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36 Table 9. Control of permeability by limiting Table 11. Control of permeability as a FM300 as a function of NMAS. function of NMAS by limiting dust-to- binder ratio to 1.0. Permeability◊105 (cm/s) at Average In-Place Air Voids Permeability◊105 (cm/s) at Average Min. 7.0 % 6.0 % In-Place Air Voids NMAS FM300 Avg. Max. Avg. Max. 7.0 % 6.0 % 9.5 mm 25 3 20 0 0 NMAS Avg. Max. Avg. Max. 12.5 mm 22 40 250 20 150 9.5 mm 30 230 10 120 19.0 mm 19 110 290 50 180 12.5 mm 40 320 20 210 Overall: 60 290 20 180 19.0 mm 130 350 60 240 Overall: 70 350 30 240 levels with increasing NMAS. This is consistent with the vari- ation of VMA and VBE with NMAS--this approach is con- ability with NMAS since as NMAS decreases, VMA and VBE sistent with the general trend of increasing fatigue resistance will increase, increasing the amount of mineral filler that and durability with decreasing NMAS. An advantage of this must be used to obtain the required minimum dust-to- approach is that it provides for mixtures with very low per- binder ratio. Table 11 summarizes the control of permeabil- meability while maintaining an overall moderate level of con- ity as a function of aggregate NMAS that results from setting trol. Another advantage is that this method of controlling a minimum dust-to-binder ratio of 1.0. This approach is aggregate specific surface tends to provide similar levels of moderately restrictive and is, in fact, identical to the resistivity regardless of aggregate NMAS. Since specific sur- dust/binder/moderate control level summarized in Table 8-- face increases with increasing NMAS, it will tend to increase the permeability values have now simply been broken down with increasing VMA. A related approach would be to control by aggregate NMAS. This approach appears to be similar to FM300 directly as a function of VMA. An example of this type that given by linking FM300 to VMA. The main advantage of of control is given in Table 10. In this case, FM300 limits were this approach is simplicity and that it is consistent with the calculated to give the same values for resistivity regardless of current Superpave system. As with linking FM300 to aggregate VMA; the FM300 values were calculated using the formula: NMAS, this method provides some control over resistivity, but not as good as does linking FM300 to VMA. FM 300 = ( 0.15 ◊ VMA 3 ) 0.5 (14) A few comments are needed concerning the analysis pre- sented above. Although the data set used is relatively large and The resulting limits are listed at the top of Table 10. This robust, the results should be considered as only guidelines. particular example is slightly more restrictive than that given Further analysis with additional data is needed to provide in Table 9, with a rejection rate of 19%. It provides slightly less more confidence in the specific degree of control exerted by control over permeability compared with the previous exam- the various approaches. A large number of approaches were ple, but has the advantage of very good control over resistivity presented here because it is not clear at this time which since specific surface is linked directly to VMA. This approach approach will be most effective and efficient for the largest would however be slightly more difficult to implement. number of users over the widest range of situations--this is a Although controlling dust-to-binder ratio was listed in decision that will be made during implementation. Further- Table 8 with several other approaches that tend to provide more, in some areas the aggregates locally available may be similar levels of specific surface and permeability regardless deficient in fines, and the cost of obtaining additional fines of aggregate NMAS, this approach does in fact tend to result may be prohibitive. Such situations require flexibility and in some variation in aggregate specific surface and perme- judgment when developing approaches for controlling aggre- gate specific surface and mixture permeability. Table 10. Control of permeability as a function of NMAS by limiting FM300 as a function of VMA. General Approaches to Improving VMA, Vol. % 11 12 13 14 15 16 17 the Durability of Mixtures Designed Min. FM300 14 16 18 20 23 25 27 According to the Superpave System Permeability◊105 (cm/s) at Average and Other HMA Mix Types In-Place Air Voids 7.0 % 6.0 % As discussed previously, there is substantial evidence that NMAS Avg. Max. Avg. Max. mixtures designed according to the Superpave system are 9.5 mm 10 160 4 50 12.5 mm 50 320 20 210 more permeable and somewhat more prone to top-down 19.0 mm 110 290 50 180 cracking compared with HMA that is designed and placed Overall: 60 320 30 210 using the Marshall system. There is therefore a desire within
