Aggregate Specific Gravity Aggregate specific gravity is useful in making weight-volume conversions and in calculating the void content in compacted HMA (Roberts et al., 1996[1]). AASHTO M 132 and ASTM E 12 define specific gravity as: “…the ratio of the mass of a unit volume of a material at a stated temperature to the mass of the same volume of gas-free distilled water at a stated temperature.” The commonly used “stated temperature” is 23° C (73.4° F). Given the structure of a typical aggregate particle, there are several different kinds of specific gravity. This section will first describe the structure of a typical aggregate particle and then discuss each type of specific gravity and its use. Aggregate Particle Structure
A typical aggregate particle consists of some amount of solid material along with a certain amount of air voids. These air voids within the aggregate particle (Figure 1) can become filled with water, binder or both (Figure 2). It takes a finite amount of time for water/binder to penetrate these pores, so specific gravity test procedures generally contain a 15 to 19-hour (for AASHTO procedures) or a 24-hour (for ASTM procedures) soak period for the purpose of allowing penetration into these pores.
Figure 1. Dry aggregate.
Figure 2. Wet aggregate.
Depending upon how aggregate voids are dealt with, calculated aggregate specific gravities can vary. If they are excluded entirely, then the specific gravity is that of the solid portion of the aggregate only, while if they are included entirely then the specific gravity essentially becomes a weighted average of the specific gravity of the solid aggregate and whatever is in its voids.
Aggregate Specific Gravities Generally, there are three different aggregate specific gravities used in association with pavements: 1. Bulk 2. Apparent 3. Effective
Coarse Aggregate Specific Gravity Overview The coarse aggregate specific gravity test (Figure 1) is used to calculate the specific gravity of a coarse aggregate sample by determining the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It is similar in nature to the fine aggregate specific gravity test.
Figure 1: Coarse Aggregate Specific Gravity (CASG). The coarse aggregate specific gravity test measures coarse aggregate weight under three different sample conditions:
Oven-dry (no water in sample). Saturated surface-dry (SSD, water fills the aggregate pores).
Submerged in water (underwater).
Using these three weights and their relationships, a sample’s apparent specific gravity, bulk specific gravity and bulk SSD specific gravity as well as absorption can be calculated. Aggregate specific gravity is needed to determine weight-to-volume relationships and to calculate various volume-related quantities such as voids in mineral aggregate (VMA), and voids filled by asphalt (VFA). Absorption can be used as an indicator of aggregate durability as well as the volume of asphalt binder it is likely to absorb. The standard coarse aggregate specific gravity and absorption test is:
AASHTO T 85 and ASTM C 127: Specific Gravity and Absorption of Coarse Aggregate
Background Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73.4°F (23°C). Therefore, by definition, water at a temperature of 73.4°F (23°C) has a specific gravity of 1. Absorption, which is also determined by the same test procedure, is a measure of the amount of water that an aggregate can absorb into its pore structure. Pores that absorb water are also referred to as “water permeable voids”. Specific Gravity Use
Aggregate specific gravity is used in a number of applications including Superpave mix design, deleterious particle identification and separation, and material property change identification. Superpave Mix Design
Superpave mix design is a volumetric process; it relies on mixing constituent materials on the basis of their volume. However, aggregate and asphalt binder volumes are difficult to measure directly, therefore a material’s weight is typically measured and then converted to a volume based on its specific gravity. Correct and accurate material specific gravity determinations are vital to proper mix design. An incorrect specific gravity value will result in incorrect calculated volumes and ultimately result in an incorrect mix design.
Material Contamination Indicator and Separator
Specific gravity can also indicate possible material contamination. For instance, deleterious particles (Figure 2) are often lighter than aggregate particles and therefore, a large amount of deleterious material in an aggregate sample may result in an abnormally low specific gravity. Figure 2: Deleterious Materials. Differences in specific gravity can also be used to separate deleterious, or bad, particles from aggregate particles using a heavy media liquid. Water absorption can also be an indicator of asphalt absorption. Material Change Indicator
Finally, specific gravity differences can be used to indicate a possible material change. A change in aggregate mineral or physical properties can result in a change in specific gravity. For instance, if a quarry operation constantly monitors the specific gravity of its output aggregate, a change in specific gravity beyond that normally expected could indicate the quarrying has moved into a new rock formation with significantly different mineral or physical properties. Aggregate Absorption Use
Aggregate absorption is the increase in mass due to water in the pores of the material. Aggregate absorption is a useful quality because: 1. High values can indicate non-durable aggregate. 2. Absorption can indicate the amount of asphalt binder the aggregate will absorb.
It is generally desirable to avoid highly absorptive aggregate in HMA. This is because asphalt binder that is absorbed by the aggregate is not available to coat the aggregate particle surface and is therefore not available for bonding. Therefore, highly absorptive aggregates (often specified as over 5 percent absorption) require more asphalt binder to develop the same film thickness as less absorptive aggregates making the resulting HMA more expensive. Aggregate Specific Gravity Types
Several different types of specific gravity are commonly used depending upon how the volume of water permeable voids (or pores) within the aggregate are addressed (Figure 3): Figure 3: Aggregate Specific Gravities.
Apparent Specific Gravity, Gsa. The volume measurement only includes the volume of the aggregate particle; it does not include the volume of any water permeable voids. The mass measurement only includes the aggregate particle. Apparent specific gravity is intended to only measure the specific gravity of the solid volume, therefore it will be the highest of
the aggregate specific gravities. It is formally defined as the ratio of the mass of a unit volume of the impermeable portion of aggregate (does not include the permeable pores in aggregate) to the mass of an equal volume of gas-free distilled water at the stated temperature.
