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A Manual for Design of Hot-Mix Asphalt with Commentary (2011)

Chapter: Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures

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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 6 - Evaluating the Performance of Asphalt Concrete Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

This chapter presents an introduction to evaluating the performance of HMA mixtures in pavement systems. It provides mixture designers with a concise compilation of information relating HMA properties to the failure mechanisms discussed in Chapter 2: • Rutting and permanent deformation • Fatigue cracking • Low-temperature cracking • Moisture damage • Durability HMA performance is strongly influenced by the composition of the mixture and the in-place density. The effect of binder properties, aggregate properties, and mixture volumetric properties on performance, which serves as the basis for the mixture design criteria in Chapters 8 and 10, are discussed. The important role of field compaction in the performance of dense-graded and SMA mixtures cannot be over emphasized. Marginally designed mixtures may perform adequately when properly compacted, but even the best designed mixture will not perform adequately when poorly compacted. In recent years there has been a growing interest in using performance testing and performance prediction models in HMA mixture design and acceptance. For higher traffic level designs, the three-level Superpave mixture design and analysis system developed during the Strategic Highway Research Program included a series of performance tests and models to evaluate the expected performance of an HMA mixture. Unfortunately due to complexity, high equipment costs, and the lack of field calibration for the models, the mixture analysis portion of this system was not fully implemented. Over the last 15 years, additional progress in performance testing has been made, and recently the Mechanistic-Empirical Pavement Design Guide (MEPDG) has been completed and made available to the profession. The MEPDG includes field-calibrated performance prediction models for rutting and cracking that can be used to predict the perfor- mance of an HMA mixture in a particular pavement section. Although performance is directly addressed by the mixture design processes presented in this manual through the material selection and volumetric criteria used in design, some agencies and mixture designers may desire confirmation through performance testing and modeling. This chapter also discusses various performance tests that can be used to assess HMA mixture per- formance and presents an introduction to the MEPDG and how this software can be used to complement the mixture design process. 65 C H A P T E R 6 Evaluating the Performance of Asphalt Concrete Mixtures

Mixture Composition and Performance The mixture design criteria for dense-graded and SMA mixtures presented in Chapters 8 and 10 place several requirements on mixture composition that are related to performance. These requirements include • Binder grade • Aggregate angularity • Nominal maximum aggregate size • Mineral filler content • Design air void content • Design compaction level, and • Design voids in the mineral aggregate (VMA) These requirements are included in the design procedure to ensure that the resulting mixtures will exhibit adequate performance when properly compacted in the field. It is important to emphasize that field compaction is a critical factor affecting every aspect of pavement performance and that the design procedures presented in this manual for dense-graded and SMA mixtures assume that the HMA mixture will receive proper compaction during construction. Table 6-1 summarizes the effect of mixture composition on pavement performance. In Table 6-1, an upward arrow indicates that a particular performance indicator improves with an increase in the compositional factor. A downward arrow indicates that the performance indicator deteriorates with an increase in the compositional factor. The relationship between binder stiffness and fatigue resistance depends on the pavement structure; for thin pavement structures, increasing binder stiffness will decrease fatigue resistance, while for thick pavement 66 A Manual for Design of Hot Mix Asphalt with Commentary Component Factor Resistance to Rutting and Permanent Deformation Resistance to Fatigue Cracking Resistance to Lo w Temperature Cracking Resistance to Moisture Damage Durability/ Resistance to Penetration by Water and Air Increasing High Temperature Binder Grade ↑↑↑ Increasing Low Temperature Binder Grade ↓↓↓Asphalt Binder Increasing Intermediate Temperature Binder Stiffness ↑↓ Increasing Aggregate Angularity ↑↑ Increasing Proportion of Flat and Elongated Particles Increasing Nominal Maximum Aggregate Size ↓ ↓ ↓ Increasing Mineral Filler Content and/or Dust/Binder Ratio ↑↑ ↑ Aggregates Increasing Clay Content ↓ Increasing Design Compaction Level ↑↑ ↑↑ Increasing Design Air Void Content ↑↑ Increasing Design VMA and/or Design Binder Content ↓↓ ↑ ↓ Volumetric Properties Increasing Field Air Void Content ↓↓ ↓↓ ↓ ↓↓ ↓↓↓ Typical Effects of Increasing Given Factor within Normal Specification Limits While Other Factors Are Held Constant within Normal Specification Limits Table 6-1. Effect of mixture composition on performance.

