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27 1/3 0.250 factors on various performance-related properties of HMA. These data, along with several data sets in the literature, were Rutting Rate, mm/m/ESALs 0.200 NCAT MN/Road used to develop semi-empirical models for estimating rut 0.150 WesTrack resistance, fatigue resistance, and mixture permeability. Mirza and Witczak's global aging system was modified to provide a 0.100 Fit 2 R = 69% more rational model for predicting age hardening, consistent with both the ChristensenAnderson model for binder mod- 0.050 ulus and the newly developed Hirsch model for estimating 0.000 the modulus of HMA. The following important findings were 4.0 6.0 8.0 10.0 12.0 14.0 made based upon these tests and analyses: Apparent Film Thickness, microns Figure 21. Relationship Between Rutting Rate and It appears reasonable to allow design air voids for Apparent Film Thickness. Superpave mixtures to vary within the range from about 3% to 5%. However, engineers and technicians that wish to deviate from the current design air void level of tionship, Figure 21 shows rutting rate as a function of AFT for 4.0% should understand how such changes can affect data from the NCAT test track, MnRoad, and WesTrack (this HMA performance. plot can be compared with Figure 3). The relationship is only A variety of models for relating mixture volumetric com- moderately strong, but rutting rate clearly increases with position to performance were identified in the literature; increasing AFT film thickness. This plot would suggest that however, these models are not well suited for evaluating HMA mixes with AFT values greater than 9 m may be prone the effect of mixture composition on performance for the to excessive rutting. Superpave system of mixture design and analysis. There- The relationships between AFT and other aspects of HMA fore, models have been developed (or existing models performance are not as straightforward. Fatigue resistance refined) during NCHRP Projects 9-25 and 9-31 for esti- increases with increasing effective binder content; therefore, mating mixture performance on the basis of volumetric if aggregate specific surface is kept constant, fatigue resistance composition. will increase with increasing AFT. Permeability decreases with Many state highway agencies have modified the require- increasing specific surface and decreasing in-place voids. ments for VMA, air voids, and related factors for Super- Therefore, at a constant value of in-place air voids, perme- pave mixtures. Three modifications are most common: (1) ability will decrease with decreasing AFT. However, as asphalt an expansion of the design air void content from 4% to a binder content is reduced for a given HMA mix, AFT will range of 3% to 5%; (2) establishing a maximum VMA decrease and the mix will become more difficult to compact, value at 1.5% to 2.0% above the minimum value; and (3) a potentially leading to higher in-place voids and greater per- slight increase in the minimum VMA values, typically by meability. Mixes with higher binder contents will be easier to about 0.5%. compact and so may often exhibit lower in-place air voids and Aggregate specific surface is very nearly proportional to lower permeability. This phenomenon is possibly the source the sum of the weight percent material passing the 75, of the proposed relationship between AFT and durability. 150, and 300 m sieves. This factor, called the fineness In summary, the results of this study suggest that AFT modulus, 300 m basis (FM300), can be used to control should relate to most aspects of HMA performance. All else aggregate specific surface in mixtures made using the being equal, rut resistance will in general increase with Superpave system to ensure adequate mixture performance decreasing AFT. Other relationships between AFT and per- and good workability. formance are indirect; therefore, the use of AFT for specifying Rut resistance as indicated by laboratory tests and as and/or controlling HMA mixes is not recommended. Instead, measured in a wide range of field test tracks/test roads such control should be exerted through the relationships pre- was predicted to within about a factor of 2 using a model sented previously, linking various aspects of HMA composi- incorporating mixture resistivity, design compaction, tion to rut resistance, fatigue resistance, and permeability. and relative field compaction. The rutting/resistivity model suggests that each 1% decrease in VMA, 1% increase in design air voids, and/or Summary 1% decrease in field air voids increases rut resistance by During NCHRP Projects 9-25 and 9-31, laboratory tests about 20%,as indicated by rutting rate in mm/m/ESALs1/3. were conducted to evaluate the effect of changes in VMA, air Increasing FM300 by 6% (at constant VMA) typically void content, VFA, aggregate specific surface, and related increases rut resistance by about a factor of 2.0 to 2.5.