Bulk Specific Gravity (Bulk Dry Specific Gravity), G sb. The volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement only includes the aggregate particle. Since it includes the water permeable void volume, bulk specific gravity will be less than apparent specific gravity. It is formally defined as the ratio of the mass of a unit volume of aggregate, including the water permeable voids, at a stated temperature to the mass of an equal volume of gas-free distilled water at the stated temperature.
Bulk
Saturated
Surface
Dry
(SSD)
Specific
Gravity.
Volume
measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement includes the aggregate particle as well as the water within the water permeable voids. It is formally defined as the ratio of the mass of a unit volume of aggregate, including the weight of water within the voids filled to the extent achieved by submerging in water for approximately 15 hours, to the mass of an equal volume of gas-free distilled water at the stated temperature.
Effective Specific Gravity, Gse. Volume measurement includes the volume of the aggregate particle plus the void volume that becomes filled with water during the test soak period minus the volume of the voids that absorb asphalt. Effective specific gravity lies between apparent and bulk specific gravity. It is formally defined as the ratio of the mass in air of a unit volume of a permeable material (excluding voids permeable to asphalt) at a stated temperature to the mass in air (of equal density) of an equal volume of gas-free distilled water at a stated temperature. Effective specific gravity is determined by a different procedure and is not covered in this section.
Relationship with Other Specific Gravities
Refer to Figure 4 for abbreviations.
Figure 4: Abbreviations.
The difference between Gsa and Gsb is the volume of aggregate used in the calculations. The difference between these volumes is the volume of absorbed water in the aggregate’s permeable voids. Both use the aggregate’s oven dry weight.
The difference between Gsb and bulk (SSD) specific gravity is the weight of aggregate used in the calculations. The difference between these weights is the weight of absorbed water in the aggregate’s permeable voids. Both use the same aggregate volume.
The difference between Gsa, Gse and Gsb is the volume of aggregate used in the calculations. All three use the aggregate’s oven dry weight.
The following relationships are always true: 1. Gsa ≥ Gse ≥ Gsb 2. Bulk (SSD) specific gravity ≥ Gsb 3. Aggregate specific gravities (Gsb, Gsa,Gse and bulk SSD specific gravity ) are all ≥ Gmm (because Gmm includes the asphalt binder, which has a lower specific gravity than the aggregate)
Test Description The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The complete procedure can be found in:
AASHTO T 85 and ASTM C 127: Specific Gravity and Absorption of Coarse Aggregate
Summary
The mass of a coarse aggregate sample is determine in SSD, oven-dry and submerged states. These values are then used to calculate bulk specific gravity(Gsb), bulk SSD specific gravity, apparent specific gravity(Gsa) and absorption. Figure 5 shows major coarse aggregate specific gravity equipment.
Figure 5: Major CASG equipment. Approximate Test Time
3 days (from sample preparation to final dry weight determination) Basic Procedure
1. Obtain a sample of coarse aggregate material retained on the No. 4 (4.75 mm) sieve (Figure 6). This sample size is based on nominal maximum aggregate size (NMAS). Sample sizes range from 2000 g for a 0.5 inch (12.5 mm) NMAS to 5000 g for a 1.5 inch (37.5 mm) NMAS.
Figure 6: No. 4 (4.75 mm) sieve. 2. Prepare the material.
Wash the aggregate retained on the No. 4 (4.75 mm) sieve. This discards small aggregate particles clinging to the retained large particles. Dry the material until it maintains a constant mass. This indicates that all the water has left the sample. Drying should occur in an oven regulated at 230°F (110°C).
Cool the aggregate to a comfortable handling temperature.
Immerse the aggregate in water at room temperature for a period of 15 to 19 hours (Figure 7).
Figure 7: Soaking the sample. WARNING
If the aggregate is not oven-dried before soaking, specific gravity values may be significantly higher. This is because in the normal procedure the water may not be able to penetrate the pores to the center of the aggregate particle during the soaking time. If the aggregate is not oven-dry to start, the existing water in the aggregate pore structure may be able to penetrate further into the pores (AASHTO, 2000c[1]). 3. Dry the sample to a saturated surface dry (SSD) condition. Rolling up the aggregate into the towel and then shaking and rolling the aggregate from side to side is usually effective in reducing the sample to a SSD condition (Video 1). It may be necessary to wipe the larger particles separately. Once there are no visible signs of water film on the aggregate particle surfaces, determine the sample mass. WARNING
Make sure to use cloth and not paper towels. Paper towels may absorb water in the aggregate pores.
Video 1: Drying a CASG sample.
4. Place the entire sample in a basket (Figure 8) and weigh it underwater (Figure 9). The basket should be pre-conditioned to the water bath temperature. Shake the container to
release any entrapped air before weighing. The container overflow needs to work properly to compensate for the water displaced by the sample.
Figure 8: The basket used for underwater weighing.
Figure 9: Weighing the sample underwater. 5. Remove the aggregate from the water and dry it until it maintains a constant mass. This indicates that all the water has left the sample. Drying should occur in an oven regulated at 230°F (110°C). 6. Cool the aggregate in air at room temperature for 1 to 3 hours then determine the mass.