structures the reverse is true—that is why there are two arrows going in different direction for this entry in Table 6-1. The relative importance of each of the factors is indicated by the number of arrows shown in the table. The information presented in Table 6-1 and the more detailed discussion on the relationships among binder properties, aggregate properties, mixture composition, and pavement performance that is given later in this chapter are based on several sources, most importantly NCHRP Report 539: Aggregate Properties and the Performance of Superpave-Designed Hot Mix Asphalt and NCHRP Report 567: Volumetric Requirements for Superpave Mix Design. Binder Characteristics and Performance Performance Grading System As discussed in more detail in Chapter 3, the Performance Grading system for asphalt binders, AASHTO M 320, controls the properties of asphalt binders that are related to pavement per- formance. This grading system includes requirements for the properties of the binder at high, intermediate, and low pavement temperatures to address rutting, fatigue cracking, and low- temperature cracking in pavements. Two conditioning procedures are used to simulate the effects of binder aging during construction and the service life of the pavement. In the Performance Grading system, asphalt binders are specified by two numbers, for example PG 64-22. The first number, 64 in this example, is called the high temperature grade. It is the pavement temperature in degrees Centigrade up to which the binder provides adequate stiffness to resist excessive rutting for a properly designed HMA mixture exposed to a moderate volume of fast-moving highway traffic. The second number, −22 in this example, is called the low temperature grade. It is the pavement temperature in degrees Centigrade down to which the binder provides adequate flexibility to resist low-temperature cracking. In the Performance Grading system, the critical value of the performance-related property remains the same, but the temperature where the binder provides the minimum or maximum property changes. A PG 70-22 must meet the minimum high temperature properties at 70°C compared to 64°C for a PG 64-22. Similarly a PG 64-28 must meet the low temperature properties at −28°C compared to −22°C for a PG 64-22. Both of these binders provide a wider performance range than the PG 64-22. The performance- related properties used in the Performance Grading system are summarized in Table 6-2. For low temperature cracking, two criteria corresponding to Table 1 and Table 2 of AASHTO M 320 are given. Table 1 of AASHTO M 320 is based on low temperatures properties from the bending beam rheometer, while Table 2 of AASHTO M 320 is based on the computed critical cracking temperature of the binder. This computation uses data from both the bending beam rheometer and the direct tension test. Most agencies use the Table 1 criteria. The test methods used in the Performance Grading system were described in Chapter 3. Rutting and Permanent Deformation The high temperature grade of the asphalt binder is one of several important factors affecting the rutting resistance of HMA. For a given pavement and HMA mixture, resistance to permanent Evaluating the Performance of Asphalt Concrete Mixtures 67 Distress Mode Performance Related Binder Property Criteria Rutting G*/sinδ Minimum of 1.00 kPa for unaged binder at 10 rad/sec Minimum of 2.20 kPa for RTFOT aged binder at 10 rad/sec Creep stiffness, S Maximum of 300 kPa for PAV aged binder at 60 s. Low Temperature Cracking, Table 1 m-value Minimum of 0.300 for PAV aged binder at 60 s Low Temperature Cracking, Table 2 Critical cracking temperature Equal to or lower than specified low temperature grade. Fatigue Cracking G*sinδ Maximum of 5,000 kPa for PAV aged binder at 10 rad/sec Table 6-2. Performance-related properties and criteria used in the performance grading system, AASHTO M 320.

deformation increases as the high temperature performance grade increases. Additionally, recent research has shown that for the same high temperature performance grade, the rutting resistance of HMA made with polymer-modified binders is significantly improved over that for neat (that is, undiluted or not mixed with other substances) asphalt binders. Rutting in asphalt pavements increases with increasing traffic volume and decreasing traffic speed. To counteract these effects, the high temperature binder grade must be increased or “bumped” for pavements exposed to high traffic levels (trucks) and slow-moving traffic. Table 6-3 presents the recommended high temperature grade changes included in the mixture design procedures presented in Chapters 8, 10, and 11. When using grade bumping for different traffic levels, it is important to avoid binders that are excessively stiff at the intermediate temperature for the binder when selected on the basis of environmental conditions alone. As the high temperature performance grade increases, the temperature where the binder is required to meet the maximum value of G*sinδ also increases. This may result in the binder being too stiff for the intermediate temperature conditions under which the pavement is expected to perform. For example, when bumping two grades from a PG 64-22 to a PG 76-22, the temperature where the intermediate stiffness is normally tested increases 6°C, from 25°C to 31°C. The appropriate intermediate temperature based on environmental con- ditions is 25°C, not 31°C, and the binder should be expected to have a value of G*sinδ that is less than or equal to 5,000 kPa at 25°C. The Performance Grading system does not ensure that bumped binders will meet the intermediate temperature conditions required for the base binder. Agencies usually place additional language in the HMA specification to require inter- mediate temperature testing at the temperature obtained on the basis of environmental con- ditions alone. Fatigue Cracking The intermediate temperature stiffness of the asphalt binder is one of several factors affecting fatigue cracking in pavements. Top-down fatigue cracking was identified as an important form of distress in thick asphalt pavements and overlays of portland cement concrete pavements in the mid 1990s. Although this form of distress is not yet completely understood, it appears that binder stiffness is at least a contributing factor. Surface courses made with binders that become excessively stiff due to rapid age hardening are more susceptible to top-down cracking. The Performance Grading system places a maximum limit on the stiffness of the binder after simulated long-term aging. Although there is much debate over this requirement and its relationship to traditional 68 A Manual for Design of Hot Mix Asphalt with Commentary Grade Adjustment for Average Vehicle Speed in kph (mph): Very Slow Slow Fast Design Traffic (MESALs) < 25 (< 15) 25 to < 70 (15 to < 45) ≥ 70 (≥ 45) < 0.3 --- --- --- 0.3 to < 3 12 6 --- 3 to < 10 18* 13 6 10 to < 30 22* 16* 10 ≥ 30 --- 21* 15* * Consider use of polymer-modified binder. If a polymer- modified binder is used, high temperature grade may be reduced one grade (6 °C) provided rut resistance is verified using suitable performance testing. Table 6-3. Recommended high temperature performance grade changes to account for traffic volume and speed.

