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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.

Table 4 AASHTO T 84 and ASTM C128 Precision Estimates
 Standard Deviation (1s)Acceptable Range of Two Results (d2s)
Single-operator precision: Bulk specific gravity (dry) 0.011 0.032
Multilaboratory precision: Bulk specific gravity (dry) 0.023 0.066

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.

Table 5 AASHTO T 84/ASTM C128 Precision Estimates Published by AMRL
Year Sample No. No. of Labs Single Operator Multilaboratory
Participated* Data Used** 1s d2s 1s d2s
2007 155/156 1025 946 0.006 0.018 0.014 0.040
2006 151/152 1044 1016 0.017 0.048 0.029 0.081
2005 147/148 965 939 0.016 0.045 0.033 0.093
2004 143/144 951 936 0.019 0.054 0.041 0.115
2003 139/140 864 850 0.017 0.048 0.037 0.105
2002 135/136 753 739 0.014 0.040 0.034 0.095
2001 131/132 656 642 0.015 0.044 0.033 0.093
2000 127/128 586 579 0.021 0.060 0.041 0.115
1999 123/124 551 540 0.013 0.038 0.028 0.079
1998 119/120 483 475 0.035 0.098 0.045 0.127
  • *Total number of laboratories participated in the program each year
  • **Number of laboratories whose data were used to determine precision estimates

Shortcomings of Standard Test Methods

Problems with the standard test methods for determining fine aggregate Gsb are:

  • The SSD condition of some fine aggregate may not be determined consistently using the cone and tamp technique because the amount of slump of the fine aggregate is not just dependent on the quantity of surface moisture but also upon the angularity and texture of the fine aggregate (6). In addition, it is suspected that the percentage of material passing the No. 100 sieve may also influence the slump condition (2). This will result in an inaccurate determination of SSD mass and thereby the calculation of Gsb.
  • Both standard test methods, including aggregate soaking time, cannot be completed in a working day. It makes the tests less effective for quality control purposes, where results typically are desired as quickly as possible.

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.

Table 6 Modifications of Standard Test Methods for Determining SSD Condition of Fine Aggregate
Method Description Advantages Disadvantages
Provisional Cone Test (AASHTO T 84 Note 2 and ASTM C128)
  • Fill cone mold and use 10 drops of tamper
  • Add more FA and use 10, 3 and 2 drops of tamper, respectively
  • Level off and lift mold vertically
  • Easy and quick to perform
  • Same shortcomings as standard test method
Kandhal and Lee Colorimetric Procedure (AASHTO T 84 Note 2 & ASTM C128)
  • FA is soaked in water containing special dye that changes color when dry
  • Upon removal from water, FA has color of wet dye
  • SSD condition reached when material changes color
  • Easy to perform
  • Dyes do not show well on dark FA particles
  • Differential drying on particle size
  • Technician judgment on color change required
Paper Towel (AASHTO T 84 Note 2 and ASTM C128)
  • Use hard-finished paper towels to surface dry FA
  • SSD condition just achieved when paper towel not picking up moisture from surface of FA
  • Easy to perform
  • Technician judgment required
California (California Test 225: Option 1)
  • Place portion of drying FA in a dry glass jar and shake
  • SSD condition is when FA ceases to adhere to dry surface
  • Easy and quick to perform
  • Technician judgment required
Texas (Tex 201-F) SSD condition is when 2 of 4 criteria below satisfied:
  • Criterion 1: drying FA slides in same manner as oven-dry FA slides down bottom of 45-deg tilted pan
  • Easy and quick to perform
  • Technician judgment required
  • Criterion 2: drying FA flows freely off a small masonry trowel in same manner as oven-dry FA when trowel tilted slowly to one side
  • Easy and quick to perform
  • Technician judgment required
  • Criterion 3: place water-soluble glue surface of wood block on drying FA for 5 seconds. SSD condition is when no more than 2 particles adhere to water soluble glue after 2 checks
  • Easy and quick to perform
  • Technician judgment required
  • Criterion 4: drying FA has same color as oven-dried FA
  • Easy and quick to perform
 
Wisconsin (Modified AASHTO T 84)
  • Minus No. 200 is removed by rinsing FA over No. 200 screen
  • More consistent results
  • Technician judgment required
  • Does not include minus No. 200 fraction
Iowa (IM 380)
  • FA is covered with water and placed under 30 mm Hg vacuum for 30 min. and then allowed to stand for another 20 min. Sample is then rinsed over No. 200 sieve. SSD condition achieved when FA grains do not adhere to steel spatula
  • Used for both combined and individual aggregate
  • No soak time required
  • More consistent results
  • Technician judgment required
  • Does not include minus No. 200 fraction

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.

Table 7 Advantages and Disadvantages of Alternative Methods for Determining Gsb of Fine Aggregate
Method AASHTO and/or ASTM Designation Advantages Disadvantages
NCAT / Dana and Peters (8) Arizona DOT Procedure Use with AASHTO T 84 or ASTM C128
  • Automated determination of SSD condition
  • More expensive than standard methods due to equipment cost
  • More effort to improve reproducibility needed
AggPlus / CoreLok System or Vacuum-Seal Method (Instrotek) None
  • SSD weight not required
  • Result in 30 min.
  • Long soaking period not required
  • Use for both coarse and fine aggregate
  • More expensive than standard methods due to equipment and bag costs
  • Precision not as good as that of AASHTO T 84
SSDetect (Thermolyne) ASTM D7172-06
  • SSD condition automatically determined
  • Result in 1 to 2 hrs.
  • Long soaking period not required
  • Improved precision compared with AASHTO T 84
  • More scientific/rational approach
  • More expensive than standard methods due to equipment cost
  • Limited research available this time
AASHTO T 84 with Langley De-airing Device Use with AASHTO T 84 or ASTM C128
  • Reproducibility improved
  • Hand agitation not required
  • Equipment cost
  • Limited research available at this time
Phunque Method (New Mexico DOT) Requesting for an AASHTO temporary test procedure
  • SSD weight not required
  • Takes 25 hrs to complete
  • Specific gravity and absorption very different from AASHTO T 84
  • No research available at this time
Rapid Water Displacement (Gilson) None
  • SSD weight not required
  • Equipment being developed; no research available at this time

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.

 
Updated: 11/22/2011
 

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