Aggregate Specific GravityAggregate 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. Aggregate absorption is a useful quality because: 1. Therefore. Differences in specific gravity can also be used to separate deleterious. or bad. Apparent Specific Gravity. Gsa. The mass measurement only includes the aggregate particle. Absorption can indicate the amount of asphalt binder the aggregate will absorb.Material Contamination Indicator and Separator Specific gravity can also indicate possible material contamination. particles from aggregate particles using a heavy media liquid. A change in aggregate mineral or physical properties can result in a change in specific gravity. 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. Material Change Indicator Finally. Figure 2: Deleterious Materials. deleterious particles (Figure 2) are often lighter than aggregate particles and therefore. Water absorption can also be an indicator of asphalt absorption. 2. For instance. Aggregate Absorption Use Aggregate absorption is the increase in mass due to water in the pores of the material. Apparent specific gravity is intended to only measure the specific gravity of the solid volume. 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. it does not include the volume of any water permeable voids. It is generally desirable to avoid highly absorptive aggregate in HMA. therefore it will be the highest of . a large amount of deleterious material in an aggregate sample may result in an abnormally low specific gravity. if a quarry operation constantly monitors the specific gravity of its output aggregate. The volume measurement only includes the volume of the aggregate particle. 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. specific gravity differences can be used to indicate a possible material change. 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. For instance. High values can indicate non-durable aggregate. Relationship with Other Specific Gravities Refer to Figure 4 for abbreviations. Effective specific gravity lies between apparent and bulk specific gravity. 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. including the water permeable voids. 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. The mass measurement includes the aggregate particle as well as the water within the water permeable voids. .the aggregate specific gravities. Bulk Specific Gravity (Bulk Dry Specific Gravity). bulk specific gravity will be less than apparent specific gravity. The volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. to the mass of an equal volume of gas-free distilled water at the stated temperature. It is formally defined as the ratio of the mass of a unit volume of aggregate. Effective Specific Gravity. The mass measurement only includes the aggregate particle. 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. including the weight of water within the voids filled to the extent achieved by submerging in water for approximately 15 hours. Volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. G sb. Since it includes the water permeable void volume. Gse. 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. It is formally defined as the ratio of the mass of a unit volume of aggregate. 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. These values are then used to calculate bulk specific gravity(Gsb). All three use the aggregate’s oven dry weight. . bulk SSD specific gravity. Aggregate specific gravities (Gsb. The following relationships are always true: 1. apparent specific gravity(Gsa) and absorption.Gse and bulk SSD specific gravity ) are all ≥ Gmm (because Gmm includes the asphalt binder. Figure 5 shows major coarse aggregate specific gravity equipment. Gsa ≥ Gse ≥ Gsb 2. The difference between these volumes is the volume of absorbed water in the aggregate’s permeable voids. Gsa. The difference between Gsa and Gsb is the volume of aggregate used in the calculations.Figure 4: Abbreviations. Both use the aggregate’s oven dry weight. oven-dry and submerged states. Both use the same aggregate volume. The difference between these weights is the weight of absorbed water in the aggregate’s permeable voids. The difference between Gsa. Bulk (SSD) specific gravity ≥ Gsb 3. Gse and Gsb is the volume of aggregate used in the calculations. It is not a complete procedure and should not be used to perform the test. The difference between Gsb and bulk (SSD) specific gravity is the weight of aggregate used in the calculations. which has a lower specific gravity than the aggregate) Test Description The following description is a brief summary of the test. 75 mm) sieve (Figure 6).Figure 5: Major CASG equipment.5 mm) NMAS to 5000 g for a 1.5 mm) NMAS. . Obtain a sample of coarse aggregate material retained on the No.5 inch (12. This sample size is based on nominal maximum aggregate size (NMAS). Approximate Test Time 3 days (from sample preparation to final dry weight determination) Basic Procedure 1. Sample sizes range from 2000 g for a 0.5 inch (37. 4 (4. Drying should occur in an oven regulated at 230°F (110°C). 4 (4. 