bottom-up fatigue cracking, the requirement serves to limit age hardening of the binder and the potential for top-down cracking. Low-Temperature Cracking The low-temperature cracking performance of asphalt pavements is almost completely con- trolled by the environmental conditions and the low temperature properties of the asphalt binder. Binder grade selection is, therefore, the most critical HMA design factor affecting the low temperature performance of asphalt pavements. Since transverse thermal cracks cannot be repaired and quickly reflect through future overlays, it is critical that binders be selected to have a high reliability against thermal cracking. Reliability as applied to binder grade selection was discussed in detail in Chapter 3. Durability Excessive age hardening of the binder during the service life of the pavement is a contributing factor to several pavement distresses including raveling, top-down fatigue cracking, thermal cracking, and moisture damage. The primary factors affecting age hardening are the environment, the permeability of the HMA, and the characteristics of the binder. Age hardening is most severe in high-temperature climates. Age hardening also occurs more rapidly in pavements that are more permeable; therefore, it is critical to ensure that a high level of in-place density is achieved to minimize the potential for interconnected air voids in the HMA. The Performance Grading system includes tests on the binder after simulated long-term aging to control the age-hardening characteristics of the binder. Moisture Damage Some combinations of asphalt binder and aggregate exhibit greater potential for moisture damage than others. For the same aggregate type, resistance to moisture damage improves marginally with the use of a stiffer binder, particularly those modified with polymers. Aggregate Characteristics and Performance Excellent-performing pavements have been constructed using a wide variety of aggregate types. Several characteristics of aggregates that are related to pavement performance are controlled in the mixture design procedures presented in Chapters 8, 10, and 11. Aggregate angularity and mineral filler content are important aggregate characteristics affecting the rutting resistance of HMA. Resistance to rutting and permanent deformation improves with increasing aggregate angularity and increasing mineral filler content—although excessive mineral filler content will tend to produce a mixture that is very stiff and sticky and difficult to compact. Rutting resistance also improves as the nominal maximum aggregate size (NMAS) of the HMA increases because the design VMA decreases with increasing NMAS and the design VMA has a major influence on the rutting resistance of HMA. Aggregate characteristics are also important factors affecting the durability of HMA and its resistance to moisture damage. Aggregates that are flat or elongated tend to break during compaction, leaving uncoated surfaces, which decrease durability and increase the potential for moisture damage. Clay particles disrupt the adhesion of the asphalt binder to the aggregates making the HMA less durable and more susceptible to moisture damage. Durability improves with decreasing NMAS because the design VMA increases with decreasing NMAS and, as discussed in the next section, increasing the design VMA increases the effective binder content of the mixture, which improves durability. Finally, increasing the mineral filler content of the HMA decreases permeability for the same in-place air void content (again, understanding that there are practical limitations to how much mineral filler can be used in HMA mixtures). Binder age hardening and Evaluating the Performance of Asphalt Concrete Mixtures 69

water infiltration are reduced in mixtures with lower permeability, leading to improved durability and greater resistance to moisture damage. Volumetric Properties and Performance The volumetric properties of HMA have a major influence on the performance of HMA. Volumetric properties affect an HMA mixture’s resistance to rutting and fatigue cracking. They also affect the durability of the mixture and its resistance to moisture damage. Rutting and Permanent Deformation Several volumetric factors affect the resistance of HMA to rutting and permanent deformation. Although individually these factors are less important than high-temperature binder grade, aggregate angularity, and mineral filler content, the volumetric factor effects are additive and, if these act together in the same way, the results can be significant. Rutting resistance tends to improve with decreasing design VMA and in-place air void content. As discussed in Chapter 5, VMA is the volume of air and asphalt binder in the mixture. These are the components of HMA that deform easily upon loading; therefore, rutting resistance improves as VMA and in-place air void content decrease. Rutting resistance also improves with increasing design compaction level. The resistance of the aggregate structure to deformation improves as the number of gyrations used in the design increases. Finally, the rutting resistance improves as the design air void content increases. At first, this effect might seem counter-intuitive, but by increasing the design air void level while maintaining the in-place air void content constant, the energy of compaction required to construct the pavement is increased significantly. Conversely, decreasing design air void content under constant in-place air void content decreases the energy required for field compaction. Even though decreasing VMA and increasing design air void content will, in general, improve rut resistance, as discussed below, VMA values that are too low and design air void values that are too high will often produce mixtures with poor durability. This is why there are both minimum and maximum values for VMA and air void content. These requirements are discussed in detail in Chapter 8 of this manual. Fatigue Cracking Several volumetric factors also affect the resistance of HMA to fatigue cracking. The most important of these is the in-place air void content of the pavement. The fatigue life of typical HMA pavements decreases with increasing in-place air void content. This occurs for several reasons. Lower air void content will tend to produce a stronger pavement more resistant to cracking. Lower air void content also will in general produce a pavement with lower permeability to both air and water. This will reduce the amount of binder age hardening in the pavement and will tend to minimize moisture damage, which can render the pavement weak and more prone to fatigue damage. The primary HMA mixture design factor affecting fatigue life is the effective volumetric binder content of the mixture (VBE). For a given pavement, fatigue life increases with increasing VBE; therefore, controlling VBE is an important consideration in mixture design. Since VBE is equal to VMA minus the air void content, the mixture design procedures presented in Chapters 8 and 10 control VBE by controlling VMA and the design air void content of the mixture. As discussed above, increasing VMA or decreasing air void content too much can significantly decrease rut resistance; therefore, the requirements given in Chapters 8 and 10 provide both upper and lower limits for VMA and design air void content. For dense-graded mixtures, the design procedure in Chapter 8 provides the flexibility to increase the design VMA requirements up to 1.0% to produce mixtures with improved fatigue resistance and durability. SMA mixtures, because they have extremely high VMA, tend to produce mixtures with excellent fatigue resistance. 70 A Manual for Design of Hot Mix Asphalt with Commentary

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 Evaluating the Performance of Asphalt Concrete Mixtures 71

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 60°C. 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 72 A Manual for Design of Hot Mix Asphalt with Commentary Figure 6-1. Photograph of a simple performance test system.