Results Parameters Measured 1. Coarse aggregate bulk specific gravity. 2. Coarse aggregate bulk SSD specific gravity. 3. Coarse aggregate apparent specific gravity. 4. Coarse aggregate absorption. Specifications
There are no minimum or maximum specific gravity or absorption values in Superpave mix design. Rather, specific gravity is an aggregate quality needed to make required volume
calculations. Some state agencies specify minimum aggregate specific gravities or maximum percent water absorption to help control aggregate quality. Typical Values
Specific gravities can vary widely depending upon aggregate type. Some lightweight shales (not used in HMA production) can have specific gravities near 1.050, while other aggregate can have specific gravities above 3.000. Typically, aggregate used in HMA production will have a bulk specific gravity between about 2.400 and 3.000 with 2.700 being fairly typical of limestone. Bulk SSD specific gravities can be on the order of 0.050 to 0.100 higher than bulk oven dry specific gravities, while apparent specific gravities can be 0.050 to 0.100 higher still. For a particular aggregate type or source, fine aggregate specific gravities can be slightly higher than coarse aggregate specific gravities because as the aggregate particles get smaller, the fraction of pores exposed to the aggregate surface (and thus excluded from the specific gravity calculation because they are water-permeable) increases. Aggregate absorption can also vary widely depending upon aggregate type. Some lightweight shales (not used in HMA production) can have absorptions approaching 30 percent, while other aggregate types can have near zero absorption. Typically, aggregate used in HMA production will have an absorption between just above zero and 5 percent. Absorptions above about 5 percent tend to make HMA mixtures uneconomical because extra asphalt binder is required to account for the high aggregate absorption. If absorption is incorrectly accounted for, the resulting HMA could be overly dry and have low durability (absorption calculated lower than it actually is) or over-asphalted and susceptible to distortion and rutting (absorption calculated higher than it actually is). Calculations (see Interactive Equation)
Three different masses are recorded during the test. Their common symbols are: A = mass of oven-dry sample in air (g) B = mass of SSD sample in air (g) C = mass of SSD sample in water (g) These masses are used to calculate the various specific gravities and absorption using the following equations:
Note that the quantity (B – C) is the mass of water displaced by the SSD aggregate sample. In the apparent specific gravity calculation the mass of the SSD aggregate sample is replaced by the mass of the oven-dry aggregate sample (A replaces B), which means that the water permeable voids within the aggregate are not included and (A – C) is the mass of water displaced by the oven-dry sample. The ratios given in the equations are then simply the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water, which is specific gravity. WARNING
Certainly, the accuracy of all measurements is important. However, of specific concern is the mass of the SSD sample. The determination of SSD conditions can be difficult. If the sample is actually still wet on the surface then the mass of the SSD sample will be higher than it ought to be, which will cause a lower calculated bulk specific gravity. Conversely, if the sample is beyond SSD and some of the pore water has evaporated (which is more likely), the mass of the SSD sample will be lower than it ought to be, which will cause a higher calculated bulk specific gravity. Either type of error will have a cascading effect on volumetric parameters in other tests that require specific gravity as an input and Superpave mix design. A quick check of the results should show that bulk specific gravity is the lowest specific gravity, bulk SSD specific gravity is in the middle and apparent specific gravity is the highest.
Footnotes (↵ returns to text) 1. American Association of State Highway and Transportation Officials (AASHTO). (2000c). AASHTO Provisional Standards, April 2000 Edition. American Association of State Highway and Transportation Officials. Washington, D.C.↵
Fine Aggregate Specific Gravity Overview The fine aggregate specific gravity test (Figure 1) is used to calculate the specific gravity of a fine aggregate sample by determining the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It is similar in nature to the coarse aggregate specific gravity test.
Figure 1: Fine aggregate specific gravity sample and pycnometer. The fine aggregate specific gravity test measures fine aggregate weight under three different sample conditions:
Oven-dry (no water in sample). Saturated surface dry (water fills the aggregate pores).
Submerged in water (underwater).
Using these three weights and their relationships, a sample’s apparent specific gravity, bulk specific gravity and bulk SSD specific gravity as well as absorption can be calculated. Aggregate specific gravity is needed to determine weight-to-volume relationships and to calculate various volume-related quantities such as voids in mineral aggregate (VMA), and voids filled by asphalt (VFA). Absorption can be used as an indicator of aggregate durability as well as the volume of asphalt binder it is likely to absorb. The standard fine aggregate specific gravity and absorption test is:
AASHTO T 84 and ASTM C 128: Specific Gravity and Absorption of Fine Aggregate
Background Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73.4°F (23°C). Therefore, by definition, water at a temperature of 73.4°F (23°C) has a specific gravity of 1. Absorption, which is also determined by the same test procefure, is a measure of the amount of water that an aggregate can absorb into its pore structure. Pores that absorb water are also referred to as “water permeable voids”. Specific Gravity Use
Aggregate specific gravity is used in a number of applications including Superpave mix design, deleterious particle indentification and separation, and material property change identification. Superpave Mix Design
Superpave mix design is a volumetric process; it relies on mixing constituent materials on the basis of their volume. However, aggregate and asphalt binder volumes are diffucult to measure directly, therefore a material’s weight is typically measured and then converted to a volume based on its specific gravity. Correct and accurate material specific gravity determinations are vital to proper mix design. An incorrect specific gravity value will result in incorrectly calculated volumes and ultimately result in an incorrect mix design. Material Contamination Indicator and Separator
Specific gravity can also indicate possible material contamination. For instance, deleterious particles (Figure 2) are often lighter than aggregate particles and therefore, a large amount of deleterious material in an aggregate sample may result in an abnormally low specific gravity. Figure 2: Deleterious Particles. Differences in specific gravity can also be used to separate deleterious, or bad, particles from aggregate particles using a heavy media liquid. Water absorption can also be an indicator of asphalt absorption. Material Change Indicator
Finally, specific gravity differences can be used to indicate a possible material change. A change in aggregate mineral or physical properties can result in a change in specific gravity. For instance, if a quarry operation constantly monitors the specific gravity of its output aggregate, a change in specific gravity beyond that normally expected could indicate the quarrying has moved into a new rock formation with significantly different mineral or physical properties.