4 (4. This indicates that all the water has left the sample.Figure 6: No. 2.75 mm) sieve. Immerse the aggregate in water at room temperature for a period of 15 to 19 hours (Figure 7). This discards small aggregate particles clinging to the retained large particles.75 mm) sieve. . Wash the aggregate retained on the No. Prepare the material. Dry the material until it maintains a constant mass. Cool the aggregate to a comfortable handling temperature. determine the sample mass. specific gravity values may be significantly higher. If the aggregate is not oven-dry to start. Paper towels may absorb water in the aggregate pores. It may be necessary to wipe the larger particles separately. Shake the container to . WARNING If the aggregate is not oven-dried before soaking. The basket should be pre-conditioned to the water bath temperature. Place the entire sample in a basket (Figure 8) and weigh it underwater (Figure 9). 4. Dry the sample to a saturated surface dry (SSD) condition. WARNING Make sure to use cloth and not paper towels. the existing water in the aggregate pore structure may be able to penetrate further into the pores (AASHTO. Video 1: Drying a CASG sample. 3.Figure 7: Soaking the sample. Once there are no visible signs of water film on the aggregate particle surfaces. 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. 2000c[1]). 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). 4. Coarse aggregate absorption. Figure 9: Weighing the sample underwater.release any entrapped air before weighing. The container overflow needs to work properly to compensate for the water displaced by the sample. 3. 6. 5. Figure 8: The basket used for underwater weighing. Specifications There are no minimum or maximum specific gravity or absorption values in Superpave mix design. This indicates that all the water has left the sample. Coarse aggregate bulk specific gravity. Coarse aggregate apparent specific gravity. specific gravity is an aggregate quality needed to make required volume . Results Parameters Measured 1. Drying should occur in an oven regulated at 230°F (110°C). Rather. 2. Remove the aggregate from the water and dry it until it maintains a constant mass. Cool the aggregate in air at room temperature for 1 to 3 hours then determine the mass. Coarse aggregate bulk SSD specific gravity. while other aggregate types can have near zero absorption.400 and 3.700 being fairly typical of limestone. Typically.calculations.050.000 with 2. fine aggregate specific gravities can be slightly higher than coarse aggregate specific gravities because as the aggregate particles get smaller. Bulk SSD specific gravities can be on the order of 0. while other aggregate can have specific gravities above 3.000. the fraction of pores exposed to the aggregate surface (and thus excluded from the specific gravity calculation because they are water-permeable) increases. while apparent specific gravities can be 0. 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). Typical Values Specific gravities can vary widely depending upon aggregate type.100 higher than bulk oven dry specific gravities. aggregate used in HMA production will have an absorption between just above zero and 5 percent. 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: .100 higher still. aggregate used in HMA production will have a bulk specific gravity between about 2. Typically. Some state agencies specify minimum aggregate specific gravities or maximum percent water absorption to help control aggregate quality. If absorption is incorrectly accounted for. Some lightweight shales (not used in HMA production) can have specific gravities near 1.050 to 0. For a particular aggregate type or source. Aggregate absorption can also vary widely depending upon aggregate type. Some lightweight shales (not used in HMA production) can have absorptions approaching 30 percent. Calculations (see Interactive Equation) Three different masses are recorded during the test.050 to 0. 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 the sample is beyond SSD and some of the pore water has evaporated (which is more likely). American Association of State Highway and Transportation Officials. 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. 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. which is specific gravity. It is similar in nature to the coarse aggregate specific gravity test. 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. Washington. the mass of the SSD sample will be lower than it ought to be.Note that the quantity (B – C) is the mass of water displaced by the SSD aggregate sample. A quick check of the results should show that bulk specific gravity is the lowest specific gravity. The determination of SSD conditions can be difficult. (2000c). April 2000 Edition. American Association of State Highway and Transportation Officials (AASHTO). Conversely. which will cause a higher calculated bulk specific gravity. of specific concern is the mass of the SSD sample. which will cause a lower calculated bulk specific gravity. 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). WARNING Certainly.↵ 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. D. Footnotes (↵ returns to text) 1. the accuracy of all measurements is important. 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. bulk SSD specific gravity is in the middle and apparent specific gravity is the highest.C. However. AASHTO Provisional Standards. Saturated surface dry (water fills the aggregate pores). a sample’s apparent specific gravity. Using these three weights and their relationships. Submerged in water (underwater). and voids filled by asphalt (VFA). The fine aggregate specific gravity test measures fine aggregate weight under three different sample conditions: Oven-dry (no water in sample). The standard fine aggregate specific gravity and absorption test is: AASHTO T 84 and ASTM C 128: Specific Gravity and Absorption of Fine Aggregate . bulk specific gravity and bulk SSD specific gravity as well as absorption can be calculated.Figure 1: Fine aggregate specific gravity sample and pycnometer. Absorption can be used as an indicator of aggregate durability as well as the volume of asphalt binder it is likely to absorb. 