SENSITIVITY EVALUATION OF THE MEPDG FOR FLEXIBLE PAVEMENTS TRB Webinar …onlinepubs.trb.org/onlinepubs/webinars/120725.pdf · TRB Webinar July 25, 2012 Presenter: Charles W. Schwartz

SENSITIVITY EVALUATION OF THE MEPDG FOR FLEXIBLE PAVEMENTS TRB Webinar July 25, 2012

Presenter:

Charles W. Schwartz University of Maryland

Moderator:

Trenton Clark Virginia Asphalt Association

2

University of Maryland

• Charles Schwartz (PI)

• Rui Li

NCHRP 1-47 Project Panel

• Kevin Hall, U AR (Chair)

• Michael Ayers, ACPA

• Imad Basheer, CA DOT

• Mohamed Elfino, VA DOT

• Laura Fenley, WI DOT

• Geoffrey Hall, MD SHA

• Kent Hansen, NAPA

• Tommy Nantung, IN DOT

• Richard Zamora, CO DOT

• Tom Yu, FHWA Liaison

• Stephen Maher, TRB Liaison

Iowa State University

• Halil Ceylan (Co-PI)

• Sung Hwan Kim

• K. Gopalakrishnan

NCHRP Staff

• Amir Hanna, Program Officer

3 3

NCHRP 1-47 Final Report

http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP01-47_FR.pdf

4

Webinar Outline

• Study Objectives

• Sensitivity Analysis Concepts

• Design Inputs for Study

• Global Sensitivity Analysis (GSA) Methodology

• Results from GSA

• Key Findings and Conclusions

• Discussion

4

5

Motivation

5

“Research is needed to determine the degree of sensitivity of the performance predicted by the MEPDG to input parameter values… for specific climatic region and traffic conditions.... Users can [then] focus efforts on those input parameters that will greatly influence the pavement design.” − NCHRP 1-47 RFP

Traffic Volume Vehicle Mix

Vehicle Speed

HMA Thickness HMA Stiffness PCC Slab Geometry PCC Strength Base Thickness

Base Modulus Subgrade Modulus SWCC Parameters

Focus on Inputs under Control of Project Designer

Definable Refinable

6 6

QUESTION FOR AUDIENCE

7

What is the Mechanistic-Empirical Pavement Design Guide (MEPDG)?

7

Empirical

TransferFunctions

INPUT

TRAFFIC

CLIMATE

PAVEMENT

Mechanistic

Analytic

International

Roughness Index

Asphalt Concr.

Rutting

Total Rutting

OUTPUTLongitudinal

Cracking

Alligator Cracking

Thermal Cracking

Mechanistic

Pavement

Analysis

Models

Empirical

Pavement

Analysis

Models

z

xy

z

y

x

z

xy

z

y

x

8

MEPDG Software

8

Version 1.100 used for study

Not available at time of study Same models Same results

✔

✗

9

Research Objective

Determine the sensitivity of the pavement performance predicted by the MEPDG to variability of the material property design input values for flexible pavements.

9

MR

MR

ν

ν

SWCC

Groundwater Depth

SWCC (Soil Water Characteristic Curve)

P200, D60, PI

P200, D60, PI

volume, speed

SSA Va, Vbe, ν, PG, E*(α, δ),

thermal properties, etc.

Climate

h1

h2

Sensitivity is high Sensitivity is Low

(Flexible Pavement)

10

Webinar Outline







• Discussion

10

11

Types of Sensitivity Analyses

Global Sensitivity Analyses

11

Local Sensitivity Analyses

Ou

tpu

t y

Input x

Fre

qu

ency

(S

I)

Sensitivity Index SI

SI ,s

SI

• Each input varied one-at-a-time • Only evaluates sensitivity around

the reference value(s) • Ignores input correlations,

interactions • Employed in most past studies

• All inputs varied simultaneously • Evaluates sensitivity over the entire

problem domain • Can include input correlations • Can quanitify input interactions • Extremely computation intensive

Model: y = f (x)

(x

0, y

0)

