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22 Because of the strong relationship between VBE and fatigue Permeability and resistance, VBE should be kept constant if design air voids are Age Hardening varied--that is, VMA should be increased or decreased in the same way as design air voids. This approach is, in fact, not at Permeability Tests all new as it is the precise methodology suggested for Mar- As discussed earlier in this report, the permeability tests shall mix designs in the Asphalt Institute's Superpave Mix performed as part of NCHRP Projects 9-25 and 9-31 were Design (SP-2 Manual) (43) where minimum VMA values unfortunately of limited value. This was for two reasons: increase 1% for each 1% increase in design air void content. (1) the air void content of the specimens was relatively low For example, the minimum VMA value for a 9.5-mm NMAS (typically from about 3% to about 7%), which, even in field aggregate blend is 14% for 3% design air voids, 15% for 4% specimens, would result in very low permeability values; and design air voids, and 16% for 5% design air voids. The effec- (2) the permeability of laboratory specimens is often much tive asphalt binder content in each case is 11%. lower than that of field cores. Therefore, the permeability of The last plot in this series is Figure 15, which summarizes most of the specimens fabricated during this research was so the effects of design air voids and in-situ air voids simulta- low as to be impractical or even impossible to measure. This neously. For every 1% increase in in-place air voids, relative does however lead to an important finding: permeability fatigue life decreases by a nearly constant amount of about testing of laboratory-fabricated specimens is usually not 22%. This means that an increase in in-place air voids of 2% effective because the permeability will be much lower than will decrease fatigue resistance by nearly 50%. However, as that of field specimens and will tend to be quite variable. For mentioned above, this probably understates the importance purpose of mix design and mix design selection, it is proba- of in-place air voids to fatigue life because it neglects the bly more practical to rely upon models for estimating per- effect of changes in air voids on permeability and age hard- meability rather than measuring permeability in the ening. This finding can be compared with those of Linden laboratory, which perhaps might show fairly low permeabil- et al. cited earlier (24). Linden et al. cited three analytical ity values for mixtures that might exhibit unacceptably high studies in which a 10% to 30% reduction in fatigue life was permeability in the field. predicted for every 1% increase in in-place air voids Because of the shortcomings of the permeability tests per- (2527). This is in good agreement with the findings of formed during NCHRP Projects 9-25 and 9-31, use has been NCHRP Projects 9-25 and 9-31. However, the rule of thumb made of the substantial permeability data set generated dur- of a 10% overall reduction in performance for every 1% ing the Florida study reported on by Choubane et al. (3). increase in in-place air void content by Linden et al. is some- Properties of this data set are summarized in Table 4. This what lower than the figure found in this analysis, but con- study involved permeability testing of a large number of field sidering the very approximate nature of the research of cores and a limited number of laboratory-fabricated speci- Linden et al., the results should not be considered to con- mens. It should be pointed out that these pavements were tradict the findings of NCHRP Projects 9-25 and 9-31. constructed relatively early during the implementation of Although an in-depth study of the effect of in-place air Superpave and their composition does not reflect that of voids on pavement performance is outside the scope of this Superpave pavements currently being constructed in Florida, research, successful implementation of the results of this which in general now have higher VMA and binder contents. research will depend in part on achieving proper field A number of approaches were used to analyze these data sta- compaction of mixtures designed according to the recom- tistically to develop an accurate and useful function for pre- mendations put forth in this report. dicting the permeability of HMA. It was determined that the In summary, the analysis presented above indicates several most effective approach was to use a relatively simple model important relationships exist between the fatigue resistance in which permeability is proportional to effective air void of HMA mixtures and volumetric composition: content (VTMEff), which in turn is a function of total air void content and aggregate specific surface: At given values for Ndesign, design air voids, and in-place air voids, fatigue resistance increases with increasing VBE. k = 108VTM Eff (10) At a given design values for VBE, design air voids, and in- place air voids, fatigue resistance will increase with increas- where ing values of Ndesign. At given design values for VBE, air void content, and Ndesign, VTM Eff = VTM - V0 and (11) fatigue resistance will increase with decreasing in-place air void content. V0 = 1.53Sa - 1.87. (12)