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28 For the types of HMA used in NCHRP Projects 9-25 and increase in air voids or 3% decrease in FM300 for air void 9-31--that is, mixtures made using good-quality, highly contents above the zero-permeability limit. angular aggregates with little or no natural sand-- The permeability of HMA specimens prepared in the lab- increasing the high temperature binder grade one level oratory tends to be significantly lower than permeability will increase rut resistance by about a factor of 2.5, as values measured on field cores of comparable mixtures. indicated by rutting rate in mm/m/ESALs1/3. For HMA For this reason and because of the highly variable nature of designed according to current Superpave requirements, permeability measurements, laboratory measurements of binder grade appears to be the most important considera- mixture permeability are not recommended for use in rou- tion in determining rut resistance of HMA; volumetrics are tine mixture design. However, the effect of air void content an important but secondary factor. It must be emphasized and aggregate fineness on permeability should be consid- that replacing the good-quality aggregates normally used ered during the mix design process. in Superpave mixes with poorly crushed gravel and/or The age hardening of the HMA studied depended not only large amounts of natural sand would almost certainly upon air void content, but also upon the specific combina- cause a substantial decrease in rut resistance and might also tion of aggregate and asphalt binder. Additional research is result in mixtures much more sensitive to changes in volu- needed to better understand the effect of aggregateasphalt metric composition. binder combinations on mixture age hardening. Increase in Ndesign by one level decreased rut resistance by A modified version of the MirzaWitczak global aging about 15% to 25%. system was used to examine the effects of air voids, aggre- A practical approach to fatigue analysis of HMA based on gate fineness, and other factors on mixture and binder continuum damage theory was developed during age hardening. For a MAAT of 15.6 C, the mixture age- NCHRP Projects 9-25 and 9-31. This technique was ini- hardening ratio decreased about 2% to 7% for every 1% tially developed through analysis of laboratory test data increase in FM300. The age-hardening ratio increased about collected during NCHRP Projects 9-25 and 9-31 and then 5% to 14% for every 1% increase in in-place air voids. verified and refined through successful application to flex- Although not extremely large effects, considered over the ural fatigue data gathered during SHRP at the University of possible range for FM300 and field air voids, these factors California at Berkeley. can significantly affect mixture age hardening. Fatigue resistance is affected by VBE, design compaction The modified global aging system predicted extreme (Ndesign), and field compaction, expressed in terms of field amounts of age hardening as indicated by binder viscos- density relative to laboratory/design density. Every 1% ity. These extreme age-hardening ratios are the result of increase in VBE increases fatigue life by about 13% to 15%. changes in binder rheology that occur during the aging Every 1% increase in field air void content (at a constant process and could significantly affect mixture performance design air void content) decreases fatigue resistance by because of the severe reduction in healing rates that might about 20%. occur with such large increases in binder viscosity. Addi- Permeability of HMA increases with increasing air tional research is needed to better understand the relation- voids and decreasing aggregate specific surface. Perme- ship among age hardening, binder viscosity, healing, and ability can be effectively modeled using the concept of fatigue cracking in HMA pavements. effective air voids--the total air void content minus The various models developed during this study suggest the air void content at zero permeability. Furthermore, that several indirect relationships exist between AFT and the zero air voids content increases with increasing aggre- various aspects of HMA performance. The most signifi- gate fineness. cant of these is between AFT and rut resistance--as AFT A simple, reasonably accurate equation has been devel- increases, rut resistance decreases. Mixtures with AFT oped based upon permeability data gathered by values above 9 m may be prone to excessive rutting. Choubane et al. in a study on the permeability of Super- However, because the relationships between AFT and per- pave mixtures in Florida (3). According to this model, per- formance are indirect, it is not recommended that AFT be meability increases by about 100 10-5 cm/s for every 1% used in specifying or controlling HMA mixtures.