SI =

f (x0+ Dx) - f (x

0- Dx)

2Dx

12

Mo

del

Ou

tpu

t Y

Model Input X

Model Output

Regression

Mo

del O

utpu

t Y j

Model Input Xk

X1

X2

Sensitivity Metrics: The Past

r=1.0 ρ=1.0 Slope=0.1

r=1.0 ρ=1.0 Slope=1.0

Instantaneous Slope

Average Slope

Correlation Coefficients - Pearson r - Spearman ρ

Multivariate Linear Regression - Regression coefficients

(average slopes)

Sensitivity ΔY/ΔX ΔOutput/ΔInput !!

13

Selected Sensitivity Metric: Design Limit Normalized Sensitivity Index (NSI)

baseline value of design input k

change in design input k about the baseline

change in predicted distress j corresponding to change of design input

the design limit of the distress j

Design Limit Normalized Sensitivity Index for Design Input k and Distress j

Sjk

DL =DY

j/ DL

j( )DX

k/ X

k( )

Example: Total Rutting Distress, Design Limit (DL) =0.75 in. Normalized Sensitivity of Total Rutting to Base MR = -0.50 What is change in Total Rutting if Base MR is increased by ΔX/X = 20%? Change in Total Rutting ΔY = (-0.50)(20%)(0.75 in) = -0.075 in.

14

Webinar Outline







• Discussion

14

15

Base Cases

• 5 pavement types x 5 climates x 3 traffic levels

• Total number of base cases = 75

Note: HMA/JPCP on Stiff Foundation is intended to represent both new construction/reconstruction on stabilized foundations or rehabilitation on underlying HMA/PCC layers.

16

Climate Categories

Climate

Category Location Weather Station Months

of Data

Binder Grade

Baseline Range

Hot-Wet Orlando FL ORLANDO

INTERNATIONAL ARPT

116 PG 70-10 PG 64-10

PG 76-10

Hot-Dry Phoenix AZ PHOENIX SKY HARBOR

INTL AP

116 PG 76-10 PG 70-10

PG 82-10

Cold-Wet Portland ME PORTLAND INTL

JETPORT ARPT

116 PG 52-28 PG 52-34

PG 52-22

Cold-Dry International

Falls MN

FALLS INTERNATIONAL

ARPT

112 PG 58-28 PG 58-34

PG 58-22

Temperate Los Angeles CA LOS ANGELES INTL

AIRPORT

108 PG 58-10 PG 52-10

PG 64-10

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Binder Grades

17

Climate Baseline PG PGHigh- PGLow+ PGLowTC PGLowTC+

Cold-Dry PG 52-40 NA PG 58-34 PG 52-28 PG 52-22

Cold-Wet PG 52-28 NA PG 52-22 PG 52-16 PG 52-10

Hot-Dry PG 76-10 PG 70-10 NA NA NA

Hot-Wet PG 70-10 PG 64-10 NA NA NA

Temperate PG 58-10 PG 52-10 NA NA NA

PGLowTC, PGLowTC+ used for Thermal Cracking sensitivity analyses.

18

Traffic Levels

Traffic

Category

Baseline Inputs

AADTT Range AADTT1

AADTT in

Design

Lane

Est. ESALs

(Flexible)5

Est. ESALs

(Rigid)6

Low 1,000 3752 2M 5M 500-5,000

Medium 7,500 2,0633 10M 25M 5,000-10,000

High 25,000 6,2504 30M 75M 20,000-30,000

1Based on MEPDG Interstate Highway TTC4 Level 3 default vehicle distribution. Potential correlations between operating speed and AADTT have been ignored.

250% directional split, 2 lanes per direction, 0.75 lane factor for design lane.



5Based on 15 year design life.

6Based on 25 year design life.

