Background

• Current ALDOT pavement thickness design

based on AASHO Road Test

– 1993 AASHTO Design Guide

• Structural coefficients (ai) are key inputs

– Express relative “strength” of component layers

– Used to determine required thicknesses of layers

• In 2009, ALDOT was using values set in 1990

– No changes between 1990 and 2009

Pavement Design in the U.S. Pierce and McGovern, 2013

NCHRP Project 20-05, Topic 44-06

Asphalt Structural Coefficients

2 10 13 1 14

5 5

1993 Design Guide Based on AASHO Road Test

HRB, 1962

HRB, 1962

HRB, 1962

HRB, 1962

PSI

ESALs

PSI

po

pt

DPSI

AASHO HMA Coefficients

Loop Layer Coefficient

(a1)

Test

Sections

R2

2 0.83 44 0.80

3 0.44 60 0.83

4 0.44 60 0.90

5 0.47 60 0.92

6 0.33 60 0.81

HRB, 1962

ALDOT Recommended Values (1990)

Holman, 1990

Flexible Pavement Design Curves

HRB, 1962

Structural Coefficient in Design SN3 SN2 SN1

SN1 = a1D1

SN2 = a1D1 + a2D2

SN3 = a1D1 + a2D2 + a3D3 D1 = SN1/a1

AASHTO Design Equation

07.8log32.2

1

10944.0

5.12.4log

20.01log36.9log

19.5

018

D

RR M

SN

PSI

SNSZW

Problem Statement

• Given new advances in mixture technology

(Superpave, SMA, polymer-modification), there

is a need to update the structural coefficient to

reflect actual performance in Alabama

Past Recalibration Efforts

• Many studies, few changes

• Most studies focus on computing a1 from deflection data

• Previous values range from 0.44 to 0.60

• Previous Test Track study found 0.59 using very thick sections from 2000 experiment

– Calibrated to deflection not performance

Recalibration Procedure

Actual Traffic

(Loads, Repetitions)

Actual Performance

(weekly IRI measurements)

SN

a1

IRIePSI 0041.05

DPSI pt

AASHTO Design

Equation

Predicted Traffic

AASHTO ESAL

Equation

Measured Traffic

Measured Traffic

Predicted Traffic

Uncalibrated

Calibrated

2003 Test Sections

0

5

10

15

20

25

30

35

40

N1 N2 N3 N4 N5 N6 N7 N8

Test Section

Depth

, in

.

Modified HMA (PG 76-22)

Unmodified HMA (PG 67-22)

SMA (PG 76-22)

Unmodified HMA (PG 67-22), Opt +0.5%

Crushed Aggregate Base CourseImproved Roadbed (A-4(0)) Soil

2006 Test Sections

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 S11

As B

uilt

Th

ickn

ess, in

.

PG 67-22 PG 76-22 PG 76-22 (SMA) PG 76-28 (SMA)

PG 76-28 PG 64-22 PG 64-22 (2% Air Voids) PG 70-22

Limerock Base Granite Base Type 5 Base Track Soil Seale Subgrade

Florida

(new)

Alabama & FHWA

(left in-place)

Oklahoma

(new)

FHWA

Missouri

(new)

Alabama

(new)

N1 PSI vs Date

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

28-Jun-03 14-Jan-04 01-Aug-04 17-Feb-05 05-Sep-05 24-Mar-06

Date

PS

I

LPSI

RPSI

AvgPSI

Pt

Pt calibration points

DPSI

N3 PSI vs. Date

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

28-Jun-03 09-Nov-04 24-Mar-06 06-Aug-07 18-Dec-08

Date

PS

I

LPSI

RPSI

AvgPSI

Actual

Modeled

N1 – Predicted and Measured Traffic

Predicted ESALs Measured ESALs Difference % Error

802,367 2,267,922 1,465,555 65% 1,126,574 2,837,091 1,710,517 60% 1,270,712 2,963,064 1,692,352 57% 1,638,661 3,212,141 1,573,480 49% 2,340,290 4,321,771 1,981,481 46%

a1 = 0.44 (R2 = 0.08)

