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Overview



The flexural fatigue test is used to characterize the fatigue life of HMA at intermediate pavement operating temperatures. This characterization is useful because it provides estimates of HMA pavement layer fatigue life under repeated traffic loading. In a well designed pavement, strains in the pavement are low enough so that fatigue is not a problem. However, when pavements are underdesigned strains are sufficiently high to cause fatigue failures under repeated loads. These failures ultimately result in fatigue cracking which will cause disintegration of the pavement if not maintained in time. The basic flexural fatigue test subjects a HMA beam to repeated flexural bending in a controlled atmosphere (Figure 1). In order to relate laboratory results to normally observed field performance, a shift factor of 10 to 20 is typically needed. Because of the testing equipment complexity and long testing times, the flexural fatigue test is primarily a research test and is not a standard test in Superpave mix design or quality assurance testing. The standard beam fatigue procedure is found in: AASHTO T 321: Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bending

Figure 1: Flexural fatigue device loaded with a HMA beam.

Background In HMA pavements, fatigue cracking occurs when repeated traffic loads ultimately cause sufficient damage in a flexible pavement to result in fatigue cracking (Figure 2). A number of factors can influence a pavement’s ability to withstand fatigue, including pavement structure (thin pavements or those that do not have strong underlying layers are more likely to show fatigue cracking than thicker

pavements or those with a strong support structure), age of the pavement, and the materials used in construction. The flexural fatiguetest is used to investigate fatigue as it relates to HMA construction materials.

Figure 2: Extensive fatigue cracking.

Fatigue Life Concept The concept of a fatigue life centers around the universal idea that most materials undergo a gradual deterioration under repeated loads that are much smaller than the ultimate strength of the material. A paper clip can be broken by repeatedly bending it just as a large pressure vessel can fail after being subject to many thousands of pressure cycles. HMA pavements are similar. A classic fatigue crack starts at the bottom of a HMA pavement layer (or structure) and grows towards the surface. It development is directly proportional to the strain level at the bottom of the layer (Carpenter, 2003[1]). This strain level changes with HMA thickness (thicker pavements give lower strain values), stiffness and other properties. Endurance Limit In 1970 Monismith et al. suggested that the relationship between strain at the bottom of the HMA layer and the number of cycles to failure seems to undergo a significant slope change at lower strain levels (in the vicinity of 70 microstrain). More recent studies seem to suggest that at low levels of strain (around 70 microstrain), HMA mixtures have, in effect, an infinite fatigue life. The theory is that a continuous physical-chemical healing reaction occurs, even during continuous loading, at low strain levels (Carpenter, 2003[1]). Therefore, a material property of HMA is its ability to recover some constant amount of damage or its “healing potential”. If damage due to loading falls below this “healing potential” then damage accumulation is virtually non-existent (Carpenter, 2003 [1]). Typical plots of flexural strain vs. loads to failure tend to look like Figure 3. Note that beyond a certain point the plot is essentially horizontal – indicating an infinite fatigue life.

Figure 3: Typical flexural fatigue graph illustrating an endurance limit.

Work on NCHRP Project 9-38, Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements, is underway to identify the existence of a fatigue endurance limit and measure it for selected HMA mixtures.

Flexural Fatigue Test Principles The flexural fatigue test is performed by placing a beam of HMA in repetitive four point loading at a specified strain level. During the test, the beam is held in place by four clamps and a repeated haversine (sinusoidal) load is applied to the two inner clamps with the outer clamps providing a reaction load (Figure 4). The load rate is variable but is normally set at 1 to 10 Hz. This setup produces a constant bending moment over the center portion of the beam (between the two inside clamps). The deflection caused by the loading is measured at the center of the beam. The number of loading cycles to failure can then give an estimate of a particular HMA mixture’s fatigue life. Another important value that can be obtained from the beam fatigue test is the dissipated energy of the specimen. Dissipated energy is a measure of the energy that is lost to the material or altered through mechanical work, heat generation, or damage to the sample.

Figure 4: Flexural fatigue test schematic.

Several key testing parameters require further explanation. Constant Strain vs. Constant Stress In recent years fatigue testing has been conducted using both constant stress or constant strain load applications. In constant strain mode the strain is maintained constant and the stress is allowed to vary. In constant stress mode the load is maintained the same and the strain is allowed to vary. Experience has shown that thick HMA pavements (> 5 inches (125 mm)) generally perform closer to a constant stress mode in the field, while thin HMA pavements (< 5 inches (125 mm)) generally perform closer to a constant strain mode in the field. Therefore, the constant strain mode favors more flexible mixtures and the constant stress mode favors stiffer materials. The constant strain mode is much more widely used because it appears to provide results that are more comparative to field observations. Test Termination The decision on when to terminate a flexural fatigue test depends on the test mode and purpose. For the constant stress mode, the test is continued until the beam actually breaks. For the constant strain mode, failure is more difficult to define because in order to keep the strain constant the applied stress is continually reduced, which results in a beam that never really breaks. Therefore, in constant strain mode, failure is normally defined as the point at which the load or stiffness reaches some predetermined value; most typically 50 percent of the original value. Number of Tests Typically it is not enough to perform only one isolated flexural fatigue test. Rather, a plot of multiple tests (typically a minimum of 10), each using a different load (for the constant stress mode) or different strain (for the constant strain mode) are produced (Kallas and Puzinauskas, 1972 [2]). These plots can reasonably estimate a particular HMA mixtures stress vs. loads to failure or strain vs. loads to failure relationship. Other plots can also be generated. Testing Temperature Beam fatigue testing is performed at intermediate temperatures, usually 68°F (20°C), because fatigue cracking is thought to be a primary HMA distress at these intermediate temperatures. At

higher in-service temperatures (above about 100°F (38°C)) rutting is usually the HMA distress of greatest concern, while at lower temperatures (below about 40 °F (4°C)) thermal cracking is usually the HMA distress of greatest concern.

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 321: Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bending

Summary Small HMA beams (15 x 2 x 2.5 inches (380 x 50 x 63 mm)) are made and placed in a 4-point loading machine, which subjects the beam to a repeated load. Tests can be run at a constant strain level or at a constant stress level. Figure 5 shows the major test equipment.

Figure 5: Flexural fatigue testing equipment.

Approximate Test Time Testing time is dependent on the strain level chosen for the test. High strain (400 – 800 microstrain) may be completed in a few hours. Low strain tests (200 – 400 microstrain) can take several days. Even lower strain levels (50 – 100 microstrain) can take upwards of a month. Typically 8 to 10 samples are used to develop results for any mix. Hence, it may take several days to several weeks to develop sufficient fatigue data to allow analysis of a given mixture.

Basic Procedure

1.

Obtain a test beam by sawing at least 0.25 inches (6 mm) from both sides of a compacted HMA specimen. The final dimensions should be 15 inches (380 mm) length by 2 inches (50 mm) height by 2.5 inches (63 mm) width (Figure 6).

Figure 6: Flexural fatigue beam.

1

2 3 4

Prepare three replicate beams. If these are laboratory-prepared or loose field samples, compact them in accordance with AASHTO PP 3 or ASTM D 3202. These compaction procedures describe the use of a linear kneading compactor (rather than a SGC) to compact square (rather than cylindrical) samples. If these are already-compacted samples obtained from the roadway, they need not be compacted in the laboratory. Measure the height and width of each beam to the nearest 0.0004 inch (0.01 mm) at three points along the middle 4 inches (100 mm) of the beam and determine the average for each dimension. Condition the beams at the test temperature (typically 68°F (20°C)) for two hours Open clamps and slide specimen into position. Close outside clamps first, then inside clamps with enough pressure to hold the specimen in place (Figure 7).

