Gyratory Locking Point

Brian D. Prowell, P.E.

Louisiana Transportation Conference 2007

On-Going Work at NCAT Related

to Gyratory Compaction Levels

• NCHRP 9-9 (1) Verification of NDesign Levels

• Determination of Design Gyration Levels for SMA – FHWA

• Verification of NDesign Levels – ALDOT

• Determination of Design Gyration Levels for SMA – ALDOT

• Determination of Design Gyration Levels for SMA – GADOT

• Verification of NDesign Levels – GADOT

Original SGC Compaction Effort

Design Average Design High Air Temperature

ESALs <39 ºC 39 - 40 ºC 41 - 42 ºC 43 - 44 ºC

(millions) Nini

Ndes

Nmax

Nini

Ndes

Nmax

Nini

Ndes

Nmax

Nini

Ndes

Nmax

0.3 7 68 104 7 74 114 7 78 121 7 82 127

0.3 - 1 7 76 117 7 83 129 7 88 138 8 93 146

1 - 3 7 86 134 8 95 150 8 100 158 8 105 167

3 - 10 8 96 152 8 106 169 8 113 181 9 119 192

10 - 30 8 109 174 9 121 195 9 128 208 9 135 220

30 - 100 9 126 204 9 139 228 9 146 240 10 153 253

100 9 143 233 10 158 262 10 165 275 10 172 288

SGC Compaction Effort 1999ESAL’s N ini N des N max App

< 0.3 6 50 75 Light

0.3 to < 3 7 75 115 Medium

3 to < 30 8 100* 160 High

10 to <30 8 100 160 High

> 30 9 125 205 Heavy

Base mix (< 100 mm) option to drop one level, unless the

mix will be exposed to traffic during construction.

What is at Stake with NDesign?

• Design asphalt content will increase with reduced gyrations for the same gradation

• Some argue VMA is the key

• For the same gradation, reducing NDesign

increases VMA

• Higher NDesign tends to force coarser mixes

• It is believed that higher gyration levels produce mixes that are harder to place and compact

Definition of Locking Point

1 2 3 4 5 6 7 8 9 10

60 111.9 111.9 111.8 111.8 111.7 111.7 111.6 111.6 111.5 111.5

70 111.4 111.4 111.3 111.3 111.2 111.2 111.2 111.1 111.1 111.0

80 111.0 110.9 110.9 110.8 110.8 110.8 110.7 110.7 110.7 110.6

Locking Point

Locking Point

• Concept developed by Illinois DOT

• Plot of Log gyrations vs. density non-linear beyond locking point

• Point where aggregate locks together –additional gyrations degrade aggregate

• Point after which change rule 25 gyrations = 1% VMA = 0.4 AC% generally true

ALDOT Specifications

ESALs Base and

Lower Binder

Surface and

Upper Binder

< 1 million 50 65

1 to 10 million 65 80

10 to 30 million 80 80

Use lesser of locking point (> 60) or specified

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

No

. of

Gyr

atio

ns

1 2 3 4 5 6 7 8

Project

Locking Point Comparison

LP-1

LP-2

LP-3

Avg. Std.

LP-1 52 8.7

LP-2 58 9.4

LP-3 82 10.3

GA DOT

• Special Provision for Locking Point

• Level I – two-way AADT < 10,000

– Lesser of first locking point or 65 gyrations

• Level II - > 10,000 AADT

– Lesser of second locking point or 80

gyrations

AL-5CO-2

AL-4MI-2

IL-3

2-1

2-2

2-3

3-1

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Pine Locking Point

2-1

2-2

2-3

3-1

Premise of NCHRP 9-9 (1)

Experimental Plan

• Laboratory compaction effort should produce sample density approximately equal to ultimate pavement density

• Ultimate pavement density believed to be reached after 2-3 years of traffic

• Typically, select laboratory density of 96% of Theoretical maximum density or 4% air voids– Too little air voids (<2%) results in rutting

