1

1

Flexible Pavement Design

1.2-2

• Experience

• Empirical

• Mechanistic-Empirical

• Mechanistic

Flexible Pavement Design Methodologies

3

Design Methods

• Highway Pavements

– AASHTO

– The Asphalt Institute

– Portland Cement Association

– Mechanistic Empirical Pavement Design

Guide (MEPDG)

4

Design Inputs and Outputs

• Inputs

– Design life (analysis period)

– Traffic (W18)

– Foundation stiffness (MR)

– Performance criterion (PSI)

– Reliability (ZR, So)

• Outputs

– Required pavement capacity: Structural

Number (SN)

2

5

AASHTO Design Equation

W18 = design traffic (18-kip ESALs)

ZR = standard normal deviate

So = combined standard error of traffic and performance prediction

PSI = difference between initial and terminal serviceability index

MR = resilient modulus (psi)

SN = structural number

10 18 10

10

10

5.19

log 9.36log 1 0.20

log4.2 1.5

2.32log 8.071094

0.401

R o

R

W Z S SN

PSI

M

SN

Structural Number

6

(AASHTO, 1993)

Rel

iab

ilit

y, %

MR

7

No Unique Solution!

(AASHTO, 1993)

1 1

2

n

i i i

i

SN a D a D m

8

Design Steps

1. Reliability (R)

2. Overall standard deviation (So)

3. Cumulative ESALs

4. Effective roadbed resilient modulus (MR)

5. Resilient moduli of pavement layers (surface, base & subbase), MRi

6. Serviceability loss (PSI)

3

9

Design Steps (Cont.) 7. Structural numbers (SNi)

8. Structural layer coefficients (ai)

9. Drainage coefficients (mi)

10. Layer thicknesses (Hi)

11. Consider freeze / thaw and swelling

12. Life-cycle cost

10

1. Reliability (R)

Chance that pavement will last for

the design period without failure

11

Reliability

(AASHTO, 1993) 12

2. Overall Standard

Deviation (So) and ZR

(AASHTO, 1993)

So = Standard Deviation

Flexible Pavements: So = 0.40 - 0.50

All variability is lumped into

a single set of parameters!

Rigid Pavements: So = 0.30 - 0.40

4

13

3. Cumulative ESAL and

Design Life

• Compute ESAL (W18) during the

design life in the design lane

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4. Effective Roadbed

Resilient Modulus

uf = 1.18 x 108 x Mreff-2.32

15

5. Resilient Moduli of Pavement

Layers (Surface, Base & Subbase)

• Lab testing

• Correlations

16

6. Serviceability Loss (PSI)

(AASHTO, 1993)

5

17

What is Serviceability? • Based upon Present

Serviceability Rating

(PSR)

• Subjective rating by

individual/panel

– Initial/post-construction

– Various times after construction

• 0 < PSR < 5

• PSR < ~2.5:

Unacceptable

(AASHO, 1961) 18

6. Serviceability Loss (PSI)

• PSI = Pavement Serviceability Index, 1 < PSI < 5

• po = Initial Serviceability Index

– Flexible pavements: 4.2

• pt = Terminal Serviceability Index

– Range from 1.5-3

o tPSI p p

(AASHTO, 1993)

19

PSI

Time

Servic

eab

ilit

y (

PS

I) p0

pt

p0 - pt

Basic Equations

20

6

22

7. Structural Numbers

• Use design nomograph three times

to determine the required SN above

subgrade, subbase, and base

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(AASHTO, 1993)

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Structural Number SN

• SN = structural number

• ai = ith layer structural coefficient

• Di = ith layer thickness (inches)

• mi = ith layer drainage coefficient

• n = number of layers (3, typically)

1 1

2

n

i i i

i

SN a D a D m

25

8. Structural Layer

Coefficients (a1, a2, a3)

7

26

What Are Layer Coefficients? • Are they fundamental engineering properties

of pavement materials?

• Can they be measured in the laboratory?

• Can they be defined easily for new materials?-

-e.g.,

– Modified HMA

– Geosynthetic reinforced unbound

materials

NO! NO! NO!

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a1: HMA

(AASHTO, 1993)

28 29

a2: Granular Base 2 100.249 log 0.977

in psi

base

base

a E

E

(AASHTO, 1993)

8

30

a3: Granular Subbase

3 100.227(log ) 0.839

in psi

subbase

subbase

a E

E

(AASHTO, 1993) 31

9. Drainage Coefficients (m2 & m3)

mi increases/decreases the effective value for ai

(AASHTO, 1993)

Captures effect of environment on material properties

32

Quality of Drainage

(AASHTO, 1993) 33

10. Layer Thicknesses • SN1 a1D1

– Solve for D1 & round off (1/2” increments)

• SN2 a1D1 + a2D2m2

– Solve for D2 & round off (1” increments)

• SN3 a1D1 + a2D2m2 + a3D3m3

– Solve for D3 & round off (1” increments)

• Consider min. practical thicknesses

• Consider material cost

9

34

Minimum Layer

Thicknesses

(Huang, 2004) 35

36

Asphalt Institute:

Mechanistic -Empirical

Traffic

Climatic

data

Design &

material

property

parameters

Pavement response

(s, e, d) calculated

using DAMA

Incremental fatigue

damage models

Transfer functions

Performance

prediction models

(rutting, % cracks,

etc….)