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). deleterious particle indentification and separation. Absorption. Therefore. . Correct and accurate material specific gravity determinations are vital to proper mix design. or bad. A change in aggregate mineral or physical properties can result in a change in specific gravity.4°F (23°C) has a specific gravity of 1. Superpave Mix Design Superpave mix design is a volumetric process. Pores that absorb water are also referred to as “water permeable voids”. if a quarry operation constantly monitors the specific gravity of its output aggregate. and material property change identification. by definition. 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.4°F (23°C). Water absorption can also be an indicator of asphalt absorption. water at a temperature of 73. Material Change Indicator Finally. it relies on mixing constituent materials on the basis of their volume. For instance. Differences in specific gravity can also be used to separate deleterious.Background Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73. However. An incorrect specific gravity value will result in incorrectly calculated volumes and ultimately result in an incorrect mix design. a large amount of deleterious material in an aggregate sample may result in an abnormally low specific gravity. For instance. therefore a material’s weight is typically measured and then converted to a volume based on its specific gravity. Figure 2: Deleterious Particles. Specific Gravity Use Aggregate specific gravity is used in a number of applications including Superpave mix design. 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. deleterious particles (Figure 2) are often lighter than aggregate particles and therefore. specific gravity differences can be used to indicate a possible material change. particles from aggregate particles using a heavy media liquid. aggregate and asphalt binder volumes are diffucult to measure directly. Material Contamination Indicator and Separator Specific gravity can also indicate possible material contamination. Since it includes the water permeable void volume. 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 absorption is a useful quality because: 1. Bulk Saturated Surface Dry (SSD) Specific Gravity. It is formally defined as the ratio of the mass of a unit volume of aggregate. Gsb. Gsa. to the mass of an equal volume of gas-free distilled water at the stated temperature. including the weight of water within the voids filled to the extent achieved by submerging in water for approximately 15 hours. Volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. it does not include the volume of any water permeable voids. It is generally desirable to avoid highly absorptive aggregate in HMA. 2. 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. Apparent specific gravity is intended to only measure the specific gravity of the solid volume. 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. 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. bulk specific gravity will be less than apparent specific gravity. High values can indicate non-durable aggregate. The mass measurement only includes the aggregate particle. Apparent Specific Gravity. Absorption can indicate the amount of asphalt binder the aggregate will absorb. Bulk Specific Gravity (Bulk Dry Specific Gravity). The volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. at a stated temperature to the mass of an equal volume of gas-free distilled water at the stated temperature. Effective Specific Gravity. 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 aggregate. The volume measurement only includes the volume of the aggregate particle.Aggregate Absorption Use Aggregate absorption is the increase in mass due to water in the pores of the material. The mass measurement only includes the aggregate particle. 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 . The mass measurement includes the aggregate particle as well as the water within the water permeable voids. including the water permeable voids. Gsa. bulk SSD specific gravity. The difference between Gsb and bulk (SSD) specific gravity is the weight of aggregate used in the calculations. The difference between Gsa and Gsb is the volume of aggregate used in the calculations. Effective specific gravity lies between apparent and bulk specific gravity.Gse and bulk SSD specific gravity ) are all ≥ Gmm(because Gmm includes the asphalt binder. 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. The difference between these weights is the weight of absorbed water in the aggregate’s permeable voids. Both use the same aggregate volume. 