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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/