Optimizing In-Place Density Through Improved Density Specifications

The objective of this research was to (1) determine critical requirement(s) for in-place density based on a review of the literature; (2) analyze density test results shared by state highway agencies (SHAs) across the country to identify state specifications that minimize density results failing the identified critical requirement; and (3) document specification parameters that are important to achieve the critical requirement to share with SHAs that are interested in improving their density specifications. Based on prior research, the minimum density of an asphalt mixture should be 92.0% of the theoretical maximum specific gravity, as density below this critical level would have a detrimental effect on the long-term performance of the mix. Twelve SHAs identified thus far in this research have successfully adopted density specifications that minimize the number of test results below the 92.0% threshold. The statewide density results below the threshold in these states ranged from 3.1 to 11.0%. The density specifications in the 12 states play an important role in achieving these results as discussed in the paper. The case study presented in this paper showed that the density results below the identified threshold for a state in the Federal Highway Administration (FHWA) Demonstration Project decreased from 20.0% to only 5.7% with an improved density specification. There are likely more states with test results like those identified, and they will be added as they are identified in the future. In addition, more states will be added as they make improvements to their density specifications through this effort.

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