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

495.2 SUPERPAVE MIX DESIGNSAs with the Marshall mixes, the Superpave mixes were designed to be applicable to localconditions. The same aggregate stockpiles and asphalt cement were used for both mix types.The Superpave mix design procedure allows for more traffic levels than the Marshall procedure.The research plan called for developing Superpave mix designs comparable to the Marshalldesigns. In addition, a 100% crushed limestone mix and a mix with the maximum amount ofnatural sand was developed for the light traffic level. Since the consensus aggregaterequirements are significantly relaxed for light traffic volume roads, it was possible to developmix designs with high natural sand contents.The dust to binder ratio controls the amount of dust is used in a Superpave mixture. This ratioaffects the gradation limits for Pan material specified by Superpave as 2.0 to 10.0 percent.However, the dust to binder ratio effectively limits this range as a function of the percenteffective binder. For example, if the effective binder content is 5 percent, the amount ofmaterial passing the #200 sieve must be in the range of 3.0 to 6.0 percent for the mix to meetthe D/B criteria. Thus, the D/B criterion is a limiting factor on the number of acceptablemixtures which can be made with available aggregate stockpiles. The Superpave low trafficmixtures which were designed to be above the restricted zone were directly affected by the D/Bratio. In all cases, the D/B criteria limited the range of material passing the #200 sieve to lessthat the aggregate specification limits. Hence, if the D/B criterion is enforced, essentiallycontrols this portion of the aggregate blend.5.2.1 Types of Mixes DevelopedEight Superpave mixes were designed for this project. The blends of stockpile materials areshown in Table 5.3. Four of these mixes mimic the Marshall mixes, two 100% limestonemixtures and two 13% natural sand mixes at traffic levels of medium and high. The two mixescontaining 13% natural sand contain the exact same blends and gradations as the comparableMarshall mixes. The blends gradations for the heavy and medium traffic Superpave mixes with100% limestone were adjusted to better meet the Superpave criteria. Four Superpave lighttraffic designs were developed. One contained the maximum amount of natural sand that couldbe used while meeting the Superpave consensus aggregate specifications. One containedapproximately half this amount of sand, but the exact proportion was determined during themix design process. Two light traffic mixes were designed with 100% limestone. One was thefinest mix that could be developed based on the stockpile gradations. One was prepared withthe same gradation as the mixes with natural sand. These were designed above the restrictedzone and two contained large amounts of natural sand. Of the two remaining mixes onecontained 100% limestone but was passed through the restricted zone due to major difficultiesin combining the research stockpiles into a 100% limestone mixture above the restricted zone.The final mixture is one that mimics the 100% natural sand mixture. This was accomplished byaltering the gradation of the limestone sand stockpile to match that of the natural sand