Figure 7: Close-up of beam with the 4 clamps in place

WARNING Raise the LVDT to its highest point to prevent damage when loading the specimen. 1 Position the LVDT onto the specimen such that the LVDT displacement reading is close to zero. 2 Allow sample to rest for 10 minutes to relax any residual stresses caused by loading. 3 Select an initial strain (250 – 750 microstrain), loading frequency (5 – 10 Hz), and interval at which the results should be recorded and enter them into the control components of the test program. 4 Apply 50 load cycles and determine the beam stiffness at the 50th cycle. This will be recorded as the initial stiffness of the beam. 5 Select a strain level that will provide an estimated 10,000 load cycles before the initial stiffness is reduced to 50 percent or less. 6 Begin the test. Test results should be monitored and recorded at the selected load cycle intervals and the test should be terminated when the beam has reached a 50 percent reduction in stiffness. It is possible that very low strain tests may not reach the 50 percent reduction in stiffness in a reasonable amount of time. In this case a maximum number of cycles should be specified as the termination point of the test.

Results Parameters Measured       

The beam fatigue test provides a measure of the fatigue life and fatigue energy of HMA pavements. To do this it can measure or calculate the following parameters: Maximum tensile stress Maximum tensile strain Flexural stiffness Phase angle Dissipated energy per cycle Cumulative dissipated energy Initial stiffness

 

Number of cycles to failure Cumulative dissipated energy to failure

Specifications There is currently no specification associated with this test procedure. The flexural fatigue test is essentially a research test and is not used in specifications.

Calculations (Interactive Equation) Calculations of maximum tensile stress, maximum tensile strain, stiffness and dissipated energy are made periodically (every load repetition, every 10th repetition, every 100th repetition, etc.).

Maximum Tensile Stress

Where:     

σt = maximum tensile stress (Pa) a = space between inside clamps (0.119 m) P = applied load (N) b = average beam width (m) h = average beam height (m) Maximum Tensile Strain

Where:  

εt = maximum tensile strain (m/m) δ = applied load (N)

  

h = average beam height (m) L = beam length between outside clamps (0.357 m) a = space between inside clamps (0.119 m) Flexural Stiffness

Where:   

  

S stiffness (Pa) σt = maximum tensile stress (Pa) εt = maximum tensile strain (m/m) Phase Angle Where: φt = phase angle (degrees) f = load frequency (Hz) s = time lag between maximum load and deflection (s) Dissipated Energy per Cycle Where:

   

D = dissipated energy per cycle (J/m3) σt = maximum tensile stress (Pa) εt = maximum tensile strain (m/m) φt = phase angle (degrees) The cumulative dissipated energy is then the sum of the dissipated energy for each load cycle. - See more at: http://www.pavementinteractive.org/article/flexural-fatigue/#sthash.n7kyywCg.dpuf

Overview Laboratory wheel-tracking devices (Figure 1) are used to run simulative tests that measure HMA qualities by rolling a small loaded wheel device repeatedly across a prepared HMA specimen. Performance of the test specimen is then correlated to actual in-service pavement performance. Laboratory wheel-tracking devices can be used to make rutting, fatigue, moisture susceptibility and stripping predictions. Some of these devices are relatively new and some have been used for upwards of 15 years like the French Rutting Tester (FRT). In general, these wheel tracking devices have potential for rut and other measurements but the individual user must be careful to establish laboratory conditions (e.g., load, number of wheel passes, temperature) that produce consistent and accurate correlations with field performance. There are several different types of wheel-tracking devices; the three most prominent devices will be covered in the Background section, while the basic procedure for the Asphalt Pavement Analyzer (APA) rut test is discussed in the Test Description section. The standard APA test is:



AASHTO TP 63: Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA) Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA)

Figure 1: Asphalt Pavement Analyzer (APA).

Background The more popular laboratory wheel tracking devices in the U.S. are generally recognized, in order of decreasing popularity, as the Asphalt Pavement Analyzer (APA), Hamburg Wheel Tracking Device (HWTD) and French Rutting Tester (FRT). These devices are all capable of proof testing HMA mixtures (i.e., providing a pass-fail test based on rutting potential, Figure 2) and can be reasonably well correlated to field rut performance. However, none should be relied on to predict field rut depths for specific projects based on laboratory wheel tracking rut depth relationships developed on other projects with different geographical locations and traffic. Additionally, due to the complex stress state of the samples, these tests cannot be used for mechanistic pavement design input. This section, taken largely from Cooley et al. (2000[1]) and Kandhal and Cooley (2003[2]), provides a brief overview of the Asphalt Pavement Analyzer (APA), Hamburg Wheel Tracking Device (HWTD) and the French Rutting Tester (FRT).

Figure 2: Comparison of a rut-resistant HMA (left) and a rut-susceptible HMA (right) after 8,000 load cycles in the APA.

Asphalt Pavement Analyzer (Figure 3)

Figure 3: Asphalt Pavement Analyzer (APA).

The Asphalt Pavement Analyzer (APA) is a second generation device that was originally developed in the mid 1980s as the Georgia Loaded Wheel Tester; a device designed for rut proof testing and field quality control. The APA tracks a loaded aluminum wheel back and forth across a pressurized linear hose over a HMA sample (Video 1). Although the APA can be used for a number of tests, it is typically used to measure and predict rutting. Most commonly, the wheel is tracked across the sample for 8,000 cycles using a 100 lb (445 N) load and a 100 psi (690 kPa) hose pressure. Test samples can be in the form of beams or cylinders. Beams are typically compacted with the asphalt vibratory compactor (Figure 4), while cylinder samples are typically compacted with the SGC. Video 1: APA operation (doors open for demonstration viewing only).

Figure 4: Asphalt Vibratory Compactor (AVC).

Kandhal and Cooley’s NCHRP Report 508: Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer (2003[2]) concludes the following about the APA: 1.

Sample compaction. Cylindrical samples compacted to 4-percent air voids and beam samples compacted to 5-percent air voids resulted in APA laboratory test results that were more closely related to field rutting performance than did cylindrical and beam samples compacted to 7-percent air voids.

2. 

Test temperature. Test temperature significantly affects measured rut depths in the APA. As test temperature increases, APA rut depths increase.



Samples tested at a test temperature corresponding to the high temperature PG specification better predicted field rutting performance than did samples tested at 6°C higher.

3. 

Hose diameter. Samples tested with both the standard- and large-diameter hoses predicted field rutting performance about equally well. However, samples tested with the standard hose produced less variability.



APA-measured rut depths were collectively higher with the standard-diameter hose than with the larger diameter hose.

4.

Field rutting prediction. Based on limited data, the APA compared well with other performance tests with respect to predicting the potential for rutting in the field. However, it is generally not possible to predict field rut depths from APA rut depths on a specific project using relationships developed on other projects with different geographical locations and traffic.



Beam and cylindrical samples predicted field rutting performance about equally well.



Laboratory rut depths measured by the APA had good correlations on an individual project basis with the field rut depths in the case of FHWA ALF, WesTrack, MnRoad, and I-80 (Nevada) projects. However, the APA-measured rut depths had a poor correlation with field rut depths in the case of 10 test sections on the NCAT Test Track, which did not develop any significant rutting after 2 years of loading.