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Baseline Flexible Pavement Sections

19

Traffic

Level

Input

Parameter

Low Traffic Medium Traffic High Traffic

Baseline Min Max Baseline Min Max Baseline Min Max

AADTT-Nominal 1000 500 5000 7500 5000 10000 25000 20000 30000

Design Lane 375 188 1875 2063 1375 2750 6250 5000 7500

HMA Thickness (in) 6.5 5 8 10 8 12 12.5 10 15

Base Thickness (in) 6 1 10 7 1 14 9 1 18

20

Design Inputs For Evaluation

Initial Triage - Literature review - Engineering judgment

OAT Local Sensitivity Analyses

(Nearly) All MEPDG Design Inputs

> Sensitive MEPDG Design Inputs

Global Sensitivity Analyses

21

Design Inputs Given Special Consideration

• Unbound Material Properties

– Gradation, plasticity inputs for soil water characteristic curve (SWCC) correlated to resilient modulus MR

• HMA Dynamic Modulus

– Synthetic Level 1 dynamic modulus |E*| data

• HMA Low Temperature Properties

– Creep compliance correlated with stiffness

21

22

Soil Water Characteristic Curve (SWCC)

22

23

Unbound Material Properties

A design input value for Mr is selected first and then gradation data giving compatible values of P200 and D60 are determined via correlations along with compatible values for PI and LL.

SWCC – function of: • Percent passing the No. 200 sieve (P200) • D60 gradation parameter

• Plasticity index (PI)

24

Unbound Material Properties

P200 D60

%Pass =100

eb+g log

10Size( )

25

HMA Dynamic Modulus

δ = lower shelf stiffness

δ+α = upper shelf stiffness

f = loading frequency (from traffic speed)

η = binder viscosity (from binder grade, temperature);

used to generate synthetic Level 1 G*, ϕ

Design Input = Synthetic Level 1 Dynamic Modulus Data

26

Design Inputs (New HMA)

27

28

Webinar Outline







• Discussion

28

29

Global Sensitivity Analyses Procedure

29

Sample the Entire Problem Domain

• Latin Hypercube

MEPDG GSA Simulations

• Over 40,000 MEPDG runs (Each run takes up to 0.5 hr on a single computer)

Respond Surface Modeling (RSM)

• Continuous response surface model (RSM) for the randomly located GSA simulation results.

Sensitivity Index Statistics

• Based on 10,000 RSM evaluations/base case

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Sampling with Complete Coverage

x1

x1

x1

x2

x2

x2

Monte Carlo n=25

Monte Carlo n=100

Latin Hypercube n=25

??

✔

31

AutoIt Windows Scripting • Automates “manual” entry of input values to MEPDG.

• Extracts results from MEPDG output spreadsheets.

31

6000+ lines of codes.

…

Cloud Storage

32

“Parallel Processing”

32

UMD Pavement Materials Lab

ISU Jelling Lab

40,000+ MEPDG Runs

33

Predicted Distresses Span Wide Range

New HMA Cold-Wet Climate 1300 Simulations

34

Respond Surface Modeling for Derivatives

34

• Continuous response surface model (RSM) fit to the randomly located GSA simulation results

• Enables computation of sensitivity index derivatives

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10

y

x

35

Respond Surface Modeling Artificial Neural Networks

35

Input Layer

23

Hidden Layer

5

Output Layer

1

Outputs

1

p1

p2

p23

…

p3

Σ

Σ

Σ

Σ

Σ

f 1

f 1

f 1

f 1

f 1

Σ f 2

iw1,1

0,1

iw23,5

0,1

b1

1

b2

1

b3

1

b4

1

b5

1

n1

1

n2

1

n3

1

n4

1

n5

1

a1

1

a2

1

a3

1

a4

1

a5

1

iw1,1

1,2

iw5,1

1,2

b1

2

n1

2

a1

2

Climate Type, Traffic Speed,

Pavement structure, etc…

Rutting Depth

36 Excellent Performance of ANN RSMs 36

MEPDG Predicted

RSM

Pre

dic

ted

Long Cracking Alligator Cracking AC Rutting Total Rutting IRI

CD

CW

T

HW

HD

37

Spreadsheet for Quick MEPDG Calculation

37

38

Webinar Outline







• Discussion

38

39 39

QUESTION FOR AUDIENCE

40

GSA Sensitivity Index Distributions

New HMA – AC Rut Depth All Climate Zones, Traffic Levels 10,000 Simulations w/ Censoring

Sensitivity Metric for Ranking: NSIμ+2σ=Mean SI + 2 Standard Deviations

41

Design Input Maximum NSIm +2s Values (ANN RSMs)

Long.

