A Review of Aggregate and Asphalt Mixture Specific Gravity Measurements and Their Impacts on Asphalt Mix Design Properties and Mix Acceptance
This Technical Brief provides an overview of the impacts of aggregate and asphalt mixture specific gravity measurements on asphalt mix design properties and mix acceptance.
Current practices for asphalt mix design and acceptance testing rely on volumetric properties. Vital to the calculation of mix volumetric properties are specific gravity measurements of the mixture and the aggregate in the mixture. In essence, the specific gravity measurements are conversion factors which allow conversion of mass percentages to volume proportions/percentages. The accuracy and reliability of the specific gravity measurements are therefore fundamental to the business of building quality hot-mix asphalt (HMA) pavements.
This Technical Brief summarizes a critical review of specific gravity measurement methods. This review was conducted as part of a task group consisting of the authors under the direction of the FHWA Asphalt Mix and Construction Expert Task Group. The objectives of this review are to summarize problems and issues with current methods, examine possible improvements and/or alternate methods, and identify areas that need further research and development.
The approach to this review had been to separately examine three specific gravity determinations, namely, the bulk specific gravity of the aggregate (Gsb), the maximum specific gravity of asphalt mixtures (Gmm), and the bulk specific gravity of compacted specimens (Gmb). The review draws upon information from recently published research studies, information from state DOTs and equipment manufacturers, and precision information cited in AASHTO and ASTM standards and published on the AASHTO Materials Reference Laboratory (AMRL) website (1). This report is organized by discussion of each of these measurements followed by a summary which considers the overall effect of the measurements on asphalt mixture volumetric properties and current criteria for mix design and acceptance. The view of the task group has been that change(s) to the current specific gravity methods may be motivated by one or more of the following three reasons:
The change(s) or new method(s) will provide specific gravity results closer to the truth (i.e. greater accuracy); the change(s) or new method(s) will yield more repeatable results (i.e. better precision); and the change(s) or new method(s) will be faster, easier, and/or less expensive.
Air Voids (Va), Voids in the Mineral Aggregate (VMA), Voids Filled with Asphalt (VFA), and Volume of Effective Binder (Vbe) are calculated from the following well known equations:
- Gmb =bulk specific gravity of the compacted sample
- Gmm =maximum specific gravity of the asphalt mixture
- Ps =percentage (by mass) of aggregate in the total mixture
- Gsb =aggregate bulk specific gravity
With these equations, the effects of the specific gravity results can be analyzed more closely and the following approximate relationships can be determined.
- From Equation 1, if Gmb is held constant, the following relationship between Air Voids and Gmm is established:
when Gmm changes by +0.01, Va changes by +0.4%. (5)
- Likewise, if Gmm is held constant in Equation 1, the following relationship between Air Voids and Gmb is established:
when Gmb changes by +0.01, Va changes by -0.4%. (6)
- From Equation 2, when Gsb and Ps are held constant, the following relationship between VMA and Gmb is established:
when Gmb changes by +0.01, VMA changes by -0.4%. (7)
- And also from equation 2, when Gmb and Ps are held constant, the following relationship between VMA and Gsb is established:
when Gsb changes by +0.01, VMA changes by +0.3%. (8)
|The Asphalt Pavement Technology Program is an integrated, national effort to improve the long-term performance and cost effectiveness of asphalt pavements. Managed by the Federal Highway Administration through partnerships with State highway agencies, Industry and academia the program's primary goals are to reduce congestion, improve safety, and foster technology innovation. The program was established to develop and implement guidelines, methods, procedures and other tools for use in asphalt pavement materials selection, mixture design, testing, construction and quality control.|