The Nation’s pavement network consists of more than 8.8 million lane-miles. The integrity of this network is critical to support the mobility of more than 282 million registered motor vehicles, with each vehicle traveling at an annual distance of, on average, about 11,000 miles. Under such a travel demand, numerous technologies and innovations have emerged to ensure the safety, resiliency, and sustainability of our transportation infrastructure. Among these innovations, density profiling technologies have gained considerable attention from highway agencies.

The density of asphalt pavement is a measurement reflecting the pavement compaction, a critical element in asphalt pavement construction. Adequately compacted asphalt pavement can meet the desired strength and reduce the risk of water penetration through excessive air voids. Adequate and uniform compaction can increase the pavement’s long‑term performance and durability, maximizing the return on investment in pavement construction.

Challenges in Pavement Density

The current state of practice for density measurement relies on drilling cores in the field. The drilling process is labor intensive and only feasible for a few random locations. Scattered measurements from random locations introduce the risk of failing to detect improperly compacted areas. Furthermore, core drilling is a destructive process, which brings added risks that may compromise the integrity of pavement structure. In some cases, highway agencies use nuclear gauges for density measurement. While nuclear gauges offer advantages in terms of nondestructiveness, they come with their own set of complexities. Specifically, the requirement of special licenses for handling nuclear gauges can add layers of regulation and potential liabilities.

All these risks translate into additional costs that asset owners need to consider, including expenses related to unnecessary pavement preservation and/or rehabilitation. The costs associated with traffic control and addressing public complaints should also be considered and not underestimated.

“Within our extensive highway infrastructure, the need for better density profiling is driven by the pursuit of safer, more resilient, and sustainable transportation networks,” says Dr. Dai Shongtao with the Office of Materials and Road Research at the Minnesota Department of Transportation. “This, in turn, contributes to pavement integrity, cost effectiveness, and a dedicated commitment to enhancing the commute of millions of travelers,” says Dr. Shongtao.

Nondestructive Innovation

A promising solution for density measurement is to use ground penetrating radar (GPR). GPR can measure the dielectric constant, a material property of asphalt material, which correlates to the pavement’s density. GPR emits short pulses of electromagnetic (EM) waves that can penetrate pavement surfaces and interact with underneath paving materials. The reflecting signals contain rich information about scanning of the asphalt pavement, and with proper correlation, the information can reflect its density. The use of GPR is nondestructive and safe for the operator’s health. The emitting energy of GPR is approximately equivalent to 1 percent of the outcoming energy from a cellular phone. One of the most prominent advantages of using GPR for density profiling is its capacity to provide continuous, real-time, or near-real-time density data across expansive pavement areas. Commercially available equipment can scan a 6-foot-wide paving area at a walking speed.

The innovative use of GPR for density profiling of asphalt pavements has interested several highway agencies. With a shared interest in advancing pavement compaction assessment, 14 State DOTs and FHWA formed a pooled fund collaboration in early 2020. The collaboration formed a working group to test, evaluate, improve, and standardize the new technology, aiming to transfer the research findings into best practices.

“Innovations in density profiling technologies hold the promise of revolutionizing how we assess and optimize pavement compaction, providing a more efficient, cost effective, and nondestructive approach to ensuring the longevity and performance of our vital transportation infrastructure,” says Dr. Jean Nehme, director of FHWA’s Office of Infrastructure Research and Development, located at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA.

Nondestructive Evaluation (NDE) Laboratory Collaboration with Federal, State Partners

The mission of the FHWA NDE Laboratory is to conduct state-of-the-art research, development, and implementation of nondestructive testing systems and technologies to improve the Nation’s highway infrastructure assets. The NDE Laboratory is committed to strengthening partnerships between Federal and State entities by leveraging its extensive knowledge and expertise in the research and development of NDE technologies. The NDE Laboratory is equipped with state-of-the-art technologies and has a research team with diverse expertise to support the laboratory’s mission and commitment.

As a participant in the pooled fund collaboration, the FHWA NDE Laboratory shares a common interest with its partners in testing, evaluating, and researching new technologies for better pavement construction. Since 2020, the NDE Laboratory has conducted a series of studies to investigate and evaluate GPR. The investigation starts with an equipment survey to review the state of practice in density profiling using GPR. The investigation also involves laboratory testing and field trials to understand the physical principle of the technology.