Hamburg Wheel Tracking Device (Figure 5)

Figure 5: Hamburg Wheel Tracking Device (HWTD) from (Stuart and Youtcheff, 2001[3]).

The Hamburg Wheel Tracking Device (HWTD), developed in Germany, can be used to evaluate rutting and stripping potential. The HWTD tracks a loaded steel wheel back and forth directly on a HMA sample. Tests are typically conducted on 10.2 x 12.6 x 1.6 inch (260 x 320 x 40 mm) slabs (although the test can be modified to use SGC compacted samples) compacted to 7 percent air voids with a linear kneading compactor. Most commonly, the 1.85 inch (47 mm) wide wheel is tracked across a submerged (underwater) sample for 20,000 cycles (or until 20 mm of deformation occurs) using a 158 lb (705 N) load. Rut depth is measured continuously with a series of LVDTs on the sample. Several modified HWTDs have been produced in the U.S. with the principal modifications being loading force or wheel type.

Data Evaluation Figure 6 shows a typical plot from a HWTD test and the key plot parameters. The following parameters are measured and reported:

Figure 6: APA samples showing rutting after 8,000 load cycles.



Post-compaction consolidation. The rut depth at 1,000 load cycles is assumed due to continued consolidation.



Creep slope. The inverse of the rutting slope after post-compaction consolidation but before the stripping inflection point. Creep slope is used to evaluate rutting potential instead of rut depth because the number of load cycles at which moisture damage begins to affect rut depth varies between HMA mixtures and cannot be conclusively determined from the plot.



Stripping inflection point. The point at which the creep slope and stripping slope intercept. This can be used to evaluate moisture damage potential. If the stripping inflection point occurs at a low number of load cycles (e.g., less than 10,000), the HMA mixture may be susceptible to moisture damage.



Stripping slope. A measure of the accumulation of moisture damage. As with flow time and flow number, this portion of the plot may contain tertiary flow as well, however it is not possible to separate out moisture damage from tertiary viscous flow. The HWTD has been found to have excellent correlation with field performance (especially in moisture damage evaluation) (Aschenbrener, 1995[4]; Izzo and Tahmoressi, 1999[5]; Williams and Prowell, 1999[6]) however, it can fail to differentiate between some mixtures (Zhou et al., 2003 [7]). The FHWA has a good concise description of the HWTD at: http://www.tfhrc.gov/pavement/asphalt/labs/mixtures/hamburg.htm.

French Rutting Tester (Figure 7)

Figure 7: The FRT (top) and associated plate compactor (bottom).

The Laboratoire Central des Ponts et Chaussées (LCPC) wheel tracker, also known as French Rutting Tester (FRT), has been used in France for over 15 years to evaluate HMA rutting characteristics (Cooley et al., 2000[1]).

The FRT tracks a loaded pneumatic tire back and forth across a HMA sample. Tests are typically conducted on 7.1 x 19.7 x 0.8-3.9 inch (180 x 50 x 20-100 mm) slabs compacted with a plate compactor (Figure 7). Most commonly, the tire is tracked across a sample for 30,000 cycles using a 1124 lb (500 N) load applied to a pneumatic tire inflated to 87 psi (600 kPa) (Cooley et al., 2000 [1]). Aschenbrener (1992[8]) showed that the FRT can be used to differentiate between good and poor field rut performance in the U.S. The FHWA has a good concise description of the FRT at: www.tfhrc.gov/pavement/asphalt/labs/mixtures/frenchr.htm. LCPC has a short video of the FRT at: www.lcpc.fr/en/produits/materiels_mlpc/fiche.dml? id=123&type=abcdaire. The FRT is reportedly not valid for HMA mixtures with NMAS greater than 0.8 inches (20 mm). The slab width is relatively small compared to the tire width and mixtures with aggregates greater than 0.8 inches (20 mm) may be inhibited from shearing outward and upward. Aggregates larger than 0.8 inches (20 mm) may also wear the tires severely, and often cannot be compacted properly using the French Plate Compactor (FHWA, 2002[9]).

Test Description The following 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 TP 63: Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA) Determining Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA)

Summary Three sets of HMA samples are loaded into the temperature controlled chamber of the APA. Gauge readings are taken initially and then again after 8,000 load cycles. The difference between the two readings is the rutting induced by the APA. An average of all samples in the APA (6 cylindrical or 3 beam) is reported as the average APA rut depth. Figure 1 shows the APA with samples pulled out.

Approximate Test Time An 8,000 cycle test takes about 8.5 hours (6 hours to preheat the samples plus about 2.5 hours for the 8,000 cycle test and rut measurements). Creation and preparation of the samples can take upwards of several days depending upon conditioning times.

Basic Procedure 1.

Prepare either 6 cylindrical or 3 beam test samples (Figure 8). Laboratory compacted cylindrical samples should be compacted to 4 percent air voids and be 3 inches (75 mm) tall. Laboratory compacted beam samples should be compacted to 5 percent air voids and be 3 inches (75 mm) tall. Field core samples should have a 6 inch (150 mm) diameter and either be 3

inches (75 mm) tall or be augmented with plaster-of-paris to produce a 3 inch (75 mm) tall sample.

Figure 8: APA beam (left and right) and cylindrical (center) samples.

1

Determine the bulk specific gravity (Gmb), maximum specific gravity (Gmm) and air void content (Va) of each sample.

2

Set the test temperature at the high temperature specification of the PG binder used.

WARNING If the high temperature PG binder grade has been increased due to traffic, do not increase the APA test temperature. 1

Preheat the samples in the APA or a calibrated oven for 6 hours.

WARNING Do not hold the samples at elevated temperature form more than 24 hours prior to testing. 1

Set the hose pressure and load cylinder pressure to the desired levels. Typically, 120 psi (827 kPa) is used for the hose and 120 lb (534 N) is used for the load.

2

Stabilize the test chamber at the desired temperature.

3

Insert the test samples into the chamber.

WARNING

Do not open the preheated APA chamber door for more than 6 minutes when inserting and securing the samples. Once the samples are secured, close the door and allow 10 min tues for the test temperature to stabilize. 1

Apply 25 cycles to seat the samples.

2

Open the chamber doors, unlock and unseat the sample, and place the rut depth measurement template over the sample.

3

Zero the gauge and take initial rut depth readings. Repeat this step for each set of cylinders of beams in the APA.

4

Push the sample holding tray in, close the APA doors and allow 10 minutes for the test temperature to stabilize.

5

Set the APA counter to 8,000 cycles.

6

Start the APA loading.

7

At the end of 8,000 cycles, repeat step 10 to get the final rut depth measurement (Figure 9).

Figure 9: APA samples showing rutting after 8,000 load cycles.

Results Parameters Measured Rutting, fatigue cracking and moisture susceptibility. Rutting prediction is the most frequent use and will be presented in this section.

Specifications Superpave mix design does not have an APA rutting specification. Typically, state and local agencies develop their own specifications based on pavement location, traffic and environment.

Hamburg Wheel Tracking Device Example Specifications The Colorado DOT recommends a maximum rut depth at 10,000 load cycles of 4 mm and 10 mm at 20,000 load cycles. The City of Hamburg, Germany uses a maximum allowable rut depth of 4 mm at 19,200 load cycles (FHWA, 2003[10]).