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. Gse and Gsb is the volume of aggregate used in the calculations. Figure 4: Abbreviations. which has a lower specific gravity than the aggregate) Test Description The following description is a brief summary of the test. apparent specific gravity and absorption. Gsa ≥ Gse ≥ Gsb 2. oven-dry and submerged states. The following relationships are always true: 1. Figure 5 shows the major equipment used to perform the FASG test. Effective specific gravity is determined by a different procedure and is not covered in this section. The difference between these volumes is the volume of absorbed water in the aggregate’s permeable voids. .absorb asphalt. It is not a complete procedure and should not be used to perform the test. These values are then used to calculate bulk specific gravity. Both use the aggregate’s oven dry weight. Relationship with Other Specific Gravities Refer to Figure 4 for abbreviations. Bulk (SSD) specific gravity ≥ Gsb 3. Aggregate specific gravities (Gsb. The difference between Gsa. All three use the aggregate’s oven dry weight. Obtain approximately 1000 g of aggregate material passing the No. o Cool the aggregate to a comfortable handling temperature. Dry the sample to a saturated surface dry (SSD) condition. The air current (typically from a blow dryer) should not blow the sample off the non-absorbent surface.Figure 5: Major equipment used in performing the FASG test. 2. Throughout this drying process. o Immerse the aggregate in water at room temperature for a period of 15 to 19 hours. . Approximate Test Time 3 days (from sample preparation to final dry weight determination). non-absorbent surface (Figure 6) and stir it occasionally to assist in homogeneous drying. o Dry the material until it maintains a constant mass.75 mm) sieve. 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. A current of warm air may be used to assist drying procedure (Figure 7). 3. Basic Procedure 1. Spread sample on a flat. Prepare the material. 4 (4. Drying should occur in an oven regulated at 230°F (110°C). o Lightly tamp the aggregate into the mold with 25 light drops of a small metal tamper (Figure 8). This indicates that all the water has left the sample. it will slump slightly. Figure 6: Wet FASG sample spread out. o Upon the first test where slumping occurs. the fine aggregate will retain its molded shape (Figure 9). record the weight of the aggregate as SSD mass. .o Remove loose aggregate from the outside of the mold and carefully lift the mold vertically. When the aggregate achieves an SSD condition. o If surface moisture is still present. Figure 7: Drying the sample with a blow dryer. Figure 8: Tamping the aggregate into the mold.Figure 9: No slump indicates surface moisture presence. it is assumed that the aggregate has already dried beyond the SSD condition (Figure 10). WARNING If the aggregate slumps on the first Cone Test. The drying process can then be resumed (AASHTO. 2000c[1]). . 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. Figure 10: Aggregate is beyond SSD. Calibrate a specific gravity flask pycnometer by filling with water at 73.4°F (23°C) to the calibration line and determine the mass. 4. . 5. Agitate the pycnometer to eliminate air bubbles and then determine total mass of the pycnometer.4°F (23°C)) to 90% of pycnometer capacity (Figure 11). Place 500 ± 10 grams of the SSD aggregate into the pycnometer and fill with water at 73. and water. Drying should occur in an oven regulated at 230°F (110°C). Determine the total weight of pycnometer.Figure 11: Pouring the SSD sample into the pycnometer.5 hours then determine the mass. 7. This process usually takes 15 to 20 minutes total. 2000a[2]). adding a few drops of isopropyl alcohol is recommended to disperse the foam (AASHTO. Agitation does not have to be constant (AASHTO. 6. Add additional water to return the pycnometer to its calibrated capacity. Remove the aggregate from the pycnometer and dry it until it maintains a constant mass. 2000a[2]). This indicates that all the water has left the sample. NOTE If bubbles prevent the proper filling of the pycnometer. specimen. NOTE This agitation procedure should be repeated several times in order to ensure that any entrapped air is eliminated.0 ± 0. Cool the aggregate in air at room temperature for 1. 8. . 9. 000. 2.Results Parameters Measured 1. Fine aggregate bulk specific gravity. aggregate used in HMA production will have an absorption between just above zero and 5 percent.050. while other aggregate can have specific gravities above 3. For a particular aggregate type or source. Fine aggregate bulk SSD specific gravity. 4.400 and 3. Rather. Typically. Calculations (see Interactive Equation) . the fraction of pores exposed to the aggregate surface (and thus excluded from the specific gravity calculation because they are water-permeable) increases. aggregate used in HMA production will have a bulk specific gravity between about 2. specific gravity is an aggregate quality needed to make required volume calculations.000 with 2.700 being fairly typical of limestone. 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. Fine aggregate absorption. fine aggregate specific gravities can be slightly higher than coarse aggregate specific gravities because as the aggregate particles get smaller.050 to 0. Some lightweight shales (not used in HMA production) can have absorptions approaching 30 percent. Typically.100 higher than bulk oven dry specific gravities. Bulk SSD specific gravities can be on the order of 0. Aggregate absorption can also vary widely depending upon aggregate type. 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). Typical Values Specific gravities can vary widely depending upon aggregate type. Specifications There are no minimum or maximum specific gravity or absorption values in Superpave mix design. Some lightweight shales (not used in HMA production) can have specific gravities near 1. Fine aggregate apparent specific gravity.100 higher still. Some state agencies specify minimum aggregate specific gravities or maximum percent water absorption to help control aggregate quality. while other aggregate types can have near zero absorption.050 to 0. 