Crack

Alligator

Crack

Thermal

Crack

AC Rut

Depth

Total Rut

Depth IRI Max

HMA E* Alpha Parameter

-29.52 -15.94 -0.58 -24.40 -8.98 -3.58 -29.52

HMA E* Delta Parameter

-23.87 -13.18 2.41 -24.43 -8.99 -2.80 -24.43

HMA Thickness -10.31 -7.46 -0.86 -4.21 -1.58 -1.11 -10.31

HMA Creep Compliance m Exponent N.A. N.A. -4.85 N.A. N.A. N.A. -4.85

Base Resilient Modulus -4.72 -2.73 -0.17 0.14 -0.15 -0.36 -4.72

Surface Shortwave Absorptivity 4.32 1.28 -0.20 4.65 1.67 0.67 4.65

HMA Air Voids 4.47 3.39 1.33 -0.05 0.03 0.29 4.47

HMA Poisson’s Ratio -2.38 -1.01 0.23 -4.33 -1.46 -0.43 -4.33

Traffic Volume (AADTT) 3.72 3.94 0.02 1.87 0.66 0.51 3.94

HMA Effective Binder Volume -3.88 -2.93 -0.17 0.05 0.06 -0.24 -3.88

Subgrade Resilient Modulus -2.07 -3.41 0.15 0.08 -0.28 -0.44 -3.41

Base Thickness -2.40 -1.02 -0.03 0.22 0.04 -0.09 -2.40

Subgrade Percent Passing No. 200 -1.71 -0.68 0.08 -0.10 -0.10 -0.12 -1.71

HMA Tensile Strength at 14oF N.A. N.A. -1.59 N.A. N.A. N.A. -1.59

Operational Speed -1.26 -0.83 -0.04 -1.06 -0.39 -0.15 -1.26

HMA Creep Compliance D Parameter N.A. N.A. -1.03 N.A. N.A. N.A. -1.03

HMA Unit Weight -0.88 0.97 -0.76 -0.88 -0.30 -0.08 0.97

Base Poisson’s Ratio 0.91 0.90 0.18 -0.19 -0.05 0.09 0.91

HMA Heat Capacity -0.76 -0.55 -0.77 -0.81 -0.28 -0.14 -0.81

Subgrade Liquid Limit -0.67 -0.79 -0.10 -0.10 0.07 0.03 -0.79 Binder Low Temperature PG 0.56 0.09 -0.74 0.25 0.09 0.02 -0.74

HMA Thermal Conductivity -0.53 -0.40 -0.67 0.20 0.04 0.02 -0.67

Binder High Temperature PG -0.60 -0.48 0.00 -0.66 -0.25 -0.09 -0.66 Subgrade Poisson’s Ratio 0.44 -0.59 0.16 0.08 0.07 0.04 -0.59

Groundwater Depth 0.20 -0.16 0.08 0.01 -0.02 -0.02 0.20

Subgrade Plasticity Index -0.15 0.11 0.03 0.01 0.02 0.00 -0.15

Aggregate Coef. Of Thermal Contraction N.A. N.A. -0.07 N.A. N.A. N.A. -0.07

Hypersensitive Very Sensitive Sensitive Non-Sensitive

42

Design Input Maximum NSIm +2s Values (ANN RSMs)

Long.

