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Asphalt Concrete Mix Design

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Title: 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
5
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)

6
Mixing
Place pre-heated aggregate in bowl and add hot
asphalt
7
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

9
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

10
Compaction
Place funnel on top of mold and place mix in
mold. Take care not to allow the mix to segregate.
11
Compaction
Place another paper on top of mix and place mold
in compactor.
12
Compaction
Example of typical full-size compactors.
13
Compaction
Key Components of Gyratory Compactor
control and data acquisition panel
height measurement
reaction frame
loading ram
mold
tilt bar
rotating base
14
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
15
Compaction
After aging, take mix and preheated mold from
oven. Place paper in bottom of mold.

16
Compaction
Once compaction is finished, extrude sample from
mold.
17
Compaction
Remove the paper and label samples.
18
SGC Results
Gmm
10 100 1000
Log Gyrations
19
Three Points on SGC Curve
Gmm
Nmax
Ndes
Nini
10 100 1000
Log Gyrations
20
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
21
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
22
Superpave Mix Design Analysis
23
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

24
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
25
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)
26
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

27
SGC Results
Gmm
Increasing asphalt cement content
96 (4 Voids)
Ndes
Nmax
Nini
10 100 1000
Log Gyrations
Each line avg. of two samples
28
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

29
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
30
Superpave Mix Design
Effective asphalt content is asphalt on surface
of aggregate (asphalt not absorbed by aggregate)
31
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

32
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
33
Moisture SensitivityAASHTO T 283
Determine the tensile strengths of both sets of 3
specimens
Calculate the Tensile Strength Ratio (TSR)
Minimum of 80 needed
34
Moisture SensitivityAASHTO T 283
Indirect tensile strength apparatus for 100 mm
specimens
35
Example of Superpave Mix Design
36
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
37
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

38
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
39
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
40
Selection of Design Asphalt Binder Content
VMA
binder
41
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