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23 Table 4. Summary of properties of Florida permeability study data set. Average Property Value Minimum Maximum Total number of tests 113 Total number of field projects 7 Total number of mixtures 13 Aggregate types Alabama limestone, Florida limestone, Georgia granite, RAP Aggregate NMAS and gradation 12.5-mm and 19-mm, all BRZ Binder grade, type PG 67-22, unmodified Estimated aggregate specific surface, m2/kg 4.47 3.57 5.34 Air void content, Vol. % 8.1 3.7 14.6 Voids in mineral aggregate, Vol. % 17.7 13.2 23.7 Effective asphalt binder content, Vol. % 9.6 8.5 10.7 Voids filled with asphalt binder, % 54.9 38.5 73.3 Permeability, 10-5 cm/s 344 5 1014 The r2 for this model, adjusted for degrees of freedom, was prepared in the laboratory. In Figure 16, the permeability for 65%. Although this does not appear to be an extremely strong the laboratory specimens is about 1/6 of the value for similar correlation, the high variability in the permeability measure- field cores. The very low permeability of laboratory prepared ments must be considered when evaluating this model. specimens suggests that testing such specimens is probably Choubane et al. did not evaluate the repeatability of their not useful since it will almost always show very low or zero measurements (3). However, as part of NCHRP Projects 9-25 permeability and when it does not, the results are likely to be and 9-31, an estimate of the standard deviation of these meas- highly variable. Instead, Equations 10 through 12 should be urements was made by grouping specimens from the same used to estimate in-place permeability, based upon aggregate project and same material and having air void contents within specific surface and measured or anticipated in-place air void 1% of each other. Variance values were then estimated for content. In specifying Superpave and other HMA types, rea- each of these groups. An overall average variance was then sonably low levels of permeability should be maintained to calculated, weighted according to the number of specimens help prevent excessive age hardening and to reduce suscepti- in each group. Because very low permeability values (below bility to moisture damage. In order to achieve such control, 50 10-5 cm/s) showed much lower variability than the other aggregate specific surface and in-place air void content must measurements, these were eliminated from the calculation. be simultaneously controlled. As with other aspects of con- The estimated variances for the remaining permeability trolling Superpave volumetric composition, a critical issue measurements appeared to fall into a similar range: the becomes how specifically to exert such control. Florida pooled estimate of the standard deviation using this method researchers have suggested that Superpave surface-course of 150 10-5 cm/s and incorporating 80 different measure- mixtures should exhibit permeability values below 100 10-5 ments. The large number of measurements incorporated into cm/s. For an in-place air void content of 7%, this corresponds this estimate means that it should be quite reliable. to an FM300 value of 26%. However, minimum FM300 values Figure 16 is a plot of measured permeability versus effec- tive air void content for several sets of data. This plot includes 1200 19 mm Low Sa data for the Florida field cores, Florida laboratory specimens, 1000 19 mm High Sa and NCHRP Projects 9-25 and 9-31 laboratory specimens. k x 10 , cm/s 800 12.5 mm Low Sa The plot includes the 95% prediction interval for new obser- 12.5 mm High Sa 600 5 vations for the Florida field cores. The width of this predic- Fit tion interval--about 310 10-5 cm/s--is very nearly equal to 400 95 % PI twice the estimated standard deviation of 150 10-5 cm/s. It 200 therefore appears most of the scatter observed in Figure 16 is 0 probably the result of experimental error rather than of lack 0.0 2.0 4.0 6.0 8.0 10.0 of fit in the model and that the proposed method of estimat- Effective Air Voids, Vol. % ing mixture permeability is significantly more accurate than Figure 16. Permeability of Specimens Tested During indicated by the r2 value of 65%. the Florida Study and During NCHRP Projects 9-25 One of the problems associated with permeability testing and 9-31 as a Function of Effective Air Void Content of HMA is that permeability values measured on field cores (Solid Line Regression Line for Predicted Perme- generally are much higher than comparable specimens ability; Dashed Lines 95% Prediction Interval).