37

Asphalt Institute

• Design Criteria 1. Limit vertical stress at top of roadbed soil

(prevent rutting)

2. Limit horizontal tensile strain at bottom of

HMA layer (prevent fatigue cracking)

Limiting Criteria: Rutting < 0.5 in

Fatigue Cracking < 20-25%

10

38

Asphalt Institute Design Criteria

et

ec

et at bottom of all bound layers (cracking)

ec at top of subgrade (rutting) 39

Asphalt Institute • Design Inputs

1. Traffic:

18-kip ESALs for Pt=2.5 & SN=5

2. Subgrade resilient modulus

40

Asphalt Institute

• Material Properties

1. High Quality HMA

2. Emulsified AC base:

a. Type I – processed dense graded aggregate

b. Type II – semi -processed graded aggregate

c. Type III – sands or silty-sands

d. Criteria for base-subbase

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Asphalt Institute

• Environmental

1. MAAT (mean annual air temp.)

– To account for changes in HMA Mr

– Note that at:

» 45°F (frost effects)

» 60°F (possible frost effects)

» 75°F (no frost effects)

11

42

Asphalt Institute • Thickness Design

1. Full depth – min. HMA = 4in

2. HMA over Emulsified Base

a. Chart TOTAL pavement thickness

3. HMA over granular base

a. Chart HMA surface thickness

b. Choose base thickness based on:

i. Drainage

ii. Frost protection

iii. Material availability/cost

iv. Agency requirements

Min. HMA Traffic

2 in ≤ 105

5 in > 107

Min. HMA Traffic

3 in ≤104

5 in ≥106

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Asphalt Institute • Design Selection

1. Full depth HMA

a. Less total required thickness

b. Relatively insensitive to frost/moisture

2. Aggregate base:

a. Inexpensive

b. Readily available

c. Shown good performance

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Asphalt Institute Method

Example

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Step 1: Traffic Calculation

• Total ESALs

– Buses + Trucks

– 2.13 million + 1.33 million = 3.46 million

12

46

Step 2: Get MR Value

• CBR tests along a Road show:

– CBR ≈ 8

• MR conversion

psiCBRMR 000,12815001500

psiCBRMR 669,982555255564.064.0

AASHTO Conversion

NCHRP 1-37A Conversion

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Step 3: Select Climate

• Determines HMA & subgrade

properties – Can select mean annual air temperature

(MAAT):

• 45°F (frost effects)

• 60°F (possible frost effects)

• 75°F (no frost effects)

– Software allows more selections

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Step 4: Calculate Design

• Decide on basic structure

– HMA

– Aggregate base (6 or 12 inches)

• Software allows for more choices

• Can also choose

– Full-depth asphalt

– HMA over emulsified asphalt base

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Step 4: Calculate Design

• Use graph Source: Asphalt Institute, MS-1, 1981

13

50

Step 4: Calculate Design

• Final Design

– 9.5 inches HMA

– 12 inches aggregate base

• 6 inches UTB

• 6 inches aggregate subbase

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MEPDG

• For free copy of the software,

climatic files, and Manual

• www.TRB.org/MEPDG

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14

Fatigue Cracking

Thermal Cracking

Rutting

MEPDG Predicted Distresses

Longitudinal Cracking

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IRI = International Roughness Index

IRI = F(Initial Roughness, Rutting, Fatigue Cracking, Transverse Cracking, and Site Factors)

MEPDG Predicted Smoothness

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MEPDG Inputs

– General Information

– Traffic

– Climate

– Structure

Four basic input categories

are required by MEPDG : Traffic Foundation Climate

Material

Properties

Trial Design Strategy

Pavement Analysis Models

Distress Prediction Models

Constructability

Issues Viable Alternatives Life Cycle Cost

Analysis

Select Strategy

Meet

Performance Criteria?

Modify

Strategy

Inputs

Analysis

No

Yes

Damage

Accumulation

Strategy Selection

Design Process Overview

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TIME

FATIGUE

CRACKING

TIME

RUT

DEPTH

Design

Period

Criterion

Criterion

Design Criteria

Color Codes

60 61

16

62 63

64

MEPDG Major Traffic Inputs

– Volume

– Classification

– Weight

– General

Four basic traffic input categories are required by

MEPDG as follows:

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MEPDG Traffic Inputs

– Base year truck traffic volume.

• AADTT

• No. of Lanes in Design Direction

• % trucks in design direction.

• % trucks in design lane

• Speed.

– Traffic volume adjustment factors

• Monthly adjustment.

• Vehicle class distribution.

• Hourly Truck distribution.

• Traffic growth factors.

– Axle load distribution factors.

– General Traffic inputs.

• Number of axles per truck.

• Axle configuration

• Wheel base.

MEPDG Lane and Directional Distribution

Factors

MEPDG Vehicle Class Distribution MEPDG Axle Load Distribution Factors

18

Change in AC Modulus with Age

70

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

0 24 48 72 96 120 144 168 192 216 240

Mo

du

lus (

psi)

Pavement Age (month)

AC1(1)h=0.5

AC1(2)h=0.5

AC1(3)h=1.0

AC1(4)h=1.0

AC1(5)h=1.2