Researchers developed sophisticated models for high-density asphalt pavement mixtures. After calibrating the model to experimental data available from 5 percent air void asphalt mixtures, the research team conducted tests on three Minnesota mixtures to further refine the model. A Phase II study will develop multiple high-density mix designs for Minnesota applications.

What Was the Need?

In asphalt pavement, air void content and density are inversely related. The more air voids in the pavement, the less dense—and less durable—the structure. While asphalt pavement density may improve over time, it typically only improves in areas where traffic compacts it, like wheelpaths. 

Variation in density throughout an asphalt mat leaves it susceptible to water infiltration and freeze-thaw damage, leading to potholes and other problems. Lower density at pavement joints, a typical location for higher-than-desired air void content, leads to premature cracking and failure at the joints.

“You have to develop something in the lab and then move it into practice. Indiana DOT has built a number of these kinds of pavements that seem to be doing quite well,” said Mihai Marasteanu, professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

In design and testing, asphalt mixtures for MnDOT may be designed for 4 percent or 5 percent air void content. But in the field, mixtures may range from 7 percent to 10 percent at areas like joints. Indiana Department of Transportation (DOT), however, designs 5 percent air void mixtures based on Superpave designs and achieves the same density in the field, where the agency’s asphalt pavements are a fairly uniform 5 percent. MnDOT has been interested in developing a design with Minnesota materials that can match this actual 5 percent density design in the field.

What Was Our Goal?

As a first step toward designing a realistic, high-density asphalt pavement, MnDOT and the Local Road Research Board sought to use Minnesota materials to develop a pavement mixture that mimics Indiana DOT’s 5 percent air void asphalt mixtures and pavement mats. Researchers would examine the ability of high-density mixtures to maintain the target density in laboratory compaction testing and computational modeling.

What Did We Do?

Researchers began with a literature review of research on compaction of asphalt pavement mixtures and compaction modeling. Based on these findings, the research team developed a fluid dynamics-based discrete element model (DEM) that was used to simulate compaction experiments.

The team then worked to improve the model by using nonspherical and spherical particles in the model to better simulate the compaction behavior of asphalt pavement mixtures. Once the team had fully developed the model, it collected three high-density mixtures: a 3 percent and a 5 percent air void mixture used in 2017 MnROAD test pavement cells, and another 5 percent mixture developed in the lab.

Using the Superpave gyratory compactor, the research team conducted several experiments on the three mixtures, including bending beam rheometer creep and strength, indirect tensile creep, semi-circular bending, dynamic modulus and flow number tests.

What Did We Learn?

Based on the review of previous research, investigators found that Superpave design assumptions lead to the higher as-constructed air voids like those experienced in Minnesota, in part because traffic loading is expected to provide final densification that cannot in practice be achieved uniformly across the pavement mat. Few researchers have used DEM to simulate compaction, and none have used a model sufficiently realistic to represent the physical interaction of aggregates and avoid significant deviation between simulated and experimental compaction results.

The research team developed a sophisticated DEM that simulates the behavior of fine aggregates and coarse aggregates in various shapes, and calibrated the model with experimental data that was available for three 5 percent Superpave asphalt mixtures. Experimentation and modeling showed that fine aggregate properties significantly impact compaction levels. Spherical shapes offer easier compaction. Angular shapes may be more difficult to compact, but once compacted, the mixtures with angular fine aggregates maintain very good density and structural stiffness.

“This study is a first step. The computational part was very strong, and the laboratory part was very supportive of the model developed in this project to study compaction,” said Ed Johnson, research project engineer, MnDOT Office of Materials and Road Research.

Lab studies also indicated that test methods and test temperatures yielded significant variation in mixture properties, and the tested properties of high-density mixtures do not differ significantly from the tested properties of conventional mixtures.

After validating and calibrating the DEM based on results from this study’s lab tests, investigators determined that the model reasonably simulates asphalt mixture compaction in the gyratory compactor.

What’s Next?

A Phase II study is underway to design 5 percent mixtures with Minnesota aggregates for various traffic levels. In addition to the Phase II work, correlations and linkages between laboratory and field compaction warrant study. Researchers may want to determine, for example, whether field compaction properties of these mixtures are related to air void design levels, design number and compaction gyrations, or a combination of these parameters.

Model simulations of compaction require significant computational time; researchers may wish to conduct experimental testing of an extensive set of mixtures and materials to develop correlations with model simulations. The model may also benefit from considering different levels of nonsphericity of aggregates in aggregate mixes.

This post pertains to Report 2019-41, Experimental and Computational Investigations of High-Density Asphalt Mixtures, published October 2019.