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CHAPTER 6 Evaluating the Performance of Asphalt Concrete Mixtures Chapter 6 discusses various means of evaluating the potential performance of HMA mixtures. It includes discussions of the relationships between mixture composition and performance and binder test properties and performance. The chapter also gives detailed practical information on various test methods being used to characterize HMA mixture performance. The last part of the chapter discusses how the Mechanistic-Empirical Pavement Design Guide (MEPDG) may be used to develop predictions of HMA pavement performance and includes specific guidelines for the number and types of tests required as input when using the higher MEPDG design levels. Much of the most important information provided in Chapter 6 is not in the form of tables, figures, and equations, but is descriptive information on various tests for characterizing HMA mixture performance. Table 3 lists references for the various performance tests discussed in the Manual. The references listed include those mentioned in the Manual and one or two additional references that will provide interested readers with more detailed information on the procedures. References for Table 3 5. Bonaquist, R., D. W. Christensen, and W. Stump, NCHRP Report 513: Simple Performance Tester for Super- pave Mix Design: First Article Development and Evaluation, Washington, DC: Transportation Research Board, 2003, 54 pp. 6. Bonaquist, R., NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester, Washington, DC: Transportation Research Board, 2008, 39 pp. Table 3. References for performance tests discussed in chapter 6 of the mix design manual. Performance Test Used to Evaluate References Asphalt Mixture Performance Rut resistance, dynamic 5, 6 Test System (AMPT) modulus Superpave shear tester, repeated Rut resistance AASHTO T 320, 7, shear at constant height test 8 High-temperature IDT strength Rut resistance 9, 10, 11 test Asphalt pavement analyzer Rut resistance AASHTO TP 63, (APA) 12 Hamburg wheel-tracking test Rut resistance and/or AASHTO T 324 moisture resistance Flexural fatigue test Fatigue resistance AASHTO T 321, 13 Low temperature IDT creep and Resistance to thermal/low AASHTO T 322, strength tests temperature cracking 14, 15 Modified Lottman procedure Resistance to moisture- AASHTO T 283 induced damage Short- and long-term oven Age-hardening AASHTO R 30 conditioning 232

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Commentary to the Mix Design Manual for Hot Mix Asphalt 233 7. Harvey, J., C. Monismith and J. Sousa, "An Investigation of Field- and Laboratory-Compacted Asphalt- Rubber, SMA, Recycled and Conventional Asphalt-Concrete Mixes Using SHRP Project A-003A Equipment," Journal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 511548. 8. Sousa, J., "Asphalt-Aggregate Mix Design Using the Repetitive Simple Shear Test (Constant Height)," Jour- nal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 298333. 9. Christensen, D. W., R. Bonaquist, and D. P. Jack, Evaluation of Triaxial Strength as a Simple Test for Asphalt Con- crete Rut Resistance, Final Report to the Pennsylvania Department of Transportation, Report No. FHWA-PA- 2000-010+97-04 (19), PTI Report 2K26, University Park: The Pennsylvania Transportation Institute, August 2000, 80 pp. 10. Christensen, D. W., et al., "Indirect Tension Strength as a Simple Performance Test," New Simple Performance Tests for Asphalt Mixes, Transportation Research Circular E-C068, http://gulliver.trb.org/publications/ circulars/ec068.pdf, Washington, D.C.: Transportation Research Board, 2004, pp. 4457. 11. Zaniewski, J. P., and G. Srinivasan, Evaluation of Indirect Tensile Strength to Identify Asphalt Concrete Rutting Potential, Morgantown, WV: West Virginia University, Department of Civil and Environmental Engineer- ing, May 2004, 65 pp. 12. Kandhal, P. S., and L. A. Cooley, NCHRP Report 508: Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer, Washington, DC: Transportation Research Board, 2003, 73 pp. 13. Tayebali, A. et al., "Mix and Mode-of-Loading Effects on Fatigue Response of Asphalt-Aggregate Mixes," Journal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 118143. 14. Hiltunen, D. R. and R. Roque, "A Mechanics-Based Prediction Model for Thermal Cracking of Asphaltic Concrete Pavements," Journal of the Association of Asphalt Paving Technologists, Vol. 63, 1994, pp. 81113. 15. Roque, R., D. R. Hiltunen and W. G. Buttlar, "Thermal Cracking Performance and Design of Mixtures Using Superpave," Journal of the Association of Asphalt Paving Technologists, Vol. 64, 1995, pp. 718733. Table 6-1 of the Manual summarizes the effects of various HMA characteristics on performance, and is reproduced here as Table 4 for the convenience of the reader. In the Manual, two NCHRP re- ports are cited as the major basis for this table: NCHRP Report 539 and NCHRP Report 567 (1, 4). Table 4. Effect of mixture composition of performance--table 6-1 in the mix design manual. Typical Effects of Increasing Given Factor within Normal Specification Limits While Other Factors Are Held Constant within Normal Specification Limits " " indicates improved performance; " " indicates reduced performance Resistance to Resistance to Durability/ Rutting and Resistance to Low Resistance to Resistance to Permanent Fatigue Temperature Penetration by Moisture Component Factor Deformation Cracking Cracking Water and Air Damage 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 Voids Increasing Design VMA and/or Design Binder Content Increasing Field Air Voids