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. Evaluating the Performance of Asphalt Concrete Mixtures 73

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 74 A Manual for Design of Hot Mix Asphalt with Commentary 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 1000 2000 3000 4000 5000 6000 Load Cycle Pe rm an en t S tra in , % 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 0.0040 0.0045 0.0050 Pe rm an en t S tra in R at e, % p er C yc le Permanent Strain Permanent Strain Rate Flow Number = Minimum Permanent Strain Rate Figure 6-2. Typical data from the flow number test. Figure 6-3. Photograph of an AMPT test specimen.

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 135°C 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 Evaluating the Performance of Asphalt Concrete Mixtures 75

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 135°C 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 10°C 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 a rubber hose that can be inflated to a specified pressure. In Hamburg Wheel-Track testing, the load is applied through a steel wheel. Conditioned air is used for temperature control in the APA, while Hamburg Wheel-Track testing uses water to control the temperature of the test specimen. 76 A Manual for Design of Hot Mix Asphalt with Commentary Figure 6-4. Photograph of the SST. Figure 6-5. Photo- graph of the asphalt pavement analyzer.

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 64°C. 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 Evaluating the Performance of Asphalt Concrete Mixtures 77 Figure 6-6. Photograph of flexural fatigue apparatus.

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 −20°C. The results of the creep tests 78 A Manual for Design of Hot Mix Asphalt with Commentary 10 100 1000 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 In iti al S tra in , μi n/ in Figure 6-7. Typical S-N diagram for HMA.

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 Evaluating the Performance of Asphalt Concrete Mixtures 79 Figure 6-8. Photograph of specimen in the IDT.

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. 80 A Manual for Design of Hot Mix Asphalt with Commentary 0 2 4 6 8 10 12 14 16 18 20 0 4000 60002000 8000 1200010000 14000 16000 18000 20000 Number of Passes R ut D ep th , m m Creep Slope Stripping Slope Stripping Inflection Point Figure 6-9. Typical rut depth versus wheel pass curve from AASHTO T 324.

The short-term conditioning procedure consists of conditioning loose mixture in a forced draft oven at 135°C 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 85°C 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 60°C. 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 Evaluating the Performance of Asphalt Concrete Mixtures 81 Property Recommended Test Design Traffic Levels for Which Property Should be Evaluated Moisture Sensitivity AASHTO T 283 All Permanent Deformation Flow Number or Dynamic Modulus, AASHTO TP 79-09 3 Million ESAL and greater Fatigue Cracking None NA Thermal Cracking None NA Table 6-4. Recommended performance tests for HMA made with conventional materials including most modified binders.

and ground roofing shingles or shingle tabs. Another class of non-conventional materials is novel additives—chemicals, compounds, or other materials—designed to provide some benefit to HMA, but which have not yet been thoroughly evaluated with laboratory tests and field trials. Care should be used in evaluating these mixtures using the criteria presented in the manual or elsewhere for HMA. Performance Predictions Using the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) Some agencies and mixture designers may be interested in using the MEPDG to predict the performance of a pavement incorporating specific HMA mixtures. Such an analysis using the MEPDG will include the effect of both pavement structure and HMA mixture properties on performance. In some cases, such as bottom-up fatigue cracking, layer thicknesses or subgrade support conditions will dominate. In other cases, such as rutting in an overlay of an existing pavement, HMA mixture effects will be more important. This section provides an introduction to the MEPDG and how it can be used to predict the performance of an HMA mixture for a specific pavement section. The MEPDG is a comprehensive, state-of-the-practice tool for the design of new and rehabilitated pavement structures. Readers interested in using this tool are encouraged to study the MEPDG user manual and other detailed documentation for the MEPDG. The MEPDG is substantially different than most pavement design procedures used in the past by highway agencies. The MEPDG is based on mechanistic-empirical pavement design principles. Critical stresses and strains from vehicle and environmental loading are computed using mechanistic theory. These critical stresses and strains are then empirically related to the occurrence of distresses such as rutting and cracking in the pavement. Most agencies have experience with the 1993 AASHTO Pavement Design Guide, which is based on limited empirical pavement performance equations from the AASHO Road Test conducted in the late 1950s and early 1960s. The distress prediction models in the MEPDG have been calibrated using data for a large number of pavement sections in the Long-Term Pavement Performance (LTPP) database. Pavement sections used in the calibration were located throughout the United States. The MEPDG is an analysis tool. The output from the MEPDG is the predicted performance of a trial pavement section, not pavement thickness design. As discussed in more detail below, some of the distress prediction models in the MEPDG provide a link between HMA material properties and pavement performance that can be used after the mixture design is completed to verify that an HMA mixture will provide acceptable performance for a specific project. MEPDG Input Levels The MEPDG requires a large amount of information about the pavement being analyzed. This includes data concerning traffic, climate, subgrade soils, the condition of existing pavements for rehabilitation design, and the thicknesses and material properties for each layer of the pavement, including existing pavement layers for rehabilitation design. To provide flexibility for users with different capabilities, the MEPDG uses a three-level hierarchical scheme of data input: • Level 1. The input parameter is measured directly. This level provides the most accurate information about the input parameter. The primary Level 1 material property input for HMA is the measured dynamic modulus of the mixture that will be used in the pavement. • Level 2. The input parameter is estimated from correlations or regression equations that are embedded in the MEPDG. For Level 2, the dynamic modulus for HMA materials is estimated from gradation, volumetric properties, and measured binder properties. 82 A Manual for Design of Hot Mix Asphalt with Commentary

