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

53The first Superpave mixes developed were those containing 13% natural sand. This was done inconjunction with Marshall mixtures of the same gradation, thus problems encountered with theMarshall development also occurred with the Superpave.VMA was a concern with the Superpave mixes because past experience, documented in Chapter2, indicated mixtures compacted in the gyratory compactor exhibit lower VMA values thanthose compacted with the Marshall compaction hammer. However, trial blends demonstratedthat VMA was not a problem with the 13% natural sand aggregate blend. The VMA was 15.0 forthe heavy traffic mixture and 15.4 for the medium. These VMA values are close to WVDOH limitof 15.0 indicating that 13% natural sand was the maximum amount that could be used for thesedesigns.Next the 100% limestone aggregate Superpave mixtures were designed. At first, trial mixeswere evaluated with gradations similar to the Marshall designs. However, the VMA was lessthan the required minimum, so the gradation was adjusted to allow for more distance betweenthe upper part of the gradation curve and the maximum density line. This increased the VMA toan acceptable level.Finally, the Superpave light-traffic volume mixtures were designed. The first mix developed wasthe Superpave light-traffic mix with 100% limestone. Initially, two problems were encountered,inadequate VMA and too much #200 material to pass the D/B criteria. Using the availablestockpile gradations, there was no combination of aggregates that passed all criteria, includingavoidance of the restricted zone. At the national level, mixes with 100% crushed material areallowed to pass through the restricted zone. Hence, it was decided to allow the 100% limestonemix for light-traffic to pass through the restricted zone. This allowed for a reasonable amount ofpan material to be in the mix while still staying away from the maximum density line to achieveproper VMA.The next mix developed was one that contained a half-and-half mixture of limestone and naturalsand. This was done by simply taking the gradation of the 100% limestone sand design andsplitting the limestone sand portion in half. Adjustments had to be made to the #9 andbaghouse fines stockpiles to allow for an acceptable gradation above the restricted zone. Thisgradation was also hard to design in that to meet the percent passing requirement of the #50sieve in the restricted zone, the percent passing the #8 sieve was pushed out of specification.This was eventually corrected by fine-tuning the gradation percentages. However, this did holdthe gradation far enough away from the maximum density line to allow for a VMA of 15.2.The next mixture designed was one that contained 100% natural sand. Various trial blends weretested that closely resembled that of the 100% limestone sand mixture using natural sandinstead of limestone. These blends were found to exhibit low VMA values and it was alsodifficult to meet the minimum percent passing the #200 sieve requirement. This was mainlyattributed to the gradation laying so closely to the upper limits on other sieves making it difficultto adjust the gradation a sufficient amount. This problem was over come by allowing a higher