Typical Values Laboratory rut depths are highly dependent on HMA mixture composition, testing temperature, hose size and applied load. Typical values can range from 0.2 to 0.8 inches (5 to 20 mm) after 8,000 loading cycles (Figure 9).

Calculations For rutting, the initial gauge reading is subtracted from the final reading to get the APA induced rutting.

Footnotes 1.

(↵ returns to text)

Cooley, L.A.; Kandhal, P.S.; Buchanan, M.S.; Fee, F. and Epps, A. (2000). Loaded Wheel Testers in the United States: State of the Practice. NCAT Report No. 2000-4. National Center for Asphalt Technology. Auburn, AL. http://www.eng.auburn.edu/center/ncat/reports/rep00-04.pdf. Accessed December 2004.↵

2.

Kandhal, P.S. and Cooley, L.A. (2003). NCHRP Report 508: Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer. Transportation Research Board, National Research Council. Washington, D.C. http://trb.org/news/blurb_detail.asp?id=2169. Accessed December 2004.↵

3.

Stuart, K.D. and Youtcheff, J.S. (2001). Understanding the Performance of Modified Asphalt Binders in Mixtures: Evaluation of Moisture Sensitivity. Report No. FHWA-RD-02-029. Federal Highway Administration. Washington, D.C. http://www.tfhrc.gov/pavement/asphalt/pavepubs/02029. Accessed December 2004.↵

4.

Aschenbrener, T. (1995). Evalulation of Hamburg Wheel-Tracking Device to Predict Moisture Damage in Hot MixAsphalt. Transportation Research Record 1492. Transportation Research Board, National Research Council, Washington, D.C. pp. 193-201.↵

5.

Izzo, R.P. and Tahmoressi, M. (1999). Use of Hamburg Wheel-Tracking Device for Evaluating Moisture Susceptibility of Hot-Mix Asphalt.Transportation Research Record 1681 . Transportation Research Board, National Research Council, Washington, D.C. pp. 76-85.↵

6.

Williams, R.C. and Prowell, B.D. (1999). Comparison of Laboratory Wheel-Tracking Test Results with WesTrack Performance. Transportation Research Record 1681. Transportation Research Board, National Research Council, Washington, D.C. pp. 121-128.↵

7.

Zhou, F.; Chen, D.H.; Scullion, T. and Bilyeu, J. (2003). Case Study: Evaluation of Laboratory Test Methods to Characterize Permanent Deformation Properties of Asphalt Mixes. International Journal of Pavement Engineering, Vol. 4, No. 3. pp. 155-164.↵

8.

Aschenbrener, T. (1992). Comparison of Results Obtained from the French Rutting Tester with Pavements of Known Field Performance. Report No. CDOT-DTD-R-92-11, Colorado DOT.↵

9.

Federal Highway Administration (FHWA). (2002). Turner-Fairbanks Highway Research Center Bituminous Mixtures Laboratory (BLM) Equipment web page. http://www.tfhrc.gov/pavement/asphalt/labs/mixtures/bmlequip.htm. Accessed December 2004.↵

10.

Federal Highway Administration (FHWA). (2003). Federal Highway Administration Administrators.http://www.fhwa.dot.gov/administrators/index.htm. Accessed 22 July 2004.↵

- See more at: http://www.pavementinteractive.org/article/laboratory-wheel-trackingdevices/#sthash.q6WLQL0h.dpuf

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 non-destructive 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.

2. 3. 4. 5. o o

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 thickwalled flasks or vacuum desiccators are used. If Weighing in Air is chosen in step 5, flasks (Figure 6) or pycnometers are used. Completely cover the sample by adding water at approximately 77°F (25°C) to the container. 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. Slowly release the vacuum. Weigh the sample in water or air: 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. 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 in-place 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) - See more at: http://www.pavementinteractive.org/article/theoretical-maximum-specificgravity/#sthash.HORkmtLv.dpuf

Bulk Specific Gravity Publish date: April 21, 2011 | Author: Pavement Interactive Print Cite

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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).

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Saturated surface dry (SSD, water fills the HMA air voids).

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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:

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AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens

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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 5: CoreLok vacuum chamber with sample inside.

Figure 6: CoreLok sample vacuum sealed in a plastic bag.

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

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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:

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AASHTO T 275: Bulk Specific Gravity of Compacted Bituminous Mixtures Using ParaffinCoated Specimens

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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. 1

Record the dry mass (Figure 8).

Figure 8: Sample weighing.

1

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).

2

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.

Footnotes

1.

(↵ returns to text)

Troxler Electronic Laboratories, Inc. (Troxler). (March 2001). Measuring Bulk Specific Gravity of Compacted Specimens Using The Troxler Model 3660 CoreReader. Web page on the Troxler web site. Troxler Electronic Laboratories, Inc. Research Triangle Park, NC. http://www.troxlerlabs.com/3660app.html. Accessed 1 July 2002.↵

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Bulk Specific Gravity Bulk specific gravity is essentially the density of a compacted (laboratory or field) HMA specimen. The bulk specific gravity is a critical HMA characteristic because it is used to calculate most other HMA parameters including air voids, VMA, and TMD. This reliance on bulk specific gravity is because mix design is based on volume, which is indirectly determined using mass and specific gravity. Bulk specific gravity is calculated as: There are several different ways to measure bulk specific gravity, all of which use slightly different ways to determine specimen volume:

1.

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. o Saturated Surface Dry (SSD). The most common method, calculates the specimen volume by subtracting the mass of the specimen in water from the mass of a saturated surface dry (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. 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 volume measurement and thus an erroneously high bulk specific gravity. o 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 (see Figure 1). 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 1. Paraffin Coated Sample



Parafilm. This method wraps the specimen in a thin paraffin film (see Figure 2) and then weighs the specimen 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 2. Parafilm Application



CoreLok. This method calculates specimen volume like the parafilm method but uses a vacuum chamber (see Figure 3) to shrink-wrap the specimen in a high-quality plastic bag (see Figure 4) rather than cover it in a paraffin film. This method has shown some promise in both accuracy and precision.

Figure 3. CoreLok Vacuum Chamber



Figure 4. CoreLok Specimen

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).

Figure 5.Gamma Ray Device





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 5 shows the Troxler device. The standard bulk specific gravity test is: AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens (this is the SSD water displacement method discussed previously)

Theoretical Maximum Specific Gravity The theoretical maximum specific gravity (often referred to as theoretical maximum density and thus abbreviated TMD) is the HMA density excluding air voids. Thus, theoretically, if all the air voids were eliminated from an HMA sample, the combined density of the remaining aggregate and asphalt binder would be the TMD – often referred to as Rice density after its inventor. TMD is a critical HMA characteristic because it is used to calculate percent air voids in compacted HMA and provide target values for HMA compaction. TMD is determined by taking a sample of oven-dry HMA in loose condition (versus compacted condition), weighing it and then completely submerging it in a 25°C water bath. A vacuum is then applied for 15 minutes (see Figure 6) to remove any entrapped air. The sample volume is then calculated by subtracting its mass in water from its dry mass. The formula for calculating TMD is:

where:

TMD

= theoretical maximum density

A

= mass of oven dry sample in air in grams

C

= mass of water displaced by sample at 25°C in grams

Figure 6. Containers Used to Agitate and Draw a Vacuum on Submerged TMD Samples

The standard TMD test is: AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures - See more at: http://www.pavementinteractive.org/article/mixture-characterizationtests/#sthash.m0STnod6.dpuf