• Level 3. The input parameter is based on default values provided by the MEPDG software. For Level 3, the dynamic modulus for HMA is estimated from gradation, volumetric properties, and the binder grade. Testing or data collection costs decrease as the hierarchical level increases from Level 1 to Level 3, but the accuracy of the input data also decreases. If performance predictions from the MEPDG are to be used to verify a specific mixture design, Level 1 inputs should be used for all of the HMA material properties. Other levels can be used for the traffic, climate, and the material properties of the other layers. The overall accuracy of the predicted performance, however, will depend on the accuracy of all of the input data, not just the HMA material properties. MEPDG Performance Models for HMA The MEPDG can analyze flexible, semi-rigid, rigid, and composite pavements. For pavements with HMA surfaces, the MEPDG includes performance models to predict the following distresses: • Rut depth for HMA layers, unbound aggregate layers, and the subgrade • Transverse thermal cracking • Alligator cracking due to bottom-initiated fatigue • Longitudinal wheel path cracking due to surface-initiated fatigue • Reflection cracking • Roughness A detailed discussion of the form of the MEPDG performance models is beyond the scope of this manual. The interested reader should refer to the MEPDG user manual and other detailed documentation for the MEPDG. The MEPDG does not include models for durability distresses such as raveling or moisture damage. It is assumed that the potential for these forms of distress will be minimized through proper HMA mixture design. The MEPDG also does not include a model to predict changes in the skid resistance of the pavement with time and traffic. HMA Material Property Inputs HMA material properties are not direct inputs to some of the distress prediction models. The reflection cracking model included in the MEPDG is an empirical function that delays the appearance of existing joints and cracks depending on the thickness of the HMA and the condition of the underlying pavement. The roughness model predicts the International Roughness Index (IRI) for the pavement based on the initial roughness, the amount of rutting and cracking obtained from the other models, and site factors including pavement age, soil type, freezing index, and precipitation. Table 6-5 summarizes the HMA materials properties used by each of the performance models. The dynamic modulus and binder grade data are used by the MEPDG to generate a dynamic modulus master curve for each HMA layer. This requires testing the dynamic modulus at multiple temperatures and frequencies as described in AASHTO PP 61-09. The dynamic modulus master curve is used for the computation of the traffic-induced strains that are used by the rutting, alligator cracking, and longitudinal cracking models. It is also a direct input into the damage models used for alligator and longitudinal cracking. The dynamic modulus testing should be conducted on short-term oven-aged mixture (4 hours at 135°C per AASHTO R30), compacted to the expected in-place air void content of the pavement. The alligator and longitudinal cracking models also require in-place volumetric properties of the HMA layers, specifically the air void content and the effective binder content. These volumetric Evaluating the Performance of Asphalt Concrete Mixtures 83

properties are direct inputs to the models and have a major effect on the predicted load-associated cracking distress. As discuss previously, the resistance of HMA to fatigue cracking increases with an increase in the effective binder content and a decrease in in-place air void content. For thermal cracking analysis, the in-place VMA of the surface mixture is needed to estimate the coefficient of thermal contraction. Creep compliance and tensile strength data for the surface mixture are obtained from AASHTO T 322. The specimens used in this testing should be compacted to the expected in-place air void content of the pavement, then long-term oven aged in accordance with AASHTO R 30 before testing. Overview of Using the MEPDG to Verify an HMA Mixture Design As discussed previously, the MEPDG is a comprehensive pavement analysis tool that can predict the performance of a given pavement section. The accuracy of the predicted performance depends, in part, on the accuracy of the input data. Detailed information on traffic, climate, subgrade soils, and unbound layers, and existing pavement conditions for rehabilitation design are needed in addition to materials properties for the HMA layers. The User Manual for the MEPDG provides guidance for the selection of specific input data. The MEPDG can be used to predict the amount of rutting, bottom-up fatigue cracking, top-down fatigue cracking, and thermal cracking in a pavement section for mixture-specific HMA properties. The change in roughness caused by these distresses can also be predicted. Any or all of these may be used as criteria to judge the acceptability of the HMA mixture for the specific pavement analyzed. Reflective cracking should not be used as a criterion because the MEPDG model for reflective cracking is empirical and is not affected by the properties of the HMA. Rut Resistance To evaluate only the rutting resistance of an HMA mixture, the E* AMPT Specification Criteria Program as described in NCHRP Report 580, not the MEPDG, should be used. This program uses the calibrated rutting model included in the MEPDG, but does not require all of the MEPDG input data for traffic, climate, and properties of the other layers of the pavement. In this approach, rutting in the HMA layer is assumed to be insensitive to underlying layer properties. The E* AMPT Specification Criteria Program provides a predicted rut depth in each HMA layer specified by the user. If rutting will be used in conjunction with other forms of distress to judge the acceptability of the HMA mixture, then a complete analysis using the MEPDG must be performed. In this case, 84 A Manual for Design of Hot Mix Asphalt with Commentary HMA Property Rutting Thermal Cracking Alligator Cracking Longitudinal Cracking Reflection Cracking Roughness Dynamic Modulus X X X Indirect* Binder Grade X X X Indirect* In-Place Air Void Content X X X Indirect* In-Place Effective Binder Content X X Indirect* In-Place VMA X Indirect* Low Temperature Creep Compliance X Indirect* Low Temperature Tensile Strength X Indirect* * The MEPDG estimates roughness from rutting and cracking predictions, which in turn depend on various physical properties as noted here. Table 6-5. Summary of HMA materials properties used by the MEPDG performance models.

