Introduction to Pavement Design Concepts by Qaiser Rafiq

Introduction to Pavement Design

Concepts

Presented By:Presented By:

Engr. Qaiser RafiqEngr. Qaiser Rafiq

07 - MS - TE - 0807 - MS - TE - 08

TOPIC:TOPIC:

PavementTypes of PavementPrincipal of Pavement DesignFailure CriteriaAspects of Pavement DesignRelative Damage ConceptPavement Thickness Design approachesEmpirical MethodMechanistic-Empirical Method

Contents:Contents:

PAVEMENTPAVEMENTThe pavement is the structure which separates the tyres of vehicles from the underlying foundation material. The later is generally the soil but it may be structural concrete or a steel bridge deck.

TYPES OF PAVEMENT

Flexible Pavements

Rigid Pavements

FLEXIBLE PAVEMENTSFLEXIBLE PAVEMENTS

Flexible Pavements are constructed from bituminous or unbound material and the stress is transmitted to the sub-grade through the lateral distribution of the applied load with depth.

Natural Soil (Subgrade)

Aggregate Subbase Course

Aggregate Base CourseAsphalt Concrete

Wheel Load

Sub-grade

Bituminous Layer

Typical Load Distribution in Flexible Pavement

Vertical stress

Foundation stress

Typical Stress Distribution in Flexible Pavement.

RIGID PAVEMENTSRIGID PAVEMENTS

Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.

In rigid pavements the stress is transmitted to the sub-grade through beam/slab effect. Rigid pavements contains sufficient beam strength to be able to bridge over localized sub-grade failures and areas of inadequate support.

Concrete Slab

Sub-grade

PRINCIPLES OF PAVEMENT PRINCIPLES OF PAVEMENT DESIGNDESIGN

The tensile and compressive stresses induced in a pavement by heavy wheel loads decrease with increasing depth. This permits the use (particularly in flexible pavements) of a gradation of materials, relatively strong and expensive materials being used for the surfacing and less strong and cheaper ones for base and sub-base.

The pavement (as a whole) limit the stresses in the sub-grade to an acceptable level, and the upper layers must in a similar manner protect the layers below.

Pavement design is the process of developing the most economical combination of pavement layers (in relation to both thickness and type of materials) to suit the soil foundation and the traffic to be carried during the design life.

WHAT IS PAVEMENT DESIGN?WHAT IS PAVEMENT DESIGN?

DESIGN LIFEDESIGN LIFE

The concept of design life has to be introduced to ensure that a new road will carry the volume of traffic associated with that life without deteriorating to the point where reconstruction or major structural repair is necessary.

• Pavements are alive structures.

• They are subjected to moving traffic loads that are repetitive in nature.

• Each traffic load repetition causes a certain amount of damage to the pavement structure that gradually

accumulates over time and eventually leads to the pavement failure.

• Thus, pavements are designed to perform for a certain life span before reaching an unacceptable degree of deterioration.

• In other words, pavements are designed to fail. Hence, they have a certain design life.

Philosophy of PavementsPhilosophy of Pavements

For roads in Britain the currently recommended design is 20 years for flexible pavements.

HOW MUCH DESIGN LIFE?HOW MUCH DESIGN LIFE?

PERFORMANCE AND FAILURE PERFORMANCE AND FAILURE CRITERIACRITERIA

A road should be designed and constructed to provide a riding quality acceptable for both private cars and commercial vehicles and must perform the functions i.e. functional and structural, during the design life.

PERFORMANCE AND FAILURE PERFORMANCE AND FAILURE CRITERIACRITERIA

If the rut depth increases beyond 10mm or the beginning of cracking occurs in the wheel paths, this is considered to be a critical stage and if the depth reaches 20mm or more or severe cracking occurs in the wheel paths then the pavement is considered to have failed, and requires a substantial overlay or reconstruction.