Crack

Alligator

Crack

Thermal

Crack

AC Rut

Depth

Total Rut

Depth IRI Max

HMA E* Alpha Parameter

-29.52 -15.94 -0.58 -24.40 -8.98 -3.58 -29.52

HMA E* Delta Parameter

-23.87 -13.18 2.41 -24.43 -8.99 -2.80 -24.43

HMA Thickness -10.31 -7.46 -0.86 -4.21 -1.58 -1.11 -10.31

HMA Creep Compliance m Exponent N.A. N.A. -4.85 N.A. N.A. N.A. -4.85

Base Resilient Modulus -4.72 -2.73 -0.17 0.14 -0.15 -0.36 -4.72

Surface Shortwave Absorptivity 4.32 1.28 -0.20 4.65 1.67 0.67 4.65

HMA Air Voids 4.47 3.39 1.33 -0.05 0.03 0.29 4.47

HMA Poisson’s Ratio -2.38 -1.01 0.23 -4.33 -1.46 -0.43 -4.33

Traffic Volume (AADTT) 3.72 3.94 0.02 1.87 0.66 0.51 3.94

HMA Effective Binder Volume -3.88 -2.93 -0.17 0.05 0.06 -0.24 -3.88

Subgrade Resilient Modulus -2.07 -3.41 0.15 0.08 -0.28 -0.44 -3.41

Base Thickness -2.40 -1.02 -0.03 0.22 0.04 -0.09 -2.40

Subgrade Percent Passing No. 200 -1.71 -0.68 0.08 -0.10 -0.10 -0.12 -1.71

HMA Tensile Strength at 14oF N.A. N.A. -1.59 N.A. N.A. N.A. -1.59

Operational Speed -1.26 -0.83 -0.04 -1.06 -0.39 -0.15 -1.26

HMA Creep Compliance D Parameter N.A. N.A. -1.03 N.A. N.A. N.A. -1.03

HMA Unit Weight -0.88 0.97 -0.76 -0.88 -0.30 -0.08 0.97

Base Poisson’s Ratio 0.91 0.90 0.18 -0.19 -0.05 0.09 0.91

HMA Heat Capacity -0.76 -0.55 -0.77 -0.81 -0.28 -0.14 -0.81

Subgrade Liquid Limit -0.67 -0.79 -0.10 -0.10 0.07 0.03 -0.79 Binder Low Temperature PG 0.56 0.09 -0.74 0.25 0.09 0.02 -0.74

HMA Thermal Conductivity -0.53 -0.40 -0.67 0.20 0.04 0.02 -0.67

Binder High Temperature PG -0.60 -0.48 0.00 -0.66 -0.25 -0.09 -0.66 Subgrade Poisson’s Ratio 0.44 -0.59 0.16 0.08 0.07 0.04 -0.59

Groundwater Depth 0.20 -0.16 0.08 0.01 -0.02 -0.02 0.20

Subgrade Plasticity Index -0.15 0.11 0.03 0.01 0.02 0.00 -0.15

Aggregate Coef. Of Thermal Contraction N.A. N.A. -0.07 N.A. N.A. N.A. -0.07

Hypersensitive Very Sensitive Sensitive Non-Sensitive

43

GSA Results: New HMA Max |NSI| Values for Longitudinal Cracking

44

GSA Results: New HMA Max |NSI| Values for Alligator Cracking

45

GSA Results: New HMA Max |NSI| Values for AC Rutting

46

GSA Results: New HMA Max |NSI| Values for Total Rutting

47

GSA Results: New HMA Max |NSI| Values for IRI

48

Most Sensitive Inputs by Property Category: New HMA

49

Webinar Outline







• Discussion

49

50

Key Findings: Methodology

• “Mean + 2 standard deviations” NSI is the best and most robust ranking metric – Relates change in design input to corresponding change in

predicted distress relative to its design limit

– Captures both the average sensitivity and its variability

– Categories: • Hypersensitive (HS): NSIμ+2σ > 5

• Very Sensitive (VS): 1 < NSIμ+2σ < 5

• Sensitive (S): 0.1 < NSIμ+2σ < 1

• Nonsensitive (NS): NSIμ+2σ < 0.1

• Artificial Neural Network response surface models provide robust and accurate relationships between design inputs and outputs

