Asphalt Review - Volume 32 Number 3 (Jun / Juy 12) - Australian ...

ASPHALT REVIEW2. Compare HMA, foamed asphalt WMA,and Evotherm WMA with regard to rutting,fatigue cracking, and other possible formsof distress;3. Compare virgin HMA, 50% RAP HMA,and 50% RAP WMA with regard to rutting,fatigue cracking, and other possible forms ofdistress; and4. Compare performance data for all GroupExperiment sections to MEPDG predictionsfor model validations.Summary of testtrack findingsThe majority of the research findings fromthis cycle can be categorized into one of thefollowing areas: (1) mix design, (2) alternativebinders and binder modifiers, (3) structuraldesign, (4) prediction testing, or (5) tirepavementinteraction. The following is asummary of the most significant findings fromthe 2009 NCAT Pavement Test Track.Mix DesignHigh Reclaimed Asphalt (RAP) Mixes.High RAP content mixes have shownexcellent rutting performance and durabilityon the test track. Two full-depth sectionscontaining 50% RAP that were constructedin 2009 have performed as well as thecontrol section after 10 million ESALs,exhibiting minimal rutting and no visible44 ROADS JUNE/JULY 2012cracking. One of the 50% RAP sections useda warm-mix asphalt (WMA) technology tolower the production temperature.Although no difference was evidentbetween the field performance of the hot50% RAP test section and the WMA-50%RAP test section, lab tests indicate thatusing WMA improved the fatigue resistanceby fivefold. The high RAP mixes increasedthe stiffness of the pavement structure,which reduced critical tensile strains by upto 31% and base pressures by up to 55%.Additionally, four sections with surfacelayers containing 45% RAP and differentvirgin binder grades (PG 52-28, PG 67-22and PG 76-22) were left in place from the2006 test track cycle, accumulating a totalof 20 million ESALs. Mixes with the stifferbinder grades exhibited minor crackingearlier than those with softer binders,indicating that using a softer virgin bindergrade improves the durability of high RAPmixes. However, all four sections hadexcellent rutting performance, with rutdepths less than 5 mm.The Mississippi DOT also sponsored asection containing a 45% RAP surface layerin the 2009 test track cycle. While the mixcontains a PG 67-22 binder, results indicatethat performance is similar to a virgin mixwith polymer-modified PG 76-22. This findingcould result in significant cost savings for theMississippi DOT and other agencies whichchoose to implement high-RAP contentmixtures with unmodified binders.Warm-Mix Asphalt (WMA).In addition to the WMA section containing50% RAP, two test sections built withdifferent WMA technologies wereconstructed for the 2009 test track cycle.One of them was built using the waterinjectionfoaming method and the other, achemical additive. Rutting was minimal inboth of these sections, but slightly increasedfrom the control section.Neither WMA section exhibited cracking,and laboratory testing indicated greaterfatigue life expectations for the WMAsections compared to the control. Structuralanalyses proved that the WMA sections haveequivalent back-calculated moduli and thesame response to loads and temperature astypical HMA.Stone-Matrix Asphalt (SMA).A study sponsored by the Georgia Departmentof Transportation for the 2009 test trackindicated that specifications for SMAaggregate properties based on early guidelinesare unnecessarily restrictive, eliminatingmaterials that may have good performanceunder heavy loading conditions. Aggregatewith up to 29% flat and elongated particles,as measured by the GDOT procedure GDT 129at the 3:1 ratio, performed well on the testtrack. The SMA containing the higher flat andelongated aggregate had excellent resistanceto rutting, ravelling, and cracking.Numerous agency-sponsored SMA testsections through the first four cycles of