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Chapter 11
Flexible Pavement Design
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Flexible Pavement Design
• Calibrated Mechanistic Design Procedure• Asphalt Institute Method
• AASHTO Method
• Design of Flexible Pavement Shoulders
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Calibrated Mechanistic Design
Procedure
• The design equations presented in the 1986 AASHTO
design guide were obtained empirically from the results
of the AASHO Road Test.
• To develop a mechanistic pavement analysis and design
procedure suitable for future versions of AASHTO guide,
a research project entitled "Calibrated MechanisticStructural Analysis Procedures for Pavements" was
awarded to the University of Illinois.
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Calibrated Mechanistic Design
Procedure
• The calibrated mechanistic procedure is a more specific
name for the mechanistic –empirical procedure.
• It contains a number of mechanistic distress models that
require careful calibration and verification to ensure that
satisfactory agreement between predicted and actual
distress can be obtained.
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Calibrated Mechanistic Design
Procedure
• The purpose of calibration is to establish transfer
functions relating mechanistically determined responses
to specific forms of physical distress.
• Verification involves the evaluation of the proposed
models by comparing results to observations in other
areas not included in the calibration exercise.
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Calibrated Mechanistic Design
Procedure• It is assumed that the materials to be used for the
pavement structure are known a priori and that only the
pavement configuration is subjected to design iterations.
• If changing the pavement configuration does not satisfy
the design requirements, it might be necessary to change
the types and properties of the materials to be used.
• Once a new material is selected, the process is repeated
until a satisfactory design is obtained .
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General methodology for
Flexible Pavement Design
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Calibrated Mechanistic Design
Procedure• Climate Models
Temperature and moisture are significant climatic inputs
for pavement design.
The modulus of the HMA depends on pavement
temperature ; the moduli of the base, sub -base, and
subgrade vary appreciably with moisture content .
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Calibrated Mechanistic Design
Procedure
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Calibrated Mechanistic Design
Procedure• Moisture Equilibrium Model
The moisture equilibrium model in the CMS model
(Dempsey et al., 1986) is based on the assumption thatthe subgrade cannot receive moisture by infiltration
through the pavement.
Any rainwater will drain out quickly through the drainage
layer to the side ditch or longitudinal drain, so the only
water in the subgrade is the capillary water caused by
the water table.
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Calibrated Mechanistic Design
Procedure
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Calibrated Mechanistic Design
Procedure• Moisture Equilibrium Model
The relationship between suction and pore pressure can be
expressed as
in which:
u is the pore pressure when soil is loaded;
S is the soil suction, which is a negative pressure;
p is the applied pressure (or overburden), which is always
positive; and
is the compressibility factor, varying from 0 for unsaturated,
cohesionless soils to 1 for saturated soils .
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Calibrated Mechanistic Design
Procedure• Moisture Equilibrium Model
For unsaturated cohesive soils, is related to the plasticity
index PI by (Black and Croney, 1957)
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Calibrated Mechanistic Design
Procedure• Moisture Equilibrium Model
The pore pressure in a soil depends solely on its distance above
the ground-water table:
z is the distance above the water table, and
w is the unit weight of water.
This simple fact can be explained by considering soils as a bundle
of capillary tubes with varying sizes.
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Calibrated Mechanistic Design
Procedure• Moisture Equilibrium Model
Water will rise in each of these capillary tubes to an elevation
that depends on the size of the tube.
At any distance z above the water table, a large number of
menisci will form at the air—water interfaces, causing a
tension at each elevation corresponding to the height of
capillary rise. Combining Eqs. 11.2 and 11 .4 yields
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Calibrated Mechanistic Design
Procedure• Moisture Equilibrium Model
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Calibrated Mechanistic Design
Procedure
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• Moisture Equilibrium Model
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Calibrated Mechanistic Design
Procedure• Distress Models
1. Fatigue Cracking Models- the number of load repetitions Nf
is related to the tensile strain at the bottom of the asphalt
layer (damages to the pavement)
2. Rutting Models
a. limiting the vertical strain on top of the subgrade
b. limiting the accumulated permanent Nd deformation
on the pavement
1. Thermal Cracking Models
a. low-temperature cracking
b. thermal fatigue cracking
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Calibrated Mechanistic Design
Procedure• Low-temperature cracking
occurs when the thermal
tensile stress in the HMA
exceeds its tensile strength .
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Calibrated Mechanistic Design
Procedure• Thermal Fatigue cracking
If the tensile stress is smaller
than the tensile strength, the
pavement will not crack
under a single daily
temperature cycle but could
still crack under a large
number of cycles.
This occurs when the fatigueconsumed by daily
temperature cycles exceeds
the HMA fatigue resistance.