50

51

Key Findings: General

• Most NSI frequency distributions showed well-defined single peaks

– Indicates that NSI did not vary significantly over problem domain

• For all pavements, design inputs for the bound layers (i.e., HMA) exhibited the highest sensitivities

• Sensitivity values did not vary substantially or systematically across climate zones

51

52

Key Findings for Designers: Flexible

• Only HMA properties were most consistently in the highest sensitivity categories:

– HMA dynamic modulus lower (δ), upper (δ+α) shelves

– HMA thickness

– Surface shortwave absorptivity

– Poisson’s ratio

• Sensitivity values were consistently higher for longitudinal cracking, alligator cracking, and AC rutting than for IRI and thermal cracking

• Little or no thermal cracking was predicted when correct binder grade used

52

53

GSA of MEPDG using Level 3 dynamic modulus data.

53

Level 1 Input Level 3 Input

Asp

hal

t M

ix

Dynamic Modulus at different

combination of Temperature and

Frequency

Cumulative % Retained 3/4 inch sieve: Cumulative % Retained 3/8 inch sieve: Cumulative % Retained #4 sieve: % Passing #200 sieve:

Asp

hal

t

Bin

der

Shear Modulus and Phase Angle at 10

rad/sec at different temperatures Superpave binder grading

Asp

hal

t G

ener

al Effective binder content (%)

Air voids (%) Total unit weight(pcf) Poisson's Ratio Thermal conductivity asphalt (BTU/hr-

ft-F) Heat capacity asphalt (BTU/lb-F)

Effective binder content (%) Air voids (%) Total unit weight(pcf) Poisson's Ratio Thermal conductivity asphalt (BTU/hr-

ft-F) Heat capacity asphalt (BTU/lb-F)

54

Level 1 vs. Level 3 HMA |E*|

-35

-30

-25

-20

-15

-10

-5

0

5

10N

SI V

alu

es

(Me

an +

2 S

tDe

v)

Level 1

Level 3

55

Additional Key Findings: HMA Over Stiff

• Thermal conductivity and heat capacity of stabilized base are additional sensitive inputs

– Not inputs for non-stabilized base in flexible pavements

• Increased sensitivity of longitudinal cracking to operating speed

55

56

Issues for Further Study

• Base, subgrade design inputs had significant sensitivities in only a small number of cases—realistic?

• Realism of unexpectedly high sensitivities for some inputs:

– |E*| lower (δ), upper (δ+α) shelves

– Poisson’s ratio for HMA

– Poisson’s ratio for base, subgrade

– HMA unit weight (especially for longitudinal cracking)

• Replace/eliminate low sensitivity inputs with internal default values?

56

57

Sensitivity Analysis in Terms of Service Life

57

y = 0.1297x0.3347 R² = 0.9878

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 24 48 72 96 120 144 168 192 216 240 264

Ru

ttin

g D

ep

th (

in)

Pavement Age (month)

Permanent Deformation: Rutting

SubTotalAC

SubTotalBase

SubTotalSG

Total Rutting

TotalRutReliability

Total Rutting Design Limit Power (Total Rutting)

AC Rutting Design Value = 0.25

Total Rutting Design Limit = 0.75

Pavement Service Life

Distress at end of analysis period

58

Overall Conclusions

• Rigorous sensitivity analysis methodology is essential

• Very few surprises in results – This is a good thing!

• There were some surprises in results – Identify MEDPG areas that may merit further investigation

• Results provide practical guidance for pavement designer – Importance of Level 1 characterization for HMA

• Results suggest areas where MEPDG/DARWin-ME could be simplified – Eliminate some inputs/replace with internal defaults

58

59

Webinar Outline







• Discussion

59

Contact Info:

Dr. Charles W. Schwartz University of Maryland [email protected] +1.301.405.1962

Documents

SENSITIVITY EVALUATION OF THE MEPDG FOR FLEXIBLE PAVEMENTS TRB Webinar …onlinepubs.trb.org/onlinepubs/webinars/120725.pdf · TRB Webinar July 25, 2012 Presenter: Charles W. Schwartz