HMA Mix Design Fundamentals Publish date: June 5, 2009 | Author: Pavement Interactive Print Cite Email This    

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HMA consists of two basic ingredients: aggregate and asphalt binder. HMA mix design is the process of determining what aggregate to use, what asphalt binder to use and what the optimum combination of these two ingredients ought to be. When aggregate and asphalt binder are combined to produce a homogenous substance, that substance, HMA, takes on new physical properties that are related to but not identical to the physical properties of its components. Mechanical laboratory tests can be used to characterize the basic mixture or predict mixture properties. HMA mix design has evolved as a laboratory procedure that uses several critical tests to make key characterizations of each trial HMA blend. Although these characterizations are not comprehensive, they can give the mix designer a good understanding of how a particular mix will perform in the field during constructionand under subsequent traffic loading. This section covers mix design fundamentals common to all mix design methods. First, two basic concepts (mix design as a simulation and weight-volume terms and relationships) are discussed to set a framework for subsequent discussion. Second, the variables that mix design may manipulate are presented. Third, the fundamental objectives of mix design are presented. Finally, a generic mix design procedure (which Hveem, Marshall and Superpave methods all use) is presented.

Concepts Before discussing any mix design specifics, it is important to understand a couple of basic mix design concepts: 

Mix design is a simulation



HMA weight-volume terms and relationships

Mix Design is a Simulation First, and foremost, mix design is a laboratory simulation. Mix design is meant to simulate actual HMA manufacturing, construction and performance to the extent possible. Then, from this simulation

we can predict (with reasonable certainty) what type of mix design is best for the particular application in question and how it will perform. Being a simulation, mix design has its limitations. Specifically, there are substantial differences between laboratory and field conditions. Certainly, a small laboratory setup consisting of several 100 – 150 mm (4 – 6 inch) samples, a compaction machine and a couple of testing devices cannot fully recreate actual manufacturing, construction and performance conditions. For instance, mix design compaction should create the same general density (void content) to which the traffic will finally compact a mix in the field under service conditions (Roberts et al., 1996 [1]). However, it is difficult to calibrate a number of tamper blows (laboratory compaction) to a specificconstruction compaction and subsequent traffic loading (field compaction). Currently used correlations between these densities are empirical in nature and extremely rough (e.g., high, medium and low traffic categories). However, despite limitations such as the preceding, mix design procedures can provide a cost effective and reasonably accurate simulation that is useful in making mix design decisions.

HMA Weight-Volume Terms and Relationships Mix design, and specifically Superpave mix design, is volumetric in nature. That is, it seeks to combineaggregate and asphalt on a volume basis (as opposed to a weight basis). Volume measurements are usually made indirectly by determining a material’s weight and specific gravity and then calculating its volume. Therefore, mix design involves several different void and specific gravity measurements. It is important to have a clear understanding of these terms before proceeding. 

See HMA Weight-Volume Terms and Relationships

Variables HMA is a rather complex material upon which many different, and sometimes conflicting, performance demands are placed. It must resist deformation and cracking, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed. In order to meet these demands, the mix designer can manipulate all of three variables: 1.

Aggregate. Items such as type (source), gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.

2.

Asphalt binder. Items such as type, durability, rheology, purity as well as additional modifying agents can be measured, judged and altered to some degree.

3.

The ratio of asphalt binder to aggregate. Usually expressed in terms of percent asphalt binder by total weight of HMA, this ratio has a profound effect on HMA pavement performance. Because of the wide differences in aggregate specific gravity, the proportion of asphalt binder expressed as a percentage of total weight can vary widely even though the volume of asphalt binder as a percentage of total volumeremains quite constant.

Objectives Before embarking on a mix design procedure it is important to understand what its objectives are. This section presents the typical qualities of a well-made HMA mix. By manipulating the variables of aggregate, asphaltbinder and the ratio between the two, mix design seeks to achieve the following qualities in the final HMA product (Roberts et al., 1996[1]): 1.

Deformation resistance (stability). HMA should not distort (rut) or deform (shove) under traffic loading. HMA deformation is related to one or more of the following: 

Aggregate surface and abrasion characteristics. Rounded particles tend to slip by one another causing HMA distortion under load while angular particles interlock with one another providing a good deformation resistant structure. Brittle particles cause mix distortion because they tend to break apart under agitation or load. Tests for particle shape and texture as well as durability and soundness can identify problem aggregate sources. These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.

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Aggregate gradation. Gradations with excessive fines (either naturally occurring or caused by excessive abrasion) cause distortion because the large amount of fine particles tend to push the larger particles apart and act as lubricating ball-bearings between these larger particles. A gradation resulting in low VMA or excessive asphalt binder content can have the same effect. Gradation specifications are used to ensure acceptable aggregate gradation.

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Asphalt binder content. Excess asphalt binder content tends to lubricate and push aggregateparticles apart making their rearrangement under load easier. The optimum asphalt binder content as determined by mix design should prevent this.

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Asphalt binder viscosity at high temperatures. In the hot summer months, asphalt binder viscosity is at its lowest and the pavement will deform more easily under load. Specifying an asphalt binder with a minimum high temperature viscosity (as can be done in the Superpave asphalt binder selection process) ensures adequate high temperature viscosity.

2.

Fatigue resistance. HMA should not crack when subjected to repeated loads over time. HMA fatigue cracking is related to asphalt binder content and stiffness. Higher asphalt binder contents will result in a mix that has a greater tendency to deform elastically (or at least deform) rather than fracture under repeated load. The optimum asphalt binder content as determined by mix design should be high enough to prevent excessive fatigue cracking. The use of an asphalt binder with a lower stiffness will increase a mixture’s fatigue life by providing greater flexibility. However, the potential for rutting must also be considered in the selection of an asphalt binder. Note that fatigue resistance is also highly dependent upon the relationship between structural layer thickness and loading. However, this section only addresses mix design issues.

3.

Low temperature cracking resistance. HMA should not crack when subjected to low ambient temperatures. Low temperature cracking is primarily a function of the asphalt binder low temperature stiffness. Specifying asphalt binder with adequate low temperature properties (as can be done in the Superpave asphalt binder selection process) should prevent, or at least limit, low temperature cracking.

4.

Durability. HMA should not suffer excessive aging during production and service life. HMA durability is related to one or more of the following: 

The asphalt binder film thickness around each aggregate particle. If the film thickness surrounding the aggregate particles is insufficient, it is possible that the aggregate may become accessible to water through holes in the film. If the aggregate is hydrophilic, water will displace the asphalt film and asphalt-aggregate cohesion will be lost. This process is typically referred to as stripping. The optimum asphalt binder content as determined by mix design should provide adequate film thickness.

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Air voids. Excessive air voids (on the order of 8 percent or more) increase HMA permeability and allow oxygen easier access to more asphalt binder thus accelerating oxidation and volatilization. To address this, HMA mix design seeks to adjust items such as asphalt content and aggregategradation to produce design air voids of about 4 percent. Excessive air voids can be either a mix design or a construction problem and this section only addresses the mix design problem.

5.

Moisture damage resistance. HMA should not degrade substantially from moisture penetration into the mix. Moisture damage resistance is related to one or more of the following: 

Aggregate mineral and chemical properties. Some aggregates attract moisture to their surfaces, which can cause stripping. To address this, either stripping-susceptible aggregates can be avoided or an anti-stripping asphalt binder modifier can be used.