Predicted ESALs Measured ESALs Difference % Error

1,314,680 2,224,691 910012 41% 2,007,491 2,806,554 799065 28% 2,332,763 2,939,906 607145 21% 3,203,489 3,207,147 3661 0% 4,996,650 4,353,456 643194 15%

a1 = 0.50 (R2 = 0.74)

a1 Summary

0.50

0.590.56

0.63 0.620.58

0.48

0.59 0.58

0.43

0.480.44

0.41

0.68

0.54

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

N1 2

003

N1 2

006

N2 2

003

N2 2

006

N3 2

003

-2006

N4 2

003

-2006

N5 2

006

N6 2

003

-2006

N7 2

003

-2006

N8 2

003

N8 2

006

N9 2

006

N10 2

00

6

S1

1 2

00

6

Ave

rag

e

La

ye

r C

oe

ffic

ien

t

Uncalibrated (a1=0.44)

0.0E+00

5.0E+06

1.0E+07

1.5E+07

2.0E+07

0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07

Measured ESALs

Pre

dic

ted

ES

AL

s

N1 2003

N1 2006

N2 2003

N2 2006

N3 2003-2006

N4 2003-2006

N5 2006

N6 2003-2006

N7 2003-2006

N8 2003

N8 2006

N9 2006

N10 2006

S11 2006

Calibrated (a1=0.54)

0.0E+00

5.0E+06

1.0E+07

1.5E+07

2.0E+07

0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07

Measured ESALs

Pre

dic

ted

ES

AL

s

N1 2003

N1 2006

N2 2003

N2 2006

N3 2003-2006

N4 2003-2006

N5 2006

N6 2003-2006

N7 2003-2006

N8 2003

N8 2006

N9 2006

N10 2006

S11 2006

Further Justification for a1 = 0.54 1993 AASHTO Design Guide

TeE

2

1

Backcalculated HMA Moduli

a1 = 0.171Ln(HMA Modulus) - 1.784

R2 = 0.998

0

0.1

0.2

0.3

0.4

0.5

0.6

0.E+00 1.E+05 2.E+05 3.E+05 4.E+05 5.E+05 6.E+05 7.E+05 8.E+05 9.E+05

HMA Modulus at 68F

Asp

ha

lt S

tru

ctu

ral C

oe

ffic

ien

t

a1 = 0.171Ln(HMA Modulus) - 1.784

R2 = 0.998

0

0.1

0.2

0.3

0.4

0.5

0.6

0.E+00 1.E+05 2.E+05 3.E+05 4.E+05 5.E+05 6.E+05 7.E+05 8.E+05 9.E+05

HMA Modulus at 68F

Asp

ha

lt S

tru

ctu

ral C

oe

ffic

ien

t

811,115 psi

a1 = 0.54

Effect on Pavement Design

0

2

4

6

8

10

12

14

1,000,000 10,000,000 100,000,000 1,000,000,000

ESALs

HM

A D

ep

th (

in)

a1 = 0.44

a1 = 0.54

18.5% Thinner

Minimum Thickness

• Not calibrated for thicknesses < 5”

• Need recommendation for thinner sections

• Lower volume recommendation

– If new coefficient (0.54) results in thickness

< 5”, use old coefficient (0.44)

• If resulting thickness > 5”; use 5”

• ALDOT Implementation

– No designs < 5”

Structural Coefficient Implementation

Conclusions

• New advances in mix design technology

warrants recalibrating structural coefficient of

HMA

• Recalibration using NCAT Test Track data

resulted in average a1 = 0.54

– Believed to be conservative estimate

• Using 0.54 instead of 0.44 yields 18.5%

reduction in AC thickness

Further Information

Available at the NCAT Website

http://www.ncat.us/

http://www.okladot.state.ok.us/index.php