Volumetric Requirements for Superpave Mix Design (2006) / Chapter Skim
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Pages 29-41
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From page 29... ...
estimates to be made regarding how changes in the composition requirements of HMA might affect rut resistance. The following factors tend to improve the fatigue resistance of Superpave and other HMA mix types: • Increasing effective asphalt binder content, at given levels of Ndesign, design air voids, and in-place air voids; • Increasing Ndesign, at given levels of VBE, design air voids, and in-place air voids; and • Decreasing in-place air voids, at given levels of design voids, VBE, and Ndesign.
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From page 30... ...
This is most likely because increasing air voids will cause an increase in permeability, in turn causing an increase in age hardening. Although not observed in the testing and analysis performed as part of NCHRP Projects 9-25 and 9-31, it is likely that increasing aggregate specific surface will also reduce age hardening because this would decrease mixture permeability.
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From page 31... ...
The significant difference in the relative performance of the two sets of Superpave mixes is due to three factors -- the NCAT mixes had substantially higher values for aggregate specific surface, were compacted much better during construction, and had higher effective binder contents. As should be expected, the relative performance of the SMA mixes is very good to excellent.
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From page 32... ...
However, if design air voids are reduced at constant VMA, VBE will increase, which will tend to improve fatigue resistance, somewhat offsetting the effect of the reduced relative density. Increasing design air voids at constant VMA will decrease VBE, decreasing fatigue resistance and, again, somewhat offsetting the effect of increased as-constructed relative density.
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From page 33... ...
However, if design air voids are increased to 5%, the maximum allowable VMA becomes 20%; if design air voids decrease to 3%, the maximum VMA becomes 12% -- lower than the minimum VMA of 14% for this aggregate size. Agencies that alter design air void levels in the Superpave system must either adjust VFA requirements to establish reasonable VMA limits, or they should eliminate VFA requirements and rely on explicit maximum VMA limits.
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From page 34... ...
Given the recent concern over top-down cracking and high permeability in mixtures designed using the Superpave system, this modification would appear to be an attempt to improve durability of HMA pavements. Increasing VMA will improve fatigue resistance, but applying the models developed during NCHRP Projects 9-25 and 9-31, this improvement is probably not as large as what many engineers might suppose -- only about 17% improvement for a 1% increase in VMA (assuming constant design air voids)
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From page 35... ...
This approach allows for a very low permeability level for mixtures with small NMAS, with gradually increasing permeability 35 Control Method Control Level Average InPlace Air Voids Vol. % Property Minimum Aggregate Specific Surface m 2/kg Minimum Percent Passing 75 μm Min.
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From page 36... ...
An advantage of this approach is that it provides for mixtures with very low permeability while maintaining an overall moderate level of control. Another advantage is that this method of controlling aggregate specific surface tends to provide similar levels of resistivity regardless of aggregate NMAS.
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From page 37... ...
2. Reduce maximum allowable in-place air voids by an amount equal to the decrease in design air voids; also, review field compaction requirements to ensure that desired level of in-place air voids will in fact be achieved.
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From page 38... ...
This is why it is best to try to mimic aggregate gradations as they come out of the plant, rather than to make adjustments in going from a laboratory mix design to a production job mix formula. Lowering Ndesign to Improve HMA Durability Some engineers may suggest that simply lowering Ndesign will provide significant improvement in durability, believing that this will increase design binder content and improve field compaction, resulting in improved fatigue resistance and lowered permeability.
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From page 39... ...
Imagine a third alternative modification, Approach 3, in which design air voids are lowered to 3% to improve fatigue resistance. At the same time, Ndesign is reduced to 50 in order to try to improve compaction, but no effort is made to make specifications for field compaction more stringent so that in-place air voids remain at 8.0%.
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From page 40... ...
Of particular significance to fatigue and rut resistance is the in-place air void content relative to the design air void content: the lower in-place air voids relative to design air voids, the higher the fatigue and rut resistance of the pavement. • There is significant evidence that the implementation of the Superpave system has resulted in an increase in the permeability and a decrease in the fatigue resistance of HMA pavements.
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From page 41... ...
Approximately one-half of the test sections should represent a variety of mixtures prepared according to the procedures given in the NCHRP Projects' 9-25 and 9-31 final report; the balance should represent mixtures made according to current Superpave specifications, but in such a way that significant contrasts are made with the NCHRP Projects' 9-25 and 9-31 mixture designs. Of particular interest are contrasts in effective binder content, VMA, aggregate fineness relative to VMA, and high temperature binder grade.
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