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The Search for the Holy GrailN-DESIGN
January 10, 2007
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SUPERPAVE DESIGN COMPATION EFFORT
• Discussion of what is the “true” N-design
• To know where to go, it helps to know where we’ve been
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GYRATORY HISTORY
• Texas Four Inch Manual 1930s• Texas Four Inch Motorized 1960• Corps of Engineers 1960 ish• French 1970 ish• Superpave 1992
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First Gyratory Compactor• 1939, Texas Highway Department• Texas 4-Inch Gyratory Press• Manually Operated
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LCPC Gyratory Compactor
• 1959 visit to Texas
• Developed Protocol– 160 mm – 1º angle– 6 gyrations/min
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SUPERPAVE GYRATORY
• N Design Experiment
– Determine number of gyrations to match the road density
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N DESIGN RECOMPACTION
85.0%
90.0%
95.0%
100.0%
1 10 100 1000Number of Gyrations
% M
ax T
heor
etic
alG
ravi
ty
119 gyrations
96.5% Existing Density
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N DESIGN EXPERIMENT
• Three levels of traffic
– Low, less than three million ESAL’s.
– Medium, more than three million, less than ten million ESAL’s.
– High, more than ten million ESAL’s
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N DESIGN EXPERIMENT
• Three high temperature environments
– Cool (monthly temperature < 90 F)
– Warm (monthly temperature > 90 F, < 100 F)
– High (monthly temperature >100 F)
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N DESIGN EXPERIMENT
• Two depths of pavement
– Surface, within upper 100 mm of pavement.
– Lower, more than 100 mm from pavement surface.
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N DESIGN EXPERIMENT
• Three ages of pavement
– Young, less than three years old.
– Middle age, more than three years, less than twelve years old.
– Old, more than 12 years old.
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N DESIGN EXPERIMENT
• In total, 108 cells were required
• Reduced the number of cells to nine and the number of sites to 18.
• In total, 15 sites were obtained and evaluated
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CORRELATION
R Square = 0.7944
88.0
90.0
92.0
94.0
96.0
98.0
100.0
88.0 90.0 92.0 94.0 96.0 98.0 100.0Density At 100 Gyrations
Den
sity
Fro
m T
he R
oad
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DESIGN GYRATION TABLEAverage High Air
TemperatureESALs <39oC
(millions) Ninitial Ndesign Nmax
< 0.3 7 68 1040.3 - 1 7 76 1171 - 3 7 86 1343 - 10 8 96 15210 - 30 8 109 17430 - 100 9 126 204> 100 9 142 233
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N-Design II Experiment
Relate Densityto
Gyrations
%Gmm
Log ESALs
%Gmm
Log Gyrations
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1.0E+06 1.0E+10 1.0E+14 1.0E+18
ESAL's
Des
ign
Gyr
atio
ns
10
100
1000
Relate Gyrations To ESAL’s
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PG 67-22 Upper Lift
PG 76-22 Upper Lift
90.0
91.0
92.0
93.0
94.0
95.0
96.0
97.0
98.0
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0ESALs (millions)
Pave
men
t Den
sity
, % G
mm
Winter
TRAFFIC COMPACTIONNdes = 115
Ndes = 70
NCAT Test Track
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So What To Do?
Look at the effect of N-design on mixes
• For Same Aggregate SkeletonN Design will cause change in – VMA– Mix Stiffness
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19 MM MIXTURES
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
-60 -40 -20 0 20 40 60
Change In Gyrations
Cha
nge
in V
MA
Effect on VMAAggregate Blend Constant
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Effect on Stiffness
-
20,000
40,000
60,000
80,000
100,000
120,000
-60 -40 -20 0 20 40 60Change from Design Gyrations
G*(
0.1H
z), k
PaVolumetric Properties Constant
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CompactionParameters
EstimatedDesign Traffic
Level(Millions1
ESALs) Ninit Ndes Nmax
< 0.3 6 50 75
0.3 to < 3 7 75 115
3 to < 30 8 100 160
≥ 30 9 125 205
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N-Design III Experiment
Relate Densityto
Compaction
%Gmm
Log Gyrations
%Gmm
Log Passes
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Relate Gyrations to Passes
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Relating Compaction to Locking Point
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7 8
Voids at Locking Point
Test
Str
ip V
oids
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BUT!!!
• Compaction depends on– Gradation– Lift thickness– Base temperature– Available rollers– Etc.
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….SO?…..
• Is our current N-design OK?• Or should we do more research?
• What is the effect of N-design on mixes??
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Influence of Ndesign on Aggregate Properties
Property Increased Ndes
Decreased Ndes
Crushed Faces
crush crush
FAA nat sand nat sand
Gradation coarser finer
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Influence of Ndesign on Volumetric Properties
Property Increased Ndes
Decreased Ndes
Air Voids none none
VMA (%AC) none* none*
VFA little little
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Influence of Ndesign on Mix Properties
Property Increased Ndes
Decreased Ndes
Stiffness increase decrease
Compaction difficult easy
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CONCLUSIONS
• Density at end of service life not rational to define N design
• Current spec based on engineering judgment (and is reasonable)
• Test strip density is more rational (would require more research)