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Evaluating the Performance of Asphalt Concrete Mixtures 71 The design compaction level and the design air void content also affect the fatigue resistance of HMA. HMA fatigue resistance increases with increasing compactive effort. Mixtures that are produced with greater compaction energy have improved performance in fatigue. For constant in-place air void content, fatigue resistance improves with increasing design air void content. This effect may at first seem counterintuitive, but for constant in-place air void content, increasing the design air void content mostly has the effect of increasing the compaction effort during construction. Because mixtures with very high air void content will be difficult to compact to an acceptably low in-place air void content and because mixtures with design air void contents that are too low may exhibit poor rut resistance, design air void contents are controlled within a narrow range for HMA mixtures, typically 4.0 0.5% for surface course mixtures. Durability Design VMA and in-place air void content have a major effect on the durability of HMA mixtures. Mixture durability improves as VBE increases. For a constant design air void content, VBE increases with increasing VMA. All else being equal, smaller NMAS mixtures have higher VMA and, therefore, have improved durability compared to larger NMAS mixtures. The dura- bility of dense-graded mixtures can be improved by increasing the design VMA (within limits) as discussed in Chapter 8. SMA mixtures have even greater durability due to their even higher design VMA. In-place air void content has a major effect on the durability of HMA. Mixture permeability increases with increasing in-place air void content. As permeability increases, binder age hardening and moisture infiltration increase, making the pavement less durable and more susceptible to moisture damage. Proper field compaction is therefore essential to producing durable pavements. Laboratory Testing Several laboratory tests have been developed to evaluate the performance of HMA. Tests have been developed to assess the resistance of HMA to rutting, fatigue cracking, thermal cracking, and moisture sensitivity. Additionally, laboratory-conditioning procedures have been developed to simulate the effects of short-term aging that occurs during construction and long-term aging that occurs during the service life of the pavement. Although numerous laboratory performance tests have been developed, only a few have been standardized and are routinely used for evaluation of HMA. Rut Resistance Testing and HMA Mix Design Several laboratory tests are available for evaluating the rutting resistance of HMA. These include tests that measure engineering properties, such as modulus or permanent deformation, and proof tests, such as the Asphalt Pavement Analyzer or Hamburg Wheel-track Test. Some specifying agencies and mixture designers have developed a level of confidence in specific tests and criteria for their local mixtures and pavements. In recent years, a major effort was undertaken to develop a rutting performance test and associated criteria that could be applied universally to HMA mixtures throughout the United States. The resulting device is the asphalt mixture performance tester (AMPT), previously called the simple performance test (SPT) system; because of its anticipated high level of future support by specifying agencies, this device is one recommended in this manual to measure rut resistance. Rut resistance can be evaluated in the AMPT using the dynamic modulus test, the flow number test, or the flow time test. Use of the dynamic modulus test to evaluate rut resistance was developed in conjunction with the MEPDG and is discussed in detail in NCHRP Report 580: Simple Performance Tests for Permanent Deformation of Hot Mix Asphalt--Volume 1: The E* Specification Criteria Program. Use of both the flow number test and

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72 A Manual for Design of Hot Mix Asphalt with Commentary flow time test to evaluate rut resistance during the mix design process are discussed in this manual. Four other tests are recommended in this manual as candidates for performance tests for the evaluation of rut resistance: The repeated shear at constant height (RSCH) test performed with the Superpave shear tester (SST). The high-temperature indirect tension (IDT) strength test. The asphalt pavement analyzer (APA). The Hamburg Wheel-track Test. These six tests for evaluating rut resistance are discussed below. Specific information on using these tests in the mix design process are given in Chapter 8. The Asphalt Mixture Performance Tester Figure 6-1 shows the AMPT. It is a relatively small, computer-controlled test machine that can perform various tests on HMA over a temperature range of 4 to 60C. The machine is available in the United States from several manufacturers who have demonstrated compliance with a detailed equipment specification prepared as part of National Cooperative Highway Research Program (NCHRP) Project 9-29 and contained in NCHRP Report 513: Simple Performance Tester for Superpave Mix Design: First Article Development and Evaluation. Two of the tests that can be performed in the AMPT have been related to the rutting performance of HMA. These are the dynamic modulus and the flow number tests. Ruggedness testing with the AMPT has demonstrated that it can control both of these tests with sufficient accuracy for use in specification testing. An interlaboratory study to establish precision statements for the dynamic modulus and flow number tests will be completed in the Fall of 2010. As this manual was being finalized, procedures for Figure 6-1. Photograph of a simple performance test system.