3. while apparent specific gravities can be 0. 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. 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. WARNING Certainly. A quick check of the results should show that bulk specific gravity is the lowest specific gravity.Four different masses are recorded during the test. April 2000 Edition. which is specific gravity. 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. (2000a). However. which will cause a higher calculated bulk specific gravity. American Association of State Highway and Transportation Officials. AASHTO Provisional Standards. (2000c). which will cause a lower calculated bulk specific gravity. The determination of SSD conditions can be difficult. the accuracy of all measurements is important.↵ 2. Washington. American Association of State Highway and Transportation Officials (AASHTO). Footnotes (↵ returns to text) 1. Conversely. 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. Standard Specifications for Transportation Materials . American Association of State Highway and Transportation Officials (AASHTO).C. 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). if the sample is beyond SSD and some of the pore water has evaporated (which is more likely). of specific concern is the mass of the SSD sample. bulk SSD specific gravity is in the middle and apparent specific gravity is the highest. D. 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. the mass of the SSD sample will be lower than it ought to be. Using these three weights and their relationships. 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. a sample’s apparent specific gravity. HMA samples in three conditions.C. . 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. 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). water fills the HMA air voids). Saturated surface dry (SSD. D. The bulk specific gravity test measures a HMA sample’s weight under three different conditions (Figure 1): Dry (no water in sample).↵ 3. Washington. American Association of State Highway and Transportation Officials. Submerged in water (underwater). Twentieth Edition: Part I – Specifications. bulk specific gravity and bulk SSD specific gravity as well as absorption can be calculated.and Methods of Sampling and Testing. 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.4°F (23°C) has a specific gravity of 1. The difference in weights can then be used to calculate the weight of water displaced. An incorrect bulk specific gravity value will result in incorrectly calculated air voids. VMA and. Methods of Determining Bulk Specific Gravity Although the Test Description section describes the standard AASHTO T 166 saturated surface dry (SSD) water displacement method. direct volume measurements are difficult. Each one uses a slightly different way to determine specimen volume and may result in different bulk specific gravity values. . water at 73. VFA. key properties are expressed in terms of volume. 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. indirectly. VMA.Background Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73. by definition. Correct and accurate bulk specific gravity determinations are vital to proper mix design. Water Displacement Methods These methods. Therefore. calculates the specimen volume by subtracting the mass of the specimen in water (Figure 2) from the mass of a SSD specimen. However. 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). 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. based on Archimedes Principle. calculate specimen volume by weighing the specimen (1) in a water bath and (2) out of the water bath. there are a number of other methods available. VFA and ultimately result in an incorrect mix design. Bulk Specific Gravity Use Superpave mix design is a volumetric process.4°F (23°C). WARNING One critical problem with this method is that if a specimen’s air voids are high. water quickly drains out of them as the specimen is removed from its water bath. theoretically. 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. a more accurate volume can be calculated. the paraffin is difficult to correctly apply and test results are somewhat inconsistent. after the wax sets there is no possibility of it draining out and. . SSD Method. In practice. and thus potentially interconnected (for dense-graded HMA this occurs at about 8 to 10 percent air voids).Figure 2. which results in an erroneously low HMA sample volume measurement and thus an erroneously high bulk specific gravity. no water can get into it and a more accurate volume measurement is theoretically possible. Parafilm In this method the specimen is wrapped in a thin paraffin film (Figure 4) and then weighed in and out of water.Figure 3. 