By conducting research, the NDE Laboratory aspires to generate practical guidelines that can help stakeholders use, evaluate, and test the technology. For example, the research team assesses the robustness and resiliency of GPR operating under harsh climatic conditions by using a temperature‑controlled chamber to simulate high temperatures during pavement construction. Another example is the study of the edge effect in dielectric measurements of cylindrical samples. Obtaining density information requires conducting tests to correlate the dielectric and the pavement’s density. The correlation could be done using field cores. An alternative approach to avoid field coring is to test gyratory samples compacted in the lab. These samples are cylindrical with limited sizes. The cylinder height is about 4.5 inches (114.3 mm), and the diameter is up to 6 inches (152.4 mm). The small size of these specimens compared to the wavelength of GPR signals can lead to signal interferences from edges, potentially affecting the accuracy of dielectric measurements. The research team investigated this issue by performing a set of numerical simulations and experimental tests. The results confirmed the presence of the edge effect. It is worth noting, however, that this effect remains within practical acceptability when conducting tests on cylindrical samples with a 6-inch (152.4 mm) diameter and a height exceeding 4.5 inches (114.3 mm).

One more noteworthy example of the study is the discovery of the scanning boundary during density profiling. Past research has shown the technology’s sensitivity to a thin layer underneath the pavement surface; however, the precise extent of this sensitivity remained unknown. The research team performed state-of-the-art numerical simulations to understand how EM propagates in asphalt pavements. The numerical model can simulate the asphalt binder, aggregates, and air voids in the asphalt mixture. This model, as shown in the figure at the top of the next page, can better capture the EM wave propagation (plotted in the black solid line) inside asphalt material, especially the wave scattering, compared to a simplified approach by modeling asphalt as a simple homogeneous material (plotted in the red dotted line). By using a classical theory of Rayleigh scattering, the research team uncovered the scanning boundary of this technology. The Rayleigh scattering describes how EM waves interact with particles that have sizes similar to the EM wavelength. The scanning region reveals the sensitivity of the technology to the density change below the pavement surface. For example, a numerical scenario with a big hollow void below the scanning region (plotted in the blue dashed line) shares similar wave characteristics to the case without the hollow void (plotted in the black solid line).

Roadmap for Next Steps

The NDE Laboratory has developed a roadmap for the next steps. One study will investigate how different asphalt mixes could potentially influence the density measurements. The study will involve a comprehensive set of numerical simulations and experimental validations. The numerical simulations may provide valuable insights to optimize the experimental tests. Another study will integrate the surface macrotexture of asphalt pavement into density profiling. Some users have expressed concerns about how the pavement surface might impact density measurements, but no studies have been done on this topic. These two studies aim to find scientific insights into how asphalt mix design and surface macrotexture may influence the density measurement and, if identified, develop practical adjustments to enhance GPR’s performance.

As the NDE Laboratory follows its research roadmap, it places a significant emphasis on the value of collaborative research. The NDE Laboratory research team believes that collaboration not only amplifies the collective research effort but also introduces a diverse range of ideas that can spark new innovations. In 2021, the NDE Laboratory participated in a round-robin program with the Minnesota DOT and the National Center for Asphalt Technology at Auburn University. The round-robin program involved testing the same sets of asphalt samples with the same set of equipment and using the same testing method but by different research groups. The collaboration tested and evaluated the consistency, precision, and bias when implementing the new density profiling technology.

In 2023, the NDE Laboratory established a collaborative partnership with the Asphalt Binder and Mixtures Laboratory, the Mobile Asphalt Technology Center, and the long-term infrastructure team at TFHRC. This collaboration is specifically aimed at driving technological innovation and knowledge transfer through the framework of the newly revamped third-generation Pavement Testing Facility. The NDE Laboratory envisions that this collaborative endeavor will give stakeholders an improved understanding of and heightened confidence in using GPR for density profiling, among other significant objectives.

For more on the Pavement Testing Facility and the Mobile Asphalt Technology Center, see the Guest Editorial and What’s New in the Summer 2023 issue of Public Roads (https://highways.dot.gov/public-roads/summer-2023/editorsnotes and https://highways.dot.gov/public-roads/summer-2023/whatsnew).

Hoda Azari is the manager of the NDE research program and NDE Laboratory. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.

Heng Liu is a contracted research engineer working in the NDE Laboratory. He holds a Ph.D. in civil engineering from the University of Maryland, College Park.

Simon Shams was a contracted subject matter expert working in the NDE Laboratory. He holds a Ph.D. in civil engineering from the University of Wisconsin-Milwaukee.

For more information, see https://highways.dot.gov/research/laboratories/nondestructive-evaluation-laboratory/ongoing-projects, or contact Hoda Azari, 202–493–3064, hoda.azari@dot.gov.

For more information about the pooled fund collaboration for continuous asphalt mixture compaction assessment, see https://www.dot.state.mn.us/materials/dps/index.html.