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Evaluating the Performance of Asphalt Concrete Mixtures 73 these tests were available in an AASHTO Provisional Standard, TP 79: Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT). A National Highway Institute (NHI) training course on the AMPT was also being planned. Dynamic Modulus Test--in the dynamic modulus test, an HMA specimen is subjected to a sinusoidal compressive load. The resulting stress and strain are recorded and used to calculate the dynamic modulus and phase angle for the mixture. The dynamic modulus is abbreviated E* (pronounced E-star; E for elastic modulus, and * for dynamic). E* is the peak stress in the test divided by the peak strain and represents the overall stiffness of the mixture. The phase angle is the amount that the strain lags the stress and is a measure of the elasticity of the mixture. The lower the phase angle, the more elastic the response. The stresses and strains in the dynamic modulus test are intentionally kept small to keep the response of the HMA in the linear range. Dynamic modulus testing can be conducted at different temperatures and loading frequencies to evaluate the effect of temperature and traffic speed on the mixture stiffness. E* data from different temperatures and loading rates can be combined into a master curve that describes the mixture stiffness for any combination of temperature and loading rate. A dynamic modulus master curve is the primary HMA materials input needed for the design of HMA pavements using the MEPDG. AASHTO TP 79-09, Determining the Dynamic Modulus and Flow Num- ber for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT), which was developed in NCHRP Project 9-29, is the standard test method for obtaining dynamic modulus measurements on HMA with the AMPT. AASHTO PP 61-09, Developing Dynamic Modulus Master Curves for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT), also developed in NCHRP Project 9-29, is the recommended practice for developing dynamic modulus master curves for pavement structural design using the AMPT. Criteria for using the dynamic modulus to judge the rutting resistance of an HMA mixture for a specific pavement can be obtained from the E* AMPT Specification Criteria Program developed in NCHRP Project 9-19 and described in NCHRP Report 580. This software uses the calibrated rutting model included in the MEPDG to determine project-specific testing conditions and dynamic modulus criteria to limit rutting to a specified level. The MEPDG is discussed in more detail later in this chapter. The E* AMPT Specification Criteria Program requires the user to enter information about the specific pavement, including HMA layer thicknesses, design traffic level, design traffic speed, environmental conditions at the project site, and the allowable rut depth in each HMA layer. The software then returns, for each HMA layer, the recommended testing conditions (temperature and frequency), and the minimum E* that the mixture must have to limit rutting to the specified level. Specifying agencies choosing to use dynamic modulus as the measure of rutting resistance can use this software to establish E* values and testing conditions based on the location of the mixture in the pavement (i.e., surface, intermediate or base), traffic level, and temperature conditions. Flow Number Test--the flow number is an alternative to the dynamic modulus test for evaluating rutting resistance. In this test, a sample of the HMA mixture at high temperature is subjected to a repeated compressive stress pulse. This repeated loading produces perma- nent strain in the specimen, which is recorded by the AMPT for each load cycle. Figure 6-2 is an example of a permanent strain curve that results from a flow number test. The point in the permanent strain curve where the rate of accumulation of permanent strain reaches a minimum value has been defined as the flow number. The flow number has been related to the rutting resistance of HMA. As the flow number increases, rutting resistance also increases. AASHTO TP 79-09 includes the standard test method for using the AMPT to obtain the flow number of HMA.

