HMA Mixture Design
Superpave
Strategic Highway Research
Program (SHRP)
• 5 years, $150 million
• Areas of Interest
– Asphalt materials ($50 million)
– Cement and concrete
– Long-term pavement performance
– Highway operations
– Maintenance effectiveness
Superpave - What is It?
• Superior Performing Asphalt
Pavements
• Refers Collectively to the Products of
SHRP Asphalt Research Program
• A System for Overcoming
– Rutting
– Low temp cracking
– Fatigue cracking
The Superpave System
• Performance Based
Binder Specification
Binder Physical Property Tests
Mixture Design
Mixture Tests
Superpave Binder
Specification
PG 64-22
Control physical properties within
this pavement temperature range.
Selection Items
• Asphalt Binder
• Aggregate
• Volumetric Properties
• Moisture Susceptibility
Aggregate Properties
• Consensus Properties
– Coarse aggregate angularity
– Fine aggregate angularity
– Flat, elongated particles
– Clay content
• Source Properties
– Toughness
– Soundness
– Deleterious materials
Aggregate Size Definitions
• Nominal Maximum Aggregate Size
– One size larger than the first sieve to
retain more than 10%
• Maximum Aggregate Size
– One size larger than nominal maximum
size
Percent Passing
100
max density line
restricted
zone
nom max
control point max size
size
0
.075 .3 2.36 4.75 9.5 12.5 19.0
Sieve Size, mm, raised to 0.45 power
Purpose of Superpave
Gradation Requirements
• Use of 0.45 Power Chart
– Achieve VMA
• Control Points
– Define mix types
• Restricted Zone
– Prevent excess of fine sand in relation
to total sand
• Develop Tough Stone Skeleton
Typical Superpave Mixes
100
80
Percent Passing
60
40
20
0
.075 .3 2.36 19 25
Sieve Size, mm, Raised to 0.45 Power
Short Term Aging
2 hours, low absorption aggregates
4 hours, high absorption aggregates
Short Term Aging
• Allows time for aggregate to absorb binder
• Helps minimize variability in volumetric
calculations
– Most terms dependent upon volumes which
vary with changes in the amount (volume)
of absorbed binder
Compaction
Key Components of Gyratory Compactor
height control and data
measurement acquisition panel
reaction
frame loading
ram
tilt bar mold
rotating
base
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
Compaction
Example of typical full-size compactors.
SGC Compaction Curve
% Gmm
Nmax
Ndes
Nini
10 100 1000
Log Gyrations
Compaction Table
20-yr.
Design Ninitial Ndesign Nmaximum
ESAL (M)
0-0.3 6 50 75
0.3-3 7 75 115
3-30 8 100 160
>30 9 125 205
% Gmm Data Presentation
100
98
Nmax = 174
96
Ndes = 109
94
Nini = 8
92
90 Specimen 1
Specimen 2
88 Average
86
84
1 10 100 1000
Number of Gyrations
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
Superpave Mixture Design
• Determine mixture properties at NDesign and compare
to criteria
– Air voids 4% (or 96% Gmm)
– VMA See table
– VFA See table
– %Gmm at Nini < 89%
– %Gmmat Nmax < 98%
– Dust proportion 0.6 to 1.8
SGC Results
% Gmm
Increasing asphalt
96% cement content
(4% Voids)
Nini Ndes Nmax
10 100 1000
Log Gyrations
Each line = avg. of two samples
Superpave Mixture 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
Superpave Mixture Design
• VFA requirements:
– Traffic (millions of ESALs) Range of VFA
< 0.3 70 to 80
1 to 3 65 to 78
> 3.0 65 to 75
Superpave Mixture Design
% Passing #200 Sieve
0.6 < < 1.8
% Effective Binder
Superpave Mixture Design
• Moisture Sensitivity
– Prepare set of 6 specimens
• 6 to 8% voids
–Represents anticipated in-service voids
– Determine tensile strength of 3 of specimens
– Saturate and condition remaining 3 in water bath
(60oC, 24 hr.)
• Option for freeze cycle
– Bring to test temperature (25oC) and determine
wet (conditioned) tensile strength
Moisture Sensitivity
AASHTO T 283
• Measured on proposed aggregate blend and
binder 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
Moisture Sensitivity
AASHTO T 283
Determine the tensile
strengths of both sets of 3
specimens
Calculate the
Tensile Strength
Ratio (TSR)
Avg. wet tensile strength
TSR =
Avg. dry tensile strength
Minimum of 80% needed
Moisture Sensitivity
AASHTO T 283
Indirect tensile
strength apparatus
for 100 mm
specimens
Selection of Design
Binder Content
Va VMA
% binder Blend 3 % binder
VFA
DP
% binder
% binder
%Gmm at Nini %Gmm at Nmax
% binder % binder
