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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
Fine Aggregate Bulk Specific Gravity
Standard Test Methods
The standard test methods for determining fine aggregate Gsb are presented in AASHTO T 84 and ASTM C128. The two procedures are similar, except for the required period in which a sample of fine aggregate is submersed in water to essentially fill the pores. The AASHTO T 84 procedure calls for immersion of fine aggregate in water for 15 to 19 hours, while the ASTM C128 method specifies a soaking period of 24 ± 4 hours. For both methods, the sample is then spread on a pan and exposed to a gentle current of warm air until approaching a free flowing condition. Periodically, the aggregate is lightly tamped into a cone-shaped mold with 25 light drops of the tamper. If the fine aggregate retains the molded shape when the mold is removed, the fine aggregate is assumed to have surface moisture, and it is dried further. When the cone of sand just begins to slump upon removal of the mold, it is assumed to have reached the SSD condition. Three masses are determined from the method using either gravimetric or volumetric methods, SSD, saturated sample in water, and oven dry. These are used to calculate Gsb.
Precision Estimates of Standard Test Methods
The precision estimates are the same for both standard methods and are shown in Table 4. Precision estimates for the current standard test methods for determining Gsb of fine aggregate are also published annually on the AMRL website (1). Table 5 shows these precision indices. Compared to the precision estimates shown in Table 4, all of the precision indices reported by the AMRL until 2006 are greater, and they also vary significantly from year to year. Since 2007, the new method of screening data has detected more outliers, resulting in smaller precision indices than those shown in Table 4. The precision estimates in the current standard test methods should be re-established.
Precision estimates for the current standard test methods for determining Gsb of fine aggregate are also published annually on the AMRL website (1). Table 5 shows these precision indices. Compared to the precision estimates shown in Table 4, all of the precision indices reported by the AMRL until 2006 are greater, and they also vary significantly from year to year. Since 2007, the new method of screening data has detected more outliers, resulting in smaller precision indices than those shown in Table 4. The precision estimates in the current standard test methods should be re-established.
Shortcomings of Standard Test Methods
Problems with the standard test methods for determining fine aggregate Gsb are:
Modifications for Determining SSD Condition of Fine Aggregate
Most modifications to the standard test methods have been undertaken in order to better pinpoint the saturated surface-dry condition of fine aggregate and thereby improve the accuracy of Gsb test results. The Gsb value is used to calculate the amount of asphalt binder absorbed by the aggregate and the VMA of the HMA mixture. These modifications along with their advantages and disadvantages are briefly described in Table 6.
Alternatives to Standard Test Methods
Several alternatives to the AASHTO T 84 and ASTM C128 procedures are available to determine fine aggregate Gsb. These alternatives along with their advantages and disadvantages when compared to the standard test methods are briefly described in Table 7.
These alternative test methods are expected to address the shortcomings of the standard test methods. A number of studies have been conducted to evaluate the reproducibility of the alternative procedures and to determine if any of the alternatives would produce results statistically similar to those produced by the standard test methods.
In 2000, Kandhal et al. (7) conducted a research project to develop a new method using automated equipment for determining the SSD condition of fine aggregate. The work was based on basic principles of thermodynamics that had been studied by Dana and Peters (8) of the Arizona Department of Transportation. The equipment measures the temperature gradient of the incoming and outgoing warm air blown into a rotating drum. The SSD condition is achieved when the thermal gradient drops suddenly. While the method shows promise, more effort is needed to improve the repeatability and reproducibility of the test.
Recently, several studies have been conducted to compare the AggPlus system using the CoreLok vacuum-sealing device to the AASHTO T 84 procedure. In 2004, Hall (4) conducted an evaluation study in which one operator performed all testing of five replicates for each of five fine aggregate materials using both test methods. He reported that Gsb results for some fine aggregates determined using the two methods were significantly different at the 95 percent confidence level. The AggPlus system was also evaluated in a round-robin study conducted with 12 laboratories by Prowell and Baker (9) using six materials, four crushed and two natural fine aggregate sources. The study found that Gsb results using the two methods were statistically different for three of six aggregates, including limestone, washed diabase, and blast furnace slag. The differences were believed to be due to over drying the aggregate. This lead to inaccurate results for angular materials with high dust contents using the AASHTO T 84 procedure. The precision indices of the CoreLok method were not as good as those of AASHTO T 84, but the authors suggested that the precision would be improved as technicians became more familiar with the CoreLok method.
