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32 Further insight into potential improvements in the volu- are allowing a design air voids range of from 3.0% to 5.0%, metric requirements for Superpave can be gained by examin- rather than using a single target value of 4.0%, and establish- ing the performance history of Superpave mixes. In general, the ing a maximum VMA 1.5% or 2.0% above minimum require- rut resistance of Superpave mixes has been very good to excel- ments. A number of states have increased minimum VMA lent. The notable exception was the extreme rutting observed requirements by 0.5% to 1.0%. An optional modification to in most of the mixes placed at WesTrack. The resistivity/rutting original Superpave requirements that already is included in model accurately predicts that these mixes should be prone to AASHTO specifications is an increase in the dust-to-binder rutting because of high VMA, low specific surface, marginal ratio from 0.61.2 to 0.81.6. The methods used above can be compaction, and a binder that would perhaps be adequate in a applied to analyze the potential effects on HMA performance normal pavement but was marginal in an accelerated loading of these modifications to Superpave requirements. environment because there was little opportunity for long- term age hardening to occur within the pavement surface. The Changing Design Air Voids most important lesson of WesTrack is that the various factors that affect pavement performance are additive; individual fac- There are two different ways in which design air void con- tors that may not result in serious degradation of performance tent can be changed: (1) at constant VMA and (2) at constant can cause premature failure if a number of them acts simulta- VBE. If design air void content is changed at constant VMA, neously. This should be kept in mind in any attempts to mod- then VBE must change an amount equal in magnitude but ify current requirements for volumetric composition. opposite in sense from the change in design air voids. For Many state highway agencies have become concerned over example, if the design air void content for a 9.5-mm NMAS the durability of Superpave mixes because of high levels of mixture is changed to 3% from 4% and no changes are made permeability and an apparent increase in the incidence of in VMA, then the minimum VBE content will be increased top-down cracking; this is evidenced by the recent large num- from 11% to 12%. If design air void content is changed at ber of research projects dealing with these topics (3, 5054). constant VBE, then VMA will change along with changes in The analysis above offers a clear explanation for the recent air void content--if design air voids are decreased 1%, then increase in HMA permeability--the decreased aggregate spe- VMA will decrease 1%, and vice versa cific surface and the increased difficulty of field compaction Changing design air voids can affect HMA performance can both act together to substantially increase the permeabil- through two mechanisms: (1) the change in design air voids ity of mixes designed using the Superpave system. Explaining relative to in-place air voids will effect relative compaction; the increase in top-down cracking is not as easy. Certainly sig- and (2) changes in design air voids will change either VBE or nificant increases in the permeability of surface-course mix- VMA, depending on the manner in which the change is tures would contribute to top-down cracking by increasing accomplished. Reducing design air voids will decrease as con- age hardening and by decreasing resistance to moisture dam- structed relative density if in-place air voids are assumed age at the pavement surface. Poor compaction would also constant--this will tend to reduce both rut resistance and tend to produce a relatively weak pavement surface, prone to fatigue resistance. Similarly, increasing design air voids will cracking under repeated loading. The lower asphalt binder increase as constructed relative density, improving rut resist- contents that have been used under the Superpave system ance and fatigue resistance. However, if design air voids are would also contribute to top-down cracking by reducing the reduced at constant VMA, VBE will increase, which will tend fatigue resistance of the HMA. However, it is not clear if these to improve fatigue resistance, somewhat offsetting the effect are the primary factors that are increasing the extent of sur- of the reduced relative density. Increasing design air voids at face cracking in HMA pavements. Other factors that could constant VMA will decrease VBE, decreasing fatigue resist- contribute to top-down cracking include increased tire-pave- ance and, again, somewhat offsetting the effect of increased ment stresses, changes in asphalt binder chemistry and flow as-constructed relative density. The net effects on perfor- properties, and a general increase in pavement modulus mance for changes in design air voids are summarized in brought about by the use of stiffer asphalt binders (50). Table 6. These estimates are based on an HMA mix with a design air void content of 4%, a VMA of 14%, a VBE of 10%, and an Ndesign of 75 gyrations. The in-place air void content is Possible Revisions in Volumetric assumed to be 7%. At constant VMA, reducing design air Requirements for Superpave voids from 4% to 3% decreases rut resistance about 18% and Mixtures and Their Effect fatigue resistance about 9%. Increasing design air voids from on Performance 4% to 5% improves rut resistance about 22% and fatigue In the Survey of State Practice, it was found that a number resistance about 8%. of highway agencies have modified volumetric requirements When air voids are changed at constant VBE, a reduction in for Superpave mixes. The most common such modifications design air void content from 4% to 3% increase rut resistance