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74 A Manual for Design of Hot Mix Asphalt with Commentary 5.0 Permanent Strain Permanent Strain Rate 0.0050 4.5 0.0045 Permanent Strain Rate, % per Cycle 4.0 0.0040 3.5 0.0035 Permanent Strain, % 3.0 0.0030 2.5 0.0025 2.0 0.0020 Flow Number = Minimum 1.5 Permanent Strain Rate 0.0015 1.0 0.0010 0.5 0.0005 0.0 0.0000 0 1000 2000 3000 4000 5000 6000 Load Cycle Figure 6-2. Typical data from the flow number test. Flow Time Test--the flow time test differs from the flow number test in that a constant rather than a repeated load is applied to the specimen and the total deformation is monitored. Thus, the flow time test is simply a static creep test and the flow time is defined as the loading time required to initiate tertiary creep, which is the point at which the rate of deformation begins to increase. The flow time test was envisioned as a simpler alternative to the flow number test. AMPT Specimens--The AMPT requires a test specimen that is 100 mm (4.0 in) in diameter by 150 mm (6.0 in) high. The specimen is sawed and cored from the middle of a 150 mm in diameter by 175-mm-high gyratory-compacted specimen. Figure 6-3 shows a completed AMPT test specimen and the original gyratory-compacted specimen from which the test specimen was cut. AASHTO PP 60-09, Preparation of Cylindrical Performance Test Specimens Using the Figure 6-3. Photograph of an AMPT test specimen.

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Evaluating the Performance of Asphalt Concrete Mixtures 75 Superpave Gyratory Compactor (SGC), also developed in NCHRP Project 9-29, is the recommended procedure for preparing AMPT specimens. There are three reasons for using smaller test specimens obtained from larger gyratory specimens in the AMPT: 1. To obtain an appropriate aspect ratio for the test specimens. Research performed during NCHRP Project 9-19 found that a minimum specimen diameter of 100 mm with a height-to-diameter ratio of 1.5 was needed. 2. To eliminate areas of the gyratory specimens with high air void content. Gyratory-compacted specimens typically have high air void content near the ends and around the circumference of the specimen. 3. To obtain relatively smooth, parallel ends for testing, which helps ensure proper stress distribution within the specimen during loading. The air void content of the AMPT specimen will have a major effect on the properties measured in the AMPT. AMPT specimens used to evaluate rutting resistance should be pre- pared to the expected average field air void content at the time of construction, not the design air void content. Mixtures should be short-term oven aged for 4 hours at 135C in accordance with the procedure for Short-Term Conditioning for Mixture Mechanical Prop- erty Testing in AASHTO R 30. A reasonable air void tolerance for AMPT specimens is 0.5%. The AMPT specimen will have a lower air void content than the larger gyratory specimen from which it is produced. AASHTO PP 60-09 contains a procedure for achieving the target air void content for AMPT specimens. The number of replicates to be tested depends on the repeatability of the test and the desired accuracy of the resulting data. Based on current estimates of coefficients of variation for the dynamic modulus and flow number tests of 13 and 20%, respectively, it is recommended that two replicate specimens be used for dynamic modulus testing and four replicate specimens for flow number testing. These numbers of replicates will result in coefficients of variation for the mean values of dynamic modulus and flow number of approximately 10%. Other Laboratory Tests for Rut Resistance Other laboratory tests are available for evaluating the rutting resistance of HMA. Those most often used are the Superpave Shear Tester (SST), the High-Temperature Indirect Tensile Test (IDT), the Asphalt Pavement Analyzer (APA), and the Hamburg Wheel-Track Test. Repeated Shear at Constant Height (RSCH) Test. This is one of several tests that can be performed with the SST. The RSCH test is designed to evaluate the rutting resistance of HMA by applying repeated shear loading to an HMA specimen at high temperatures. The test has been standardized as AASHTO T 320. The RSCH test is performed on 150-mm-diameter specimens that are 38 to 50 mm thick, depending on the nominal maximum aggregate size. The test specimens are glued to loading platens and subjected to repeated direct shear loading while the vertical load is varied to maintain the specimen at a constant height. In September 1997, during the Superpave implementation effort, the Mixtures Expert Task Group established preliminary guidelines for using the RSCH test to evaluate the rutting resistance of HMA. In the standard performance test--discussed in detail in Chapter 8 of this manual--the RSCH test is performed at the maximum, 7-day average pavement temperature found 20 mm below the pavement surface, as given in LTPPBind Version 3.0. The SST, a relatively expensive device, is available in few laboratories in the United States. Figure 6-4 is a photograph of an SST. The RSCH test is not recommended for routine evaluation of the rut resistance of HMA mixtures in the laboratory--the other tests discussed in this chapter are generally easier to perform and less expensive to conduct and so more widely used than the RSCH test. For those laboratories that have an SST device, specific