Another evaluation study was conducted by Sholar et al. (6) of the Florida Department of Transportation. One operator tested two replicates for each of seven aggregates using the CoreLok method and AASHTO T 84. The study found that the CoreLok and AASHTO T 84 gave similar Gsb results for three low absorptive granite aggregates but different Gsb values for four high absorptive limestone aggregates. The CoreLok method produced slightly higher Gsb values for the granite aggregate and lower Gsb values for limestone aggregate. For the limestone aggregate, the average difference in Gsb between the two test methods was 0.040, which would result in a change in VMA of 1.4 percent. The repeatability of Gsb results using the CoreLok was judged to be slightly better than that of AASHTO T 84.
The most recent evaluation study was conducted by Cross et al. (10) in 2006 using 14 fine aggregates of various types, including limestone, sandstone, granite, rhyolite, and natural sand. They reported that Gsb results using the CoreLok and AASHTO T 84 methods were significantly different, and the CoreLok tended to produce higher Gsb values.
In summary, studies have shown that Gsb results using the CoreLok method are statistically different from those of the AASHTO T 84 procedure for a variety of aggregate sources. Some studies have shown that the precision of the CoreLok is not as good as that of AASHTO T 84, whereas other studies have shown repeatability of the CoreLok method to be better.
Like the AggPlus system, the SSDetect system does not require the material be immersed in water for at least 15 hours or for the operator to determine SSD condition. The SSDetect system was compared to the AASHTO T 84 procedure in two projects, one conducted by Prowell and Baker (9) and the other by Cross et al. (10). Materials used and research plans implemented in these studies were previously described. Prowell and Baker (9) reported that Gsb results using the two methods were significantly different for three aggregates, including washed diabase, rounded natural sand, and angular natural sand. However, these differences were less than those between the CoreLok and AASHTO T 84 Gsb results. The precision of the SSDetect method was better than that of AASHTO T 84. Cross et al. (10) also found significant differences between Gsb results determined by the SSDetect and AASHTO T 84 methods. In addition, the SSDetect method produced the highest Gsb results, followed by the CoreLok and AASHTO T 84. However, the SSDetect system has better reproducibility than the other two methods. In summary, the two studies showed the significant differences between Gsb results determined by the SSDetect and AASHTO T 84 methods. In addition, the precision of the SSDetect system was better than that of AASHTO T 84. However, the studies had different conclusions on the differences in Gsb results using the CoreLok, SSDetect, and AASHTO T 84 methods. These different conclusions may be due to different materials used in the two studies.
For the method using the Langley de-airing device with AASHTO T 84, one study was conducted by Cross et al. (10). The study compared Gsb results using the AASHTO T 84 procedure with hand agitation and with a Langley de-airing device for 20 minutes to remove air bubbles from the sample in the flask. They reported that the use of the Langley de-airing device improved the reproducibility of the test results; however, the results were not statistically different in most cases.
The Phunque method has been approved for use in New Mexico since July 1, 2006. However, no published evaluation study is available at the time of this writing. A preliminary comparison shown in an electronic presentation received by the authors showed that Gsb results using the Phunque method were higher than those using the AASHTO T 84 method.
Equipment for the Gilson Water Rapid Displacement method is currently being developed and therefore no study results are available for the method.
In summary, most recent studies of alternative test methods for determining fine aggregate Gsb focus on evaluating two recently developed test procedures, including the AggPlus and SSDetect methods. These methods have been developed to avoid the determination of SSD condition manually and to reduce the aggregate soaking time. The studies show that Gsb results determined using alternative test methods are statistically different from those using AASHTO T 84. The differences appear to be greater for more angular fine aggregate with higher dust contents. Among the alternative methods evaluated, the SSDetect has better precision than AASHTO T 84.