Superior Performing Asphalt Pavements

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Superior Performing Asphalt Pavements

• Performance based design system
• testing and design procedures simulate closer to
  the actual loading and climatic conditions
• performance graded asphalt binders
• aggregate specifications
• hot mix asphalt design and analysis system
• Level 1 material selection and volumetric design
• Level 2 Level 1 design performance testing
• Level 3 Level 1 design more performance
  testing
• Enhanced mix performance
• Permanent deformation
• Fatigue Cracking and Low temperature cracking

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Superpave

• SHRP established 1987 - 5 yrs.
• Binder specs in use by 100 of US, Ont., Quebec
  in Canada
• Level 1 Mix design - partial use
• Still to come
• simple performance tests
• performance prediction models

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Superpave Mixture Design

• Key features are laboratory compaction and
  performance testing
• Laboratory compaction
• Superpave gyratory compactor (SGC)
• compacts test specimens to simulate actual
  pavement and loading conditions
• provides information about the compactability of
  a particular mixture
• design mixtures that do not exhibit tender mix
  behavior and do not densify to low air voids
  under traffic

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Superpave Mixture Design

• Performance testing and performance prediction
  models
• detailed predictions of actual pavement
  performance in terms of ESALs
• Superpave Shear tester (been replaced by
  triaxial)
• Indirect Tensile tester
• Short term aging protocol
• performance of HMA immediately after construction
  is influenced by mix properties from hot mixing
  and compaction
• oven aged mix at 135 C 4 hours to simulate delays
  that can occur during construction

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Superpave Mixture Design
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Level 1 Mix Design

• Volumetric mix design
• Select asphalt and aggregate materials
• develop several aggregate trial blends (3) that
  meet Superpave aggregate specs
• blend asphalt, compact, analyze
• use best blend with several asphalt contents to
  determine the design asphalt content at 4 air
  voids and compare to other criteria

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Gyratory Compactor

• Better simulates field compaction than hammer
• Compacts specimen but also measures density
  during compaction
• Measures compactibility so tender mixes are
  identifies
• Larger size aggregate blends can be used (50 mm
  max size) because of larger mold sizes

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• Amount of compaction depends on traffic and
  climate
• Ndes is the design number of gyrations used for
  volumetric properties
• Nmax lt 98, Nini lt 89 of max. density - used to
  indicate compactibility
• Log Nmax 1.1 Log Ndes
• Log Nini .45 Log Ndes

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Trial Blends

• The available aggregate stockpiles are combined
  in varying percentages, 3 blends are usually
  considered
• Aggregate properties are determined (either on
  blends or estimated from individual aggregates)
• Compact each blend and estimate volumetric
  properties

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Trial Blends

• For each aggregate trial blend
• Asphalt binder grade selected
• Mixing temperature (non modified)
• viscosity .15 - .19 Pa-s (150 190 mm2/s)
• Compaction temperature
• viscosity 0.25 - .31 Pa-s (250 310 mm2/s)
• Prepare aggregates
• 2 specimens for each trial blend 2 x 4700g for
  compaction
• 1 approx. 2000g for max density (uncompacted mix)
• 1 x 3700 g for moisture sensitivity

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Trial Blends

• Heat, mix with asphalt and short term age for 4
  hours at 135C representing possible delays in
  the field
• Compact specimens for Nmax gyrations, recording
  height

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Data Analysis

• Measure density at Nmax, using volume of cylinder
  calculate estimated density at Nmax, Nini, Ndes
• Errors in estimated densities due to surface
  irregularities of briquette are corrected using
  ratio of measured to estimated density at Nmax
• Measure Max. theoretical density of the
  uncompacted mix
• Calculate air voids, VMA, VFA at Ndes

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Data Analysis

• When you calculate your air voids you want to be
  at 4..
• Estimate asphalt content at 4 using
• Pb,est Pbi - (.4x(4-AV))
• Calculate corrected values of design VMA and VFA
  for 4 air voids using
• VMAest VMAinitialC x (4-AV) where C
• .1 if AV lt 4 or .2 if AV gt 4
• VFAest ( VMAest- 4) / VMAest

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Blend Selection

• Check blend properties against design criteria
• VMA VFA meets table 6-14,6-15 in text
• Nini lt 89, Nmax lt 98 of max density
• passes moisture sensitivity (T283)
• 80 minimum indirect tensile strength
• dust content ( passing 0.075 mm sieve divided by
  effective asphalt content) between .6 and 1.2
• Choose best blend - new samples ( 2 compaction 2
  max density) are made and checked
• Pb _at_ 4 AV
• one at 0.5 less
• 2 more at 0.5 and 1 higher
• volumetric properties calculated at each a/c
  content
• a/c content at 4 chosen other criteria checked