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33 Table 6. Effect of change in design air Table 7. Minimum and maximum VMA void content on HMA performance. values for mixes designed using the Superpave system at 4% design air Design Change in Change in voids and for traffic levels of 3 million Air Void Rut Fatigue Approach Content Resistance Resistance ESALs and higher (AASHTO M323-04). 3% 18% 9% Constant Aggregate Minimum Maximum VMA 4% -- -- VMA NMAS VMA VMA Range 5% +22% +8% 3% +5% 22% Mm % % % Constant 19.0 13.0 16.0 3.0 4% -- -- VBE 12.5 14.0 16.0 2.0 5% 3% +29% 9.5 15.0 16.7 1.7 4.75 16.0 18.2 2.2 a small amount (5%) while fatigue resistance is decreased by about 22%. An increase in design air void content from 4% to ance. However, it is extremely important to realize that if 5% has almost no effect on rut resistance, but increases fatigue design air void levels are allowed to change, and no other resistance by about 29%. changes are made in volumetric requirements, the indirectly When contemplating increases in design air void content, specified VMA values inherent in the Superpave system can engineers should consider that this will increase the difficulty change dramatically. For example, for a 12.5-mm mixture for of field compaction, which in some cases might increase in- heavy traffic, the maximum allowable VFA is currently 75%. place air voids. This would reduce or eliminate all of the At 4% design air voids, this translates to 4/[1-(75/100)] = 16% potential benefits of a higher design air void level and would maximum VMA. However, if design air voids are increased to also increase the permeability of the pavement surface, ren- 5%, the maximum allowable VMA becomes 20%; if design air dering it more susceptible to age hardening and moisture voids decrease to 3%, the maximum VMA becomes 12%--lower damage. On the other hand, even though decreasing design than the minimum VMA of 14% for this aggregate size. Agen- air void content to 3% reduces performance when in-place air cies that alter design air void levels in the Superpave system void contents are held constant as discussed in Chapter 2, the must either adjust VFA requirements to establish reasonable effect on performance would be negligible if the reduction in VMA limits, or they should eliminate VFA requirements and design air voids were to be accompanied with a similar rely on explicit maximum VMA limits. decrease in allowable in-place air void contents. This might Indirect maximum VMA values are higher at design traffic be an effective approach to reducing permeability of surface- levels below 3 million ESALs because maximum VFA limits course mixtures for some agencies. increase to 78% and 80%, depending on the traffic level. At design traffic levels below 0.3 million ESALs, the maximum VFA of 80% results in a maximum allowable VMA of 20% for Establishing Maximum Limits for VMA all aggregate sizes. At design traffic levels of from 0.3 million A number of state highway agencies have established max- ESALs, the maximum VFA is 78%, resulting in maximum imum limits for VMA, generally either 1.5% or 2.0% above the VMA of 18.2%. Some of the complexity and confusion inher- minimum VMA for a given aggregate size. AASHTO M323-04 ent in this system could be avoided by eliminating VFA includes a note warning that mixtures made with VMA values requirements and relying solely on VMA and air voids to con- more than 2.0% above the specified minimums might be trol this aspect of mixture composition. As discussed previ- prone to rutting and flushing. For surface-course mixtures ously, this has an advantage in that it is effective even when designed for higher traffic levels (3 million ESALs and above), design air void levels vary. A reasonable scheme would be to capping VMA at 2.0% above current minimum values is allow maximum VMA to vary according to design traffic largely a matter of practicality and does not have a significant level: effect on performance. The reason for this is that current Superpave requirements include indirect limits on VMA 0.3 million ESALs: maximum VMA of 4.0% above min- that result from the interaction of design air void requirements imum value; and maximum values for VFA. Table 7 lists the current, indi- 0.3 to 3 million ESALs: maximum VMA of 3.0% above rect limits on VMA as given in AASHTO M323-04. The max- minimum value; and imum VMA values range from 1.7% to 3.0% above the 3 million ESALs: maximum VMA of 2.0% above mini- minimum values. For mixtures made using 12.5-mm NMAS mum value. and smaller, the maximum values are 1.7% to 2.2% above the minimum. For most surface-course mixtures designed at high Other similar schemes are possible. Establishing explicit traffic levels, establishing explicit maximum VMA values has limits on VMA is simpler than the current system and little effect on allowable ranges for VMA and HMA perform- involves less possibility of misinterpretation. It is also more