Failure Mechanism (Fatigue and Rut)Failure Mechanism (Fatigue and Rut)

Bitumen Layer

Unbound Layer

Nearside Wheel Track

Fatigue Crack

Rut Depth

Granular base/Sub-base

Sub-grade

Bituminous bound Material

Elastic Modulus ’E1’

Poison’s Ratio ‘ v1’

Thickness ‘H1’



Thickness ’H2’

Maximum Tensile Strain at Bituminous Layer

Maximum Compressive on the top of the sub-grade

Er

Ez



log N = -9.38 - 4.16 logr (Fatigue, bottom of bituminous layer)log N = - 7.21 - 3.95 logz (Deformation, top of the sub-grade)

r = is the permissible tensile strain at the bottom of the bituminous layer

z = is the permissible Compressive strain at the top of the sub-grade.

The following relationship can be used to calculate permissible tensile and compressive strains by limiting strain criterion for 85% probability of survival to a design life of N repetition of 80 kN axles and an equivalent pavement temperature of 20C;

ASPECTS OF DESIGN

Functional Structural

Safety Riding QualityCan sustain Traffic Load

Structural PerformanceStructural Performance

Strength

Safety

Comfort

Functional PerformanceFunctional Performance

RUDIMENTARY DEFINITION

Pavement Thickness Design is the determination of required thickness of various pavement layers to protect a given soil

condition for a given wheel load.

Pavement Thickness Design is the determination of required thickness of various pavement layers to protect a given soil

condition for a given wheel load.

Given Wheel Load

150 Psi

3 Psi

Given In Situ Soil Conditions

Asphalt Concrete Thickness?Base Course Thickness?Subbase Course Thickness?

PAVEMENT DESIGN PROCESS

Climate/Environment

Load Magnitude

VolumeTraffic

Material Properties

Asphalt Concrete

Roadbed Soil (Subgrade)

Base

Subase

• Pavement Design Life = Selected

• Structural/Functional Performance = Desired

• Design Traffic = Predicted

?

Asphalt Concrete Thickness ?

Base Course Thickness ?

Sub-base Course Thickness ?

Truck

WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?

SELECTED DESIGN LIFE

DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?


DESIRED STRUCTURAL AND FUNCTIONAL PERFORMANCE

FUNCTIONAL PERFORMANCE CURVE

STRUCTURAL PERFORMANCE CURVE

RehabilitationUnacceptable

limitR

ide

Qu

alit

yPerfect

Traffic/ Age

Str

uc

tura

l C

ap

acit

y

Perfect Traffic/ Age

Rehabilitation Structural Failure


PREDICTED DESIGN TRAFFIC

Traffic Loads Characterization

Pavement Thickness Design Are Developed To Account For The Entire Spectrum Of Traffic Loads

Cars Pickups Buses Trucks Trailers

Failure = 10,000 Repetitions13.6 Tons

Failure = 100,000 Repetitions

11.3 Tons

Failure = 1,000,000 Repetitions

4.5 Tons

Failure = 10,000,000 Repetitions

2.3 Tons

11.3 TonsFailure = Repetitions ?

13.6 Tons4.5 Tons

2.3 Tons

Equivalent

Standard ESAL

Axle Load

18000 - Ibs

(8.2 tons)

Damage per Pass = 1

• Axle loads bigger than 8.2 tons cause damage greater than one per pass

• Axle loads smaller than 8.2 tons cause damage less than one per pass

• Load Equivalency Factor (L.E.F) = (? Tons/8.2 tons)4

RELATIVE DAMAGE CONCEPT

Consider two single axles A and B where:

A-Axle = 16.4 tons

Damage caused per pass by A -Axle = (16.4/8.2)4 = 16

This means that A-Axle causes same amount of damage per pass as caused by 16 passes of standard 8.2 tons axle i.e,

8.2 Tons Axle

16.4 Tons Axle

=

B-Axle = 4.1 tons

Damage caused per pass by B-Axle = (4.1/8.2)4 = 0.0625

This means that B-Axle causes only 0.0625 times damage per pass as caused by 1 pass of standard 8.2 tons axle.