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Air voids. When HMA air voids exceed about 8 percent by volume, they may become interconnected and allow water to easily penetrate the HMA and cause moisture damage through pore pressure or ice expansion. To address this, HMA mix design adjusts asphalt binder content and aggregategradation to produce design air voids of

about 4 percent. Excessive air voids can be either a mix design or a construction problem and this section only addresses the mix design problem. 6.

Skid resistance. HMA placed as a surface course should provide sufficient friction when in contact with a vehicle’s tire. Low skid resistance is generally related to one or more of the following: 

Aggregate characteristics such as texture, shape, size and resistance to polish. Smooth, rounded or polish-susceptible aggregates are less skid resistant. Tests for particle shape and texture can identify problem aggregate sources. These sources can be avoided, or at a minimum, aggregatewith good surface and abrasion characteristics can be blended in to provide better overall characteristics.



Asphalt binder content. Excessive asphalt binder can cause HMA bleeding. Using the optimumasphalt binder content as determined by mix design should prevent bleeding.

7.

Workability. HMA must be capable of being placed and compacted with reasonable effort. Workability is generally related to one or both of the following: 

Aggregate texture, shape and size. Flat, elongated or angular particles tend to interlock rather than slip by one another making placement and compaction more difficult (notice that this is almost in direct contrast with the desirable aggregate properties for deformation resistance). Although no specific mix design tests are available to quantify workability, tests for particle shape and texture can identify possible workability problems.

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Aggregate gradation. Gradations with excess fines (especially in the 0.60 to 0.30 mm (No. 30 to 50) size range when using natural, rounded sand) can cause a tender mix. A gradation resulting in low VMA or excess asphalt binder content can have the same effect. Gradation specifications are used to ensure acceptable aggregate gradation.



Asphalt binder content. At laydown temperatures (above about 120 C (250 F)) asphalt binder works as a lubricant between aggregate particles as they are compacted. Therefore, low asphaltbinder content reduces this lubrication resulting in a less workable mix. Note that a higher asphaltbinder content is generally good for workability but generally bad for deformation resistance.



Asphalt binder viscosity at mixing/laydown temperatures. If the asphalt binder viscosity is too high at mixing and laydown temperatures, the HMA becomes difficult to dump, spread and compact. The Superpave rotational viscometer specifically tests for mixing/laydown temperature asphalt binder viscosity.

Knowing these objectives, the challenge in mix design is then to develop a relatively simple procedure with a minimal amount of tests and samples that will produce a mix with all the above HMA qualities.

Basic Procedure

HMA mix design is the process of determining what aggregate to use, what asphalt binder to use and what the optimum combination of these two ingredients ought to be. In order to meet the demands placed by the preceding desirable HMA properties, all mix design processes involve three basic steps: 1.

Aggregate selection. No matter the specific method, the overall mix design procedure begins with evaluation and selection of aggregate and asphalt binder sources. Different authorities specify different methods of aggregate acceptance. Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source. Then, for each mix design, gradation and size requirements are checked. Normally, aggregate from more than one source is required to meet gradation requirements.

2.

Asphalt binder selection. Although different authorities can and do specify different methods of asphaltbinder evaluation, the Superpave asphalt binder specification has been or will be adopted by most State DOTs as the standard (NHI, 2000 [2]).

3.

Optimum asphalt binder content determination. Mix design methods are generally distinguished by the method with which they determine the optimum asphalt binder content. This process can be subdivided as follows: 

Make several trial mixes with different asphalt binder contents.



Compact these trial mixes in the laboratory. It is important to understand that this step is at best arough simulation of field conditions.



Run several laboratory tests to determine key sample characteristics. These tests represent a starting point for defining the mixture properties but they are not comprehensive nor are they exact reproductions of actual field conditions.



Pick the asphalt binder content that best satisfies the mix design objectives.

Job Mix Formula The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder. This recommended mixture, which also includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF) or recipe.

Summary HMA mix design is a laboratory process used to determine the appropriate aggregate, asphalt binder and their proportions for use in HMA. Mix design is a process to manipulate three variables: (1) aggregate, (2) asphaltbinder content and (3) the ratio of aggregate to asphalt binder with the objective of obtaining an HMA that is deformation resistant, fatigue resistant, low temperature crack

resistant, durable, moisture damage resistant, skid resistant and workable. Although mix design has many limitations it has proven to be a cost-effective method to provide crucial information that can be used to formulate a high-performance HMA. Static Creep Tests A static creep test (see Figure 1) is conducted by applying a static load to an HMA specimen and then measuring the specimen’s permanent deformation after unloading (see Figure 2). This observed permanent deformation is then correlated with rutting potential. A large amount of permanent deformation would correlate to higher rutting potential. Creep tests have been widely used in the past because of their relative simplicity and availability of equipment. However, static creep test results do not correlate well with actual in-service pavement rutting (Brown et al., 2001[1]).

Figure 1. Unconfined Static Creep Test

Figure 2. Static Creep Test Plot

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Unconfined Static Creep Test The most popular static creep test, the unconfined static creep test (also known as the simple creep test or uniaxial creep test), is inexpensive and relatively easy. The test consists of a static axial stress of 100 kPa (14.5 psi) being applied to a specimen for a period of 1 hour at a temperature of 40°C (104°F). The applied pressure is usually cannot exceed 206.9 kPa (30 psi) and the test temperature usually cannot exceed 40C (104F) or the sample may fail prematurely (Brown et al., 2001[1]). Actual pavements are typically exposed to tire pressures of up to 828 kPa (120 psi) and temperatures in excess of 60C (140F). Thus, the unconfined test does not closely simulate field conditions (Brown et al., 2001[1]). Confined Static Creep Test The confined static creep test (also known as the triaxial creep test) is similar to the unconfined static creep test in procedure but uses a confining pressure of about 138 kPa (20 psi), which allows test conditions to more closely match field conditions. Research suggests that the static confined creep test does a better job of predicting field performance than the static unconfined creep test (Roberts et al., 1996[2]). Diametral Static Creep Test A diametral static creep test uses a typical HMA test specimen but turning it on its side so that it is loaded in its diametral plane. Some standard static creep tests are: AASHTO TP 9: Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device Repeated Load Tests A repeated load test applies a repeated load of fixed magnitude and cycle duration to a cylindrical test specimen (see Figure 3). The specimen’s resilient modulus can be calculated using the its horizontal deformation and an assumed Poisson’s ratio. Cumulative permanent deformation as a

function of the number of load cycles is recorded and can be correlated to rutting potential. Tests can be run at different temperatures and varying loads. The load varies is applied in a short pulse followed by a rest period. Repeated load tests are similar in concept to the triaxial resilient modulus test for unconfined soils and aggregates. Repeated load tests correlate better with actual in-service pavement rutting than static creep tests (Brown et al., 2001[1]).

Figure 3. Repeated Load Test Schematic

Note: this example is simplified and shows only 6 load repetitions, normally there are conditioning repetitions followed by a series of load repetitions during the test at a determined load level and possibly at different temperatures. Most often, results from repeated load tests are reported using a cumulative axial strain curve like the one shown in Figure 4. The flow number (FN) is the load cycles number at which tertiary flow begins. Tertiary flow can be differentiated from secondary flow by a marked departure from the linear relationship between cumulative strain and number of cycles in the secondary zone. It is assumed that in tertiary flow, the specimen’s volume remains constant. The flow number (FN) can be correlated with rutting potential.

