Asphalt Concrete Mix Design

1
Asphalt Concrete Mix Design

• Superpave

2
Superpave Volumetric Mix Design

• Goals
• Compaction method which simulates field
• Accommodates large size aggregates
• Measure of compactibility
• Able to use in field labs
• Address durability issues
• Film thickness
• Environmental

3
Specimen Preparation

• Mechanical mixer
• 0.170 Pa-s binder viscosity
• Short term oven aging
• 4 hours at 135C
• 2 hours at 135C (optional)

4
Mixing/Compaction Temps
Viscosity, Pa s
10
5
1
.5
Compaction Range
.3
.2
Mixing Range
.1
100
110
120
130
140
150
160
170
180
190
200
Temperature, C
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Specimen Preparation

• Specimen Height
• Mix Design - 115 mm (4700 g)
• Moisture Sens. - 95 mm (3500 g)
• Loose Specimen for Max. Theor. (Rice)
• varies with nominal max size
• 19 mm (2000 g)
• 12.5 mm (1500 g)

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Mixing
Place pre-heated aggregate in bowl and add hot
asphalt
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Mixing
Place bowl on mixer and mix until aggregate is
well-coated
8
Short Term Aging
Empty mix into pan and place in oven to simulate
short term aging 2 hours for low absorption
aggregates 4 hours for high absorption aggregates

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Short Term Aging Important

• Allows time for aggregate to absorb asphalt
• Helps minimize variability in volumetric
  calculations
• Most terms dependent upon volumes which change
  with changes in the amount (volume) of absorbed
  asphalt

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Compaction
Place funnel on top of mold and place mix in
mold. Take care not to allow the mix to segregate.
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Compaction
Place another paper on top of mix and place mold
in compactor.
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Compaction
Example of typical full-size compactors.
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Compaction
Key Components of Gyratory Compactor
control and data acquisition panel
height measurement
reaction frame
loading ram
mold
tilt bar
rotating base
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Compaction

• Gyratory compactor
• Axial and shearing action
• 150 mm diameter molds
• Aggregate size up to 37.5 mm
• Height measurement during compaction
• Allows densification during compaction to be
  evaluated

Ram pressure 600 kPa
1.25o
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Compaction
After aging, take mix and preheated mold from
oven. Place paper in bottom of mold.

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Compaction
Once compaction is finished, extrude sample from
mold.
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Compaction
Remove the paper and label samples.
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SGC Results
Gmm
10 100 1000
Log Gyrations
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Three Points on SGC Curve
Gmm
Nmax
Ndes
Nini
10 100 1000
Log Gyrations
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Design Compaction
Gmm
Nmax

• Ndes based on
• average design high air temp
• traffic level
• Log Nmax 1.10 Log Ndes
• Log Nini 0.45 Log Ndes

Ndes
Nini
10 100 1000
Log Gyrations
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Data Presentation
Gmm
100
98
Nmax 174
96
Ndes 109
94
Nini 8
92
Specimen 1
90
Specimen 2
88
Average
86
84
1
10
100
1000
Number of Gyrations
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Superpave Mix Design Analysis
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Superpave Testing

• Specimen heights
• Mixture volumetrics
• Air voids
• Voids in mineral aggregate (VMA)
• Voids filled with asphalt (VFA)
• Mixture density characteristics
• Dust proportion
• Moisture sensitivity

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Superpave Mix Design

• Gmb (estimated) Wm

gw Vmx
Where Wm mass of specimen, g Vmx volume of
compaction mold (cm3) gw density of water,
g/cm3
Assumption specimen is smooth-sided cylinder
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Superpave Mix Design

• However, surface irregularities cause the volume
  of the specimen to be slightly less than volume
  of cylinder
• Actual bulk specific gravity measurement of
  compacted sample used to determine correction
  factor, C

Gmb (measured) Gmb (estimated)
C
Gmm Gmb (estimated) C / Gmm (measured)
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Superpave Mix Design

• Determine mix properties at NDesign and compare
  to criteria
• Air voids 4 (or 96 Gmm)
• VMA See table
• VFA See table
• Gmm at Nini lt 89
• Gmmat Nmax lt 98
• Dust proportion 0.6 to 1.2

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SGC Results
Gmm
Increasing asphalt cement content
96 (4 Voids)
Ndes
Nmax
Nini
10 100 1000
Log Gyrations
Each line avg. of two samples
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Superpave Mix Design

• VMA requirements
• Nominal max agg size Min. VMA
• 9.5 mm 15
• 12.5 mm 14
• 19 mm 13
• 35 mm 12
• 37.5 mm 11

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Superpave Mix Design

• VFA requirements
• Traffic (millions of ESALs) Range of VFA

lt 0.3 70 to 80 1 to 3 65 to 78 gt
3.0 65 to 75
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Superpave Mix Design
Effective asphalt content is asphalt on surface
of aggregate (asphalt not absorbed by aggregate)
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Superpave Mix Design

• Moisture Sensitivity
• Prepare set of 6 specimens
• 6 to 8 voids
• Represents anticipated in-service voids
• Determine tensile strength of 3 of specimens
• Condition remaining 3 in water bath (60oC, 24
  hr.)
• Option for freeze cycle
• Bring to test temperature (25oC) and determine
  wet (conditioned) tensile strength

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Moisture SensitivityAASHTO T 283

• Measured on proposed aggregate blend and asphalt
  content
• Reduced compactive effort to increase voids

3 Conditioned Specimens
3 Dry Specimens
Vacuum saturate specimens Soak at 60oC for 24
hours Soak at 25oC for 2 hours
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Moisture SensitivityAASHTO T 283
Determine the tensile strengths of both sets of 3
specimens
Calculate the Tensile Strength Ratio (TSR)
Minimum of 80 needed
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Moisture SensitivityAASHTO T 283
Indirect tensile strength apparatus for 100 mm
specimens
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Example of Superpave Mix Design
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Trial Gradations
19.0 mm Nominal Mixture
100.0
90.0
80.0
70.0
Trial Blend 3
Trial Blend 1
60.0
PASSING
50.0
40.0
Trial Blend 2
30.0
20.0
10.0
0.0
19.0
2.36
0.075
Sieve Size (mm) raised to 0.45 power
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Aggregate Consensus Properties

• Blended Aggregate properties are determined
• Property Criteria Blend 1 Blend 2
  Blend 3
• Coarse Ang. 95/90 min. 96/92 95/92
  97/93
• Fine Ang. 45 min. 46 46
  48
• FLat/Elong. 10 max. 0
  0 0
• Sand Equiv. 45 min. 59
  58 54
• Combined Gsb n/a 2.699 2.697
  2.701
• Combined Gsa n/a 2.768 2.769
  2.767

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Compaction Characteristics
Gmm
Blend AC Nini Ndes
Nmax 1 4.3 86.9 96.0
97.4 2 4.5 85.9 96.0
97.7 3 4.7 87.1 96.0
97.3
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Volumetric Properties
Blend AC Air VMA VFA DP 1
4.3 4.0 12.7 68.5 0.86 2
4.5 4.0 13.0 69.2 0.78 3
4.0 4.0 13.5 70.1 0.88
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Selection of Design Asphalt Binder Content
VMA
binder
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Questions - ?