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37 some highway agencies to improve the durability of HMA been implemented by highway agencies in attempts to designed using Superpave methods, while still maintaining improve the performance of HMA designed using the Super- the excellent rut resistance that these materials have exhibited. pave system: Before discussing how this goal might be achieved, it must be noted that much evidence suggests it is not advisable to make ∑ Approach 1: large changes in the current requirements for HMA designed 1. Increase minimum VMA from 0.5% to 1.0% while main- using the Superpave system--implementing drastic changes taining design air voids at 4.0%; this produces an increase may have the effect of significantly decreasing rut resistance in minimum VBE of 0.5% to 1.0%. of HMA mixtures or causing other unforeseen problems. 2. Apply optional increased dust-to-binder ratio of 0.8 to 1.6 Furthermore, the differences in HMA composition between or even further to 1.0 to 2.0. Alternately, one of the other materials designed under the Superpave system and materi- methods presented earlier for controlling aggregate spe- als designed using the traditional Marshall system, although cific surface can be used. significant, are not so large to suggest that complete revision 3. Review field compaction requirements to ensure that in- of the Superpave system is needed. Changes to the Superpave place air voids are sufficiently low to provide for low per- system implemented by various highway agencies support the meability and overall good performance. advisability of measured changes in current specifications. Therefore, any modifications to current requirements in the ∑ Approach 2: Superpave system should be kept relatively minor. 1. Maintain current minimum VMA values while decreasing Based upon the findings given in Chapter 2 and the dis- design air voids to 3.0% to 3.5%; this produces an increase cussion presented above, there are four critical aspects to in minimum VBE of 0.5% to 1.0%. improving HMA durability while maintaining good rut 2. Reduce maximum allowable in-place air voids by an resistance: amount equal to the decrease in design air voids; also, review field compaction requirements to ensure that 1. Effective binder content should be increased to provide desired level of in-place air voids will in fact be achieved. better fatigue resistance. 3. Consider applying optional increased dust-to-binder ratio 2. Aggregate fineness should be increased to decrease mix- of 0.8 to 1.6. Alternately, one of the other methods pre- ture permeability. sented earlier for controlling aggregate specific surface can 3. Design air voids can be decreased to improve compaction-- be used. However, reducing in-place air void requirements lowering in-place air voids and decreasing permeability-- should reduce the need to increase minimum require- but unless in-place air voids are in fact significantly ments for aggregate specific surface since mixture perme- decreased, both rut resistance and fatigue resistance will ability will be significantly lower because of the improved decrease if design air void content is reduced. field compaction. 4. Requirements for in-place air voids can be decreased, improving both rut resistance and fatigue resistance. The resulting improvements in performance for these two approaches, as estimated using the various models presented These aspects to improving HMA durability can be com- in this report, are summarized in Table 12. This example is for bined in a number of reasonable ways. Listed below are two a 12.5-mm NMAS design, with Ndesign = 75. The "current promising approaches, some aspects of which have already HMA" design assumes a VMA of 14.0% and a design air void Table 12. Relative performance of 12.5-mm NMAS mix designs modified using different approaches. Current 12.5-mm Characteristic HMA Approach 1 Approach 2 Composition Ndesign 75 75 75 VMA, Vol. % 14.0 15.0 14.0 VTMdesign, Vol. % 4.0 4.0 3.0 VTMin-place, Vol. % 8.0 7.0 6.0 Dust/binder 0.6 0.8 0.6 Sa, m2/kg 3.8 4.6 3.9 Estimated Performance Relative Rut Resistance, % 100 150 140 Relative Fatigue Resistance, % 100 150 150 Permeability ◊ 105, cm/s 440 200 200
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38 content of 4.0, resulting in a VBE of 10.0%. The dust-to- tory mix designs should be carefully considered when modi- binder ratio is 0.6, corresponding to the current minimum fying requirements for aggregate fineness. For example, the value for the standard range for this characteristic in the data set used in the analyses of aggregate specific surface pre- Superpave system. Field air voids are assumed to be 8.0%, sented earlier in Chapter 3 included a total of 94 points. Of slightly above the standard assumed field air void content of these, only 1 point had a dust-to-binder ratio below 1.60, and 7.0%. The amount of aggregate finer than the 75-m sieve only 3 had a dust-to-binder ratio below 0.80. On the other was estimated from the mix composition and the dust-to- hand, 42 mixtures had dust-to-binder ratios above 1.20, and binder ratio and the aggregate specific surface then estimated 19 had values above 1.60. Although in some cases the