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76 A Manual for Design of Hot Mix Asphalt with Commentary Figure 6-4. Photograph of the SST. recommendations for using it to evaluate mixture rut resistance during the mix design process are given in Chapter 8. The High-Temperature IDT Strength Test. This test was developed as a quick and inexpensive procedure for evaluating rut resistance using equipment currently available in many HMA design and quality assurance laboratories. Christensen, Bonaquist, and Jack reported an excellent relationship between rutting resistance and the indirect tensile strength at high temperature in a 2000 publication. Additional work confirming these results was reported in 2004 by Zaniewski and Srinivasan. The test is conducted on standard gyratory specimens produced for mixture design or quality assurance with the indirect tensile strength equipment used in AASHTO T 283. Specimens should be compacted to the design gyration level. When testing specimens as part of mixture design, the mixture should be short-term oven aged for 4 hours at 135C in accordance with the procedure for Short-Term Conditioning for Mixture Mechanical Property Testing in AASHTO R 30. When testing quality assurance specimens from plant production, the short- term aging is not required. The testing temperature is 10C less than the 50% reliability, 7-day average maximum pavement temperature obtained from LTPPBind Version 3.0. The Asphalt Pavement Analyzer and Hamburg Wheel-Track Test. The APA (see Figure 6-5) and the Hamburg Wheel-Track tests are proof tests for rutting resistance that are used by some specifying agencies. Both tests attempt to simulate the effect of traffic loading by rolling a small loaded wheel over an HMA specimen at high temperature. In the APA, the load is applied through Figure 6-5. Photo- a rubber hose that can be inflated to a specified pressure. In Hamburg Wheel-Track testing, the graph of the asphalt load is applied through a steel wheel. Conditioned air is used for temperature control in the APA, pavement analyzer. while Hamburg Wheel-Track testing uses water to control the temperature of the test specimen.

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Evaluating the Performance of Asphalt Concrete Mixtures 77 The Hamburg Wheel-Track test can be used to assess both rutting resistance and moisture sensitivity. Originally, these tests were performed on rectangular specimens; however, in recent years the equipment has been modified to use 150-mm-diameter gyratory-compacted specimens. The standard test method for the APA is AASHTO TP 63-09; the standard test method for Hamburg Wheel-Track testing is AASHTO T 324. Agencies that specify these tests have established criteria for rutting resistance based on the deformation or impression depth after a specified number of load cycles. For example, for high traffic levels, the Georgia Department of Transportation specifies a maximum deformation of 5 mm in the APA after 8,000 cycles at a test temperature of 64C. The Texas Department of Transporta- tion specifies the minimum number of wheel passes in the Hamburg Wheel-Track test to reach an impression depth of 12.5 mm when tested at a temperature determined by the performance grade of the asphalt binder. These values are >10,000 for mixes produced with PG 64-XX binder, >15,000 for mixes produced with PG 70-XX binder, and >20,000 for mixes produced with PG 76-XX binder. Additional information on the use of the APA and Hamburg Wheel-Track testing as performance tests for use in the mix design process is given in Chapter 8. Fatigue Testing The only standard test method available for fatigue testing of HMA is the flexural fatigue test, AASHTO T 321. In this test a beam sample, 380 mm long by 63 mm wide by 50 mm high, is subjected to strain-controlled, repeated four-point bending. The beam samples are prepared using either a kneading or rolling wheel compaction; there are no AASHTO standards for either of these methods of laboratory compaction. Figure 6-6 shows a device for flexural fatigue testing. The number of laboratories in the United States that can fabricate and test flexural fatigue spec- imens is limited. During a flexural fatigue test, the beam is damaged by the repeated flexing. This damage results in a decrease in the modulus of the beam. The beam is considered failed when the modulus decreases to 50% of its initial value. The number of loading cycles applied to the beam can range Figure 6-6. Photograph of flexural fatigue apparatus.

