introduction to pavement design concepts by qaiser rafiq
TRANSCRIPT
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Introduction to Pavement Design
Concepts
Presented By:Presented By:
Engr. Qaiser RafiqEngr. Qaiser Rafiq
07 - MS - TE - 0807 - MS - TE - 08
TOPIC:TOPIC:
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PavementTypes of PavementPrincipal of Pavement DesignFailure CriteriaAspects of Pavement DesignRelative Damage ConceptPavement Thickness Design approachesEmpirical MethodMechanistic-Empirical Method
Contents:Contents:
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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.
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TYPES OF PAVEMENT
Flexible Pavements
Rigid Pavements
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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.
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Natural Soil (Subgrade)
Aggregate Subbase Course
Aggregate Base CourseAsphalt Concrete
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Wheel Load
Sub-grade
Bituminous Layer
Typical Load Distribution in Flexible Pavement
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Vertical stress
Foundation stress
Typical Stress Distribution in Flexible Pavement.
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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.
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Concrete Slab
Sub-grade
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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.
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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?
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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.
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• 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
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For roads in Britain the currently recommended design is 20 years for flexible pavements.
HOW MUCH DESIGN LIFE?HOW MUCH DESIGN LIFE?
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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.
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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.
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Failure Mechanism (Fatigue and Rut)Failure Mechanism (Fatigue and Rut)
Bitumen Layer
Unbound Layer
Nearside Wheel Track
Fatigue Crack
Rut Depth
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Granular base/Sub-base
Sub-grade
Bituminous bound Material
Elastic Modulus ’E1’
Poison’s Ratio ‘ v1’
Thickness ‘H1’
Elastic Modulus ’E2’
Poison’s Ratio ‘ v2’
Thickness ’H2’
Maximum Tensile Strain at Bituminous Layer
Maximum Compressive on the top of the sub-grade
Er
Ez
Elastic Modulus ’E3’
Poison’s Ratio ‘ v3’
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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;
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ASPECTS OF DESIGN
Functional Structural
Safety Riding QualityCan sustain Traffic Load
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Structural PerformanceStructural Performance
Strength
Safety
Comfort
Functional PerformanceFunctional Performance
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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?
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PAVEMENT DESIGN PROCESS
Climate/Environment
Load Magnitude
VolumeTraffic
Material Properties
Asphalt Concrete
Roadbed Soil (Subgrade)
Base
Subase
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• Pavement Design Life = Selected
• Structural/Functional Performance = Desired
• Design Traffic = Predicted
?
Asphalt Concrete Thickness ?
Base Course Thickness ?
Sub-base Course Thickness ?
Truck
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WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?
SELECTED DESIGN LIFE
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DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?
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WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?
DESIRED STRUCTURAL AND FUNCTIONAL PERFORMANCE
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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
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WHAT DO WE MEAN BY ?WHAT DO WE MEAN BY ?
PREDICTED DESIGN TRAFFIC
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Traffic Loads Characterization
Pavement Thickness Design Are Developed To Account For The Entire Spectrum Of Traffic Loads
Cars Pickups Buses Trucks Trailers
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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
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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
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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
=
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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
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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)
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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.
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PAVEMENT THICKNESS DESIGN APPROACHES
EMPIRICALPROCEDURE
MECHANISTIC-EMPIRICAL
PROCEDURE
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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
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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.
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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
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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.
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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
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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
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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
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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
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LIMITATIONS OF THE AASHTO EMPIRICAL PROCEDURE
AASHTO being an EMPIRICAL procedure is applicable to the AASHO Road TEST conditions under which it was developed.
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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)
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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
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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
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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
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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.
Benefits of Mechanistic - Empirical Design Approach
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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.
Benefits of Mechanistic - Empirical Design Approach
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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
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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.
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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
Construction Practices&
Quality Control
Material D
esign
Thic
knes
s D
esig
n
Construction Practices&
Quality Control
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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
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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
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Plastic Flow Rutting
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Rutting in Sub-grade or Base
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