Volumetric Requirements for Superpave Mix Design (2006)

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S U M M A R Y During NCHRP Projects 9-25 and 9-31, laboratory tests were conducted to evaluate the effect of changes in voids in the mineral aggregate (VMA), air void content, voids filled with asphalt (VFA), aggregate specific surface, and related factors on various performance related properties of hot mix asphalt (HMA). These data, along with several data sets in the literature, were used to develop semi-empirical models for estimating rut resistance, fatigue resistance, and mixture permeability. The Mirza–Witczak global aging system was modified to provide a more rational model for predicting age hardening (1), consistent with both the Christensen–Anderson model for binder modulus (2) and the newly devel- oped Hirsch model for estimating the modulus of HMA (3). The following important findings were made based upon these tests and analyses: • It appears reasonable to allow design air voids for Superpave® mixtures to vary within the range from about 3% to 5%; however, engineers and technicians who wish to deviate from the current design air voids level of 4.0% should understand how such changes can affect HMA performance. • A variety of models for relating mixture volumetric composition to performance was iden- tified in the literature; however, these models are not well suited for evaluating the effect of mixture composition on performance for the Superpave system of mixture design and analysis. Therefore, as part of NCHRP Projects 9-25 and 9-31, models have been developed (or existing models refined) for estimating mixture performance on the basis of volumetric composition. • Many state highway agencies have modified the requirements for VMA, air voids, and related factors for Superpave mixtures. Three modifications are most common: (1) an expansion of the design air void content from 4% to a range of 3% to 5%; (2) establish- ing a maximum VMA value at 1.5% to 2.0% above the minimum value; and (3) a slight increase in the minimum VMA values, typically by about 0.5%. • Aggregate specific surface is very nearly proportional to the sum of the weight percent material passing the 75-, 150-, and 300-μm sieves. This factor—called the fineness mod- ulus 300 μm basis (FM300)—can be used to control aggregate specific surface in mixtures made using the Superpave system to ensure adequate mixture performance and good workability. • FM300 is somewhat more effective in controlling aggregate specific surface than using either the percent finer than 75 μm or the dust-to-binder ratio. • Rut resistance as indicated by laboratory tests and as measured in a wide range of field test tracks/test roads was predicted to within about a factor of 2 using a model incorporating mixture resistivity, design compaction, and relative field compaction. VOLUMETRIC REQUIREMENTS FOR SUPERPAVE MIX DESIGN 1

2• The rutting/resistivity model suggests that each 1% decrease in VMA at constant design air voids, 1% increase in design air voids at constant VMA, or 1% decrease in field air voids also at constant VMA increases rut resistance by about 20%, as indicated by rutting rate in mm/m/ESALs1/3 (equivalent single axle loads). • In this project, increasing FM300 by 6% (at constant VMA) typically increased rut resist- ance by about a factor of 2.0 to 2.5. • For the types of HMA used in NCHRP Projects 9-25 and 9-31—that is, mixtures made using good quality, highly angular aggregates with little or no natural sand—increasing the high temperature binder grade one level will increase rut resistance by about a factor of 2.5, as indicated by rutting rate in mm/m/ESALs1/3. For HMA designed according to current Superpave requirements, binder grade appears to be the most important consid- eration in determining rut resistance of HMA; volumetrics are an important but second- ary factor. It must be emphasized that replacing the good quality aggregates normally used in Superpave mixes with poorly crushed gravel and/or large amounts of natural sand would almost certainly cause a substantial decrease in rut resistance and might also result in mixtures that are much more sensitive to changes in volumetric composition. • Increase in Ndesign by one level increased rut resistance by about 15% to 25%. • A practical approach to fatigue analysis of HMA based on continuum damage theory was developed during NCHRP Projects 9-25 and 9-31. This technique was initially developed through analysis of laboratory test data collected during the projects and was then verified and refined through successful application to flexural fatigue data gathered during the Strategic Highway Research Program (SHRP) at the University of California at Berkeley. • Fatigue resistance of the HMA analyzed during NCHRP Projects 9-25 and 9-31 