Figure 4. Repeated Load Test Results Plot

Unconfined Repeated Load Test The unconfined repeated load test is comparatively more simple to run than the unconfined test because it does not involve any confining pressure or associated equipment. However, like the unconfined creep test, the allowable test loads are significantly less that those experience by inplace pavement. Confined Repeated Load Test The confined repeated load test is more complex than the unconfined test due to the required confining pressure but, like the confined creep test, the confining pressure allows test loads to be applied that more accurately reflect loads experienced by in-place pavements. Diametral Repeated Load Test A diametral repeated load test uses a typical HMA test specimen but turning it on its side so that it is loaded in its diametral plane. Diametral testing has two critical shortcomings that hinder its ability to determine permanent deformation characteristics (Brown et al., 2001 [1]): 1. The state of stress is non-uniform and strongly dependent on the shape of the specimen. At high temperature or load, permanent deformation produces changes in the specimen shape that significantly affect both the state of stress and the test measurements. 2. During the test, the only relatively uniform state of stress is tension along the vertical diameter of the specimen. All other states of stress are distinctly nonuniform. Shear Repeated Load Test The Superpave shear tester (SST), developed for Superpave, can perform a repeated load test in shear. This test, known as the repeated shear at constant height (RSCH) test, applies a repeated haversine (inverted cosine offset by half its amplitude – a continuous haversine wave would look like a sine wave whose negative peak is at zero) shear stress to an axially loaded specimen and records axial and shear deformation as well as axial and shear load. RSCH data have been shown to have high variability (Brown et al., 2001[1]). Some standard repeated load tests are:

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AASHTO TP 7: Determining the Permanent Deformation and Fatigue Cracking Characteristics of Hot Mix Asphalt (HMA) Using the Superpave Shear Tester (SST) – Procedure F AASHTO TP 31: Determining the Resilient Modulus of Bituminous Mixtures by Indirect Tension ASTM D 4123: Indirect Tension Test for Resilient Modulus of Bituminous Mixtures Dynamic Modulus Tests Dynamic modulus tests apply a repeated axial cyclic load of fixed magnitude and cycle duration to a test specimen (see Figure 1). Test specimens can be tested at different temperatures and three different loading frequencies (commonly 1, 4 and 16 Hz). The applied load varies and is usually applied in a haversine wave (inverted cosine offset by half its amplitude – a continuous haversine wave would look like a sine wave whose negative peak is at zero). Figure 5 is a schematic of a typical dynamic modulus test.

Figure 5. Dynamic Modulus Test Schematic

Dynamic modulus tests differ from the repeated load testsin their loading cycles and frequencies. While repeated load tests apply the same load several thousand times at the same frequency, dynamic modulus tests apply a load over a range of frequencies (usually 1, 4 and 16 Hz) for 30 to 45 seconds (Brown et al., 2001[1]). The dynamic modulus test is more difficult to perform than the repeated load test since a much more accurate deformation measuring system is necessary. The dynamic modulus test measures a specimen’s stress-strain relationship under a continuous sinusoidal loading. For linear (stress-strain ratio is independent of the loading stress applied) viscoelastic materials this relationship is defined by a complex number called the “complex modulus” (E*) (Witczak et al., 2002[3]) as seen in the equation below:

where E = complex modulus

:

* = dynamic modulus phase angle – the angle by which εo lags behind σo. For a pure elastic material, φ = 0, and the complex modulus (E*) is equal to the φ = absolute value, or dynamic modulus. For pure viscous materials, φ = 90°.

i = imaginary number The absolute value of the complex modulus, |E*|, is defined as the dynamic modulus and is calculated as follows (Witczak et al., 2002[3]):

where :

= dynamic modulus peak stress amplitude

o = (applied load / sample cross sectional area) peak amplitude of recoverable axial strain =  L/L. Either measured directly with strain gauges or calculated from displacements measured with linear variable eo = displacement transducers (LVDTs). L

= gauge length over which the sample deformation is measured

 L = the recoverable portion of the change in sample length due to the applied load The dynamic modulus test can be advantageous because it can measure also measure a specimen’s phase angle (φ), which is the lag between peak stress and peak recoverable strain. The complex modulus, E*, is actually the summation of two components: (1) the storage or elastic modulus component and (2) the loss or viscous modulus. It is an indicator of the viscous properties of the material being evaluated. Unconfined Dynamic Modulus Test The unconfined dynamic modulus test is performed by applying an axial haversine load to a cylindrical test specimen. Although the recommend specimen size for the test is 100 mm (4 inch) in diameter by 200 mm (8 inches) high, it may be possible to use smaller specimen heights with success (Brown et al., 2001[1]). Unconfined dynamic modulus tests do not permit the determination of phase angle (φ). Confined Dynamic Modulus Test The confined dynamic modulus test is basically the unconfined test with an applied lateral confining pressure. Confined dynamic modulus tests allow for the determination of phase angle (φ). Although the recommend specimen size for the dynamic modulus test is 100 mm (4 inch) in diameter by 200 mm (8 inches) high, it may be possible to use smaller specimen heights with success (Brown et al., 2001[1]). Figures 6 and 7 show a prototype Superpave Simple Performance Test (SPT). The SPT will provide a performance test for the Superpave mix design method.

Figure 6. A Prototype Superpave SPT

Figure 7. The SPT is a Confined Dynamic Modulus Test

Shear Dynamic Modulus Test The shear dynamic modulus test is known as the frequency sweep at constant height (FSCH) test. Shear dynamic modulus equations are the same as those discussed above although traditionally the term E* is replace by G* to denote shear dynamic modulus and o ando are replaced by 0 and 0 to denote shear stress and axial strain respectively. The shear dynamic modulus can be accomplished by two different testing apparatuses: 1. Superpave shear tester (SST). The SST FSCH test is a is a constant strain test (as opposed to a constant stress test). Test specimens are 150 mm (6 inches) in diameter and 50 mm (2 inches) tall (see Figure 8). To conduct the test the HMA sample is essentially glued to two plates (see Figures 9 through 11) and then inserted into the SST. Horizontal strain is applied at a range of frequencies (from 10 to 0.1 Hz) using a haversine loading pattern, while the specimen height is maintained constant by compressing or pulling it vertically as required. The SST produces a constant strain of about 100 microstrain (Witczak et al., 2002[3]). The SST is quite expensive and requires a highly trained operator to run thus making it impractical for field use and necessitating further development. 2. Field shear tester (FST). The FST FSCH test is a is a constant stress test (as opposed to a constant strain test). The FST is a derivation of the SST and is meant to be less expensive and easier to use. For instance, rather than compressing or pulling the sample to maintain a constant height like the SST, the FST maintains constant specimen height using rigid spacers attached to the specimen ends. Further, the FST shears the specimen in the diametral plane.

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Figure 8. Superpave Shear Tester (SST)

Figure 9. Loading Chamber

Figure 10. Prepared Sample

Figure 11. Prepared Sample (left) and Sample After Test.

