The secret to a durable highway lies beneath the driving surface.
“The base layer just beneath the pavement largely contributes to the structural integrity of the highways and makes them strong enough to withstand the traffic loads to which they will be subjected,” explained South Dakota State University assistant professor Rouzbeh Ghabchi of the Department of Civil and Environmental Engineering. He is working with the South Dakota Department of Transportation to evaluate an effective, quick and inexpensive method of determining the quality of the base layer in the field.
To form the base layer, stones, known as aggregates, with different particle sizes, are mixed with gravel/sand and then compacted in the construction site. “To reach maximum compaction, we need a certain level of moisture,” said Ghabchi, who began working on the S.D. DOT-funded project in May 2020.
The goal of the project is to verify whether compaction curves developed in a previous SDSU research project can be used in the field to produce maximum density using at the optimum moisture content,” Ghabchi said. “If we can evaluate the materials to spot any inconsistencies during construction, that can help mitigate any problems before placing the surface course. This will result in an improved durability and longevity of the highway.”
The challenge is the materials vary based on the road’s location because the aggregates come from the nearest quarry to reduce material hauling costs. In addition, the moisture content of aggregates can vary, even during a day’s work, based on the weather conditions.
Correlating strength with density
Ghabchi is figuring out whether the penetration index value calculated through a tool called a dynamic cone penetrometer, or DCP, can reliably determine the in-situ compaction quality of the base layer.
“The DCP test provides us an index which correlates with the strength characteristics of the geomaterials. We are trying to verify if the DCP index has a strong correlation with in-situ density,” Ghabchi explained.
The DCP used in the study has an 8-kilogram drop hammer that drives a shaft with a cone-shaped tip into the base layer. The penetration distance of the shaft is measured after the specified number of hammer drops, as prescribed by American Society of Testing and Materials, or ASTM, standards.
“Specifically, we are identifying which types of materials can be evaluated using this device,” Ghabchi said, noting inconsistences and variability, such as segregation of the aggregates, may affect the results. “If, by chance, the tip hits some big stones, the penetration will be low; if it lands in a soft spot, the value will be high.”
Ghabchi is working in collaboration with S.D. DOT office of research personnel to gather samples and perform testing at 10 highway construction sites.
“We want to cover as many types of materials as possible,” he said. Last year, they completed work at six sites.
The site environment can vary dramatically, Ghabchi pointed out. “On a sunny, windy day, the moisture in the base material can evaporate quickly. Within a couple of hours, you are dealing with a different moisture content that affects the material properties.” This can create variability in the quality of the base layer.
To monitor how moisture changes, the crew collects samples as the base layer is being compacted. “We look at five levels of compaction and take samples to test moisture, density and amount of penetration using the DCP after each pass of the roller,” Ghabchi explained. Sand cone testing is also used to measure density at each compaction level.
Ghabchi expressed appreciation for this opportunity to collaborate with S.D. DOT personnel: “They are very experienced people and their input is of vital importance to success of the project. This is a great experience. Working with people who are building the highways will help our students connect their knowledge to the real world and make them better prepared to join the workforce after graduation.”
The S.D. DOT collects samples of uncompacted aggregates from each construction site. The collected materials will then be tested in the laboratory under controlled conditions. “We will build a test box and will compact unbounded aggregates while precisely controlling testing parameters—density and moisture content—and do a number of tests done in the field,” Ghabchi explained.
The lab tests will allow the researchers to “establish benchmarks with which they can compare the field results to identify sources of variability.” Based on this comparison and researchers’ input, S.D. DOT officials will decide whether the penetration index value along with the compaction curves are a reliable means of evaluating base layer density.