– Too many air voids tend to cause durability problems

NCHRP 9-9 (1): Field Project Locations

Legend

: Project Site

Distribution of Design Traffic

0

1

2

3

4

5

6

7

8

9

10

300,000 1,000,000 3,000,000 10,000,000 30,000,000 > 30,000,000

20-Year Design ESALs

Num

ber

of

Pro

jects

Experimental Plan

• Roadway cores taken at construction, 3

months, 6 months, 1 year and 2 years

after construction from right wheel path

• Project extended to monitor projects 4

years after construction

• Goal: predict gyrations to match field

density

Cumulative Frequency of Construction

Densities

0102030405060708090

100

84 85 86 87 88 89 90 91 92 93 94 95 96

Percent Maximum Density, %

Cu

mu

lati

ve F

req

uen

cy,

%

55%

78%

Cores indicate adequate

field compaction not being

achieved

y = 1.3832x1.0041

R2 = 0.9584

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0

Brand 1 SGC

Bra

nd

2 S

GC

Line of Equality

Comparison of Number of Gyrations to

Match Field Density for Two Compactors

Brand 1 angle = 1.24

Brand 2 angle = 1.15

y = 0.9748x + 3.1728

R2 = 0.98

0

10

20

30

40

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110 120

Pine 3-2-2 Locking Point

Tro

xle

r 3

-2-2

Lo

ckin

g P

oin

t

Line of Equality

y = 0.7201x + 23.883

R2 = 0.2637

y = 0.6905x + 27.214

R2 = 0.2474

87.0

88.0

89.0

90.0

91.0

92.0

93.0

94.0

95.0

96.0

97.0

87.0 88.0 89.0 90.0 91.0 92.0 93.0 94.0 95.0 96.0 97.0

Locking Point 2-1 Density, %

As-C

on

str

ucte

d D

esn

ity,

%

Pine Troxler Linear (Pine) Linear (Troxler)

3-2-2 Locking Point

Histogram

0

2

4

6

8

10

12

14

50 65 80 100 More

Bin

Fre

qu

en

cy

Frequency

Pine

y = 0.6114x + 36.101

R2 = 0.21

Troxler

y = 0.6561x + 32.323

R2 = 0.29

91.0

92.0

93.0

94.0

95.0

96.0

97.0

98.0

99.0

91.0 92.0 93.0 94.0 95.0 96.0 97.0 98.0 99.0

3-2-2 Locking Point Density, %

2-Y

ear

In-P

lace D

ensity,

%

Pine Troxler Linear (Pine) Linear (Troxler)

0

20

40

60

80

100

120

Granite Gravel Dolomite Limestone Sandstone Slag

3-2

-2 L

ockin

g P

oin

t

SMA Locking Point

Xie 2006

Xie 2006

Xie 2006

NCHRP 9-9(1)

Recommendations

y = 0.9752x + 2.6026

R2 = 0.84

y = 1.1005x + 10.837

R2 = 0.75

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

Predicted Gyrations at 2 Years for Pine Compactor

Pre

dic

ted

Gyra

tio

ns a

t T

wo

Ye

ars

fo

r T

roxle

r

Co

mp

acto

r

1.16 Raw Linear (1.16) Linear (Raw)

Line of Equality

SGC Densities Adjusted to DIA of 1.16 Degrees

Model Developed Based on %

of Lab Density

• Model developed to predict % of lab density,

based on as-constructed density, HGP and

traffic (R2=0.53)

• Number of gyrations to match % of lab

density similar for all mixes (STD

approximately 8)

ESALsYearLogHPGNdesign 201.2027.18.16

HPG = High temperature binder grade

R2=0.97, SE = 3.54

Proposed Ndesign Levels for an SGC DIA of 1.16 0.02 Degrees

20-Year Design

Traffic, ESALs

2-Year Design

Traffic, ESALs

Ndesign

Unmodified

Ndesign PG

76-22

< 300,000 < 30,000 50 NA

300,000 to

3,000,000

30,000 to 230,000 65 50

3,000,000 to

10,000,000

230,000 to 925,000 80 65

10,000,000 to

30,000,000

925,000 to

2,500,000

80 65

> 30,000,000 > 2,500,000 100 80

Based on equation to predict Ndesign at 92% ACD

Recommended Ndesign Table

Effect of Design Compaction

Property Increased Ndesign Decreased Ndesign

Coarse Aggregate

Angularity

Increased demand for

crushed aggregate

Reduced demand for

crushed aggregate or no

change

Fine aggregate

angularity

Reduce natural sand Reduced need for

manufactured sand or no

change

Gradation Change to increase VMA Change to reduce VMA

or no change

Air Voids No effect No effect

Voids in Mineral

Aggregate

No effect after mix

adjustment

No effect after mix

adjustment

Voids filled with asphalt Little or no change Little or no change

Compaction on road More difficult Less difficult

Mixture stiffness Increased stiffness Decreased stiffness

Huber and Anderson AAPT

Why Change?

• 23 states have altered Ndesign levels

(Gibson)

• Examples:

– Georgia, Ohio, and Virginia all using 65 gyrations;

– Many states continue to use AASHTO levels (e.g.

50, 75, 100), but have altered traffic – New Jersey

is an example of this

• One of the goals of Superpave was

unification

Ninitial and Nmax

• 12 of 40 Projects had at least one sample which failed Ninitial

– One of these projects reported as being tender

– No conclusive evidence to keep or eliminate

• 25 of 40 Projects had at least one sample which failed Nmax

– Maximum rutting 9 mm after 4 years (one site at one project)

– Nmax does not appear to be related to field rutting

Summary

• Need to balance rut resistance, durability and constructability

• Pavements studied have been rut resistant

• Locking point appears to be a tool to identify aggregate interlock point

• Gyratory type or internal angle effect recommendations

Questions?