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34 flexible, but maintains a similar degree of control over VMA, NCHRP Projects 9-25 and 9-31 suggest that including an ade- VFA, and related factors. quate amount of fines--mineral filler and material in the 75- to 300-m-size range--is one of the most important fac- tors affecting HMA performance. It should be emphasized Increasing Minimum VMA Limits that a general increase in the aggregate specific surface for all A number of agencies have increased minimum VMA lim- mixtures does not appear to be needed--what is needed is an its by 0.5% to 1.0%. Given the recent concern over top-down increase in the minimum requirements for aggregate fineness cracking and high permeability in mixtures designed using in order to avoid designing and producing HMA mixtures the Superpave system, this modification would appear to be deficient in fines. Such mixtures will potentially exhibit poor an attempt to improve durability of HMA pavements. rut resistance and high permeability, which will in turn Increasing VMA will improve fatigue resistance, but applying increase their susceptibility to age hardening and moisture the models developed during NCHRP Projects 9-25 and 9-31, damage. this improvement is probably not as large as what many engi- There are three possible specification modifications related neers might suppose--only about 17% improvement for a to this aspect of HMA composition. First, the dust-to-binder 1% increase in VMA (assuming constant design air voids). ratio could be increased either to the currently optional range Furthermore, unless aggregate specific surface is increased of 0.8 to 1.6 or to some other value. Second, the requirements along with minimum VMA, there is a risk that rut resistance for dust-to-binder ratio could be replaced with requirements will be decreased. This decrease could be as much as 19% or for minimum FM300 as a function of VMA or aggregate roughly equal to the gain in fatigue resistance. To avoid a NMAS. Although this approach may at first seem somewhat reduction in rut resistance, agencies should consider increas- more complicated than the current system, dust-to-binder ing the minimum dust-to-binder ratio when increasing min- ratios are generally calculated on the basis of effective asphalt imum VMA requirements. content, the determination of which is not simple. Further- Although only a 17% improvement in fatigue resistance is more, controlling FM300 as a function of VMA or aggregate predicted for an increase of 1% in VMA, other factors may be NMAS would be a somewhat more effective approach since at work that will improve the durability of HMA made with FM300 is a better indicator of aggregate specific surface com- higher VMA and binder contents. An important issue is the pared with the percent passing 75 m alone. A third approach ease of compaction of the mixture. It is possible that higher would be to eliminate the requirements for dust-to-binder VMA and binder contents might improve field compaction ratio and increase the requirements for percent finer than for HMA, which would substantially improve durability 75 m in the aggregate gradation specifications. This above and beyond the improvement resulting from increased approach is the simplest of all and probably is just as effective binder content alone. It is also possible that alternate cycles of as controlling dust-to-binder ratio. fatigue damage and healing could mean that relatively small Before these alternate approaches are evaluated, it is useful improvements in fatigue resistance might significantly to try to determine some rational limits for aggregate specific improve field performance. The fact that top-down cracking surface. With the exception of the WesTrack mixtures, there is in HMA pavements has recently been observed to increase at little evidence that mixtures designed according to the Super- the same time VMA requirements have been reduced sup- pave system are prone to rutting. On the other hand, recent ports the use of increased VMA to improve HMA durability. research suggests that there is significant concern about the Further research is needed to better understand the relation- relatively high permeability of HMA mixtures being placed in ship between HMA mixture characteristics, binder rheology, North America. Furthermore, increasing minimum require- laboratory fatigue test data, and fatigue performance in situ. ments for aggregate specific surface can only improve rut resistance because of its beneficial effect on resistivity. There- fore, permeability requirements will control any modification Increasing Dust-to-Binder Ratio to the dust-to-binder ratio requirements. Control