• cycles . Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
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SEATWORK
Use a ¼ sheet of paper to answer the problemon the next slide. You have ten minutes to work
on the answer with complete solution.
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Asphalt Institute Method
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Asphalt Institute Method
•
From 1954 to 1969, eight editions of Manual Series No. 1(MS-1) were published by the Asphalt Institute for the
thickness design of asphalt pavements .
• The procedures recommended in these manuals
were empirical.
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Asphalt Institute Method
• As has been explained in previous chapters, two types of
strains have frequently been considered the most critical
for the design of asphalt pavements.
• One is the horizontal tensile strain t at the bottom of
the asphalt layer, which causes fatigue cracking ;
• The other is the vertical compressive strain c on thesurface of the subgrade, which causes permanent
deformation or rutting.
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Asphalt Institute Method
Design Criterion
• Fatigue Criterion - allowable number of load repetitions
to control fatigue cracking
• Permanent Deformation Criterion – allowable number
of load repetitions to control permanent deformation
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Asphalt Institute Method
Traffic Analysis
• Determination of Design ESAL
• Simplified Procedure for Determining Design ESAL
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Asphalt Institute Method
Traffic Analysis
• Simplified Procedure for Determining Design ESAL
If detailed traffic data are not available, the AsphaltInstitute recommends the use of Table 11.3 for
estimating the design ESAL (AI, 1981b).
This simplified procedure separates traffic into six
classes, each associated with a type of highway or street
and an average number of heavy trucks expected on the
facility during the design period.
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Asphalt Institute Method
Design Procedure
• The DAMA computer program was used to determine the
minimum thickness required to satisfy both fatigue cracking
and rutting criteria.
• For any given material and environmental conditions, two
thicknesses were obtained, one by each criterion, and the
larger of the two was used to prepare the design charts .
• For this reason, many of the design curves represent shapes
associated with two different criteria .
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Asphalt Institute Method
Design Procedure
• The DAMA Model
It can be used to analyze a multilayer elastic pavement
structure by cumulative-damage techniques for a single-
or dual-wheel system.
Any pavement structure comprised of hot-mix asphalt,
emulsified asphalt mixtures, untreated granular
materials, and subgrade soils can be analyzed, provided
that the maximum number of layers does not exceed
five.Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
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Asphalt Institute Method
Design Procedure
• Full Depth HMA
Figure 11 .11 is the design chart for full-depth asphalt
pavements
Given the subgrade resilient modulus MR and theequivalent 18-kip single-axle load, ESAL, the total HMA
thickness, including both surface and base courses, can
be read directly from the chart .
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Asphalt Institute Method
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Asphalt Institute MethodMaterial Characterization
Emulsified Asphalt Mixtures
It is a suspension of small asphalt cement globules in water,
which is assisted by an emulsifying agent (such as soap).
The emulsifying agent assists by imparting an electrical charge
to the surface of the asphalt cement globules so that they
do not coalesce.
Emulsions are used because they effectively reduce viscosity
for lower temperature uses.Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
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Asphalt Institute Method
Material Characterization
Emulsified Asphalt Mixtures
It is permissible to use emulsified asphalt mixtures for base courses.
1. Type I: mixes with processed dense graded aggregates, which
should be mixed in a plant and have properties similar to HMA.
2. Type II: mixes with semiprocessed, crusher run, pit run, or bank
run aggregates .
3. Type III: mixes with sands or silty sands .
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Design Procedure
• HMA over
Emulsified Asphalt Base
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Asphalt Institute Method
Design Procedure
• HMA over Emulsified Asphalt Base
– Emulsified Asphalt Type I – Emulsified Asphalt Type II
– Emulsified Asphalt Type III
The chart gives the combined thickness of HMA surface
course and emulsified asphalt base course.
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Design Procedure
• HMA over
Untreated Aggregate Base
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Asphalt Institute Method
Design Procedure
• HMA over untreated aggregate base
The thickness of the aggregate base to be used must firstbe determined and then the HMA thickness can then be
found on the appropriate design chart
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
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Asphalt Institute Method
Design Procedure
HMA and Emulsified Asphalt Mix over untreated aggregate
baseDesign Charts for this type of pavement are currently not
available.
The following method has been recommended by the
Asphalt Institute:
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Asphalt Institute Method
HMA and Emulsified Asphalt Mix over untreated aggregate base
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Asphalt Institute Method
Problems
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AASHTO Method
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AASHTO Method
• The design procedure recommended by the American
Association of State Highway and Transportation Officials
(AASHTO) is based on the results of the extensive AASHO
Road Test conducted in Ottawa, Illinois, in the late 1950s
and early 1960s.
• The AASHO Committee on Design first published an interim
design guide in 1961 .