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 . in practice the paraffin film application is quite difficult and test results are inconsistent. Since the specimen is completely wrapped when it is submerged. Figure 4: Covering a HMA sample with Parafilm. However. Parafin-covered HMA sample. When a gamma ray source of primary energy in the Compton range is placed near a material. it is often inaccurate because it assumes a perfectly smooth surface. Figure 6: CoreLok sample vacuum sealed in a plastic bag. Dimensional This method. and an energy selective gamma ray detector is used for gamma ray counting.than cover it in a paraffin film (Video 1). With proper calibration. thereby ignoring surface irregularities (i. the rough surface texture of a typical specimen). the simplest. Although it avoids problems associated with the SSD condition. the scattered and unscattered gamma rays with energies in the Compton range can be counted exclusively.e. . Figure 5: CoreLok vacuum chamber with sample inside. calculates the volume based on height and diameter/width measurements. Figure 7 shows the Troxler device. the gamma ray count is directly converted to the density or bulk specific gravity of the material (Troxler. This method has shown promise in both accuracy and precision. Video 1: CoreLok device. 2001[1]).. Gamma Ray The gamma ray method is based on the scattering and absorption properties of gamma rays with matter. It is not a complete procedure and should not be used to perform the test. Test Description The following description is a brief summary of the test.Figure 7: Troxler Model 3660 CoreReader. saturated surface dry (SSD) and submerged (Figure 1). 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. These . Submerge sample in 77°F (25°C) water for 4 minutes and record the submerged mass . Record the dry mass (Figure 8). 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). Approximate Test Time Each test takes approximately 7 minutes to conduct excluding preparation time. Considerable preparation time may be necessary if contamination must be removed from the bottom of the sample. . Basic Procedure 1. Figure 8: Sample weighing. Dry specimen to a constant mass and cool to room temperature. however.weights are used to calculate specific gravity and the percentage of water absorbed by the sample. NOTE Laboratory samples are typically dry at the beginning of the test. When several samples are tested the test time per sample can be reduced. 3. 2. field samples will typically be damp. 200 to 2. Typical Values Typical values for bulk specific gravity range from 2. VMA and VFA. Bulk Specific Gravity and Density of Compacted Asphalt Mixtures Using Automatic Vacuum Sealing Method. use AASHTO T 275. the asphalt binder content.500 depending upon the bulk specific gravity of the aggregate.4. Calculations (Interactive Equation) Three different masses are recorded during the test. and the amount of compaction. In this case. but it is used to calculate other specified parameters such as air voids. If more than 2 percent water by volume is absorbed by the sample then this method is not appropriate. Absorption should typically be below 2 percent. Quickly blot the sample with a damp towel and record the surface dry mass. If this water is not weighed. significant error can result. Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens or AASHTO TP 69. 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: . Specifications There is no specification for bulk specific gravity. Results Parameters Measured Bulk specific gravity (Gmb) and the percentage of water absorbed by volume. WARNING Any water that escapes from the sample during weighing is considered part of the saturated specimen. Theoretical maximum specific gravity can be multiplied by the density of water (62. As mentioned in the background section. Theoretical Maximum Specific Gravity Overview The theoretical maximum specific gravity (Gmm) of a HMA mixture is the specific gravity excluding air voids. Theoretical maximum specific gravity is determined by taking a sample of loose HMA (i. theoretically. who developed the test procedure). which leads to an erroneously low HMA sample volume measurement and thus an erroneously high bulk specific gravity. Thus. the accuracy of all measurements is important. water quickly drains out of them as the specimen is removed from its water bath.. the combined specific gravity of the remaining aggregate and asphalt binder would be the theoretical maximum specific gravity. Theoretical maximum specific gravity is a critical HMA characteristic because it is used to calculate percent air voids in compacted HMA. which results in an erroneously low SSD weight. not compacted). if all the air voids were eliminated from an HMA sample. of specific concern is the mass of the SSD sample.e.WARNING Certainly. Theoretical maximum specific gravity is then the sample weight divided by its volume. However. if a specimen’s air voids are high. This calculation is used both in Superpave mix design and determination of in-place air voids in the field. weighing it and then determining its volume by calculating the volume of water it displaces (Figure 1).4 lb/ft3 or 1000 g/L) to obtain a theoretical maximum density (TMD) or “Rice” density (named after James Rice. 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 . and thus potentially interconnected (for dense-graded HMA this occurs at about 8 to 10 percent air voids). 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). 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. which is then used in determining the effective asphalt content (Pbe). Maximum theoretical specific gravity sample. . 