of in-place and Related Modifications permeability is relatively straightforward since permeability AASHTO M323-04 already includes an option for an is mostly a function of in-place air voids and aggregate spe- increase in dust-to-binder ratio from 0.612 to 0.81.6 for cific surface (Equations 1012). If in-place air voids are mixtures made using coarse aggregate gradations. It is not assumed constant, at an average value of 7.0%, permeability clear why this requirement should only be applied to such then becomes only a function of aggregate specific surface, mixtures although in most cases, fine and dense gradations which in turn can be estimated from either percent finer than would probably meet this stricter requirement anyway, so 75 m or FM300 (see Figures 1 and 2). enforcing this increased dust-to-binder ratio for such mix- A complicating factor in this analysis is the variability in tures might be redundant. In any case, the findings of the relationships between the percent finer than 75 m and

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35 FM300 and specific surface (referring again to Figures 1 and 2). Equations 1012. Because Choubane et al. recommended The relationship between FM300 and specific surface is signif- lowering target in-place air voids from 7.0% to 6.0%, both of icantly better than that between the percent finer than 75 m these values were used in the analysis. Using this analysis, the and specific surface, and neither is close to perfect. To account more effective a given approach, the lower will be the average for variability in these relationships, the following approach and maximum values for permeability. The results are sum- was used in analyzing the effectiveness of different means of marized in Table 8. With no control, the average permeability controlling aggregate specific surface and permeability. The for the data set is 100 cm/s and the maximum value is 510 data set used was the same as that plotted in Figures 1 and 2 cm/s. The high degree of control would most likely be con- and included data from eight projects: NCHRP Project 9-9, sidered too restrictive to be practical: the lowest level of con- the NCAT Test Track, Pooled Fund Study 176, the Florida per- trol appears to yield only modest decreases in permeability at meability study, MN/Road, FHWA's ALF rutting study, Wes- 7% in-place air voids, although the reduction at 6% in-place Track, and data from NCHRP Projects 9-25 and 9-31 (2, 3, 5, air voids is significant. It would appear that to gain significant 6, 2931). Four approaches to controlling specific surface reductions in in-place permeability, either the moderate level were initially considered: (1) direct control of specific surface of control is needed while maintaining current levels of in- (minimum value for calculated Sa), (2) minimum value for place air voids, or the low level of control can be applied along percent finer than 75 m, (3) minimum value for FM300, and with a 1% reduction in target in-place air voids. (4) minimum dust-to-binder ratio (by weight, calculated Rather than control permeability at a constant level, an using effective binder content). It was decided (somewhat alternative approach is to try to control permeability as a arbitrarily) that three levels of control would be examined, function of aggregate NMAS. For example, FM300 limits can corresponding to rejection rates of about 30%, 20%, and be established as a function of aggregate NMAS; an example 10%. "Rejection rate" in this case means the percentage of of this type of control is given in Table 9. In this case, maxi- mixtures that fail to meet the given criteria. The minimum mum FM300 values range from 19 for 19.0-mm NMAS to values for the various control parameters were varied until 25 for 9.5-mm NMAS. The rejection rate for this scheme using the rejection rate matched the target value as close as possi- the same data set used in the previous analysis was 15%. This ble, then, the average permeability and maximum permeabil- approach allows for a very low permeability level for mixtures ity for the mixes meeting this criteria were estimated using with small NMAS, with gradually increasing permeability Table 8. Effectiveness of various approaches to controlling HMA permeability and aggregate specific surface, 7% in-place air voids. Average Control Method In- Minimum Minimum Min. Place Aggregate Percent Dust/ Control Air Specific Passing Min. Binder Level Voids Property Surface 75 m FM300 Ratio Vol. % m2/kg Avg. k 105, cm/s 100 7.0 Max. k 105, cm/s 510 None Avg. k 105, cm/s 50 6.0 Max. k 105, cm/s 400 Min. Value 4.5 4.5 24 1.1 N/A % Failing Spec. 28 28 28 27 Avg. k 105, cm/s 30 40 30 50 High 7.0 Max. k 105, cm/s 170 250 230 320 Avg. k 105, cm/s 4 10 10 20 6.0 Max. k 105, cm/s 60 140 120 210 Min. Value 4.4 4.3 22 1.0 N/A % Failing Spec. 19 21 20 17 Avg. k 105, cm/s 50 50 50 70 Moderate 7.0 Max. k 105, cm/s 230 320 270 350 Avg. k 105, cm/s 20 20 20 30 6.0 Max. k 105, cm/s 120 210 160 240 Min. Value 4.0 4.0 20 0.9 N/A % Failing Spec. 9 5 12 8 Avg. k 105, cm/s 70 80 60 80 Low 7.0 Max. k 105, cm/s 290 370 280 370 Avg. k 105, cm/s 30 40 30 40 6.0 Max. k 105, cm/s 180 260 170 260