2. 2
Pavement Behavior and Performance Factors
Subgrade
•saturation
•pumping
•freeze/thaw
Pavement Structure
•surface
•base
•subbase
Traffic Loading
•time opened to traffic
•loading rate
Climate
•temperature
•moisture
3. 3
Nature of Performance Models
Empirical Mechanistic
More computing
power needed
Completeness of
theory?
Theory based
prediction
Less computing
power needed
Completeness of
observed data?
Observation
based prediction
4. 4
Limitations of Early AASHTO Design Guide
Representative of
the AASHTO Road
Test
No consideration
for pavement
rehabilitation
design
No consideration
of stress within
the pavement to
design for rutting
resistance
Effects of
different climate
conditions on
performance
Using 2-year
period of AASHTO
Road Test to
design
pavements for
20 years
Only one type of
subgrade used on
the AASHTO Road
Test
Vehicle
suspension, axle
configuration,
and tire types no
longer
representative
Limited
consideration for
treated base on
asphalt
pavements
5. 5
Evolution to AASHTO Pavement ME Design
(PMED)
AASHTO 1986 Guide
includes a section on the
state of knowledge on ME
design concepts
AASHTO Joint Technical
Committee on Pavements
recommends development
of ME design method (1996)
NCHRP 1-37A Development of the
2002 Guide for the Design of New and
Rehabilitated Pavement Structures
(1998-2004)
MEPDG v1.1
(2004)
AASHTOWare
DARWin-ME
(2011)
AASHTOWare
PMED
v2.2 – v3.0
(2015-2022)
6. 6
PMED Version Release Notes – Asphalt
Pavements
Date Build Note
Jul 2016 2.3 • Fixed reflection cracking without base layer
Jul 2018 2.5
Aug 2018 2.5.2
• Allowable range for existing layer fatigue cracking changed from 1-80 percent to 0-80 percent
• Asphalt fatigue damage f1 updated
• Bottom-up fatigue cracking C2 calibration coefficient updated
Oct 2018 2.5.3
Apr 2019 2.5.4
Jul 2019 2.5.5 • Master transverse cracking model gives very different results in v2.5.5 vs v2.5.4
Jul 2020 2.6 • Top-down cracking added
Aug 2021 2.6.1 • Fixed top-down cracking error in the AC over JPCP
7. 7
Inputs for a Asphalt Pavement Response
Models
Pavement
Geometry
•Layer
thicknesses
•Discontinuities
Environment
•Temperature
vs. depth
•Moisture vs.
depth
Material
Properties
•Elastic
properties
•Nonlinear
properties
Traffic
•Load spectrum
•Tire contract
pressure
distributions
and areas
8. 8
Asphalt Pavement Response – Typical Critical
Locations
Tensile horizontal strain at
the bottom of the asphalt
layer (fatigue cracking)
Compressive vertical
stresses/strains within the
asphalt layer (rutting)
Compressive vertical
stresses/strains within the
base/subbase layers
(rutting of unbound layers)
Compressive vertical
stresses/strains at the top
of the subgrade (subgrade
rutting)
– AASHTO 2002 Design Guide
9. 9
Example Methods for Determining
Stresses, Strains, and Deformations
Analytical (e.g., Burmister solution)
Multilayer elastic theory
» Rate-independent
» Viscoelastic
Finite difference methods
Finite element methods
» General purpose
» Pavement-specific
Boundary element methods
Hybrid methods
Stress
Strain
– AASHTO 2002 Design Guide
Deforming
force per
unit area
Relative change
in length due to
deforming force
Yield
Strength
Ultimate
Strength
Fracture
10. 10
Required Capabilities of Asphalt Layer
Response Model
Linear material model for asphalt, other bound, and unbound
layers
Stress-dependent material model for unbound materials
Loads from single or multiple wheel configurations
Interface conditions (e.g., fully bonded, full slip, intermediate
condition) between layers
– AASHTO 2002 Design Guide
13. 13
NCHRP 1-37A Fatigue Models Evaluated
Shell Oil Model
•Two separate fatigue relationships:
•Constant stress (asphalt layer > 8”)
•Constant strain (asphalt layer < 2”)
Asphalt Institute (MS-1) Model
•Found to have better trends and less
scatter in the data
•Essentially a constant stress model
•Corrections for the thinner sections
Constant strain – strain level is
maintained, and load (stress) varies; thin
pavements in the field generally perform