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234 A Manual for Design of Hot Mix Asphalt with Commentary Table 5 gives a more complete list of references supporting the information given in Table 6-1 of the Manual, including several notes explaining complex relationships between HMA char- acteristics and performance. For convenience, references for Table 4 appear below the table and are given again at the end of the Commentary. In many cases, the relative strength of these relationships--indicated by the number of arrows in Table 6-1--is to some degree a matter of engineering judgment. Table 6-2 in the Manual, which lists performance-related test properties used in specifying asphalt binders in AASHTO M 320, is directly based on M 320 itself. However, as noted above, the use of G* sin to limit susceptibility to fatigue damage is controversial and at this writing much effort is underway to develop a more effective means of specifying the fatigue-related properties of asphalt binders. Table 5. References for table 6-1 in the manual, on the effect of mixture composition of performance. Resistance to Resistance to Durability/ Rutting and Resistance to Low Resistance to Resistance to Component Factor Permanent Fatigue Temperature Penetration by Moisture Deformation Cracking Cracking Water and Air Damage Increasing High M 320, 16, 17, Temperature Binder Grade 18 Increasing Low M 320, 19 Asphalt Binder Temperature Binder Grade Increasing Intermediate M 320, Note 1 Temperature Binder Stiffness Increasing Aggregate M 323, 1, 20, 21 Angularity Increasing Proportion of Aggregates Flat and Elongated Particles Increasing Nominal Notes 2, 3 Notes 2, 3 Note 4 Maximum Aggregate Size Increasing Mineral Filler 4, 16 4 Content and/or Dust/Binder Ratio Increasing Clay Content M 323, 20 Increasing Design M 323, 16 M 323, 16 Volumetric Compaction Level Properties Increasing Design Air 16 Voids Increasing Design VMA 16, 17, 18 16, 22, 13, 24 20 and/or Design Binder Content Increasing Field Air Voids 16, 17, 18 17, 23, 24 20 16, 25, 26 Note 5 1 Most research suggests that the fatigue resistance of an HMA mixture shows a complex relationship with its stiffness and indirectlywith binder stiffness; for thin pavements, fatigue resistance decreases with increasing modulus, whereas for thick pavements, fatigue resistance increases with increasing modulus (see references 7 and 8). 2 In current and previous HMA mix design systems, VMA and binder content typically decrease with increasing aggregate NMAS. In the interest of clarity, these two factors have been separated, and the effect of design VMA and binder content on mixture performance are listed separately in this table. However, it should be kept in mind that increasing aggregate NMAS will usually decrease VMA and binder content, which in turn will affect mixture performance in various ways. Decreasing aggregate NMAS will, in general, increase VMA and binder content, which will also affect mixture performance. 3 Although there is currently little research linking HMA fatigue properties and resistance or low temperature cracking to aggregate NMAS, the strength properties and fatigue resistance of most particulate composites like HMA increase with decreasing particle size because the size of flaws and magnitude of internal stress concentrations tend to decrease with decreasing particle size. This is the reason for the thoroughly documented increase in compressive strength with decreasing aggregate size in portland cement concrete mixtures. It is highly likely that similar relationships exist for HMA mixtures. 4 As discussed in Note 2 above, increasing aggregate NMAS will tend to result in an overall increase in the number of large flaws in an HMA mixture. This is partly a result of occasional poor bonding at the asphalt-aggregate interface. Such large flaws will tend to result in a significant increase in permeability to both air and water, reducing the durability of HMA mixtures made with large-sized aggregates. 5 The current test procedure used widely to evaluate the resistance of HMA to moisture damage, AASHTO T 283, is performed at a constant air void content of 7 1%. Therefore, little information concerning the effect of air void content on moisture resistance is available. However significant research shows permeability increases with increasing air voids, so it should be expected that as air voids and permeability increase, resistance to moisture damage will decrease.