was found to be affected by effective asphalt content (VBE), design compaction (Ndesign), and field compaction, expressed in terms of field density relative to laboratory/design density. Every 1% increase in VBE increased fatigue life by about 13% to 15%. Every 1% increase in field air void content (at a constant design air void content) decreased fatigue resistance by about 20%. • Data analyzed during NCHRP Projects 9-25 and 9-31 showed that permeability of HMA increases with increasing air voids and decreasing aggregate specific surface. Permeability can be effectively modeled using the concept of effective air voids—the total air void con- tent minus the air void content at zero permeability. Furthermore, the zero air voids con- tent increases with increasing aggregate fineness. • A simple, reasonably accurate equation has been developed based upon permeability data gathered by Choubane et al. in a study on the permeability of Superpave mixtures in Florida (4). According to this model, permeability increases by about 100 × 10-5 cm/s for every 1% increase in air voids or 3% decrease in FM300, for air void contents above the zero- permeability limit. • The permeability of HMA specimens prepared in the laboratory tends to be significantly lower than permeability values measured on field cores of comparable mixtures. For this reason and because of the highly variable nature of permeability measurements, labora- tory measurements of mixture permeability are not recommended for use in routine mix- ture design. However, the effect of air void content and aggregate fineness on permeability should be considered during the mix-design process. • The age hardening of the HMA studied during NCHRP Projects 9-25 and 9-31 depended not only upon air void content, but also upon the specific combination of aggregate and asphalt binder. Additional research is needed to better understand the effect of aggregate/ asphalt binder combinations on mixture age hardening. • A modified version of the Mirza–Witczak global aging system was used to examine the effects of air voids, aggregate fineness and other factors on mixture and binder age hard-

ening (1). For a mean annual air temperature (MAAT) of 15.6°C, the mixture age hard- ening ratio decreased about 2% to 7% for every 1% increase in FM300. The age hardening ratio increased about 5% to 14% for every 1% increase in in-place air voids. Although not extremely large effects, when considered over the possible range for FM300 and field air voids, these factors can significantly affect mixture age hardening. • The modified global aging system predicted extreme amounts of age hardening as indi- cated by binder viscosity. These extreme age hardening ratios are the result of changes in binder rheology that occur during the aging process and could significantly affect mixture performance because of the severe reduction in healing rates that might occur with such large increases in binder viscosity. Additional research is needed to better understand the relationship among age hardening, binder viscosity, healing, and fatigue cracking in HMA pavements. • The various models developed during NCHRP Projects 9-25 and 9-31 suggest that sev- eral indirect relationships exist between apparent film thickness (AFT) and various aspects of HMA performance. The most significant of these is between AFT and rut resistance— as AFT increases, rut resistance decreases. Mixtures with AFT values greater than 9 μm may be prone to excessive rutting. However, because the relationships between AFT and performance are indirect, it is not recommended that AFT be used in specifying or con- trolling HMA mixtures. The results of NCHRP Projects 9-25 and 9-31 suggest that current Superpave require- ments for volumetric design of HMA do not need major revision; however, there appears to be some need for refinements in the system because many highway agencies have recently funded research and engineering projects dealing with both top-down cracking and perme- ability of HMA. It appears that current HMA mixtures tend to be somewhat leaner (lower in asphalt binder content) compared with mixtures designed and placed prior to the implementation of Superpave; this may be a contributing factor to the observed frequency of raveling and surface cracking in Superpave mixtures. Because the Superpave system has encouraged the use of coarse aggregate gradations—below the maximum density gradation—they also contain relatively few fines, which, in combination with relatively high in-place air voids, can result in mixtures with high permeability and less resistance to age hardening. The potentially low fines content, when combined with high VMA values, can also lead to poor rut resistance, although this problem is relatively uncommon in HMA that has been designed using the Superpave system. Many