Aggregate Absorption Use
Aggregate absorption is the increase in mass due to water in the pores of the material. Aggregate absorption is a useful quality because: 1. High values can indicate non-durable aggregate. 2. Absorption can indicate the amount of asphalt binder the aggregate will absorb.
It is generally desirable to avoid highly absorptive aggregate in HMA. This is because asphalt binder that is absorbed by the aggregate is not available to coat the aggregate particle surface and is therefore not available for bonding. Therefore, highly absorptive aggregates (often specified as over 5 percent absorption) require more asphalt binder to develop the same film thickness as less absorptive aggregates making the resulting HMA more expensive. Aggregate Specific Gravity Types
Several different types of specific gravity are commonly used depending upon how the volume of water permeable voids (or pores) within the aggregate are addressed (Figure 3): Figure 3: Aggregate specific gravities.
Apparent Specific Gravity, Gsa. The volume measurement only includes the volume of the aggregate particle; it does not include the volume of any water permeable voids. The mass measurement only includes the aggregate particle. Apparent specific gravity is intended to only measure the specific gravity of the solid volume, therefore it will be the highest of the aggregate specific gravities. It is formally defined as the ratio of the mass of a unit volume of the impermeable portion of aggregate (does not include the permeable pores in aggregate) to the mass of an equal volume of gas-free distilled water at the stated temperature. Bulk Specific Gravity (Bulk Dry Specific Gravity), Gsb. The volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement only includes the aggregate particle. Since it includes the water permeable void volume, bulk specific gravity will be less than apparent specific gravity. It is formally defined as the ratio of the mass of a unit volume of aggregate, including the water permeable voids, at a stated temperature to the mass of an equal volume of gas-free distilled water at the stated temperature.
Bulk Saturated Surface Dry (SSD) Specific Gravity. Volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement includes the aggregate particle as well as the water within the water permeable voids. It is formally defined as the ratio of the mass of a unit volume of aggregate, including the weight of water within the voids filled to the extent achieved by submerging in water for approximately 15 hours, to the mass of an equal volume of gas-free distilled water at the stated temperature.
Effective Specific Gravity, Gse. Volume measurement includes the volume of the aggregate particle plus the void volume that becomes filled with water during the test soak period minus the volume of the voids that
absorb asphalt. Effective specific gravity lies between apparent and bulk specific gravity. It is formally defined as the ratio of the mass in air of a unit volume of a permeable material (excluding voids permeable to asphalt) at a stated temperature to the mass in air (of equal density) of an equal volume of gas-free distilled water at a stated temperature. Effective specific gravity is determined by a different procedure and is not covered in this section. Relationship with Other Specific Gravities
Refer to Figure 4 for abbreviations. Figure 4: Abbreviations.
The difference between Gsa and Gsb is the volume of aggregate used in the calculations. The difference between these volumes is the volume of absorbed water in the aggregate’s permeable voids. Both use the aggregate’s oven dry weight. The difference between Gsb and bulk (SSD) specific gravity is the weight of aggregate used in the calculations. The difference between these weights is the weight of absorbed water in the aggregate’s permeable voids. Both use the same aggregate volume.
The difference between Gsa, Gse and Gsb is the volume of aggregate used in the calculations. All three use the aggregate’s oven dry weight.
The following relationships are always true: 1. Gsa ≥ Gse ≥ Gsb 2. Bulk (SSD) specific gravity ≥ Gsb 3. Aggregate specific gravities (Gsb, Gsa,Gse and bulk SSD specific gravity ) are all ≥ Gmm(because Gmm includes the asphalt binder, which has a lower specific gravity than the aggregate)
Test Description The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The complete fine aggregate specific gravity procedure can be found in:
AASHTO T 84 and ASTM C 128: Specific Gravity and Absorption of Fine Aggregate
Summary
The mass of a fine aggregate sample is determine in SSD, oven-dry and submerged states. These values are then used to calculate bulk specific gravity, bulk SSD specific gravity, apparent specific gravity and absorption. Figure 5 shows the major equipment used to perform the FASG test.
Figure 5: Major equipment used in performing the FASG test. Approximate Test Time
3 days (from sample preparation to final dry weight determination). Basic Procedure 1. Obtain approximately 1000 g of aggregate material passing the No. 4 (4.75 mm) sieve. 2. Prepare the material. o
Dry the material until it maintains a constant mass. This indicates that all the water has left the sample. Drying should occur in an oven regulated at 230°F (110°C).
o
Cool the aggregate to a comfortable handling temperature.
o
Immerse the aggregate in water at room temperature for a period of 15 to 19 hours.