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Commentary to the Mix Design Manual for Hot Mix Asphalt 235 Table 6. Recommended changes to high temperature performance grade to account for traffic volume and speed--table 6-3 in the mix design manual. 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. Table 6-3 in the Manual lists recommended high-temperature performance grade adjust- ments to account for traffic volume and speed. These grade adjustments should be applied to the base high-temperature binder grade in order to ensure that the resulting HMA mixture will have adequate resistance to rutting for the given traffic conditions. In AASHTO M 320, these adjustments are given without explanation. In LTPPBind Version 3.1, the adjustments given follow directly from the rational approach taken in the development of the software-- that is, the adjustments are based on predicted damage under different traffic levels and traffic speeds. However, the problem encountered with LTPPBind Version 3.1 is that the traf- fic speeds are limited to fast and slow--there is no adjustment for very slow traffic. Further- more, it is not clear what range in average traffic speeds were assumed in the calculation of grade adjustments. Table 6-3 in the Manual (reproduced as Table 6) is presented to provide grade adjustments for the full range of traffic speeds, and with a full rational derivation, as given below. References for Tables 4 and 5 1. Prowell, B. D., J. Zhang and E. R. Brown, NCHRP Report 539: Aggregate Properties and the Perfor- mance of Superpave-Designed Hot Mix Asphalt, Washington DC: Transportation Research Board, 2005, 90 pp. 16. Christensen, D. W., and R. F. Bonaquist, "Rut Resistance and Volumetric Composition of Asphalt Concrete Mixtures," Journal of the Association of Asphalt Paving Technologists, Vol. 74, 2005. 17. Leahy, R. B., and M. W. Witczak, "The Influence of Test Conditions and Asphalt Concrete Mix Parameters on Permanent Deformation Coefficients Alpha and Mu," Journal of the Association of Asphalt Paving Tech- nologists, Vol. 60, 1991, p. 333. 18. Kaloush, K. E., and M. W. Witczak, Development of a Permanent to Elastic Strain Ratio Model for Asphalt Mixtures, NCHRP 1-37 A Inter-Team Technical Report, College Park, MD: University of Maryland, March 1999, 106 pp. 19. Anderson, D. A., et al. Asphalt Behavior at Low Service Temperatures, Final Report to the Federal Highway Administration. PTI Report No. 8802. University Park, PA: The Pennsylvania Transportation Institute, March 1990, 337 pp. 20. Kandhal, P. S. and F. Parker, Jr., NCHRP Report 405: Aggregate Tests Related to Asphalt Concrete Performance in Pavements, Washington, DC: Transportation Research Board, 1998, 103 pp. 21. White, T. D., J. E. Haddock and E. Rismantojo, NCHRP Report 557: Aggregate Tests for Hot-Mix Asphalt Mixtures Used in Pavements, Washington, DC: Transportation Research Board, 2006, 38 pp.