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AASHTO Method
• It should be kept in mind that the original equations were
developed under a given climatic setting with a specific set of
pavement materials and subgrade soils.
• The climate at the test site is temperate with an average annual
precipitation of about 34 in. (864 mm).
• The average depth of frost penetration is about 28 in. (711 mm).
• The subgrade soils consists of A-6 and A-7-6 that are poorly
drained, with CBR values ranging from 2 to 4 .
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AASHTO Method: Design Variables
DESIGN VARIABLES
• Time Constraints
• Traffic
•Reliability
• Environmental Effects
• Serviceability
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AASHTO Method: Design Variables
• Time Constraints
To achieve the best use of available funds, the AASHTO
design guide encourages the use of a longer analysis periodfor high-volume facilities including at least one
rehabilitation period .
Thus, the analysis period should be equal to or greater thanthe performance period.
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Design Variables: Time Constraints
• Performance Period
The performance period refers to the time that an initial
pavement structure will last before it needs rehabilitationor the performance time between rehabilitation operations.
It is equivalent to the time elapsed as a new, reconstructed,
or rehabilitated structure deteriorates from its initialserviceability to its terminal serviceability.
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Design Variables: Time Constraints
• Analysis Period
The analysis period is the period of time that any design strategy
must cover. It may be identical to the selected performance
period.
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Design Variables: Traffic
• Traffic
The design procedures are based on cumulative expected
18-kip (80-kN ) equivalent single-axle load (ESAL).
If a pavement is designed for the analysis period without
any rehabilitation or resurfacing, all that is required is the
total ESAL over the analysis period.
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Design Variables: Reliability
• Reliability
• Basically, reliablity is a means of incorporating some degree
of certainty into the design process to ensure that thevarious design alternatives will last the analysis period.
• The level of reliability to be used for design should increase
as the volume of traffic, difficulty of diverting traffic, andpublic expectation of availability increase.
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Design Variables: Reliability
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Design Variables:
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Design Variables:
Environmental Effects• Environmental Effects
The AASHO design equations were based on the results of
traffic tests over a two-year period.
The long-term effects of temperature and moisture on the
reduction of serviceability were not included.
The shape of these curves indicates that the serviceability
loss due to environment increases at a decreasing rate.
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Design Variables:
Environmental Effects
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Design Variables: Serviceability
• Serviceability
Initial and terminal serviceability indexes must be established to
compute the change in serviceability, PSI, to be used in the
design equations.
The initial serviceability index is a function of pavement type
and construction quality.
Typical values from the AASHO Road Test were 4.2 for flexible
pavements and 4.5 for rigid pavements.
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Design Variables: Serviceability
• The terminal serviceability index is the lowest index that will be
tolerated before rehabilitation, resurfacing, and reconstruction
become necessary.
• An index of 2.5 or higher is suggested for design of major
highways and 2.0 for highways with lower traffic.
• For relatively minor highways where economics dictate a
minimum initial capital outlay, it is suggested that this be
accomplished by reducing the design period or total traffic
volume, rather than by designing a terminal serviceability index
less than 2.0.
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AASHTO Method: Design Equations
DESIGN EQUATIONS
• Original Equations
• Modified Equations
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Design Equations:
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Design Equations:
Original Equations• The original equations were based purely on the results of
the AASHO Road Test but were modified later by theory and
experience to take care of subgrade and climatic conditions
other than those encountered in the Road Test.
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Design Equations:
Original Equations
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Design Equations:
Original Equations
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Design Equations:
Original Equations
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Design Equations:
Modified Equations
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Design Equations:
Modified Equations• Modified Equations
To take local precipitation and drainage conditions into account,
Eq. 11.32 was modified to
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es g quat o s
Modified Equations• Modified Equations
To achieve a higher level of reliability, W18 must be samller than
Wt18 by a normal deviate ZR, as shown in Figure 11.24:
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2nd Edition, Prentice Hall, Inc., 2004
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g q
Modified Equations• Modified Equations
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2nd Edition, Prentice Hall, Inc., 2004
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g q
Modified Equations• Modified Equations
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g q
Modified Equations• Modified Equations
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g q
Modified Equations• Modified Equations
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g q
Modified Equations
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g q
Modified Equations
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g q
Modified Equations• Modified Equations
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g q
Modified Equations• Modified Equations
Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
AASHTO Method:
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Effective Roadbed Soil Resilient Modulus ERSRM
EFFECTIVE ROADBED SOIL RESILIENT MODULUS ERSRM
• Relative Damage
• Computation of ERSRM
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Effective Roadbed Soil Resilient Modulus
• The effective roadbed soil resilient modulus MR is anequivalent modulus that would result in the same damage
if seasonal modulus values were actually used.