3. Dry the sample to a saturated surface dry (SSD) condition. Spread sample on a flat, non-absorbent surface (Figure 6) and stir it occasionally to assist in homogeneous drying. A current of warm air may be used to assist drying procedure (Figure 7). The air current (typically from a blow dryer) should not blow the sample off the non-absorbent surface. Throughout this drying process, the aggregate should be repeatedly tested for a SSD condition using the Cone Test as follows: o
Fill a cone-shaped metal mold to overflowing with drying aggregate.
o
Lightly tamp the aggregate into the mold with 25 light drops of a small metal tamper (Figure 8).
o
Remove loose aggregate from the outside of the mold and carefully lift the mold vertically.
o
If surface moisture is still present, the fine aggregate will retain its molded shape (Figure 9). When the aggregate achieves an SSD condition, it will slump slightly.
o
Upon the first test where slumping occurs, record the weight of the aggregate as SSD mass.
Figure 6: Wet FASG sample spread out.
Figure 7: Drying the sample with a blow dryer.
Figure 9: No slump indicates surface moisture presence. Figure 8: Tamping the aggregate into the mold. WARNING
If the aggregate slumps on the first Cone Test, it is assumed that the aggregate has already dried beyond the SSD condition (Figure 10). The aggregate can be restored by thoroughly mixing in a small amount of water and allowing the aggregate to stand in a covered container for 30 minutes. The drying process can then be resumed (AASHTO, 2000c[1]).
Figure 10: Aggregate is beyond SSD. 4. Calibrate a specific gravity flask pycnometer by filling with water at 73.4°F (23°C) to the calibration line and determine the mass. 5. Place 500 ± 10 grams of the SSD aggregate into the pycnometer and fill with water at 73.4°F (23°C)) to 90% of pycnometer capacity (Figure 11). Agitate the pycnometer to eliminate air bubbles and then determine total mass of the pycnometer.
Figure 11: Pouring the SSD sample into the pycnometer. NOTE
This agitation procedure should be repeated several times in order to ensure that any entrapped air is eliminated. This process usually takes 15 to 20 minutes total. Agitation does not have to be constant (AASHTO, 2000a[2]). 6. Add additional water to return the pycnometer to its calibrated capacity. NOTE
If bubbles prevent the proper filling of the pycnometer, adding a few drops of isopropyl alcohol is recommended to disperse the foam (AASHTO, 2000a[2]). 7. Determine the total weight of pycnometer, specimen, and water. 8. Remove the aggregate from the pycnometer and dry it until it maintains a constant mass. This indicates that all the water has left the sample. Drying should occur in an oven regulated at 230°F (110°C). 9. Cool the aggregate in air at room temperature for 1.0 ± 0.5 hours then determine the mass.
Results Parameters Measured 1. Fine aggregate bulk specific gravity. 2. Fine aggregate bulk SSD specific gravity. 3. Fine aggregate apparent specific gravity. 4. Fine aggregate absorption. Specifications
There are no minimum or maximum specific gravity or absorption values in Superpave mix design. Rather, specific gravity is an aggregate quality needed to make required volume calculations. Some state agencies specify minimum aggregate specific gravities or maximum percent water absorption to help control aggregate quality. Typical Values
Specific gravities can vary widely depending upon aggregate type. Some lightweight shales (not used in HMA production) can have specific gravities near 1.050, while other aggregate can have specific gravities above 3.000. Typically, aggregate used in HMA production will have a bulk specific gravity between about 2.400 and 3.000 with 2.700 being fairly typical of limestone. Bulk SSD specific gravities can be on the order of 0.050 to 0.100 higher than bulk oven dry specific gravities, while apparent specific gravities can be 0.050 to 0.100 higher still. For a particular aggregate type or source, fine aggregate specific gravities can be slightly higher than coarse aggregate specific gravities because as the aggregate particles get smaller, the fraction of pores exposed to the aggregate surface (and thus excluded from the specific gravity calculation because they are water-permeable) increases. Aggregate absorption can also vary widely depending upon aggregate type. Some lightweight shales (not used in HMA production) can have absorptions approaching 30 percent, while other aggregate types can have near zero absorption. Typically, aggregate used in HMA production will have an absorption between just above zero and 5 percent. Absorptions above about 5 percent tend to make HMA mixtures uneconomical because extra asphalt binder is required to account for the high aggregate absorption. If absorption is incorrectly accounted for, the resulting HMA could be overly dry and have low durability (absorption calculated lower than it actually is) or over-asphalted and susceptible to distortion and rutting (absorption calculated higher than it actually is). Calculations (see Interactive Equation)
Four different masses are recorded during the test. Their common symbols are: A = mass of oven-dry sample in air (g) B = mass of pycnometer filled with water (g) C = mass of pycnometer filled with SSD sample & water (g) S = mass of SSD sample (g) These masses are used to calculate the various specific gravities and absorption using the following equations: Note that the quantity (B + S – C) is the mass of water displaced by the SSD aggregate sample. In the apparent specific gravity calculation the mass of the SSD aggregate sample is replaced by the mass of the oven-dry aggregate sample (A replaces S), which means that the water permeable voids within the aggregate are not included and (B + A – C) is the mass of water displaced by the oven-dry sample. The ratios given in the equations are then simply the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water, which is specific gravity. WARNING
Certainly, the accuracy of all measurements is important. However, of specific concern is the mass of the SSD sample. The determination of SSD conditions can be difficult. If the sample is actually still wet on the surface then the mass of the SSD sample will be higher than it ought to be, which will cause a lower calculated bulk specific gravity. Conversely, if the sample is beyond SSD and some of the pore water has evaporated (which is more likely), the mass of the SSD sample will be lower than it ought to be, which will cause a higher calculated bulk specific gravity. Either type of error will have a cascading effect on volumetric parameters in other tests that require specific gravity as an input and Superpave mix design. A quick check of the results should show that bulk specific gravity is the lowest specific gravity, bulk SSD specific gravity is in the middle and apparent specific gravity is the highest.