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24 should vary both with air void content and with application-- 2 = average effect for aggregate/binder "2" (Virginia that is, mixtures in protected layers of the pavement can have limestone and PG 76-16); higher permeability values. Specific guidelines for minimum Xi2 = indicator variable for aggregate/binder "2" and FM300 values are given in Chapter 3. = 1 for aggregate/binder "2" and 0 otherwise; 3 = average effect for aggregate/binder "3" (Pennsylva- nia gravel and PG 64-22); Age-Hardening Tests Xi3 = indicator variable for aggregate/binder "3" and As part of NCHRP Projects 9-31 and 9-25, a variety of mix- = 1 for aggregate/binder "3" and 0 otherwise; tures were subjected to long-term oven conditioning and the 4 = average effect for aggregate binder "2" (Kentucky extent of the resulting age hardening was measured using the limestone and PG 64-22); field-shear test to measure the complex modulus before and Xi4 = indicator variable for aggregate/binder "4" and after conditioning. It was expected that the results of this = 1 for aggregate/binder "4" and 0 otherwise; experiment could be related to the permeability of the mix- 5 = average effect for aggregate binder "5" (California tures. The results in part did confirm a relationship between granite and PG 64-22); permeability and age hardening, in that the amount of age Xi5 = indicator variable for aggregate/binder "5" and hardening clearly increased with increasing air voids. How- = 1 for aggregate/binder "5" and 0 otherwise; ever, equally clear was that the extent of age hardening also 6 = average effect for aggregate binder "6" (California depended strongly on the specific aggregate and binder used granite and PG 58-28); in a mixture. The age-hardening data was analyzed using a Xi6 = indicator variable for aggregate/binder "6" and multiple regression model with indicator variables to account = 1 for aggregate/binder "6" and 0 otherwise; for the effects of aggregate/binder combinations and using air 7 = coefficient for effect of air void content (VTMi) on void content as a covariate: age-hardening ratio; and i = error term for ith observation. AHRi = 0 + 1 X i1 + 2 X i 2 + 3 X i 3 + 4 X i 4 The r2 value for this model was 90.5%. All coefficients were +5 X i 5 + 6 X i 6 + 7VTM i + i (13) highly significant, with the exception of the coefficient for the indicator variable for the Virginia limestone/PG 76-16 binder where combination. Figure 17 shows the average effect of different AHRi = age-hardening ratio for ith observation; aggregate/binder combinations on age hardening--that is, on 0 = intercept (average response for aggregate/binder the vertical axis are the values of the constant 0 and the coeffi- "0" [Virginia limestone and PG 64-22 binder]); cients i1 through i5. The differences are significant and cannot 1 = average effect for aggregate/binder "1" (Virginia be easily interpreted in terms of mineralogy or binder grade. limestone and PG 58-28); Figure 18 shows the effect of air void content on age hardening, Xi1 = indicator variable for aggregate/binder "1" and after removing the effect of different aggregate/binder combi- = 1 for aggregate/binder "1" and 0 otherwise; nations. As an example of this adjustment, consider mixtures 1.6 Age Hardening Ratio 1.4 1.2 1.0 0.8 64 64 58 64 58 76 64 G G G G G G G /P /P /P /P /P /P l/ P ne ne te te ne ne ve ni ni to to to to ra ra ra es es es es G G G m m m m PA A A Li Li Li Li C C Y VA VA VA K Figure 17. Average Age Hardening for Various Mixtures Subjected to Long-Term Oven Conditioning, as Calculated from Dynamic Modulus at 25 C and 5 Hz Using the Field Shear Test (Error Bars are for Bonfer- roni Joint 95% Confidence Intervals).