mixes using the relationship shown earlier in Figure 2. For may have been purposely designed with gradations outside Approach 1, the VMA was increased 1.0% to 15.0% while Superpave limits, much of this discrepancy between the maintaining the design air voids at 4.0%, resulting in an observed dust-to-binder ratios and current specification lim- increase in VBE to 11.0%. The dust-to-binder ratio was its is probably due to an increase in fines during batching, increased to 0.8, corresponding to the minimum value for the mixing, transport, and placement. The best approach to deal- optional, higher range for this characteristic. It is assumed ing with this problem would be to try to obtain information that a review of field compaction data has shown that field air concerning the changes that occur in aggregate gradation voids are not as low as desired, and that modification in within a specific plant during production and to adjust stock- acceptance plans and enforcement result in achieving the pile aggregates to try to mimic these changes in laboratory desired level of 7.0% air voids in-place. Approach 2 keeps the mix designs. Alternately, requirements for dust-to-binder VMA at 14.0% while decreasing design air voids to 3.0%, ratio (or FM300 or percent finer than 75 m) could be relaxed increasing VBE to 11.0%. The dust-to-binder ratio is kept at somewhat during the mix design process: dust-to-binder 0.6. It is assumed that field compaction is significantly ratio could be set at 1.0 for production purposes, but allowed improved by revising acceptance plans, resulting in an aver- to go to 0.80 during the mix design process. It should how- age in-place air void content of 6.0%. The resulting improve- ever be understood that the increase in fine material that ments in estimated performance are significant. Both rut occurs in actual plant production will cause other changes in resistance and fatigue resistance improve by 40% to 50% for HMA characteristics--typically, air void content and VMA both modifications, while permeability is roughly cut in half. will decrease. This is why it is best to try to mimic aggregate These approaches are only meant as general examples as to gradations as they come out of the plant, rather than to make the type and magnitude of modifications that might prove adjustments in going from a laboratory mix design to a pro- successful in improving the durability of HMA designed duction job mix formula. according to the Superpave system while maintaining good rut resistance. Other approaches are possible and will be Lowering Ndesign to Improve HMA Durability effective if proper consideration is given to how each specifi- cation change will affect various aspects of performance. Some engineers may suggest that simply lowering Ndesign Highway agencies must consider their local climate, traffic will provide significant improvement in durability, believing levels, and materials characteristics when attempting to mod- that this will increase design binder content and improve field ify requirements for HMA. Furthermore, although evaluating compaction, resulting in improved fatigue resistance and specification changes with performance models is a useful lowered permeability. However, lowering Ndesign will not nec- tool, engineers should note that existing performance mod- essarily increase design binder content--in this situation, els (including the ones developed as part of this research) pro- many producers will adjust their aggregate gradation so that vide only approximate results and should be used with the design binder content remains as low as possible since this discretion. will minimize the cost of the HMA and maximize profits. When making adjustments in the requirements for aggre- Paying for asphalt binder as a separate item removes the gate fineness--be it through dust-to-binder ratio, FM300, per- incentive to minimize binder content, but in no way guaran- cent finer than 75 m, or some other means--it should be tees that binder contents will be sufficient for good fatigue kept in mind that the analyses presented here were based on resistance. If an agency believes that current minimum binder aggregate gradations from QC data--that is, these were contents are too low for adequate fatigue resistance and/or aggregates that had gone through most or all of the batching, durability, the most effective and efficient remedy is simply to mixing, and transport processes. The amount of fine mate- increase these minimum values. A similar situation exists for rial, and the specific surface, and related parameters will field compaction. Lowering Ndesign values will tend to make therefore be somewhat higher than for aggregates taken from HMA mixtures easier to compact, but will not guarantee that stockpiles without going through an HMA plant. This in-place air voids will decrease. Assuming most successful increase in fines during production compared with labora- contractors are motivated not by maximizing losses but by