Footnotes (↵ returns to text) 1. American Association of State Highway and Transportation Officials (AASHTO). (2000c). AASHTO Provisional Standards, April 2000 Edition. American Association of State Highway and Transportation Officials. Washington, D.C.↵ 2. American Association of State Highway and Transportation Officials (AASHTO). (2000a). Standard Specifications for Transportation Materials
and Methods of Sampling and Testing, Twentieth Edition: Part I – Specifications. American Association of State Highway and Transportation Officials. Washington, D.C.↵ 3.
Bulk Specific Gravity
Overview The bulk specific gravity test is used to determine the specific gravity of a compacted HMA sample by determining the ratio of its weight to the weight of an equal volume of water. The bulk specific gravity test measures a HMA sample’s weight under three different conditions (Figure 1):
Dry (no water in sample). Saturated surface dry (SSD, water fills the HMA air voids).
Submerged in water (underwater).
Using these three weights and their relationships, a sample’s apparent specific gravity, bulk specific gravity and bulk SSD specific gravity as well as absorption can be calculated. HMA bulk specific gravity is needed to determine weight-volume relationships and to calculate various volume-related quantities such as air voids and voids in mineral aggregate (VMA). The standard bulk specific gravity test is:
AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens ASTM D 2726: Bulk Specific Gravity and Density of Non-Absorptive Compacted Bituminous Mixtures
Figure 1. HMA samples in three conditions.
Background Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73.4°F (23°C). Therefore, by definition, water at 73.4°F (23°C) has a specific gravity of 1. Bulk Specific Gravity Use
Superpave mix design is a volumetric process; key properties are expressed in terms of volume. However, direct volume measurements are difficult, therefore weight measurements are usually made and then converted to a volume based on material specific gravities. Bulk specific gravity is involved in most key mix design calculations including air voids, VMA and, indirectly, VFA. Correct and accurate bulk specific gravity determinations are vital to proper mix design. An incorrect bulk specific gravity value will result in incorrectly calculated air voids, VMA, VFA and ultimately result in an incorrect mix design. Methods of Determining Bulk Specific Gravity
Although the Test Description section describes the standard AASHTO T 166 saturated surface dry (SSD) water displacement method, there are a number of other methods available. Each one uses a slightly different way to determine specimen volume and may result in different bulk specific gravity values. Water Displacement Methods
These methods, based on Archimedes Principle, calculate specimen volume by weighing the specimen (1) in a water bath and (2) out of the water bath. The difference in weights can then be used to calculate the weight of water displaced, which can be converted to a volume using the specific gravity of water.
Saturated Surface Dry (SSD) The most common method (and the one described in the Test Description section), calculates the specimen volume by subtracting the mass of the specimen in water (Figure 2) from the mass of a SSD specimen. SSD is defined as the specimen condition when the internal air voids are filled with water and the surface (including air voids connected to the surface) is dry. This SSD condition allows for internal air voids to be counted as part of the specimen volume and is achieved by soaking the specimen in a water bath for 4 minutes then removing it and quickly blotting it dry with a damp towel.
Figure 2. SSD Method. WARNING
One critical problem with this method is that if a specimen’s air voids are high, and thus potentially interconnected (for dense-graded HMA this occurs at about 8 to 10 percent air voids), water quickly drains out of them as the specimen is removed from its water bath, which results in an erroneously low HMA sample volume measurement and thus an erroneously high bulk specific gravity.
Paraffin This method determines volume similarly to the water displacement method but uses a melted paraffin wax instead of water to fill a specimen’s internal air voids (Figure 3). Therefore, after the wax sets there is no possibility of it draining out and, theoretically, a more accurate volume can be calculated. In practice, the paraffin is difficult to correctly apply and test results are somewhat inconsistent.
Figure 3. Parafin-covered HMA sample.
Parafilm In this method the specimen is wrapped in a thin paraffin film (Figure 4) and then weighed in and out of water. Since the specimen is completely wrapped when it is submerged, no water can get into it and a more accurate volume measurement is theoretically possible. However, in practice the paraffin film application is quite difficult and test results are inconsistent.
Figure 4: Covering a HMA sample with Parafilm.
CoreLok This method calculates specimen volume like the parafilm method but uses a vacuum chamber (Figure 5) to shrink-wrap the specimen in a high-quality plastic bag (Figure 6) rather
than cover it in a paraffin film (Video 1). This method has shown promise in both accuracy and precision.
Figure 6: CoreLok sample vacuum sealed in a plastic bag.
