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The Search for the Holy GrailN-DESIGN

January 10, 2007

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

GYRATORY HISTORY

• Texas Four Inch Manual 1930s• Texas Four Inch Motorized 1960• Corps of Engineers 1960 ish• French 1970 ish• Superpave 1992

First Gyratory Compactor• 1939, Texas Highway Department• Texas 4-Inch Gyratory Press• Manually Operated

LCPC Gyratory Compactor

• 1959 visit to Texas

• Developed Protocol– 160 mm – 1º angle– 6 gyrations/min

SUPERPAVE GYRATORY

• N Design Experiment

– Determine number of gyrations to match the road density

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

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

N DESIGN EXPERIMENT

• Three high temperature environments

– Cool (monthly temperature < 90 F)

– Warm (monthly temperature > 90 F, < 100 F)

– High (monthly temperature >100 F)

N DESIGN EXPERIMENT

• Two depths of pavement

– Surface, within upper 100 mm of pavement.

– Lower, more than 100 mm from pavement surface.

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.

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

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

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

N-Design II Experiment

Relate Densityto

Gyrations

%Gmm

Log ESALs

%Gmm

Log Gyrations

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

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

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

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

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

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

N-Design III Experiment

Relate Densityto

Compaction

%Gmm

Log Gyrations

%Gmm

Log Passes

Relate Gyrations to Passes

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

BUT!!!

• Compaction depends on– Gradation– Lift thickness– Base temperature– Available rollers– Etc.

….SO?…..

• Is our current N-design OK?• Or should we do more research?

• What is the effect of N-design on mixes??

Influence of Ndesign on Aggregate Properties

Property Increased Ndes

Decreased Ndes

Crushed Faces

crush crush

FAA nat sand nat sand

Gradation coarser finer

Influence of Ndesign on Volumetric Properties

Property Increased Ndes

Decreased Ndes

Air Voids none none

VMA (%AC) none* none*

VFA little little

Influence of Ndesign on Mix Properties

Property Increased Ndes

Decreased Ndes

Stiffness increase decrease

Compaction difficult easy

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)

SO!!!! There is no TRUE N-

Design