The document discusses various methods for flexible pavement design, including empirical, mechanistic, and mechanistic-empirical methods. It provides details on the AASHTO, Asphalt Institute, and MEPDG design methods, including required inputs like traffic, materials properties, and environmental factors as well as outputs like structural number and layer thicknesses. It also explains concepts fundamental to flexible pavement design like structural number, layer coefficients, serviceability, reliability, and distresses like rutting and cracking.

1. 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. 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
log
4.2 1.5
2.32log 8.07
1094
0.40
1
R o
R
W Z S SN
PSI
M
SN
   
 
    


Structural Number
6
(AASHTO, 1993)
Reliability,%
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. 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. 4
13
3. Cumulative ESAL and
Design Life
• Compute ESAL (W18) during the
design life in the design lane
14
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. 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
Serviceability(PSI)
p0
pt
p0 - pt
Basic Equations
20

6. 6
22
7. Structural Numbers
• Use design nomograph three times
to determine the required SN above
subgrade, subbase, and base
23
(AASHTO, 1993)
24
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. 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!
27
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. 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. 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. 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
41
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. 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
43
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
44
Asphalt Institute Method
Example
45
Step 1: Traffic Calculation
• Total ESALs
– Buses + Trucks
– 2.13 million + 1.33 million = 3.46 million

12. 12
46
Step 2: Get MR Value
• CBR tests along a Road show:
– CBR ≈ 8
• MR conversion
    psiCBRMR 000,12815001500 
    psiCBRMR 669,9825552555
64.064.0

AASHTO Conversion
NCHRP 1-37A Conversion
47
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
48
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
49
Step 4: Calculate Design
• Use graph
Source: Asphalt Institute, MS-1, 1981

13. 13
50
Step 4: Calculate Design
• Final Design
– 9.5 inches HMA
– 12 inches aggregate base
• 6 inches UTB
• 6 inches aggregate subbase
51
52
MEPDG
• For free copy of the software,
climatic files, and Manual
• www.TRB.org/MEPDG
53

14. 14
Fatigue
Cracking
Thermal
Cracking
Rutting
MEPDG Predicted Distresses
Longitudinal
Cracking
54
IRI = International Roughness Index
IRI = F(Initial Roughness, Rutting, Fatigue
Cracking, Transverse Cracking, and Site Factors)
MEPDG Predicted Smoothness
55
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

15. 15
TIME
FATIGUE
CRACKING
TIME
RUT
DEPTH
Design
Period
Criterion
Criterion
Design Criteria
Color Codes
60 61

16. 16
62 63
64
MEPDG Major Traffic Inputs
– Volume
– Classification
– Weight
– General
Four basic traffic input categories are required by
MEPDG as follows:

17. 17
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. 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
Modulus(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