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236 A Manual for Design of Hot Mix Asphalt with Commentary 22. Bonnaure, F. P., A. H. J. J. Huibers, and A. Boonders, "A Laboratory Investigation of the Influence of Rest Periods on the Fatigue Characteristics of Bituminous Mixes," Proceedings, the Association of Asphalt Paving Technologists, Vol. 51, 1980, p. 104. 23. Shook, J. F., et al., "Thickness Design of Asphalt Pavements--The Asphalt Institute Method," Proceedings, Fifth International Conference on the Structural Design of Asphalt Pavements, Vol. 1, The University of Michi- gan and The Delft University of Technology, August 1982. 24. Tayebali, A. A., J. A. Deacon, and C. L. Monismith, "Development and Evaluation of Surrogate Fatigue Models for SHRP A-003A Abridged Mix Design Procedure," Journal of the Association of Asphalt Paving Technologists, Vol. 64, 1995, pp. 340364. 25. Choubane, B., G. Page, and J. Musselman, "Investigation of Water Permeability of Coarse Graded Superpave Pavements," Journal of the Association of Asphalt Paving Technologists, Vol. 67, 1998, p. 254. 26. Huang, B., et al., "Fundamentals of Permeability in Asphalt Mixtures," Journal of the Association of Asphalt Paving Technologists, Vol. 68, 1999, pp. 479496. Estimating grade adjustments such as those shown in Table 6 is somewhat complicated, given that three different factors must be considered: traffic volume, traffic speed, and design compaction. It must be remembered that when designing HMA mixtures following this method (or the Superpave system), the design compaction level changes along with traffic level, so the properties of the mix, including rut resistance, will change significantly. This will affect the required binder grade. One other piece of information is needed to develop binder grade adjustments: the typical change in binder G* /sin values with temperature. Recent research performed for the Airfield Asphalt Pavement Technology Program (AAPTP) Project 4-2 described the calculation of high-temperature binder grade adjustments in detail; the development given here closely follows that given in the Final Report for AAPTP Project 4-2 (27). The effect on rut resistance of differences in mixture properties can be estimated using the resistivity-rutting model initially developed during NCHRP Projects 9-25 and 9-31, and further refined as part of NCHRP Project 9-33 and AAPTP Project 4-2 (4, 16, 27). The most recent ver- sion of the resistivity/rutting equation gives allowable traffic as a function of mixture composi- tion, compaction, and air voids: TR = 9.85 10-5 ( PNK s ) 1.373 1.5185 -1.4727 VQC VIP M (1) where TR = million ESALs to an average rut depth of 7.2 mm (50% confidence level) = million ESALs to a maximum rut depth of 12 mm (95% confidence level) = resistivity, s/nm = ( G * sin ) Sa2Ga2 49VMA3 G* /sin = Estimated aged performance grading parameter at high temperatures, deter- mined at 10 rad/s and at the yearly, 7-day average maximum pavement temper- ature at 20 mm below the pavement surface, as determined using LTPPBind, Ver- sion 3.1 (units of Pa); aged value can be estimated by multiplying the RRTFOT value by 4.0 for long-term projects (10 to 20 year design life), and by 2.5 for short- term projects of 1 to 2 years. Sa = specific surface of aggregate in mixture, m2/kg the sum of the percent passing the 75, 150, and 300 micron sieves, divided by 5.0 2.05 + (0.623 percent passing the 75 micron sieve)

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Commentary to the Mix Design Manual for Hot Mix Asphalt 237 Ga = the bulk specific gravity of the aggregate blend VMA = voids in the mineral aggregate for the mixture, volume%, as determined during QA testing N = design gyrations Ks = speed correction = (v/70)0.8, where v is the average traffic speed in km/hr VQC = air void content, volume%, determined during QA testing at design gyrations VIP = air void content, volume%, in-place M = 7.13 for mixtures containing typical polymer-modified binders, 1.00 otherwise The equation for resistivity can be inserted into Equation 1 and the results simplified to give an alternate form for allowable traffic: TR = 4.71 10-7 ( G * sin ) 1.373 2.746G 2.746VMA -4.119 N 1.373 K 1.373V 1.5185V -1.4727 M Sa a eq s QC IP (2) In order to develop high-temperature binder grade adjustments, Equation 2 must be manip- ulated into a form that allows the direct calculation of the temperature adjustment needed to off- set a specified change in a given property or combination of properties: TR2 ( G * sin )2 1.373 2.746 2.746 -4.199 1.373 Sa 2 Ga 2 VMA2 N2 = TR1 ( G * sin )1 Sa1 Ga1 VMA1 N1 1.098 1.5158 -1.4727 vs2 VQC 2 VIP 2 M2 v s1 VQC 1 VIP1 (3) M1 In Equation 3, subscripts 1 and 2 refer to two different sets of conditions--binder G* /sin , aggregate surface area, mix VMA, design gyrations, and so forth. Because