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66 A Manual for Design of Hot Mix Asphalt with Commentary 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 Table 6-1. Effect of mixture composition on performance. Typical Effects of Increasing Given Factor within Normal Specification Limits While Other Factors Are Held Constant within Normal Specification Limits Resistance to Resistance to Durability/ Rutting and Resistance to Low Resistance to Resistance to Permanent Fatigue Temperature Moisture Penetration by Component Factor Deformation Cracking Cracking Damage Water and Air Increasing High Temperature Binder Grade Increasing Low Asphalt Binder Temperature Binder Grade Increasing Intermediate Temperature Binder Stiffness Increasing Aggregate Angularity Increasing Proportion of Aggregates Flat and Elongated Particles Increasing Nominal Maximum Aggregate Size Increasing Mineral Filler Content and/or Dust/Binder Ratio Increasing Clay Content Increasing Design Volumetric Compaction Level Properties Increasing Design Air Void Content Increasing Design VMA and/or Design Binder Content Increasing Field Air Void Content

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Evaluating the Performance of Asphalt Concrete Mixtures 67 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 70C compared to 64C for a PG 64-22. Similarly a PG 64-28 must meet the low temperature properties at -28C compared to -22C 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 Table 6-2. Performance-related properties and criteria used in the performance grading system, AASHTO M 320. Distress Mode Performance Related Criteria Binder Property Rutting G*/sin 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 Low Temperature Creep stiffness, S Maximum of 300 kPa for PAV aged binder at 60 s. Cracking, Table 1 m-value Minimum of 0.300 for PAV aged binder at 60 s Low Temperature Critical cracking Equal to or lower than specified low temperature grade. Cracking, Table 2 temperature Fatigue Cracking G*sin Maximum of 5,000 kPa for PAV aged binder at 10 rad/sec

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68 A Manual for Design of Hot Mix Asphalt with Commentary 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*sin 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 6C, from 25C to 31C. The appropriate intermediate temperature based on environmental con- ditions is 25C, not 31C, and the binder should be expected to have a value of G*sin that is less than or equal to 5,000 kPa at 25C. 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 Table 6-3. Recommended high temperature performance grade changes to account for traffic volume and speed. Grade Adjustment for Average Vehicle Speed in kph (mph): Design Very Slow Slow Fast Traffic < 25 25 to < 70 70 (MESALs) (< 15) (15 to < 45) ( 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.

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Evaluating the Performance of Asphalt Concrete Mixtures 69 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

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70 A Manual for Design of Hot Mix Asphalt with Commentary 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.