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78 A Manual for Design of Hot Mix Asphalt with Commentary 1000 Initial Strain, in/in 100 10 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 Cycles to Failure Figure 6-7. Typical S-N diagram for HMA. from 1,000 to 10,000,000 or more. The results of fatigue tests are presented in the form of S-N diagrams, which are simply plots of the applied strain and the corresponding number of cycles to failure. Figure 6-7 presents a typical S-N diagram for HMA generated from laboratory test data. The point where the fatigue life becomes indefinite is called the fatigue endurance limit. Because of its extreme importance in the structural design of perpetual pavements, research is in progress to better define the endurance limit for HMA. Generating an S-N curve for HMA requires testing several beams at different strain levels. Due to the high variability of fatigue testing, each strain level requires testing a number of replicate specimens. Because of the high level of effort required to generate S-N curves for HMA, fatigue testing is rarely performed in practice. Instead, relationships between mixture compositional factors and fatigue life that have been developed from databases of tests on a number of mixtures are used. These relationships show that the most important mixture design factor affecting the fatigue life of HMA is the effective volumetric binder content of the mixture, VBE. By controlling VBE, the mixture design process controls the fatigue life of the mixture. As discussed previously, VBE, is controlled in the design method described in this manual by controlling both the VMA and the design air void content. Thermal Cracking The MEPDG can predict the amount of thermal cracking that will occur in an asphalt pavement. To perform this analysis, information on the creep and strength properties of the HMA at low temperatures are needed. These properties are measured using the Indirect Tensile Tester (IDT), AASHTO T 322. Low-temperature tests on HMA require an expensive environmental chamber and the capacity to impose high loads on the test specimens. Figure 6-8 shows a specimen being tested in the IDT. Only a few laboratories in the United States have IDT equipment for low-temperature testing. AASHTO T 322 involves preparing nine IDT test specimens and performing creep and strength tests on three specimens each at temperatures of 0, -10, and -20C. The results of the creep tests

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Evaluating the Performance of Asphalt Concrete Mixtures 79 Figure 6-8. Photograph of specimen in the IDT. are used to generate a compliance master curve for the mixture, which governs the buildup of thermal stresses in the pavement. The potential for thermal fracture depends on the magnitude of the estimated thermal stress relative to the tensile strength of the mixture. IDT test specimens are 150 mm in diameter by 38 mm thick. They are formed by sawing the test specimen from the middle of a gyratory-compacted specimen. Sawed ends are needed to attach the deformation measuring equipment. IDT testing is usually conducted on specimens compacted to the anticipated in-place air void content and exposed to long-term oven aging in accordance with AASHTO R 30. Because the resistance to thermal cracking is almost completely governed by the properties of the binder, IDT testing is usually only performed when the binder cannot be tested using the bending beam rheometer and direct tension device. When modifiers are added to the mixture rather than the binder, it may be necessary to conduct IDT testing to evaluate the low temperature properties of the resulting mixtures. Moisture Sensitivity Testing Two tests have received acceptance in the United States to evaluate the moisture sensitiv- ity of HMA: the Lottman procedure (AASHTO T 283) and the Hamburg Wheel-Track test (AASHTO T 324). In many cases, the two tests provide different results, likely because they simulate different moisture damage processes. Recent efforts to improve moisture sensitivity testing using the Environmental Conditioning System (ECS) developed during the Strategic Highway Research Program have not yet resulted in a standard test method used by state agencies in the routine design of HMA. In AASHTO T 283, six laboratory specimens are prepared to an air void content of 7.0 0.5%, then divided into two subsets with approximately equal average air void contents. The tensile strength of one subset is measured dry. The tensile strength of the second subset is measured