Figure 5: CoreLok vacuum chamber with sample inside. Video 1: CoreLok device. Dimensional
This method, the simplest, calculates the volume based on height and diameter/width measurements. Although it avoids problems associated with the SSD condition, it is often inaccurate because it assumes a perfectly smooth surface, thereby ignoring surface irregularities (i.e., the rough surface texture of a typical specimen). Gamma Ray
The gamma ray method is based on the scattering and absorption properties of gamma rays with matter. When a gamma ray source of primary energy in the Compton range is placed near a material, and an energy selective gamma ray detector is used for gamma ray counting, the scattered and unscattered gamma rays with energies in the Compton range can be counted exclusively. With proper calibration, the gamma ray count is directly converted to the density or bulk specific gravity of the material (Troxler, 2001[1]). Figure 7 shows the Troxler device.
Figure 7: Troxler Model 3660 CoreReader.
Test Description The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The complete procedure can be found in:
AASHTO T 166: Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens ASTM D 2726: Bulk Specific Gravity and Density of Non-Absorptive Compacted Bituminous Mixtures
Other standard tests available to determine bulk specific gravity that are not described in this section are:
AASHTO T 275: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens AASHTO TP 69: Bulk Specific Gravity and Density of Compacted Asphalt Mixtures Using Automatic Vacuum Sealing Method
Summary
A compacted HMA sample (usually a SGC compacted laboratory sample or a field-obtained HMA core) is weighed dry, saturated surface dry (SSD) and submerged (Figure 1). These
weights are used to calculate specific gravity and the percentage of water absorbed by the sample. Approximate Test Time
Each test takes approximately 7 minutes to conduct excluding preparation time. When several samples are tested the test time per sample can be reduced. Considerable preparation time may be necessary if contamination must be removed from the bottom of the sample. Basic Procedure 1. Dry specimen to a constant mass and cool to room temperature. NOTE
Laboratory samples are typically dry at the beginning of the test; however, field samples will typically be damp. 2. Record the dry mass (Figure 8).
Figure 8: Sample weighing. 3. Submerge sample in 77°F (25°C) water for 4 minutes and record the submerged mass . This can be done with a water-filled container on top of a scale or with a basket suspended in water under a scale (Figure 2).
4. Quickly blot the sample with a damp towel and record the surface dry mass. WARNING
Any water that escapes from the sample during weighing is considered part of the saturated specimen. If this water is not weighed, significant error can result.
Results Parameters Measured
Bulk specific gravity (Gmb) and the percentage of water absorbed by volume. Specifications
There is no specification for bulk specific gravity, but it is used to calculate other specified parameters such as air voids, VMA and VFA. Typical Values
Typical values for bulk specific gravity range from 2.200 to 2.500 depending upon the bulk specific gravity of the aggregate, the asphalt binder content, and the amount of compaction. Absorption should typically be below 2 percent. If more than 2 percent water by volume is absorbed by the sample then this method is not appropriate. In this case, use AASHTO T 275, Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens or AASHTO TP 69, Bulk Specific Gravity and Density of Compacted Asphalt Mixtures Using Automatic Vacuum Sealing Method. Calculations (Interactive Equation)
Three different masses are recorded during the test. Their common symbols are: A = mass of sample in air (g) B = mass of SSD sample in air (g) C = mass of sample in water (g) These masses are used to calculate bulk specific gravity and water absorption using the following equations:
WARNING
Certainly, the accuracy of all measurements is important. However, of specific concern is the mass of the SSD sample. As mentioned in the background section, if a specimen’s air voids are high, and thus potentially interconnected (for dense-graded HMA this occurs at about 8 to 10 percent air voids), water quickly drains out of them as the specimen is removed from its water bath, which results in an erroneously low SSD weight, which leads to an erroneously low HMA sample volume measurement and thus an erroneously high bulk specific gravity.
Theoretical Maximum Specific Gravity Overview The theoretical maximum specific gravity (Gmm) of a HMA mixture is the specific gravity excluding air voids. Thus, theoretically, if all the air voids were eliminated from an HMA sample, the combined specific gravity of the remaining aggregate and asphalt binder would be the theoretical maximum specific gravity. Theoretical maximum specific gravity can be multiplied by the density of water (62.4 lb/ft3 or 1000 g/L) to obtain a theoretical maximum density (TMD) or “Rice” density (named after James Rice, who developed the test procedure). Theoretical maximum specific gravity is a critical HMA characteristic because it is used to calculate percent air voids in compacted HMA. This calculation is used both in Superpave mix design and determination of in-place air voids in the field. Theoretical maximum specific gravity is determined by taking a sample of loose HMA (i.e., not compacted), weighing it and then determining its volume by calculating the volume of water it displaces (Figure 1). Theoretical maximum specific gravity is then the sample weight divided by its volume. The standard theoretical maximum specific gravity test is:
AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures
Figure 1. Maximum theoretical specific gravity sample.
Background The theoretical maximum specific gravity test is integral to Superpave mix design as well as field quality assurance. Theoretical maximum specific gravity is used along with bulk specific gravity values from field cores and laboratory compacted specimens to calculate air voids and the in-place air voids of a HMA pavement. It is also used to calculate the amount of asphalt absorbed in a HMA mixture (Vba) , which is then used in determining the effective asphalt content (Pbe). Basic Premise
The basic premise of the maximum specific gravity is to divide the mass of the sample by the volume of the sample excluding the air voids. The mass is determined by measuring the dry mass of the sample either at the beginning of the test or after it has been dried at the end of the test. The volume is calculated by weighing the mass of the water displaced by the sample and dividing by the unit weight of water. In-place Density Measurement
As previously discussed, theoretical maximum specific gravity is needed to calculate air void content; therefore, it is involved in in-place air void determination during HMA pavement construction. In-place air void measurements are used as a measure of compaction (Figure 2).