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Effective Roadbed Soil Resilient Modulus
• Relative Damage
From Eq. 11.37, the effect of MR on W18 can be expressed as
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Effective Roadbed Soil Resilient Modulus
• Relative Damage
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Effective Roadbed Soil Resilient Modulus
• Relative Damage
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Effective Roadbed Soil Resilient Modulus
• Computation of Effective Roadbed Soil Resilient Modulus
Figure 11.26 is a worksheet for estimating effective roadbed soil
resilient modulus, in which Eq. 11 .43, together with a vertical
scale for graphical solution of u f , is also shown .
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AASHTO Method: Structural Number
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AASHTO Method: Structural Number
Structural number is a function of:
• layer thicknesses,
•
layer coefficients, and• drainage coefficients
This can be computed from Eq. 11.35 .
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AASHTO Method: Structural Number
• Layer Coefficient
The layer coefficient, a, is a measure of the relative ability
of a unit thickness of a given material to function as a
structural component of the pavement .
Layer coefficients can be determined from test roads or
satellite sections, as was done in the AASHO Road Test, orfrom correlations with material properties.
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AASHTO Method: Structural Number
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AASHTO Method: Structural Number
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AASHTO Method: Structural Number
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AASHTO Method: Structural Number
• Layer Coefficient
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Structural Number
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Structural Number
• It is recommended that the layer coefficient be based on the
resilient modulus, which is a more fundamental material
property.
• In following the AASHTO design guide, the notation MR, as
used herein, refers only to roadbed soils, whereas E l , E 2 ,
and E 3
apply to the HMA, base, and subbase, respectively.
Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
Structural Number
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Structural Number
• Asphalt —
Concrete Surface Course
The layer coefficient a1 for the dense-graded HMA used in
the AASHO Road Tests is 0.44, which corresponds to a
resilient modulus of 450,000 psi (3.1 GPa).
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Structural Number
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Structural Number
• Untreated and Stabilized Base Courses
In lieu of Figure 7.15a, the following equation can also be used to
estimate a2 for an untreated base course from its resilient
modulus E2:
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Structural Number
Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
•Untreated and Stabilized Base Courses
The layer coefficient a2 for the granular base material used in the
AASHO Road Test is 0 .14, which corresponds to a base resilient
modulus of 30,000 psi (207 GPa) .
Structural Number
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Structural Number
• Untreated and Stabilized Base Courses
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Structural Number
• Untreated and Stabilized Base Courses
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Structural Number
• Granular Subbase Course
Figure 7.16 provides the chart that may be used to estimate layer
coefficient a3 of granular subbase courses. The relationship
between a3 and E3 can be expressed as
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Structural Number
• Granular Subbase Course
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Structural Number
• Granular Subbase Course
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Structural Number
• Drainage Coefficient
Depending on the quality of drainage and the availability of
moisture, drainage coefficients m2 and m3 should be applied to
granular bases and sub-bases to modify the layer coefficients.
At the AASHTO Road Test site, these drainage coefficients are all
equal to 1.
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St uctu a u be
• Drainage Coefficient
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AASHTO Method: Selection of Layer Thicknesses
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y
SELECTION OF LAYER THICKNESSES• Minimum Thickness
• General Procedure
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y
• Once the design structural number SN for an initial pavementstructure is determined, it is necessary to select a set of
thicknesses so that the provided SN, as computed by Eq. 11.35,
will be greater than the required SN.
• Note that Eq. 11.35 does not have a single unique solution.
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y
• Many combinations of layer thicknesses are acceptable, so theircost effectiveness along with the construction and maintenance
constraints must be considered to avoid the possibility of
producing an impractical design.
• The optimum economical design is to use a minimum base
thickness by increasing the HMA thickness .
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y
• Minimum Thickness
It is generally impractical and uneconomical to use layers of
material that are less than some minimum thickness.
Furthermore, traffic considerations may dictate the use of a
certain minimum thickness for stability .
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y
• Minimum Thickness
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y
• General ProcedureThe procedure for thickness design is usually started from the
top, as shown in Figure 11 .28 and described as follows :
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General Proced re
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General Procedure
• General Procedure
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General Procedure
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General Procedure
• General Procedure
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Selection of Layer Thicknesses:
General Procedure
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General Procedure
• General Procedure
Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
Design Equations:
M difi d E ti
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Modified Equations
• Modified Equations
Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
Design Equations:
M difi d E ti
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Modified Equations
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Design Equations:
Modified Equations
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Modified Equations
Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004
Comparison with Asphalt Institute Method
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FINAL EXAM
Coverage: Chapter 11 – Flexible Pavement Design
• Open Notes (no sharing of notes)
• Nomograph on SN Determination (Fig 11.25) will be provided