Standard complex modulus tests are: Unconfined dynamic modulus. ASTM D 3497: Dynamic Modulus of Asphalt Mixtures Shear dynamic modulus. AASHTO TP 7: Determining the Permanent Deformation and Fatigue Cracking Characteristics of Hot Mix Asphalt (HMA) Using the Simple Shear Test (SST) Device, Procedure E – Frequency Sweep Test at Constant Height. Empirical Tests

The Hveem stabilometer and cohesiometer and Marshall stability and flow tests are empirical tests used to quantify an HMA’s potential for permanent deformation. They are discussed in their mix design sections. Simulative Tests – Laboratory Wheel-Tracking Devices

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Laboratory wheel-tracking devices (see Video 1) measure rutting by rolling a small loaded wheel device repeatedly across a prepared HMA specimen. Rutting in the test specimen is then correlated to actual in-service pavement rutting. Laboratory wheel-tracking devices can also be used to make moisture susceptibility and stripping predictions by comparing dry and wet test results Some of these devices are relatively new and some have been used for upwards of 15 years like the Laboratoire Central des Ponts et Chausées (LCPC) wheel tracker – also known as the French Rutting Tester (FRT). Cooley et al. (2000[4]) reviewed U.S. loaded wheel testers and found: Results obtained from the wheel tracking devices correlate reasonably well to actual field performance when the in-service loading and environmental conditions of that location are considered. Wheel tracking devices can reasonably differentiate between binder performance grades. Wheel tracking devices, when properly correlated to a specific site’s traffic and environmental conditions, have the potential to allow the user agency the option of a pass/fail or “go/no go” criteria. The ability of the wheel tracking devices to adequately predict the magnitude of the rutting for a particular pavement has not been determined at this time. A device with the capability of conducting wheel-tracking tests in both air and in a submerged state, will offer the user agency the most options of evaluating their materials. In other words, wheel tracking devices have potential for rut and other measurements but the individual user must be careful to establish laboratory conditions (e.g., load, number of wheel passes, temperature) that produce consistent and accurate correlations with field performance.

Video 1: Asphalt Pavement Analyzer - A Wheel Tracking Device

Fatigue Life

HMA fatigue properties are important because one of the principal modes of HMA pavement failure is fatigue-related cracking, called fatigue cracking. Therefore, an accurate prediction of HMA fatigue properties would be useful in predicting overall pavement life. Flexural Test One of the typical ways of estimating in-place HMA fatigue properties is the flexural test (see Figures 12 and 13). The flexural test determines the fatigue life of a small HMA beam specimen (380 mm long x 50 mm thick x 63 mm wide) by subjecting it to repeated flexural bending until failure (see Figure 14). The beam specimen is sawed from either laboratory or field compacted HMA. Results are usually plotted to show cycles to failure vs. applied stress or strain.

Figure 12 (left). Flexural Testing Device

Figure 13 (right). Flexural Testing Device

Figure 14. Flexural Test Schematic (click picture to animate)

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The standard fatigue test is: AASHTO TP 8: Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bending

Tensile Strength

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HMA tensile strength is important because it is a good indicator of cracking potential. A high tensile strain at failure indicates that a particular HMA can tolerate higher strains before failing, which means it is more likely to resist cracking than an HMA with a low tensile strain at failure. Additionally, measuring tensile strength before and after water conditioning can give some indication of moisture susceptibility. If the water-conditioned tensile strength is relatively high compared to the dry tensile strength then the HMA can be assumed reasonably moisture resistant. There are two tests typically used to measure HMA tensile strength: Indirect tension test Thermal cracking test Indirect Tension Test

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The indirect tensile test uses the same testing device as the diametral repeated load test and applies a constant rate of vertical deformation until failure. It is quite similar to the splitting tension test used for PCC. Standard indirect tension test is a part of the following test: AASHTO TP 9: Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device Thermal Cracking Test The thermal cracking test determines the tensile strength and temperature at fracture of an HMA sample by measuring the tensile load in a specimen which is cooled at a constant rate while being restrained from contraction. The test is terminated when the sample fails by cracking.

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The standard thermal cracking test is: AASHTO TP 10: Method for Thermal Stress Restrained Specimen Tensile Strength

Stiffness Tests Stiffness tests are used to determine a HMA’s elastic or resilient modulus. Although these values are fairly well-defined for many different mix types, these tests are still used to verify values, determine values in forensic testing or determine values for new mixtures or at different temperatures. Many repeated load tests can be used to determine resilient modulus as well. Of particular note, temperature has a profound effect on HMA stiffness. Table 1 shows some typical HMA resilient modulus values at various temperatures. Figure 15 shows that HMA resilient modulus changes by a factor of about 100 for a 56 °C (100 °F) temperature change for “typical” dense-graded HMA mixtures. This can affect HMA performance parameters such as rutting and shoving. This is one reason why the Superpave PG binder grading system accounts for expected service temperatures when specifying an asphaltbinder. Material Resilient Modulus (MR) MPa

psi

HMA at 32°F (0 °C)

14,000

2,000,000

HMA at 70°F (21 °C)

3,500

500,000

HMA at 120°F (49 °C)

150

20,000

Compare to other materials Table 1: Typical Resilient Modulus Values for HMA PavementMaterials

Figure 15. General Stiffness-Temperature Relationship for Dense-Graded Asphalt Concrete

Moisture Susceptibility Numerous tests have been used to evaluate moisture susceptibility of HMA; however, no test to date has attained any wide acceptance (Roberts et al., 1996[2]). In fact, just about any performance test that can be conducted on a wet or submerged sample can be used to evaluate the effect of moisture on HMA by comparing wet and dry sample test results. Superpave recommends the modified Lottman Test as the current most appropriate test and therefore this test will be described. The modified Lottman test basically compares the indirect tensile strength test results of a dry sample and a sample exposed to water/freezing/thawing. The water sample is subjected to vacuum saturation, an optional freeze cycle, followed by a freeze and a warm-water cycle before being tested for indirect tensile strength (AASHTO, 2000a[5]). Test results are reported as a tensile strength ratio:

where:

TSR

=

tensile strength ratio

S1

=

average dry sample tensile strength

S2

=

average conditioned sample tensile strength

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Generally a minimum TSR of 0.70 is recommended for this method, which should be applied to field-produced rather than laboratory-produced samples (Roberts et al., 1996). For laboratory samples produced in accordance with AASHTO TP 4 (Method for Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor), AASHTO MP 2 (Specification for Superpave Volumetric Mix Design) specifies a minimum TSR of 0.80. In addition to the modified Lottman test, some state agencies use the Hamburg Wheel Tracking Device (HWTD) to test for moisture susceptibility since the test can be carried out in a warm water bath. The standard modified Lottman test is: AASHTO T 283: Resistance of Compacted Bituminous Mixture to Moisture-Induced Damage - See more at: http://www.pavementinteractive.org/article/hma-performancetests/#sthash.tJqLLYSU.dpuf

Absolute Viscosity Publish date: August 4, 2008 | Author: Pavement Interactive Print Cite Email This       

Viscosity is simply a measure of a fluid’s resistance to flow and is described by the following equation:

Asphalt binder viscosity is typically measured at 60 C (140 F) because it approximates the maximum HMA pavement surface temperature during summer in the U.S. The basic absolute viscosity test measures the time it takes for a fixed volume of asphalt binder to be drawn up through a capillary tube by means of vacuum, under closely controlled conditions of vacuum and temperature (ASTM, 2003 [1]). Although absolute viscosity is an improvement over the penetration test, it still only measures viscosity at one temperature and thus does not fully characterize an asphalt binder’s consistency over the expected range of construction and service conditions.

Standard Test Methods 

AASHTO T 202 and ASTM D 2171: Viscosity of Asphalts by Vacuum Capillary Viscometer - See more at: http://www.pavementinteractive.org/article/absolute-viscosity/#sthash.PeT3ZfkP.dpuf