the rut depth in each layer of the pavement will be predicted as a function of time for the pavement section. Fatigue Cracking To evaluate the potential for bottom-up (alligator) and top-down (longitudinal) cracking, a complete analysis must be performed using the MEPDG. The user is cautioned that fatigue cracking is more sensitive to traffic, subgrade support conditions, and pavement layer thicknesses than to HMA properties. It may not be possible to adjust HMA properties to obtain acceptable cracking performance if the pavement is not thick enough or the subgrade support is poor. The MEPDG provides separate predictions of alligator and longitudinal cracking with time for the pavement section being analyzed. Alligator cracking is expressed as the percent of the total lane area. Longitudinal cracking is expressed as feet of longitudinal cracking per lane mile. Thermal Cracking To evaluate a mixture for resistance to low temperature cracking, a thermal cracking analysis must be performed with the MEPDG. All climatic data needed for this analysis are stored within a database supplied with the MEPDG; therefore, the user need only specify the longitude and latitude of the pavement and the required material properties. The MEPDG provides a prediction of transverse thermal cracking with time for the pavement section being analyzed. Thermal cracking is expressed as feet of transverse cracking per lane mile. Surface Roughness Within the MEPDG, the change in roughness in a pavement section depends on the initial roughness, the predicted rutting and cracking, and site factors including pavement age, soil type, freezing index, and precipitation. To analyze changes in roughness, a complete analysis must be performed with the MEPDG. In many cases, the initial roughness and site factors, which are not associated with the HMA mixtures used in the pavement, will dominate the prediction. MEPDG Predictions Using the MEPDG as an analysis tool for HMA mixtures is conceptually simple. The basic steps are summarized below. Additional detail for each step is provided in the MEPDG User Manual. 1. Select a trial pavement section. For verification of a mixture design, the pavement section will usually be specified based on the original design of the pavement. 2. Select the performance criteria that will be used. As discussed above the MEPDG predicts the development of various distresses with time for the trial pavement section. The performance criteria used to evaluate the HMA mixture will generally be determined by the specifying agency based on its pavement management policies. 3. Obtain the necessary inputs for the trial pavement section. This is the most time-consuming step. The MEPDG requires a large amount of information about the pavement being analyzed. This includes data concerning traffic, climate, subgrade soils, the condition of existing pavements for rehabilitation design, and the thicknesses and material properties for each layer of the pavement, including existing pavement layers for rehabilitation design. For verification of an HMA mixture, Level 1 material property inputs should be used for the HMA mixture being analyzed. For the remaining inputs, other level data can be used, keeping in mind that the accuracy of the distress predictions depends on the accuracy of the input data. 4. Run the MEPDG software and examine the inputs and outputs for engineering reason- ableness. The software summarizes the inputs. This summary should be examined to ensure that no errors were made during the data input process. If input errors are discovered, fix the errors and rerun the analysis. The software also summarizes the pavement layer moduli Evaluating the Performance of Asphalt Concrete Mixtures 85

for each month of the analysis. These summaries should be examined to ensure that they are reasonable. Temperature and aging change the modulus of HMA layers over time. Frost and moisture content change the modulus of unbound materials on a seasonal basis. Finally, the software summarizes all distresses by month over the design life of the pavement. These should be examined carefully to see if they appear reasonable and then compared to the performance criteria. 5. Modify the HMA mixture properties to improve performance. The evolution of distress over the design life of the pavement should be studied carefully to identify potential adjustments that can be made to the mixture to improve the predicted performance. The next section presents guidance for adjusting HMA mixtures based on the distresses predicted by the MEPDG. Mixture Adjustments Based on MEPDG Predictions The distress prediction models in the MEPDG are driven by the stiffness of the HMA layers (dynamic modulus and creep compliance), the low-temperature strength of the HMA surface layer, and the in-place volumetric properties of the HMA layers. Therefore, to change the predicted level of distress by changing HMA mixture properties, the change must affect the properties listed above. The effects of changing HMA mixture properties are specific to the distress type, and it is not uncommon that actions taken to improve HMA rutting performance result in an adverse effect on cracking performance. The exception to this rule is the in-place air void content. Decreasing the in-place air void content of the mixture will improve the performance for all distresses. Recommended mixture adjustments are presented below for rutting, alligator cracking, longitudinal cracking, and thermal cracking. It is recommended that users of the MEPDG perform a sensitivity analysis for the pavement section to determine the magnitude of adjustment needed. In some cases, the adjustments may not be possible with the mixture types and binder grades available. In such cases, changes to the pavement structure may be needed to obtain acceptable performance predictions. HMA Layer Rutting Within the MEPDG, the only way to decrease the predicted rutting within the HMA layers of a pavement is to increase the dynamic modulus of the mixture. The E* AMPT Specification Criteria Program, described in NCHRP Report 580, can be used to determine the minimum dynamic modulus needed to keep the predicted rutting below a specified level. The MODULUS spreadsheets in the EXCEL™ workbook that accompanies this manual provide tools for estimating mixture dynamic modulus values from mixture composition. These tools should be used to estimate mixture properties needed to meet the minimum dynamic modulus determined from the E* AMPT Specification Criteria Program. The mixture design factors that affect the dynamic modulus are listed below in order of importance: • High-Temperature Binder Grade. The high-temperature binder grade has the greatest effect on the dynamic modulus of the HMA mixture. Increasing the high temperature binder grade one level will increase the dynamic modulus of the mixture approximately 25%. • Design VMA. VMA is the sum of air void content and effective binder content in the mixture, which are the components of HMA that deform under load. The modulus of HMA increases with decreasing VMA. A 1% decrease in design VMA will increase the dynamic modulus approximately 5%. • Nominal Maximum Aggregate Size (NMAS). Larger NMAS mixtures have lower design VMA. Increasing the nominal maximum aggregate size of the mixture one level will increase the dynamic modulus approximately 5%. • In-Place Air Void Content. In-place air void content affects the in-place VMA of the mixture. The MEPDG predicts performance based on the properties of the in-place mixture. Decreasing in-place air void content 1% will increase the dynamic modulus approximately 5%. 86 A Manual for Design of Hot Mix Asphalt with Commentary