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25 1.4 apparent. This should not be taken as definitive proof that Adjusted A.H. Ratio 1.3 VA Limestone/PG 64 such a relationship does not exist, only that it could not be VA Limestone/PG 58 statistically detected in this particular experiment. 1.2 VA Limestone/PG 76 1.1 PA Gravel/PG64 KY Limestone/PG 64 Effect of Mixture Composition 1.0 CA Granite/PG 64 on Age Hardening 0.9 CA Granite/PG 58 The only model identified in the literature review for esti- 0.8 mating the effect of mixture volumetrics on age hardening is 0.0 2.0 4.0 6.0 8.0 10.0 the global aging system developed by Mirza and Witczak Air Voids, Vol. % (22). Unfortunately, this model makes use of traditional Figure 18. Age-Hardening Ratio after Removing measurements such as penetration, softening-point temper- Aggregate/Binder Effect as a Function of Air ature, and capillary viscosity, which are then converted to Void Content. apparent viscosity values. This makes the model difficult to apply in a meaningful way to the Superpave system. Further- made using the California granite and the PG 64-22 binder. more, the age hardening is predicted only in terms of age- The average age-hardening ratio for all such mixtures was 1.28, hardening ratios and not in terms of binder master curve while the average age-hardening ratio for the Virginia limestone/ parameters, which means that the global aging system is also PG 64-22 binder (the "0" aggregate) was 1.00 (see Figure 17). difficult to apply in developing models and plots to illustrate Therefore, the California granite/PG 64-22 mixtures had an the effect of changes in mixture composition on age harden- average effect on age-hardening ratio of +0.28. To remove this ing. For these reasons, a modification of Mirza and Witczak's effect from the data plotted in Figure 18, 0.28 was subtracted global aging system was developed, which provides results from the observed age-hardening ratios for all California very similar with the original system but makes use of granite/PG 64-22 mixtures. Figure 18 then shows the effect of rational rheological measurements and binder master curve air voids along with all errors. The effect of increased air void parameters. content on age hardening is significant (p < 0.001), but the The modified global aging system was used to analyze effect of different aggregate/binder combinations appears to be several hypothetical situations to evaluate the effect of air stronger than the effect of air void content. It can be concluded voids and aggregate specific surface on age hardening. Age- that control of air voids in HMA can only partially control the hardening ratios were predicted at an age of 60 months for extent of age hardening in flexible pavements. This might MAAT values of 7.2 C, 15.6 C, and 23.9 C. In-place air mean, for example, that surface cracking in some mixtures void contents were assumed to be 5%, 7%, and 9%, while might be the result of a particular combination of aggregate assumed values for FM300 were 20, 30, and 40. Age-hardening and asphalt binder being especially susceptible to age harden- ratios were calculated for both mixture complex modulus ing and not necessarily the result of an inappropriate mix (|E*|) at 10 Hz and binder steady-state viscosity at temper- design or poor construction. Additional research is needed to atures of 0, 25, 40 and 60 C. The analysis was performed better understand the relationship between aggregate mineral- for the PG 58-28 and PG 76-16 binders used in various ogy, asphalt-binder chemistry, and age hardening of HMA. In other parts of NCHRP Projects 9-25 and 9-31. The age study of mixtures prone to surface cracking, evaluation of the hardening for binder viscosity was estimated because high age-hardening resistance of specific aggregate/binder combi- binder viscosities could contribute significantly to pave- nations should be considered along with other tests. ment distress by preventing healing of surface cracks dur- Some additional comments on Figures 17 and 18 are war- ing hot weather. Two examples of this analysis are shown in ranted. It appears from examining this plot that the amount Figures 19 and 20. Figure 19 shows mixture age hardening of age hardening increases more rapidly at higher air void at 25 C and 10 Hz for the PG 58-28 binder for a MAAT of contents than at lower. However, the amount of variability 15.6 C. Figure 20 shows binder age hardening at 40 C, also makes such a hypothesis difficult to evaluate with certainty. for a MAAT of 15.6 C. Estimated age-hardening ratios, as Increased age hardening at air void contents above 4% is con- should be expected, increase dramatically with increasing sistent with the concept of effective air voids--that is, that MAAT. The mixture age-hardening ratios were generally permeability of HMA is effectively zero below a certain air highest for the PG 58-28 binder at "test" temperatures of void content, which varies from mixture to mixture. An 25 C and/or 40 C; age-hardening ratios for binder viscos- attempt was made to relate the age hardening of the different ity decrease with increasing "test" temperature. Also, the mixtures to the estimated zero permeability air voids content age-hardening ratios for the PG 58-28 binder were always (related to aggregate fineness), but no such relationship was higher than for the PG 76-16 binder. Several important,