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. In-place Density Measurement As previously discussed. It is also used to calculate the amount of asphalt absorbed in a HMA mixture (Vba) . theoretical maximum specific gravity is needed to calculate air void content. Background The theoretical maximum specific gravity test is integral to Superpave mix design as well as field quality assurance. The volume is calculated by weighing the mass of the water displaced by the sample and dividing by the unit weight of water. therefore.Figure 1. WARNING If percent air voids is used as a primary quality assurance characteristic. increasing asphalt binder content will fill more voids with asphalt binder and thus lower the air void content for the same amount of compaction. During HMA production and pavement construction. if adequate compaction is not being achieved. This bulk specific gravity is then compared to the most current theoretical maximum specific gravity to determine air voids. . portable non-destructive devices can be used to measure HMA density in-place. the fundamental parameter of concern is always percent air voids. Therefore. Although this is not wrong.This is because compaction reduces the volume of air in HMA. 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. The terms “percent air voids” and “density” are often used interchangeably. 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”. For instance. Once Gmm is known. However. since density is used to calculate percent air voids. increased asphalt binder content can also potentially make a HMA mixture more likely to rut or shove. 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. 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. there can be a tendency to control this characteristic at the expense of others. 1. The difference in volume is the volume of air in the compacted HMA mixture. 2. Relationship with Other Specific Gravities Refer to Figure 3 for abbreviations. The weights are identical. Gse and bulk SSD specific gravity ) are all ≥ Gmm .Figure 2: HMA compaction. Gsa. The difference between Gmm and Gmb is volume. The following relationships are always true: o Gmm ≥ Gmb o Aggregate specific gravities (Gsb. Figure 3: Typical weight-volume variables. 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.25 inches (6. Approximate Test Time 45 minutes per test after samples are prepared (2 samples per test typically). The theoretical maximum specific gravity is then the sample’s mass divided by its volume. .25 mm) taking care not to fracture aggregate (Figure 4). Test Description The following description is a brief summary of the test. Basic Procedure Test samples may be representative of a mixture prepared in the laboratory or in a HMA plant. It is not a complete procedure and should not be used to perform the test. The mixture should be loose and broken up so that the fine aggregate is separated into particles smaller than 0. 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. Completely cover the sample by adding water at approximately 77°F (25°C) to the container. If Weighing in Water is chosen in step 5. WARNING In highly absorptive aggregate. glass.To determine whether significant seepage has occurred.7 kPa) to the pycnometer or flask for 15 minutes.75 mm Hg (3. Slowly release the vacuum. Determine the mass of the completely filled container within 10 minutes of releasing the vacuum. 2. If Weighing in Air is chosen in step 5. o Weighing in air. Place a loose sample at room temperature into a vacuum container and record the dry mass. 5. Take several of the larger pieces of aggregate and . 1. decant the sample through a towel (so that the fines are retained) held over the top of the container. The container should be agitated continuously by mechanical means (Video 1) or shaken vigorously by hand every two minutes. flasks (Figure 6) or pycnometers are used. Weigh the sample in water or air: o Weighing in water. Fill the container completely with water at 77°F (25°C). water may seep in between the absorbed asphalt and the aggregate particle resulting in an erroneous dry weight measurement. 4. Remove entrapped air in the sample by applying a vacuum of 27. 3.Figure 4: Loose HMA sample. plastic or metal bowls (Figure 5) as well as thick-walled flasks or vacuum desiccators are used. This procedure is accomplished by spreading the wet sample in front of a fan and weighing at 15 minute intervals. This is often called a “dry-back” procedure. If seepage is detected. Video 1: Mechanical agitation. but it is used to calculate other specified parameters such as air voids (Va) in laboratory compacted mixtures and inplace density in the field. Specifications There is no specification for theoretical maximum specific gravity. Results Parameters Measure Maximum specific gravity.05 percent. Examine the broken faces for wetness. Generally. Wetness indicates seepage. . This dry mass should be used for calculations. Figure 5: Vacuum assembly loaded with a metal bowl (left).5 percent the supplemental procedure is not needed. Figure 6: Vacuum assembly loaded with a flask (right). if the aggregate has a water absorption of less than 1.break them. a supplemental procedure needs to be run on the sample at the end of the test. When the mass loss between weighings is less than 0. the sample is said to be dry. 700 depending on the aggregate specific gravity and asphalt binder content. Calculations (Interactive Equation) Calculate and report Gmm to the nearest thousandth.Typical Values Typical values for theoretical maximum specific gravity range from approximately 2. 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) . Unusually light or heavy aggregates may result in a value outside this typical range.400 to 2.
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