• Filler Content. Increasing the filler content of the mixture will increase the dynamic modu- lus. Within the filler contents allowed for dense-graded mixtures, a 1% increase in the percent passing the #200 sieve will increase the dynamic modulus approximately 1.5%. Alligator Cracking (Bottom-Up Fatigue Cracking) Within the MEPDG, alligator cracking depends on pavement thickness, subgrade support conditions, and the properties of the lowest asphalt-bound layer in the pavement. Fatigue cracking can be decreased by increasing the pavement thickness, improving subgrade support, or enhancing the HMA properties for the lowest layer. Alligator cracking is generally more sensitive to changes in thickness and subgrade support than to changes in the properties of the bottom HMA layer. The effective binder content, VBE, and in-place density are direct inputs to the fatigue cracking model in the MEPDG. Increasing the effective binder content and decreasing the in-place air void content of the lowest HMA layer will substantially decrease the predicted alligator cracking in the pavement. As discussed previously, use of dense-graded mixtures with higher design VMA will decrease the predicted alligator cracking compared to that for mixtures with more typical VMA values. The effective binder content of these mixtures is up to 1% higher than that for the standard mixtures. The effective binder content can also be increased by decreasing the nominal maximum aggregate size of the lowest HMA layer. A one-level decrease in nominal maximum aggregate size also increases the effective binder content by 1%. The effect of decreasing air void content is similar to that of increasing effective binder content. Alligator cracking is affected to a lesser degree by the dynamic modulus of the lowest HMA layer. For pavements with 5 inches or more of HMA, alligator cracking predicted by the MEPDG decreases with an increase in the dynamic modulus. For pavements with 3 inches or less of HMA, the predicted alligator cracking decreases with a decrease in the dynamic modulus. As discussed above for rutting, changing binder grade is the most efficient way of changing the dynamic modulus of HMA. Longitudinal Cracking (Top-Down Fatigue Cracking) Within the MEPDG, top-down cracking depends heavily on the properties of the surface HMA layer. Since effective binder content and in-place density are direct inputs to the fatigue model in the MEPDG, these properties for the surface HMA layer have a major effect on the predicted longitudinal cracking. Increasing the effective binder content and decreasing the in-place air void content of the surface HMA layer will substantially decrease the predicted longitudinal cracking in the pavement. As discussed previously, the use of dense-graded mixtures with higher design VMA will decrease the predicted longitudinal cracking compared to that for the standard mixtures. The effective binder content of these mixtures is up to 1% higher than that for the standard mixtures. The effective binder content can also be increased by decreasing the nominal maximum aggregate size of the surface HMA layer. A one-level decrease in nominal maximum aggregate size also increases the effective binder content by 1%. The effect of decreasing air void content is similar to that for increasing effective binder content. Longitudinal cracking is affected to a lesser degree by the dynamic modulus of the surface HMA layer. Decreasing the dynamic modulus of the surface layer will decrease the amount of longitudi- nal cracking that occurs in the pavement. As discussed above for rutting and alligator cracking, changing binder grade is the most efficient way of changing the dynamic modulus of HMA. Thermal Cracking Within the MEPDG, the predicted thermal cracking depends on the environment at the project location, the total thickness of the HMA, and the properties of the surface HMA layer. Evaluating the Performance of Asphalt Concrete Mixtures 87

For a given project, the predicted thermal cracking can be decreased by increasing the HMA thickness or improving the low-temperature properties of the surface layer. The predicted amount of thermal cracking will decrease with an increase in either the tensile strength or creep compliance of the surface mixture. Low-temperature tensile strength increases with increasing voids filled with asphalt, VFA. A 5% increase in VFA will increase the low-temperature tensile strength approximately 85 psi. In-place properties of the HMA layer are used in the MEPDG predictions; therefore, the in-place air void content also affects the tensile strength of the mixture. For a given binder content, decreasing the in-place air void content increases VFA and the tensile strength of the mixture. Polymer-modified binders exhibit approximately 8% higher low-temperature strengths compared to neat binders. The creep compliance of the mixture is affected by the same properties that affect the dynamic modulus of the mixture. Low temperature binder grade is by far the most important factor affecting the creep compliance of the mixture. Decreasing the low temperature grade by one level increases the creep compliance by approximately 25%. VMA and in-place air void content have smaller effects. Increasing the VMA or in-place air void content 1% will increase the creep compliance by approximately 5%. Summary Table 6-6 summarizes the effects of mixture composition on performance predictions using the MEPDG. The properties highlighted in bold for each distress have the greatest effect on the predicted performance. A Note on Modulus, HMA Mix Design, and Pavement Design Using the MEPDG It is likely that many state highway agencies will eventually adopt the MEPDG for designing flexible pavements. In most cases, pavement designs—at least preliminary designs—will be done well in advance of developing or selecting HMA mix designs for a given pavement. This will involve making assumptions about the type of mixture used, and most importantly, its E* values as a function of temperature. In such situations, the potential performance of the HMA mixture should be verified by comparing the E* value assumed in the pavement design with that developed by the mix design. These latter E* values can be determined in two ways. For pavements subject to relatively low traffic levels (below 3 million ESALs), estimated values for E* 88 A Manual for Design of Hot Mix Asphalt with Commentary HMA Property Rutting Thermal Cracking Alligator Cracking HMA ≥ 5 in Alligator Cracking HMA < 3 in Longitudinal Cracking High Temperature Binder Grade Increase to improve Increase to improve Decrease to improve Decrease to improve Low Temperature Binder Grade Decrease to improve Design VMA Decrease to improve Increase to improve Increase to improve Increase to improve Design VFA Increase to improve Filler Content Increase to improve In-Place Air Void Content Decrease to improve Decrease to improve Decrease to improve Decrease to improve Decrease to improve Table 6-6. Summary of effect of mixture composition on performance predictions.