This is because compaction reduces the volume of air in HMA. Therefore, the characteristic of concern in compaction is the volume of air within the compacted HMA. This volume is typically quantified as a percentage of air voids by volume and expressed as “percent air voids”. Percent air voids is calculated by comparing a test specimen’s bulk specific gravity (Gmb) with its theoretical maximum specific gravity (Gmm) and assuming the difference is due to air. Once Gmm is known, portable non-destructive devices can be used to measure HMA density in-place. The terms “percent air voids” and “density” are often used interchangeably. Although this is not wrong, since density is used to calculate percent air voids, the fundamental parameter of concern is always percent air voids. Percent air voids is typically calculated using Gmm and Gmb in the following equation:
Each time density is to be determined a measure of bulk specific gravity is made by either coring the pavement and determining bulk specific gravity on the sample or using a nondestructive testing method. This bulk specific gravity is then compared to the most current theoretical maximum specific gravity to determine air voids. During HMA production and pavement construction, theoretical maximum specific gravity should be determined at regular intervals because it may change over time as the asphalt binder content and properties as well as aggregate properties vary over time. WARNING
If percent air voids is used as a primary quality assurance characteristic, there can be a tendency to control this characteristic at the expense of others. For instance, if adequate compaction is not being achieved, increasing asphalt binder content will fill more voids with asphalt binder and thus lower the air void content for the same amount of compaction. However, increased asphalt binder content can also potentially make a HMA mixture more likely to rut or shove.
Figure 2: HMA compaction. Relationship with Other Specific Gravities
Refer to Figure 3 for abbreviations. 1. The difference between Gmm and Gmb is volume. The weights are identical. The difference in volume is the volume of air in the compacted HMA mixture. 2. The following relationships are always true: o
Gmm ≥ Gmb
o
Aggregate specific gravities (Gsb, Gsa, Gse and bulk SSD specific gravity ) are all ≥ Gmm
Figure 3: Typical weight-volume variables. Test Description
The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The complete test procedure can be found in:
AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures
Summary
A loose sample of either laboratory or plant produced HMA is weighed while dry (to determine its dry mass) and then a short procedure is used to determine the sample’s volume. The theoretical maximum specific gravity is then the sample’s mass divided by its volume. Approximate Test Time
45 minutes per test after samples are prepared (2 samples per test typically). Basic Procedure
Test samples may be representative of a mixture prepared in the laboratory or in a HMA plant. The mixture should be loose and broken up so that the fine aggregate is separated into particles smaller than 0.25 inches (6.25 mm) taking care not to fracture aggregate (Figure 4).
Figure 4: Loose HMA sample. 1. Place a loose sample at room temperature into a vacuum container and record the dry mass. If Weighing in Water is chosen in step 5, glass, plastic or metal bowls (Figure 5) as well as thick-walled flasks or vacuum desiccators are used. If Weighing in Air is chosen in step 5, flasks (Figure 6) or pycnometers are used. 2. Completely cover the sample by adding water at approximately 77°F (25°C) to the container. 3. Remove entrapped air in the sample by applying a vacuum of 27.75 mm Hg (3.7 kPa) to the pycnometer or flask for 15 minutes. The container should be agitated continuously by mechanical means (Video 1) or shaken vigorously by hand every two minutes. 4. Slowly release the vacuum. 5. Weigh the sample in water or air: o
Weighing in water. Suspend the container (which is filled with the sample and water) in a water bath at 77°F (25°C) for 10 minutes and record the mass.
o
Weighing in air. Fill the container completely with water at 77°F (25°C). Determine the mass of the completely filled container within 10 minutes of releasing the vacuum.
WARNING
In highly absorptive aggregate, water may seep in between the absorbed asphalt and the aggregate particle resulting in an erroneous dry weight measurement.To determine whether significant seepage has occurred, decant the sample through a towel (so that the fines are retained) held over the top of the container. Take several of the larger pieces of aggregate and
break them. Examine the broken faces for wetness. Wetness indicates seepage. If seepage is detected, a supplemental procedure needs to be run on the sample at the end of the test. Generally, if the aggregate has a water absorption of less than 1.5 percent the supplemental procedure is not needed. This procedure is accomplished by spreading the wet sample in front of a fan and weighing at 15 minute intervals. When the mass loss between weighings is less than 0.05 percent, the sample is said to be dry. This dry mass should be used for calculations. This is often called a “dry-back” procedure.
Figure 5: Vacuum assembly loaded with a metal bowl (left).
Figure 6: Vacuum assembly loaded with a flask (right).
Video 1: Mechanical agitation.
Results Parameters Measure
Maximum specific gravity. Specifications
There is no specification for theoretical maximum specific gravity, but it is used to calculate other specified parameters such as air voids (Va) in laboratory compacted mixtures and inplace density in the field.
Typical Values
Typical values for theoretical maximum specific gravity range from approximately 2.400 to 2.700 depending on the aggregate specific gravity and asphalt binder content. Unusually light or heavy aggregates may result in a value outside this typical range. Calculations (Interactive Equation)
Calculate and report Gmm to the nearest thousandth. Weighing in Water Method
Where:
A = sample mass in air (g) C =mass of water displaced by the sample (g)
Weighing in Air Method
Where:
A = sample mass in air (g) D = mass of flask filled with water (g)
E = mass of flask and sample filled with water (g)
