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A Review of Aggregate and Asphalt Mixture Specific Gravity Measurements and Their Impacts on Asphalt Mix Design Properties and Mix Acceptance
Bulk Specific Gravity of Aggregate
Bulk specific gravity is defined as the ratio of the weight of a given volume of aggregate, including the permeable and impermeable voids in the particles, to the weight of an equal volume of water. Bulk specific gravity of aggregate is important information for designing HMA because it is used to calculate VMA and VFA. Since different procedures are used to determine the Gsb of coarse and fine aggregate, this section is divided into two parts, one for coarse aggregate and one for fine aggregate.
Coarse Aggregate Bulk Specific Gravity
Standard Test Methods
The standard test methods used for the determination of specific gravity of coarse aggregate are described in AASHTO T 85 and ASTM C127. The methods are essentially the same, except for the required time in which a sample of aggregate is submersed in water to essentially fill the pores. While the AASHTO standard requires the sample be immersed for a period of 15 to 19 hours, the ASTM method specifies an immersed period of 24 ± 4 hours. After the specimen is removed from the water, it is rolled in an absorbent towel until all visible films of water are removed. This is defined as the saturated surface-dry (SSD) condition. Three mass measurements are obtained from a sample: SSD mass, water submerged mass, and oven dry mass. Using these mass values, the Gsb of an aggregate can be determined.
Precision Estimates of Standard Test Methods
Even though the two standard methods require different saturation periods, the precision indices are the same, as shown in Table 1.
Precision estimates for the standard coarse aggregate Gsb test methods are also determined annually by the Proficiency Sample Programs and reported on the AMRL website (1). These precision indices are shown in Table 2. The precision estimates from 1998 through 2005 vary significantly from year to year due partially to the use of different aggregate sources in the program. The precision estimates from the proficiency program are greater than the precision estimates cited in the standard test methods (Table 1). Since 2006, the Proficiency Sample Programs have used a different method of screening data (2) that detects more outliers, resulting in precision estimates that are smaller than those cited in the current standards. Due to these differences, the precision estimates in the standard test methods should be re-established.
Shortcomings of Standard Test Methods
Problems with the current standard test methods are:
Alternatives to Standard Test Methods
Alternatives to the standard test methods of determining the bulk specific gravity of coarse aggregate are available. Table 3 summarizes the advantages and disadvantages of the alternatives to the standard test methods.
The two alternative methods shown in Table 3 are expected to address the shortcomings of the current standard test methods. A number of investigators have attempted to evaluate the AggPlus system against the current AASHTO method for determining the specific gravity and absorption of coarse aggregate. For the Gilson Rapid Water Displacement method, equipment is currently being developed, so no comparison is available at this time. However, the AggPlus/CoreLok system or vacuum-seal method has been studied by several researchers. The objectives of these studies were to evaluate the reproducibility of the AggPlus system and to determine if it would produce results statistically different from those produced by the current standard test methods.
In 2004, Hall (4) measured bulk specific gravity of six coarse aggregates from various mineralogy sources in Arkansas using the AASHTO T 85 and vacuum-seal (CoreLok) method. To minimize sources of variability, one operator conducted all testing of five replicates for each aggregate using both test methods. Hall reported that Gsb values determined using the two test procedures were significantly different. The AggPlus system tended to produce higher Gsb values for coarse aggregate with absorptions of more than one percent regardless of mineralogy. More effort was recommended to improve the test consistency and produce test results comparable to those resulting from the standard test methods if the results are to be used for specification purposes.
In 2005, Mgonella (5) evaluated the AggPlus system against the AASHTO T 85 method using eight coarse aggregates representing four basic aggregate types in Oklahoma. The tests were performed by two operators to determine the interaction between the test methods and operators. Mgonella reported that Gsb values determined using the two methods were statistically different. The AggPlus system produced higher Gsb values. No interactions between Gsb values and operators were found for either test method. The AggPlus system and the AASHTO T 85 method had similar reproducibility. The research did not recommend the alternative procedure for replacement of the current AASHTO T 85 method.
Another evaluation of the AggPlus system using the CoreLok vacuum-seal device was performed by Sholar et al. (6) and compared to the Florida Department of Transportation FM 1-T 085 procedure, which is similar to the AASHTO T 85 method. The test plan used 11 coarse aggregates from six sources in Florida and Georgia. One operator tested two replicates for individual coarse aggregates using both test methods. Sholar et al. reported that the AggPlus method produced higher Gsb values and the difference was greater for higher absorptive aggregate. The difference was approximately 0.165 for absorptive aggregate, which would result in a VMA change of 5.5 percent. In most HMA applications, such a difference in VMA would be significant. Influence of aggregate gradation on aggregate Gsb was not significant. The repeatability of the AggPlus system was slightly better than the standard test method with respect to bulk specific gravity. The research team did not recommend the AggPlus system for use as a test procedure for determining coarse aggregate Gsb in Florida.
In summary, all studies found that Gsb values determined using the AggPlus and AASHTO T 85 procedures were significantly different. The AggPlus system produced higher specific gravity values with greater differences for highly absorptive coarse aggregate. In one study, the difference in Gsb would result in a VMA change of 5.5 percent, which would be significant in most HMA applications. Test results using the AggPlus system were not sensitive to nominal maximum aggregate size, gradation, or mineralogy. All studies recommended that the AggPlus system not be used for determining specific gravity and absorption of coarse aggregate in existing specifications.