Recalibration of the Asphalt Layer Coefficient
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Transcript of Recalibration of the Asphalt Layer Coefficient
8/18/2019 Recalibration of the Asphalt Layer Coefficient
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Recalibration of the Asphalt
Layer Coefficient
Dr. David H. Timm, P.E.
Mrs. Kendra Peters-Davis
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Overview
• Current ALDOT pavement design based on
AASHO Road Test
• Structural coefficients (ai) are key inputs
– Express relative “strength” of component layers
– Used to determine required thicknesses of layers
• Current ALDOT asphalt coefficients wereofficially set in 1990
– No changes since then
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Structural Coefficient in DesignSN3 SN2 SN1
SN1 = a1D1
SN2 = a1D1 + a2D2
SN3 = a1D1 + a2D2 + a3D3 D1 = SN1/a1
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AASHTO Design Equation
07.8log32.2
1
10944.0
5.12.4log
20.01log36.9log
19.5
018
R R M
SN
PSI
SN S Z W
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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 0.926 0.33 60 0.81
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Current ALDOT Asphalt
Coefficients
Pavement Material Structural Coefficient
Hot Mix Asphalt 0.44
Sand Asphalt 0.40
Road Mix (Low Stability) 0.20
Limestone Agg. Base 0.14
Granite Agg. Base 0.12
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Problem Statement
• Given new advances in mixture technology
(Superpave, SMA, polymer-modification), there
is a need to update the structural coefficient toreflect actual performance in Alabama
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Objectives
1. Quantify sensitivity of design equation
2. Recalibrate equation to match observed
performance
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Scope of Work
• Literature Review
– Past recalibration efforts
• Sensitivity Analysis
– Rank variables from most to least important• Recalibration using NCAT Test Track
performance data
– 2003 and 2006 Test Sections
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Past Recalibration Efforts
• Many studies, few changes
• Most studies focus on computing a1
fromdeflection data
• Previous values range from 0.44 to 0.60
• Previous Test Track study found 0.59
using very thick sections from 2000experiment
– Calibrated to deflection not performance
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Sensitivity Analysis
Parameter Range
Layer coefficient (a1) 0.20 – 0.60
Traffic level (W18) 1e6 – 1e9 ESALs
Resilient modulus (MR
) 3,000 – 30,000 psi
Reliability (R) 50% – 99%
Change in serviceability (ΔPSI) 1 – 2.5
Variability (So
) 0.20 – 0.60
• 3-layer pavement (HMA, aggregate base, soil)
• 5,120 thicknesses calculated
• Determined correlation coefficients betweenHMA thickness and input parameters
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Results of Sensitivity AnalysisParameter Correlation Coefficient
Layer coefficient (a1) -0.518Traffic level (W18) 0.483
Resilient modulus (MR ) -0.425
Reliability (R) 0.157Change in serviceability (ΔPSI) -0.141
Variability (So) 0.083
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Recalibration Procedure
Actual Traffic
(Loads, Repetitions)
Actual Performance
(weekly IRI measurements)
IRI ePSI 0041.05
IRI ePSI 0041.05
SN
a1
IRI ePSI
0041.05
PSIpt
AASHTO Design
Equation
Predicted Traffic
AASHTO ESALEquation
Measured Traffic
Measured Traffic
Predicted Traffic
Uncalibrated
Calibrated
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2006 Test Sections
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.022.0
24.0
N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 S11
A s B u i l t T h i c k n e s s ,
i n .
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)
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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-06Date
P S I
LPSI
RPSI
AvgPSI
Pt
Pt calibration points
PSI
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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-08Date
P S I
LPSI
RPSI
AvgPSI
Actual
Modeled
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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 = 0.55 (R2 = 0.74)
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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
N 1
2 0 0 3
N 1
2 0 0 6
N 2
2 0 0 3
N 2
2 0 0 6
N 3
2 0 0 3 - 2 0 0 6
N 4
2 0 0 3 - 2 0 0 6
N 5
2 0 0 6
N 6
2 0 0 3 - 2 0 0 6
N 7
2 0 0 3 - 2 0 0 6
N 8
2 0 0 3
N 8
2 0 0 6
N 9
2 0 0 6
N 1 0
2 0 0 6
S 1 1
2 0 0 6
A v e r a g e
L a y e r C o e f f i c i e n
t
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Calibrated
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
P r e d i c
t e d E S A L s
N1 2003
N1 2006
N2 2003
N2 2006N3 2003-2006
N4 2003-2006
N5 2006
N6 2003-2006
N7 2003-2006
N8 2003
N8 2006
N9 2006
N10 2006S11 2006
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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
H M A
D e p t h ( i n )
a1 = 0.44
a1 = 0.54
18.5% Thinner
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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”
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Conclusions
• New advances in mix design technologywarrants recalibrating structural coefficient of
HMA
• Structural coefficient has greatest impact of
all design variables on pavement thickness
• Recalibration using NCAT Test Track dataresulted in average a1 = 0.54
– Believed to be conservative estimate
• Using 0.54 instead of 0.44 yields 18.5%
reduction in HMA thickness
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Structural Coefficient Status
• ALDOT has implemented new coefficient
for rehabilitation/overlay design
• ALDOT will soon implement new
coefficient on all new construction
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Washington State DOT Design
Table
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Acknowledgements