Featured in Technology Today (Volume 33, Issue 1), a quarterly publication of the Louisiana Transportation Research Center.
The density of soil and asphalt layers is often considered the most important component in the construction of durable, long-lasting roads. To meet density requirements, contractors and transportation agencies follow quality control (QC) and quality assurance (QA) procedures to ensure specifications are met and performance is achieved. However, many contractors and DOTD personnel have voiced concerns over the unsafe, expensive, and time-consuming issues regarding the current density measures.
As a result, local geotechnical and asphalt research groups came together to find solutions. David Mata, P.E., and LTRC researchers Nicholas Ferguson, E.I., and Saman Salari, E.I., launched a study to test new non-destructive methods.
Current Concerns
The current methods used to measure density include placing a nuclear density gauge (NDG) on a pavement or taking asphalt cores from new pavement. However, these procedures typically show problems such as: nuclear radiation, decreased safety, additional training requirements, special storage and handling, damage to the fresh pavement, long testing times, and small sample sizes.
Ferguson explained, “Non-destructive methods are trying to fix these issues. In the geotechnical group’s point of view, the research was to see if we could use a device that reduces the utilization of radiation.”
Advantages of New Methods
“Low- to non-nuclear gauge methods offer advantages of economic savings, faster data measurement, no intense federal regulations, decreased concerns for safety, no extra licensing and intense training, improved calibration techniques, non-destructive testing, faster testing times, and increased density measurements throughout the entire paving project,” explained Mata.
Research + Results
To compare the current methods against the new non-destructive methods, a validation study was conducted. The geotech and asphalt groups evaluated two types of density gauges along with typical coring (for asphalt) and density measurements. For asphalt, the density gauges were nuclear and non-nuclear (electric pulses); for geotech, the density gauges were nuclear and low nuclear (smaller gamma source).
Researchers in the asphalt group determined that the non-destructive alternatives to measuring density, such as the non-nuclear gauges, are accurate and economically feasible. Mata explained, “Based on the results of the asphalt research, we recommend the use of the non-destructive testing for both QC and QA testing, provided the manufacturer’s and AASHTO T-343’s recommendation is followed to calibrate the device daily by applying a core-calibration offset.”
However, while still hopeful of future technological improvements, the geotech group decided to continue with the conventional methods due to a concern with the low nuclear density gauge’s probe depth and short life cycle.
Please visit Final Report 600 to learn more about this project. You may also contact Saman Salari at Saman.Salari@la.gov and Nicholas Ferguson at Nicholas.Ferguson@la.gov.