Comparison of 9.5 mm SuperPave and Marshall Wearing I Mixes in ...

32theoretically correct in that the farther the gradation curve is kept from the maximum densityline, the more voids that would be in the mix.Twenty-four laboratory samples were also made up for this project to study the effects ofgradation, quantity of material passing the #200 sieve and the FAA value of the fine aggregate.The researchers hypothesized that these variables have the greatest effect on VMA. Gradationhad the largest effect on VMA. The quantity of material passing the #200 and FAA hadsignificant affects on VMA. One interesting finding of this study was that FAA affected the VMAof coarse mixes more than the fine mixes, but the amount of minus #200 material affected theVMA of the fine mixes more than the coarse mixes.Coree and Hislop examined the historical aspect of the VMA criteria (Coree, 1999). Coree foundthat a minimum VMA value was not used until the 1950s and in the 1960s it was specified as amix design parameter in Marshall mix design. They stated that many researchers haveidentified problems meeting the current Superpave VMA specifications. These researchers feelthat they often have to “fail” mixes that would have been acceptable in pavement performancebecause of the VMA criteria. Also changing the gradation to get an improved VMA value willsometimes increase the optimum asphalt content leading to higher construction costs.During Coree’s research several problems were encountered with the VMA, including:no field data could be located that was used to set the original VMA values,the original and current VMA values are based on Marshall mix design, andthe precision of tests used to determine the VMA values is not good enough to allow forstrict enforcement of the minimum VMA values.Recommendations of this research are: (a) the minimum VMA values need to be validatedagainst pavement performance, including factors such as particle shape, texture and gradation,(b) since there is a lack of precision of the tests used to determine the VMA criteria, rigidenforcement should be discouraged at this time, and (c) minimum average film thickness needsto be verified and correlated to pavement performance.There has also been concern expressed about the relationship between the VMA values, theSuperpave restricted zone and asphalt film thickness. It was found that the decreased VMAvalues, as compared to Marshall VMA values, while designing under Superpave could beattributed to the higher compactive effort of the Superpave gyratory compactor as compared tothe Marshall compaction hammer. This in turn has led to the use of coarser mixes in order toachieve the required VMA percentage. The problem arises in that this VMA requirement forSuperpave’s coarser mixes is the same as those for Marshall’s mixtures.Experiments during this research indicate that average film thickness yields a better picture ofthe mix being developed and encompasses more mix types (Kandal, 1998). This allows forhigher pavement durability since film thickness is optimized. The method for achieving the