we are only interested in changes in three of the properties included in Equation 3 ( G* /sin , N, and v), Equation 3 can be simplified by removing the other variables: TR2 ( G * sin )2 1.373 1.373 1.098 N2 vs2 = N1 v s1 (4) TR1 ( G * sin )1 An analysis was performed on a set of nine different binders from various accelerated pave- ment tests. Eight of the binders were from projects included in development of the AMPT: the FHWA ALF rutting test; MnRoad; and Westrack (28, 29, 30). One binder tested was a PG 64-22 used in NCHRP Projects 9-25 and 9-31 (4). These binders were chosen for this analysis because they have been included in well-known studies, and their flow properties have been thoroughly documented. As shown in Figure 1, the relationship between temperature and modulus ( G* /sin in this case) is exponential: (G* sin )1 = exp [ A (T1 - T2 )] (5) (G* sin )2 The value of constant A in Equation 5 varies somewhat among the binders included in Fig- ure 1, but is typically very close to -0.135, as shown in Figure 9. Equation 5 can be substituted

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238 A Manual for Design of Hot Mix Asphalt with Commentary 100,000 ALF AC-5 ALF AC-10 G*/sin delta, Pa ALF AC-20 10,000 ALF SBS ALF PE 1,000 y = 3356e-0.1349x2 PG 64-22 R = 98 % MN/Road 120 Pen MN//Road AC-20 100 WesTrack PG 64-22 -30.0 -20.0 -10.0 0.0 10.0 Relative Temperature Fit Figure 1. Temperature dependence of nine asphalt binders relative to the performance grading temperature. into Equation 4 and rearranged, giving the following relationship between binder grade adjust- ment T, traffic level, design gyrations, and traffic speed: TR 0.728 N1 v s1 0.800 T = T2 - T1 = 7.41 ln 2 N2 Ns2 (6) TR1 Equation 6 can then be used to estimate the high-temperature binder grade adjustments given in Table 6, keeping in mind the design gyration levels for various traffic levels: 50 gyrations for less than 0.3 million ESALs, 75 gyrations for 0.3 million to less than 10 million ESALs, 100 gyrations for 10 million to less than 30 million ESALs, and 125 gyrations for traffic levels of 30 million ESALs or more. The traffic level used to calculate the grade adjustments was the highest in the given range, and 100 million for traffic levels of 30 million ESALs or more. Traffic speeds used in the calcula- tions were 70 kph for fast traffic, 25 kph for slow traffic, and 10 kph for very slow traffic. Table 6-4, in the Manual (included here as Table 7) summarizes the performance testing rec- ommended for routine, dense-graded HMA mix designs. As in the Superpave system, moisture resistance testing is required for all mix designs. The only other performance testing normally required for any routine mix design is rut-resistance testing. This is because of the high level of com- plexity and cost for performing tests to characterize resistance to fatigue cracking and thermal crack- ing. This table is largely based on engineering judgment. It is assumed that the reliability for HMA mixtures intended for pavements at lower traffic levels--below 3 million ESALs--does not need to be as high as that for mixtures intended for higher traffic levels, and so rut resistance testing is not warranted for these cases. Table 6-5 in the Manual summarizes mixture properties used as input in the MEPDG. This table is straightforward and is based directly on the MEPDG User Manual (31). Table 6-6 in the Manual, reproduced as Table 8 below, summarizes the effect of changes in mixture composition and high-temperature binder grade on MEPDG performance predictions. Like Table 6-5, this is based on information contained in the MEPDG User Manual (31). Table 7. Recommended performance tests for HMA mixtures made with conventional materials including most modified binders-- table 6-4 in the mix design manual. Design Traffic Levels for Which Property Recommended Test Property Should be Evaluated Moisture AASHTO T 283 All Sensitivity Permanent Flow Number or Dynamic Modulus, 3 Million ESAL and greater Deformation NCHRP 9-29 PT 01 Fatigue Cracking None NA Thermal Cracking None NA

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Commentary to the Mix Design Manual for Hot Mix Asphalt 239 Table 8. Summary of effect of mixture composition on performance predictions--table 6-6 in the mix design manual. Alligator Alligator Thermal Cracking Cracking Longitudinal HMA Property Rutting Cracking HMA 5 in HMA < 3 in Cracking High Temperature Increase to Increase to Decrease to Decrease to Binder Grade improve improve improve improve Low Temperature Decrease Binder Grade to improve Design VMA Decrease to Increase to Increase to Increase to improve improve improve improve Design VFA Increase to improve Filler Content Increase to improve In-Place Air Voids Decrease to Decrease to Decrease to Decrease to Decrease to improve improve improve improve improve