ce522a 05 flexible pavement design

8/19/2019 CE522A 05 Flexible Pavement Design

1/133

Chapter 11

Flexible Pavement Design

Yang H. Huang, Pavement Analysis and Design 2nd Edition, Prentice Hall, Inc., 2004


2/133


• Calibrated Mechanistic Design Procedure• Asphalt Institute Method

• AASHTO Method

• Design of Flexible Pavement Shoulders



3/133

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.



4/133


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.



5/133


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.



6/133


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 .



7/133

General methodology for


Yang H. Huang, Pavement Analysis and Design 2nd Edition,Prentice Hall, Inc., 2004


8/133


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 .



9/133


Procedure


10/133


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.



11/133


Procedure



12/133


13/133



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 .



14/133



For unsaturated cohesive soils, is related to the plasticity

index PI by (Black and Croney, 1957)



15/133



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.



16/133



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



17/133


18/133





19/133


Procedure



20/133

• Moisture Equilibrium Model


21/133


22/133


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



23/133


Procedure• Low-temperature cracking

occurs when the thermal

tensile stress in the HMA

exceeds its tensile strength .



24/133


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


25/133

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.


26/133



27/133

Asphalt Institute Method



28/133


•

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.



29/133


• 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.



30/133


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



31/133


Traffic Analysis

• Determination of Design ESAL

• Simplified Procedure for Determining Design ESAL



32/133


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.



33/133



34/133


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 .



35/133


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


36/133


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 .



37/133




38/133


39/133

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


40/133


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 .



41/133

Design Procedure

• HMA over

Emulsified Asphalt Base


42/133


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.



43/133




44/133




45/133




46/133




47/133




48/133

Design Procedure

• HMA over

Untreated Aggregate Base


49/133


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



50/133




51/133




52/133




53/133




54/133




55/133




56/133




57/133


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:



58/133


HMA and Emulsified Asphalt Mix over untreated aggregate base



59/133



60/133


Problems



61/133

AASHTO Method



62/133

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 .



63/133

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 .



64/133

AASHTO Method: Design Variables

DESIGN VARIABLES

• Time Constraints

• Traffic

•Reliability

• Environmental Effects

• Serviceability



65/133

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.



66/133

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.



67/133

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.



68/133

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.



69/133

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.



70/133

Design Variables: Reliability

Yang H. Huang, Pavement Analysis and Design2nd Edition, Prentice Hall, Inc., 2004


71/133

Design Variables:


72/133

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.


Design Variables:


73/133

Design Variables:

Environmental Effects



74/133

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.



75/133

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.



76/133

AASHTO Method: Design Equations

DESIGN EQUATIONS

• Original Equations

• Modified Equations


Design Equations:


77/133

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.


Design Equations:


78/133

Design Equations:

Original Equations


Design Equations:


79/133

Design Equations:

Original Equations


Design Equations:


80/133

Design Equations:

Original Equations



81/133

Design Equations:


82/133

Design Equations:

Modified Equations


Design Equations:


83/133

Design Equations:

Modified Equations• Modified Equations

To take local precipitation and drainage conditions into account,

Eq. 11.32 was modified to


Design Equations:


84/133

es g quat o s


To achieve a higher level of reliability, W18 must be samller than

Wt18 by a normal deviate ZR, as shown in Figure 11.24:

Yang H. Huang, Pavement Analysis and Design

2nd Edition, Prentice Hall, Inc., 2004

Design Equations:


85/133

g q




Design Equations:


86/133

g q



Design Equations:


87/133

g q



Design Equations:


88/133

g q



Design Equations:


89/133

g q

Modified Equations


Design Equations:


90/133

g q

Modified Equations


Design Equations:


91/133

g q



Design Equations:


92/133

g q



AASHTO Method:


93/133

Effective Roadbed Soil Resilient Modulus ERSRM

EFFECTIVE ROADBED SOIL RESILIENT MODULUS ERSRM

• Relative Damage

• Computation of ERSRM


Effective Roadbed Soil Resilient Modulus


94/133


• The effective roadbed soil resilient modulus MR is anequivalent modulus that would result in the same damage

if seasonal modulus values were actually used.




95/133


• Relative Damage

From Eq. 11.37, the effect of MR on W18 can be expressed as




96/133


• Relative Damage




97/133


• Relative Damage




98/133


• 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 .



99/133




100/133




101/133

AASHTO Method: Structural Number


102/133


Structural number is a function of:

• layer thicknesses,

•

layer coefficients, and• drainage coefficients

This can be computed from Eq. 11.35 .




103/133


• 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.




104/133





105/133





106/133





107/133


• Layer Coefficient


Structural Number


108/133

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.


Structural Number


109/133

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).


Structural Number


110/133

Structural Number


Structural Number


111/133

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:


Structural Number


112/133

Structural Number


•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


113/133

Structural Number



Structural Number


114/133

Structural Number



Structural Number


115/133

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


Structural Number


116/133

Structural Number



Structural Number


117/133

Structural Number



Structural Number


118/133

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.


Structural Number


119/133

St uctu a u be

• Drainage Coefficient


AASHTO Method: Selection of Layer Thicknesses


120/133

y

SELECTION OF LAYER THICKNESSES• Minimum Thickness

• General Procedure


Selection of Layer Thicknesses


121/133

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.




122/133

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 .




123/133

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 .




124/133

y

• Minimum Thickness




125/133

y

• General ProcedureThe procedure for thickness design is usually started from the

top, as shown in Figure 11 .28 and described as follows :


Selection of Layer Thicknesses:

General Proced re


126/133

General Procedure




General Procedure


127/133

General Procedure




General Procedure


128/133

General Procedure



Design Equations:

M difi d E ti


129/133

Modified Equations

• Modified Equations


Design Equations:

M difi d E ti


130/133

Modified Equations


Design Equations:

Modified Equations


131/133

Modified Equations


Comparison with Asphalt Institute Method


132/133


AASHTO Method


133/133

FINAL EXAM

Coverage: Chapter 11 – Flexible Pavement Design

• Open Notes (no sharing of notes)

• Nomograph on SN Determination (Fig 11.25) will be provided

ce522a 05 flexible pavement design

Documents