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80 A Manual for Design of Hot Mix Asphalt with Commentary 20 18 16 Stripping Slope 14 Rut Depth, mm 12 10 8 Creep Slope 6 4 2 Stripping Inflection Point 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Number of Passes Figure 6-9. Typical rut depth versus wheel pass curve from AASHTO T 324. after conditioning by vacuum saturation followed by a freeze-thaw cycle and a warm water soak. The ratio of the average tensile strength of the conditioned to unconditioned subsets and a visual assessment of stripping is used to measure moisture sensitivity. A mixture is considered to have an acceptable level of moisture sensitivity if the tensile strength ratio is equal to or greater than 80% and there is no visual evidence of stripping in the conditioned test specimens. Since the Hamburg Wheel-Track test (AASHTO T 324) tests HMA submerged in water, it can also be used to evaluate the resistance of a mixture to moisture damage. Moisture sensitivity is evaluated by computing the stripping inflection point, which is defined as the intersection of the slopes from the creep and stripping portions of the rut depth versus wheel pass curve as shown in Figure 6-9. The recommended air void content of laboratory-prepared specimens for AASHTO T 324 is 7.0 2.0%. Criteria for evaluating moisture sensitivity based on AASHTO T 324 place a minimum limit on the stripping inflection point. For example, Aschenbrener et al. suggested for Colorado conditions that mixtures with good performance with respect to moisture damage (life of 10 to 15 years) should have a stripping inflection point greater than 14,000 passes. Short- and Long-Term Oven Conditioning When conducting performance tests on HMA, it is important to simulate the effects of (1) short-term aging that occurs during plant mixing and construction and (2) long-term aging that occurs during the service life of the pavement. During production and laydown, some of the binder is absorbed into the aggregate, decreasing the effective binder content, and the binder is aged by the high temperatures that occur in the overall construction process. Further oxidative aging of the binder occurs during the service life of the pavement. During the Strategic Highway Research Program, procedures for short-term and long-term conditioning of mixtures were developed. These were subsequently standardized in AASHTO R 30.

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Evaluating the Performance of Asphalt Concrete Mixtures 81 The short-term conditioning procedure consists of conditioning loose mixture in a forced draft oven at 135C for 4 hours. The mixture is evenly spread in a pan to a thickness between 25 and 50 mm and stirred every hour. In the long-term conditioning procedure, test specimens prepared from loose mix that was previously short-term conditioned as described above are further conditioned in a forced draft oven before testing. The temperature for this conditioning is 85C for a period of 120 hours Because rutting is a distress that occurs early in the life of a pavement, performance testing to assess rutting resistance should be conducted on specimens that have been short-term conditioned. Fatigue and thermal cracking tests should be performed on specimens that have been long-term conditioned. AASHTO T 283 has a different conditioning procedure of 16 hours in a forced draft oven at 60C. Evaluating the Need for Performance Testing It is neither practical nor necessary to perform the full suite of performance tests discussed above when designing HMA using conventional materials, including most modified binders. The test methods for fatigue and thermal cracking require a high level of effort and complex equipment, and substantial research has shown that resistance to these forms of distress can be controlled by controlling the effective binder content of the mixture and the low temperature binder grade, respectively. Rutting resistance is somewhat more difficult to control since several compositional factors affect rutting resistance, and if these act in the same direction, the resulting HMA may exhibit poor performance. Fortunately, tests for rutting resistance using the AMPT are relatively easy to perform and criteria differentiating various levels of performance are available. There is general agreement in the industry that testing the specific combinations of binder, aggregate, and additives used in an HMA mixture is the only way to assess the potential for moisture sensitivity. Table 6-4 summarizes the performance testing recommended in this manual for HMA made from conventional materials, including most modified binders. It is recommended that all mixtures be evaluated for moisture sensitivity using AASHTO T 283. Equipment for this test is widely available. Rutting resistance should be evaluated for mixtures designed for traffic levels greater than 3 million ESALs. Rutting resistance can be evaluated using either the dynamic modulus test in conjunction with the E* AMPT Specification Criteria Program, or the flow number test and the criteria given in Chapter 8. Performance testing is not recommended for fatigue cracking when the mixture design criteria given in Chapters 8 and 10 or 11 are met. Performance testing for thermal cracking is not recommended when binders are selected using the Performance Grading system. For HMA made with non-conventional materials, performance tests for rutting, fatigue cracking, thermal cracking, and moisture sensitivity should be performed and compared to results from mixtures made with conventional materials. Non-conventional materials might include recycled materials such as ground glass, ground tire rubber, ground or shredded plastic Table 6-4. Recommended performance tests for HMA made with conventional materials including most modified binders. Design Traffic Levels for Which Property Recommended Test Property Should be Evaluated Moisture Sensitivity AASHTO T 283 All Permanent Deformation Flow Number or Dynamic 3 Million ESAL and greater Modulus, AASHTO TP 79-09 Fatigue Cracking None NA Thermal Cracking None NA