In other words, 16 passes (1/0.625) of B-Axle cause same amount of damage as caused by 1 pass of standard 8.2 tons axle i.e.,

Consider two single axles A and B where:

=4.1 Tons Axle 8.2 Tons Axle

AXLE LOAD & RELATIVE DAMAGE

1.0

1.1 2.3 3.3

4.7 6.5 8.

711

.5 14.9 18

.9 23.8 29

.5 36.3 44

.1 53.1

63.4 75

.2

0

10

20

30

40

50

60

70

80D

AM

AG

E P

ER

PA

SS

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

SINGLE AXLE LOAD (Tons)

PAVEMENT THICKNESS DESIGNComprehensive Definition

Pavement Thickness Design is the determination of thickness of various pavement layers (various paving materials) for a given soil condition and the predicted design traffic in terms of equivalent standard axle load that will provide the desired structural and functional performance over the selected pavement design life.

PAVEMENT THICKNESS DESIGN APPROACHES

EMPIRICALPROCEDURE

MECHANISTIC-EMPIRICAL

PROCEDURE

EMPIRICAL PROCEDURES• These procedures are derived from experience (observed field performance) of in-service pavements and or “Test Sections”

• These procedures are only accurate for the exact conditions for which they were developed and may be invalid outside the range of variables used in their development.• EXAMPLE

•AASHTO Procedure (USA)•Road Note Procedure (UK)

between

Pavement performance

, traffic loads &

pavement thickness

for

A given set of paving materials

and soils, geographic location and

climatic conditions

• These procedures define the interaction

EMPIRICAL PROCEDURES

These methods or models are generally used to determine the required pavement thickness, the number of load applications required to cause failure, or the occurrence of distress due to pavement material properties, sub-grade type, climate, and traffic conditions.

One advantage in using empirical models is that they tend to be simple and easy to use. Unfortunately they are usually only accurate for the exact conditions for which they have been developed. They may be invalid outside of the range of variables used in the development of the method

EMPIRICAL PROCEDURES

AASHTO PROCEDURE

Empirical Procedure developed through statistical analysis of the observed performance of AASHTO Road Test Sections.

AASHTO Road Test was conducted from 1958 to 1960 near Ottawa, Illinois, USA.

234 “Test Sections” (160 feet long), each incorporating a different combination of thicknesses of Asphalt Concrete, Base Course and Subbase Course were constructed and trafficked to investigate the effect of pavement layer thickness on pavement performance.

178

Utica

Uti

ca R

oad

23

2371

71US

6

North

US

6Ottawa

Loop 4Loop 5

Loop 6Loop 3

Frontage Road

Frontage Road

Maintenance Building

AASHO Adm’n

12

Proposed FA 1 Route 80

Army Barracks

Pre-stressed / Reinforced Concrete

Typical Loop

XX

X X

XX

X X

Test Tangent

Test Tangent

Rigid

Flexible

Steel I-Beam

AASHO ROAD TEST CONDITIONS

ENVIRONMENT•Climate -4 to 24oC•Average Annual Precipitation 34 Inches (864 mm)•Average Frost Penetration Depth 28 Inches

Soil•Classification A-6/A-7-6 (Silty-Clayey)•Drainage Poorly Drained•Strength 2-4 % CBR (Poor)

Pavement Layer Materials•Asphalt Concrete AC a1 = 0.44•Base Course Crushed Stone a2 = 0.14•Subbase Course Sandy Gravel a3 = 0.11

AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST

LOOP LANE

LOAD LOAD

1

FRONT LOAD

2

2

WEIGHT IN TONS

0.9 0.9FRONT AXLE LOAD AXLE GROSS WEIGHT

1.80.9 2.7 3.6

FRONT LOAD

1

FRONT LOAD

LOAD

LOAD

4

FRONT LOAD

1

FRONT LOAD

LOAD

LOAD

3

FRONT LOAD

1

FRONT LOAD

LOAD

LOAD

6

FRONT LOAD

1

FRONT LOAD

LOAD

LOAD

5

1.8 5.5 12.7

2.7 10.9 24.6

2.7 8.2 19.1

4.1 14.6 33.2

2.7 10.2 23.2

4.1 18.2 40.5

4.1 13.6 31.4

5.5 21.8 49.1

AASHO ROAD TEST

• “Test Sections” were subjected to 1.114 million applications of load.