highway agencies have already modified volumetric requirements in the Superpave system, the most common changes being establishing maximum VMA values 1.5% to 2.0% above the minimum values, increasing minimum VMA by 0.5% to 1.0%, and/or a broad- ening of design air void content from 4.0% to a range of 3.0% to 5.0%. Establishing maxi- mum VMA values and eliminating VFA requirements make the Superpave system simpler and more direct and reduce the chances of designing HMA with poor rut resistance. Increas- ing VMA while maintaining design air voids at 4.0% will improve fatigue resistance because this will increase VBE. However, unless care is taken to ensure that adequate aggregate spe- cific surface is maintained while increasing VMA, rut resistance will be reduced when increasing VMA. Increasing aggregate specific surface while increasing minimum VMA will improve both fatigue resistance and rut resistance and will tend to decrease permeability. Changing design air voids in essence has the effect of changing the design compaction level because this changes the amount of compaction energy that will be required in the field to reach the target air void levels. Since most agencies specify minimum VMA rather than min- imum VBE, changing design air voids will also change VBE. Design air void contents below 4.0% reduce the required field compaction effort and will tend to decrease both fatigue 3

4resistance and rut resistance; increasing design air voids to levels above 4.0% has the oppo- site effect—increasing the required field compaction effort and improving both fatigue resistance and rut resistance. The effect on fatigue resistance of changing design air voids at constant in-place voids may surprise some engineers. Decreasing design air voids at constant VMA increases VBE, which normally would increase fatigue resistance; increasing design air voids at constant VMA decreases VBE, which normally would decrease fatigue resistance. However, in this case the effect of changing VBE is less than the effect of changing relative compaction. This illustrates the importance of field compaction on pavement performance and also emphasizes that care is needed when changing requirements in HMA in an attempt to address specific performance issues. Decreasing design air voids to from 4.0% to 3.0% while decreasing the target air voids in the field a similar amount will improve both fatigue resistance and rut resistance while decreasing permeability. Other approaches are possible to improving the fatigue resistance of HMA while maintaining or improving rut resistance and decreasing permeability. Agencies contemplating modification in Superpave specifications should first evaluate the level of in-place air voids being achieved during flexible pavement construction and should verify that acceptable levels of field compaction are being achieved—poor field compaction will have a significant negative impact on pavement performance that can only be partially offset by proper mix design. Any changes in current Superpave requirements should be care- fully evaluated using performance models tempered with engineering judgment and expe- rience with local conditions and materials. Although performance models are useful tools for evaluating the effects of modifications in HMA specifications, they should be used with caution because such models provide only approximate results. Care is also needed when instituting multiple changes in Superpave specifications or in specifications for any other HMA mix type; changes in volumetric requirements, compaction levels, materials specifi- cations, and other mixture characteristics are additive, and unless such changes are carefully evaluated and implemented, significant and unanticipated reductions in pavement per- formance can result. Chapter 3 of this report includes an Extended Work/Validation Plan, which is described at the end of Chapter 3. This plan has been devised to extend the results of this research to mixtures made with larger aggregate sizes (25- and 37.5-mm) and also to validate the results of this research using accelerated pavement testing and other field data. Summary References 1. Mirza, M.W. and M.W. Witczak.“Development of a Global Aging System for Short- and Long-Term Aging of Asphalt Cements,” Journal of the Association of Asphalt Paving Technologists, Vol. 64, 1995, pp. 393–424. 2. Christensen, D.W., and D.A. Anderson. “Interpretation of Dynamic Mechanical Test Data for Paving Grade Asphalt Cements,” Journal of the Association of Asphalt Paving Technologists, Vol. 61, 1992. 3. Christensen, D.W., T. Pellinen and R.F. Bonaquist. “Hirsch Model for Estimating the Modulus of Asphalt Concrete,” Journal of the Association of Asphalt Paving Technologists, Vol. 72, 2003, pp. 97–121. 4. 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.