can be used; the HMA Tools spreadsheet can be used to provide such estimates at virtually any combination of frequency and temperature. For more critical pavements, an E* master curve should be measured and compared with the E* values assumed during development of the pavement design. If there are discrepancies, the mix design should be modified—in general, the most effective way of modifying E* values for an HMA mix design is to change the binder: the stiffer the binder, the higher the E* values will be. The HMA Tools modulus prediction tool can be used to estimate modulus early in the mix design process, even for critical mixtures that will eventually require E* testing. Thus, potential mix designs that do not provide proper E* values need not be evaluated further. As with other aspects of the MEPDG and related performance testing, there will likely be many differences in the way E* values will be used in both the pavement design and mix design process and differences in when these requirements will be implemented. Engineers and technicians responsible for mix design should refer to the appropriate state standards for details of verifying mix design modulus. Bibliography AASHTO Standards M 320, Performance-Graded Asphalt Binder R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA) PP 60-09, Preparation of Cylindrical Performance Test Specimens Using the Superpave Gyratory Compactor (SGC) PP 61-09, Developing Dynamic Modulus Master Curves for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT) T 320, Determining the Permanent Shear Strain and stiffness of Asphalt Mixtures Using the Superpave Shear Tester (SST) T 321, Determining the Fatigue Life of Compacted Hot Mix Asphalt (HMA) Subjected to Repeated Flexural Bending. T 322, Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device T 324, Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA) TP 63-09, Determining the Rutting Susceptibility of Asphalt Paving Mixtures Using the Asphalt Pavement Analyzer (APA) TP 79-09, Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT) Other Publications Aschenbrener, T., R. B. McGennis, and R. L. Terrel (1995) “Comparison of Several Moisture Susceptibility Tests to Pavements of Known Field Performance,” Journal of the Association of Asphalt Paving Technologists, Vol. 64. Bonaquist, R. (2008) NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester, TRB, National Research Council, Washington, DC, 137 pp. Bonaquist, R., D. W. Christensen, and W. Stump (2003) NCHRP Report 513: Simple Performance Tester for Super- pave Mix Design: First Article Development and Evaluation TRB, National Research Council, Washington, DC, 169 pp. Bonaquist, R. (2008) NCHRP Report 614: Refining the Simple Performance Tester for Use in Routine Practice, TRB, National Research Council, Washington, DC, 153 pp. Bukowski, J. R., and T. Harman (1997) Minutes of the Superpave Mixture Report Task Group, Meeting of September 1997. Christensen, D. W., and R. F. Bonaquist (2006) NCHRP Report 567: Volumetric Requirements for Superpave Mix Design, TRB, National Research Council, Washington, DC, 57 pp. Christensen, D. W., R. Bonaquist, and D. Jack (2000) Evaluation of Triaxial Strength as a Simple Test for Asphalt Concrete Rut Resistance, Final Report, PennDOT University-Based Research, Education and Technology Transfer Program, The Pennsylvania State University, State College, PA, August. NAPA (2001) Moisture Susceptibility of HMA Mixtures: Identification of Problems and Solutions, Lanham, MD, 24 pp. Evaluating the Performance of Asphalt Concrete Mixtures 89

The Asphalt Institute, Causes and Prevention of Stripping in Asphalt Pavements (ES-10), 2nd Ed., 8 pp. The Asphalt Institute, Moisture Sensitivity (MS-24), 1st Ed., 48 pp. User Manual for the M-E Pavement Design Guide (2007) March. Von Quintus, H., J. Mallela, and M. Buncher, Transportation Research Record 2001 “Quantification of Effect of Polymer-Modified Asphalt on Flexible Pavement Performance,” TRB, National Research Council, Wash- ington, DC Witczak, M. W., et al. (2000) “Specimen Geometry and Aggregate Size Effects in Uniaxial Compression and Constant Height Shear Tests,” Journal of the Association of Asphalt Paving Technologists, Vol. 69. Witczak, M. W. (2007) NCHRP Report 580: Specification Criteria for Simple Performance Tests for Rutting, TRB, National Research Council, Washington, DC, 108 pp. Zaniewski, J. P., and G. Srinivasan (2004) Evaluation of Indirect Tensile Strength to Identify Asphalt Concrete Rutting Potential, Asphalt Technology Program, Department of Civil and Environmental Engineering, West Virginia University, Morgantown, WV, May. 90 A Manual for Design of Hot Mix Asphalt with Commentary

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary incorporates the many advances in materials characterization and hot-mix asphalt (HMA) mix design technology developed since the conclusion of the Strategic Highway Research Program (SHRP).

The final report on the project that developed NCHRP Report 673 and Appendixes C through F to NCHRP Report 673 were published as NCHRP Web-Only Document 159. The titles of the appendixes are as follows:

• Appendix C: Course Manual

• Appendix D: Draft Specification for Volumetric Mix Design of Dense-Graded HMA

• Appendix E: Draft Practice for Volumetric Mix Design of Dense-Graded HMA

• Appendix F: Tutorial

The companion Excel spreadsheet HMA tool and the training course materials described in NCHRP Report 673 are available for download from the Internet.

In January 2012, TRB released NCHRP Report 714: Special Mixture Design Considerations and Methods for Warm Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot Mix Asphalt with Commentary. The report presents special mixture design considerations and methods used with warm mix asphalt.

In January 2012, TRB released an errata to NCHRP Report 673: Page 41, Table 4-7, and page 123, Table 8-10, respectively, should be replaced with a new table.

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