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Level 2 3 Design

• Performance tests for permanent deformation
  fatigue cracking and low temperature cracking
• Superpave Shear Tester (SST) (been replaced by
  triaxial)
• Indirect Tensile Tester (IDT)
• Lab testing has not yet been validated with field
  results and is still in experimental stage
• Current expectations are that Level 2 3 will be
  combined into one and Level 1 will include an
  additional simplified stability test
• At present research - triaxial testing for
  rutting and fatigue and indirect tension test for
  thermal cracking - presently being evaluated

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Example - Level 1

• Interstate 43 in Milwaukie, Wisconsin
• Nominal max aggregate size 19mm
• Design ESALs 18,000,000
• this would actually require a level 3 design

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• Select a PG 58-34
• mix temp 165 to 172C
• compaction temp 151 to 157C
• 5 stockpiles - measure bulk and apparent specific
  gravities

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• Measure 4 consensus properties
• can measure blends but will use individual in
  this case to estimate blend properties which has
  an advantage in choosing blends
• final design blends must meet criteria not
  individual stockpiles

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• 3 blends are produced
• 1 - Intermediate, 2 - Coarse, 3 - Fine

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• Blends are checked against gradation requirements
  and for consensus properties - all 3 pass

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• Trial asphalt content
• absorption water calculated from bulk and
  apparent specific gravities of aggregate blends
• Absorption water 100 (1/Gsb-1/Gsa)
• Blend 1 .9 water absorption
• Blend 2 1.0, Blend 3 .9
• From table 6-12 in text, initial binder content
  is 4.4 for all 3 blends
• Two samples of each blend are compacted using
  Nini 8, Ndes 109, Nmax 174 (from design
  table table 6-13 text)
• Maximum and bulk specific gravities (RD) (Gmm,
  Gmb) are measured and compactor data obtained

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Trial Blend 1
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Trial Blend 1
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Trial Blend 2
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Trial Blend 2
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Trial Blend 3
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Trial Blend 3
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• The max density at Nini , Ndes, Nmax is
  calculated from an average of the two samples of
  each blend

• Calculated air voids and VMA at Ndes
• AV 100 - Gmm
• 1 100-96.2 3.8, 2 4.3, 34.8
• VMA 100 - (Gmm x Gmm x (1 - Pbi))/Gsb
• 1 100-(96.2x2.563x.956)/2.699 12.7
• 2 13.0
• 3 13.5

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• Estimate properties _at_ 4 air voids
• Pb,est Pbi - (.4 x(4-AV))
• 1 4.4-(.4x(4-3.8)) 4.3
• 2 4.5
• 3 4.7
• VMAest VMAinitial C x (4 - AV)
• 1 12.7 (.1 x (4.0 - 3.8)) 12.7
• 2 13.0 (.2 x (4.0 - 4.3)) 13.0
• 3 13.5 (.2 x (4.0 - 4.8)) 13.4
• VFAest 100x (VMAest - 4)/VMAest
• 1 100 x (12.7 - 4.0)/12.7 68.5
• 2 69.2
• 3 70.1

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• Estimate properties at 4 Air Voids
• Gmm,est _at_ Nini Gmm,trial - (4-Av,trial)
• 1 87.1-(4-3.8) 86.9
• 2 85.9
• 3 87.1
• Similarly for Nmax
• 1 97.4
• 2 97.7
• 3 97.3
• Results for 4 air voids for 3 blends

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• Dust ration P0.75/Pbe,est
• 1 3.1/3.6 .86
• 2 2.9/3.7 .78
• 3 3.5/4.0 .88
• All pass with values between .6 and 1.2
• Blend 1 fails VMA, blend 2 just meets VMA,
  therefore select blend 3
• If no blends were acceptable start over with new
  stockpile ratios
• Evaluate blend 3 as before by mixing 2 specimens
  with 4.2, 4.7, 5.2 and 5.7 AC contents
• Note this time aggregate consensus properties
  must be measured on the blends and not calculated
  from stockpile values

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Calculating mix volumetric properties (not
estimating) for the final blend use

• Calculated air voids, VMA, VFA at NdesAV 100
  - GmmVMA 100 - (Gmm x Gmm x (1 -
  Pbi))/GsbVFA 100 x ((VMA - AV)/VMA)

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• Blend is tested for moisture sensitivity and has
  tensile strength ratio of 82.6 which exceed the
  80 minimum requirement
• This blend meets all required design criteria and
  is approved for use!