Extending the Life of Asphalt Mixes
David Lee, P.E. - ARAC Chair, Salem District Materials
Kevin McGhee, P.E. – ARAC Secretary, VCTIR
OUTLINE
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Background on Need for Research Study
Results from Previous Research Project Phases
Results from Latest Phase
Potential Steps Forward in 2015
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Background
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There is a clear perception by both VDOT and Industry leaders that our
present Superpave mixes do not have enough liquid asphalt content
and, as such, are not lasting as long as they could.
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This perception is supported by some recent data suggesting that our
mixes are lasting 1-3 years less than previously determined.
(McGhee/Clark)
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There is a clear perception by both VDOT and Industry leaders that
premature fatigue cracking due to low AC content is the reason for a
significant amount of pavement failures around the state.”
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2010 Research Report by G. W. Maupin “Investigation of Optimized
Mixture Design for Superpave Surface Mixtures” recommended
additional research on the subject.
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RESEARCH QUESTION
• Are dense graded mixes designed with SUPERPAVE
system providing consistently longer service life
compared to previous mixes?
• Criteria for Life
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Durable or Fatigue resistant
Rutting resistant
No flushing
Skid resistant (surface mixes)
• Research Objective – Maximize the service life of dense
graded asphalt mixes
PREVIOUS RESEARCH PHASE RESULTS
Phase I – Superpave Designed Mix Analysis
• Comparisons
• Volumetrics for 50 and 65 Gyrations
• Volumetric results for 50 and 75 blow Marshall
• Mix gradations
• Bag samples for future phases
VTM – SM 9.5 Mixes
Marshall
Superpave
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VMA – SM 9.5 Mixes
Min. (Sup. & Marshall
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RESULTS FROM PHASE I
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VTM – Superpave Gyratory (SGC) produces approximately 2% lower
VTM than the Marshall hammer.
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VMA – SGC produces approximately 1-2% lower VMA than the Marshall
hammer.
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Review of the data did not produce a definitive “simple solution” (i.e.,
just reduce the number of gyrations).
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LATEST PHASE RESULTS
Sampled plant produced SM-9.5 mixes
Targeted mixes with 23% or less passing #30 sieve and at least 58%
passing #4 sieve
Collected materials to do:
1. Determine AC content, gradation and volumetrics of Spec produced
mixes
2. Determine VTM and VMA for mixes at 50 gyrations
3. Calculate binder needed to meet 3.5% VTM at 50 gyrations (new
optimum AC)
4. Performance test initial mix, mix at optimum, mix at optimum plus 0.2%
binder, and mix at optimum plus 0.4% binder
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GRADATIONS FOR MIXES TESTED
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VTM vs. Added Binder @ 50 Gyrations
6.5
Trial Added Asphalt Content for Branscome
VTM at 50 Gyrations (%)
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5.5
5
4.5
4
3.5
3
2.5
2
0
0.2
0.4
0.6
0.8
Added Binder Content (%)
1
1.2
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VMA vs. Added Binder @ 50 Gyrations
Trial Added Asphalt Content for Branscome
VMA at 50 Gyrations (%)
19
18
17
16
15
14
13
0
0.2
0.4
0.6
0.8
Added Binder Content (%)
1
1.2
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FLOW NUMBER vs. ADDED BINDER (VTM = 7%)
2500
Flow Number
2000
1607
1500
1450
1000
873
513
500
0
0
0.65
0.85
Added Binder Content (%)
1.05
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FE INDEX vs. ADDED BINDER (VTM = 7%)
Fracture Energy Index
10
9
8
7
6
5
4
3
2
1
0
0
0.65
0.85
Added Binder Content (%)
1.05
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The Bottom Line
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We know we need mixes that have a longer fatigue life to prolong the
development of cracking.
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We know we need to ensure that any added AC does not lead to rutting
or bleeding.
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We believe the aggregate gradations need to be tighter on certain
sieves as well as the addition of one or more control sieves (#30 and #4)
to combat mix tenderness and rutting.
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Ensuring we have the AC content and gradation correct is even more
critical with increasing use of RAP.
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POTENTIAL NEXT STEPS
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Finalize select use special provision for SM-9.5 with changes to:
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Solicit demonstration projects from across state
Design and produce experimental mixes
Pave control (65 gyrations and 4% VTM) and experimental mixes
Retrieve mix (control and experimental) for additional lab performance
testing
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Design Gradations (#30 and #4 sieves)
Design Gyrations (50)
Flow Number and APA for rutting
Fracture Energy for cracking
Collect density data from control and experimental sites
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