closer to a constant strain mode
Constant stress – load is maintained,
and strain varies; thick pavements in
the field generally perform closer to a
constant stress mode
14. 14
NCHRP 1-37A Final Fatigue Cracking Model
Bottom-up cracking final calibration model:
Top-down cracking final calibration model:
D = damage (%)
Where,
C1 = 1.0
C2 = 1.0
C2 = -2.40874 – 39.748*(1+hac)-2.856
C1 = -2* C2
15. 15
NCHRP 1-37A Final Fatigue Cracking Model
(continued…)
Damage
» “Distress (or damage) is estimated and accumulated for each analysis
interval (NCHRP 1-37A Final Report)”
» Miner’s Law:
» Considers:
• Changes in dynamic modulus due to hardening of the asphalt binder
• Monthly variation temperature and moisture changes in pavement layers
• Loading frequency and axle configuration (singles, doubles, tridem, and quads)
𝑫 =
𝒊=𝟏
𝑻
𝒏𝒊
𝑵𝒊
Where,
D = damage
T = total number of periods
ni = actual traffic for period i
Ni = traffic allowed for period i
16. 16
PMED Load-Related Cracking Prediction Model
Alligator and longitudinal cracking
𝐍𝐟 = 𝐤𝐟𝟏 𝐂 𝐂𝐇 𝛃𝐟𝟏 𝛆𝐭
𝐤𝐟𝟐𝛃𝐟𝟐 𝐄𝐀𝐂 𝐤𝐟𝟑𝛃𝐟𝟑
𝑀 = 4.84
𝑉𝑏𝑒
𝑉
𝑎 + 𝑉𝑏𝑒
− 0.69
𝑉𝑏𝑒 = effective asphalt content by volume (%)
𝑉
𝑎 = air voids (%)
Where:
Nf = allowable number of axle load applications
C = 10M
CH = thickness correction term, dependent on type of cracking
εt = tensile strain at critical locations and calculated by the structural
response model (inch/inch)
EAC = AC dynamic modulus (psi)
kf1, kf2, kf3 = global laboratory-derived model coefficients for dense-graded neat asphalt mixtures
(kf1= 3.75, kf2 = 2.87, kf3 = 1.46)
βf1, βf2, βf3 = local or mixture specific field shift or adjustment constants.
if hAC less than 5 in. : βf1 = 0.02054
if hAC is 5 to 12 in. : βf1 = 5.014 hAC
−3.416
if hAC more than 12 in. : βf1 = 0.001032
, βf2 = 1.38, βf3 = 0.88
17. 17
Thickness Correction Term (CH)
For bottom-up or alligator cracking:
For top-down or longitudinal cracking
» CH term was removed in the 2021 MOP Supplement and replaced
with…
𝐶𝐻 =
1
0.000398 +
0.003602
1 + 𝑒 11.02−3.49𝐻𝐴𝐶
18. 18
MOP 2021 Supplement
Top-down Cracking Prediction Model (NCHRP
1-52)
Fracture mechanics model based on Paris’ Law
» f(loading and temperature)
𝒅𝒄
𝒅𝑵
= 𝑨′ ×
𝟏 − 𝒗𝟐
𝑬𝑹
× 𝑲𝑰
𝟐
+ 𝑲𝑰𝑰
𝟐
+
𝟏 + 𝒗
𝑬𝑹
× 𝑲𝑰𝑰𝑰
𝟐
𝒏′
Where,
dc = change or growth in crack length
dN = increase in loading cycles during a time increment
A’, n’ = fracture properties of asphalt mixture
v = Poisson’s ratio
ER = representative elastic modulus
KI = stress intensity factor in Mode I (crack opening)
KII = stress intensity factor in Mode II (in-plane shear)
KIII = stress intensity factor in Mode III (out-of-plane shear)
J-integral
Mode I
Mode II
Mode III
23. 23
NCHRP 1-47 Sensitivity of MEPDG
(v1.1) Inputs
Alligator cracking
»Hypersensitive – E* Alpha, E*
Delta, hac
»Very sensitive – air voids,
surface shortwave absorptivity,
effective binder volume,
Poisson’s ratio
»Sensitive – unit weight, heat
capacity, low and high
temperature, thermal
conductivity
Longitudinal cracking
»Hypersensitive – E* Alpha, E*
Delta, hac
»Very sensitive – air voids,
surface shortwave absorptivity,
effective binder volume,
Poisson’s ratio
»Sensitive – unit weight, heat
capacity, high temperature,
thermal conductivity
24. 24
PMED Calibration Factors
PMED distress models calibrated using a large set of sections
from multiple experiments—primarily the LTPP database
User has the option to adjust calibration factors based on local or
regional data sets
Guidance for agency-specific adjustment factor in Guide for the
Local Calibration of the Mechanistic-Empirical Pavement Design
Guide
25. 25
Transfer Function Calibration – Factors to Adjust
Distress Eliminate Bias
Reduce Standard
Error
Total Rutting: Unbound
materials and HMA layers
kr1, s1, or r1 kr2, kr3 and r2, r3
Alligator cracking C2 or kf1 kf2, kf3, and C1
Longitudinal cracking C2 or kf1 kf2, kf3, and C1
Semi-rigid pavements C2 or c1 C1, C2, and C4
Transverse cracking t3 t3
IRI C4 C1, C2, and C3
– AASHTO 2010 Local Calibration Guide
