi-lane highway in Minnesota took place during the summer of 2022. MnDOT identified this project as a case study to test DPS technology and its ability to improve construction practices in the field. A rolling DPS unit was used to accurately measure the pavement’s density. DPS results were interpreted by MnDOT engineers using the on-site software to give feedback to the contractor about the density achieved during paving. The data collected by the DPS unit generally indicated a median pavement density between 94 and 95 percent. However, there were sections of lower density along the left lane longitudinal joint. Mapping the pavement’s density provided a more complete picture that allowed for the determination of specific construction practices that could be improved. Comparing the DPS data with information collected from other intelligent construction (IC) technologies, engineers verified the lower density locations were often caused by a reduced number of vibratory roller breakdown passes. MnDOT presented the findings to the contractor, who diagnosed potential causes and proposed plans for improving operations.
Ohio case study
Like most other DOTs, the Ohio Department of Transportation (ODOT) deals with pavement performance issues resulting from segregation and other defects during placement. In 2014 ODOT completed a pilot study of Paver Mounted Thermal Profiling (PMTP) Systems on six projects. DPS-type density gauges were used, in conjunction with the PMTP, to study how well the PMTP system could predict areas of low density.
ODOT has found that despite the new Material Transfer Vehicle (MTV) specification requirements, segregation issues persist. An example of these issues can be found in a recent pilot project utilizing PMTP and DPS on the same project. The 12.5mm surface was placed using an MTV yet load-to-load density dips were still evident.
The PMTP detected colder material between truckloads and the DPS detected lower mat density at those same locations. ODOT is working to implement both PMTP and DPS for quality control and quality assurance on construction projects. ODOT believes that the use of these technologies can both improve construction practices in real-time and eliminate many of the hidden defects that cause pavement performance issues.
Pooled fund status
After two years of work, the pooled fund has developed and refined the various processes for using DPS to properly collect data, as well as a range of supporting documents and training videos that detail and demonstrate the steps involved. The efforts have provided hundreds of miles of DPS data that pooled fund members, consultants, vendors and other industry stakeholders have shared and discussed at DPS pooled fund project updates, peer exchanges, trainings, vendor updates, technical working group meetings and an in-person training event at MnROAD.
In March 2022, the pooled fund study delivered draft specifications to AASHTO, which aim to prescribe dielectric testing of gyratory compacted specimens – a process detailed in “Calculating Asphalt Density Using Dielectric Measurements.” This specification is in the process of being updated to reflect additional work by NCAT and the University of New Hampshire, simplifying the document based on feedback from the committee chair and is anticipated to be published as a draft specification in 2023. This is the next step toward cataloging all the procedures that must be followed to measure the dielectric constant of gyratory-compacted production mix samples. The specification covers equipment setup and configuration, data collection, analysis and verification.
The pooled fund continues to work toward its other goals by conducting analysis for equipment and operator certification, developing an initial precision and bias statement for the technology, and continuing to improve DPS technology and promote its benefits.
Agencies in the United States spend billions of dollars each year to maintain, repair and reconstruct pavements. Improving compaction can equate to immense savings. The FHWA Asphalt Pavement Technology Program in 2016 brought forth research data that showed increasing compaction to reduce in-place air voids by one percent can increase the fatigue and rutting performance of asphalt pavements by a conservatively estimated 10 percent.
Craig E. Landefeld of ODOT summarized his thoughts at a recent DPS meeting stating that “Adding a year of service life to a pavement can provide any agency with significant savings. The Ohio DOT devotes approximately $650 million each year to pavement preservation. If we could get just one more year out of all our pavements, we’d save $50 to $65 million annually.”
A dielectric profiling system is just the right tool to measure our efforts to increase pavement life for a more sustainable future.
Blow is an Asphalt Institute Senior Regional Engineer based in South Dakota.