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Asphalt Concrete Mix Design

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Asphalt Concrete Mix
Design
Superpave
1
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
2
Specimen Preparation
• Mechanical mixer
–
0.170 Pa-s binder viscosity
• Short
–
–
term oven aging
4 hours at 135В°C
2 hours at 135В°C (optional)
3
Mixing/Compaction Temps
Viscosity, Pa s
10
5
1
.5
.3
.2
.1
Compaction Range
Mixing Range
100
110
120
130
140
150 160
170 180 190 200
Temperature, C
4
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
•
•
150 mm
19 mm (2000 g)
12.5 mm (1500 g)
5
Mixing
Place pre-heated aggregate in
bowl and add hot asphalt
6
Mixing
Place bowl on mixer and mix until
aggregate is well-coated
7
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
8
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
9
Compaction
Place funnel on top of mold and place mix in mold.
Take care not to allow the mix to segregate.
10
Compaction
Place another paper on top of mix
and place mold in compactor.
11
Compaction
Example of typical full-size compactors.
12
Compaction
Key Components of Gyratory Compactor
height
measurement
reaction
frame
tilt bar
control and data
acquisition panel
loading
ram
mold
rotating
base
13
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
14
Compaction
After aging, take mix and preheated mold
from oven. Place paper in bottom of mold.
15
Compaction
Once compaction is finished, extrude
sample from mold.
16
Compaction
Remove the paper and label samples.
17
SGC Results
% Gmm
10
100
1000
Log Gyrations
18
Three Points on SGC Curve
% Gmm
Nmax
Ndes
Nini
10
100
1000
Log Gyrations
19
Design Compaction
•
•
•
% Gmm
Ndes based on
– average design high
air temp
– traffic level
Log Nmax = 1.10 Log Ndes
Log Nini = 0.45 Log Ndes
Nmax
Ndes
Nini
10
100
1000
Log Gyrations
20
% Gmm
Data Presentation
100
98
Nmax = 174
Ndes = 109
Nini = 8
96
94
92
Specimen 1
Specimen 2
Average
90
88
86
84
1
10
100
1000
Number of Gyrations
21
Superpave Mix Design Analysis
22
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
23
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
24
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)
C=
Gmb (estimated)
% Gmm = Gmb (estimated) C / Gmm (measured)
25
Superpave Mix Design
•
Determine mix properties at NDesign and compare to
criteria
– Air voids
– VMA
– VFA
– %Gmm at Nini
– %Gmmat Nmax
– Dust proportion
4% (or 96% Gmm)
See table
See table
< 89%
< 98%
0.6 to 1.2
26
SGC Results
% Gmm
Increasing asphalt
cement content
96%
(4% Voids)
Ndes
Nini
10
Nmax
100
1000
Log Gyrations
Each line = avg. of two samples
27
Superpave Mix Design
•
VMA requirements:
– Nominal max agg size
В» 9.5 mm
В» 12.5 mm
В» 19 mm
В» 35 mm
В» 37.5 mm
Min. VMA
15
14
13
12
11
28
Superpave Mix Design
•
VFA requirements:
– Traffic (millions of ESALs) Range of VFA
< 0.3
1 to 3
> 3.0
70 to 80
65 to 78
65 to 75
29
Superpave Mix Design
% weight of - 0.075 material
0.6 <
< 1.2
% weight of effective asphalt
Effective asphalt content is asphalt on surface of aggregate
(asphalt not absorbed by aggregate)
30
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
31
Moisture Sensitivity
AASHTO 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
32
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
33
Moisture Sensitivity
AASHTO T 283
Indirect tensile
strength apparatus
for 100 mm
specimens
34
Example of Superpave Mix Design
35
Trial Gradations
% PASSING
19.0 mm Nominal Mixture
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
Trial Blend 3
Trial Blend 1
Trial Blend 2
0.075
2.36
19.0
Sieve Size (mm) raised to 0.45 power
36
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
37
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%
38
Volumetric Properties
Blend
1
2
3
%AC
4.3%
4.5%
4.0%
%Air
4.0%
4.0%
4.0%
%VMA
12.7%
13.0%
13.5%
%VFA
68.5%
69.2%
70.1%
DP
0.86
0.78
0.88
39
Selection of Design Asphalt
Binder Content
Va
VMA
% binder
% binder
Blend 3
VFA
DP
% binder
% binder
%Gmm at Nini
%Gmm at Nmax
% binder
% binder
40
Questions - ?
41
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