By Garrett Winkelmaier, Ph.D
Asphalt compaction is widely recognized as the most significant indicator of roadway quality because it directly affects the performance and lifespan of pavement. Compaction is influenced by a range of factors, from the asphalt mix design to the rolling pattern used during construction, making it a reliable measure of actual in-place air voids, referred to later as simply “air voids”, present in the finished roadway.
Decades of research have demonstrated a clear link between air void content and pavement longevity. Key studies collectively show that improper air void levels can result in decreased stiffness and strength, reduced fatigue life, accelerated aging, diminished durability, increased rutting, raveling and heightened susceptibility to moisture damage (Kennedy et al., 1984; Pell and Taylor, 1969; Epps and Monismith, 1969; Linden et al., 1989; McLeod, 1967; Kandhal and Koehler, 1984; Scherocman, 1984; Cooley et al., 2002). As a result, air void measurement has become central to quality control and assurance practices in road construction.
The gold standard for measuring air voids is core sampling, which involves extracting cylindrical samples from the roadway to directly assess the air void content. While core sampling remains the benchmark, this article will explore modern, commercially available devices for in-place density measurement by reviewing the technologies behind them.
The nuclear gauge is one of the most widely recognized devices for in-place density measurement tools for contractors. After standardization of the device, the measurement process follows a specific sequence: First, the nuclear gauge is positioned directly on the asphalt surface. The device then emits radioactive particles that penetrate the pavement. Some of these particles interact with the asphalt and are scattered, meaning they are reflected back to the device — a phenomenon known as particle scattering. The gauge detects and counts these returning particles to determine the “wet density” of the material. Wet density refers to the density of the asphalt, including its moisture content, as measured by the nuclear gauge. Finally, the determined wet density is correlated with results from core sampling to calculate the percentage of air voids present in the tested area, providing a reliable assessment of pavement quality.
More modern gauges, known as non-nuclear gauges, have eliminated the use of radioactive materials to provide increased safety for operators and reduce operational complexity. These gauges emit an electric field through the sample and measure its electrical resistance. The measured resistance is then analyzed to estimate the proportion of air voids within the material, which helps determine its overall composition. The process involves placing the gauge on the asphalt surface, generating an electric field and recording the resistance response from the sample. This resistance value is compared to reference data obtained from core samples to calculate how much of the sample consists of air voids. By eliminating radioactive materials, non-nuclear gauges reduce health risks for operators and simplify regulatory compliance requirements.
An emerging alternative technology, known as Ground Penetrating Radar (GPR), has been developed to provide complete coverage mapping of roadway density. GPR works on a similar principle to that of non-nuclear gauges by emitting an electromagnetic signal and assessing how the signal interacts with the material. Specifically, it measures the dielectric, which is a property that affects how electromagnetic signals pass through materials and is directly related to the material’s composition and density.
A key advantage of GPR technology is that the device does not need to be in direct contact with the asphalt; instead, it can be mounted on movable structures, such as vehicles, allowing it to collect continuous readings over large pavement areas. Because GPR can be operated from moving vehicles and without touching the pavement, it enables faster and safer data collection while capturing a more comprehensive and detailed density profile compared to traditional methods. In recent years, arrays of GPR devices have made it possible to construct complete coverage mappings and detailed density profiles of entire roadways.
The final technology for in-place measurements can be categorized as vibrational analysis. In this method, accelerometers are used to measure the physical forces interacting between vibratory rollers and the asphalt. Many roller manufacturers offer accelerometers on newer roller models, and this data can sometimes be displayed to roller operators as a Compaction Measurement Value (CMV) number or similarly named quantity specific to the roller manufacturer. Traditionally, the challenge with CMV has been the difficulty of correlating it in the field with the density data obtained from core samples. Machine learning algorithms address this by analyzing large datasets from roller sensors and core samples to identify patterns, enabling more accurate real-time predictions of asphalt density. After proper calibration and training, these algorithms can perform in-field correlations, offering a percentage density number directly to roller operators during construction, thereby improving decision-making on the spot.
There are many solutions available today that can be used as a contractor’s quality control tool based on different technologies and the physical properties of asphalt. Each option is supported by research, but choosing the best fit for your company depends on factors such as density readout, who will receive and use the information (such as project managers, site supervisors or roller operators) and your current paving procedures.
Garrett Winkelmaier, Ph. D, is the General Manager of G4 Technologies. He can be reached at garrett.winkelmaier@G4tech.com
References
Kennedy, T. W., McGennis, R. B., & Roberts, F. L. (1984). Effects of compaction temperature and effort on the engineering properties of asphalt concrete mixtures. West Conshohocken, Pa.: ASTM International.
Pell, P. S., & Taylor, I. F. (1969, February). Asphaltic road materials in fatigue. In Association of Asphalt Paving Technologists Proc.
Epps, J. A., Monismith, C. L., Warden, W. B., Pell, P. S., Kallas, B. F., Terrell, R. L., … & Mcleod, N. W. (1969, February). Influence of mixture variables on the flexural fatigue properties of asphalt concrete. In Association of Asphalt Paving Technologists Proc.
Linden, R. N., Mahoney, J. P., & Jackson, N. C. (1989). Effect of compaction on asphalt concrete performance. Transportation research record, (1217).
McLeod, N. W. (1967). Influence of viscosity of asphalt-cements on compaction of paving mixtures in the field and discussion. Highway Research Record, (158).
Kandhal, P. S., & Koehler, W. C. (1984). Pennsylvania’s experience in the compaction of asphalt pavements. West Conshohocken, Pa.: ASTM International.
Scherocman, J. A. (1984). Guidelines for compacting asphalt concrete pavement. Better Roads, 54(3).
Cooley, L. A., Prowell, B. D., & Brown, E. (2002). Issues pertaining to the permeability characteristics of coarsegraded superpave mixes.