• Performance measurements (roughness, rutting, cracking etc.) were taken at regular intervals and were used to develop statistical performance prediction models that eventually became the basis for the current AASHTO Design procedure.

• AASHTO performance model/procedure determines for a given soil condition, the thickness of Asphalt Concrete, Base Course and Subbase Course needed to sustain the predicted amount of traffic (in terms of 8.2 tons ESALs) before deteriorating to some selected level of ride quality.

ESALs

Terminal

Initial

RID

E

QU

AL

ITY Asphalt Concrete = ?

Base = ?

Subbase = ?

Soil

LIMITATIONS OF THE AASHTO EMPIRICAL PROCEDURE

AASHTO being an EMPIRICAL procedure is applicable to the AASHO Road TEST conditions under which it was developed.

MECHANISTIC-EMPIRICAL PROCEDURES These procedures, as the name implies, have two parts:

=> A mechanistic part in which a structural model (theory) is used to calculate stresses, strains and deflections induced by traffic and environmental loading.

=> An empirical part in which distress models are used to predict the future performance of the pavement

structure.

The distress models are typically developed from the laboratory data and calibrated with the field data.

EXAMPLES• Asphalt Institute Procedure (USA) • SHRP Procedure (USA)

Mechanistic - Empirical Methods

The mechanistic–empirical method of design is based on the mechanics of materials that relates an input (such as a wheel load) to an output or pavement response (such as stress or strain). The response values are used to predict distress based on laboratory test and field performance data. Dependence on observed performance is necessary because theory alone has not proven sufficient to design pavements realistically

Researchers assumes that mechanistic - empirical design procedures will model a pavement more accurately than empirical equations. The primary benefits that could result from the successful application of mechanistic empirical procedures include:

Mechanistic - Empirical Design Approach

The ability to predict the occurrence of specific types of distress.

Stress dependency of both the subgrade and base course.

The time and temperature dependency of the asphaltic layers.

Benefits of Mechanistic - Empirical Design Approach

Estimates of the consequences of new loading conditions can be evaluated. For example, the damaging effects of increased loads, high tire pressures, and multiple axles, can be modeled by using mechanistic processes.

Better utilization of available materials can be accomplished by simulating the effects of varying the thickness and location of layers of stabilized local materials.

Seasonal effects can be included in performance estimates.


One of the most significant benefits of these methods is the ability to structurally analyze and extrapolate the predicted performance of virtually any flexible pavement design from limited amounts of field or laboratory data prior to full scale construction applications. This offers the potential to save time and money by initially eliminating from consideration those concepts that have been analyzed and are judged to have little merit.


One of the biggest drawbacks to the use of mechanistic design methods is that these methods require more comprehensive and sophisticated data than typical empirical design techniques.

Draw Back of Mechanistic - Empirical Design Procedures

However, the potential benefits are believed to far outweigh the drawbacks. In summary, mechanistic-empirical design procedures offer the best opportunity to improved pavement design technology for the next several decades.

SOURCES OF PREMATURE PAVEMENT FAILURE

Thic

knes

s D

esig

n

Construction Practices&

Quality ControlM

aterial Design

Inadequately Designed Pavements Will Fail Prematurely Inspite

Of Best Quality Control & Construction Practices

Thic

knes

s D

esig

n Material D

esign


Quality Control

Material D

esign

Thic

knes

s D

esig

n


Quality Control

Causes of Premature Failure in Pakistan

Causes of premature failure of pavements in Pakistan

Rutting due to high variations in ambient temperature

Uncontrolled heavy axle loads Limitations of pavement design procedures

to meet local environmental conditions

COMPARISON OF TRUCK DAMAGEPAKISTAN Vs USA

1

2 8

7

6

5

4

3

14

13

12

11

10

9

18

17

16

15

22

21

20

19

Plastic Flow Rutting

Rutting in Sub-grade or Base

Introduction to Pavement Design Concepts by Qaiser Rafiq

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