TOPIC:
Introduction to Pavement Design Concepts
Presented By: Engr. Qaiser Rafiq 07 - MS - TE - 08
Contents: ❧ Pavement ❧ Types of Pavement ❧ Principal of Pavement Design ❧ Failure Criteria ❧ Aspects of Pavement Design ❧ Relative Damage Concept ❧ Pavement Thickness Design approaches ❧ Empirical Method ❧ Mechanistic-Empirical Method
PAVEMENT The 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 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.
Asphalt Concrete
Aggregate Base Course
Natural Soil (Subgrade) Aggregate Subbase Course
Typical Load Distribution in Flexible Pavement
Wheel Load Bituminous Layer
Sub-grade
Typical Stress Distribution in Flexible Pavement.
Vertical stress Foundation stress
RIGID PAVEMENTS 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. Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.
Concrete Slab
Sub-grade
PRINCIPLES OF PAVEMENT DESIGN
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 subgrade to an acceptable level, and the upper layers must in a similar manner protect the layers below.
WHAT IS PAVEMENT DESIGN? 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.
DESIGN 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.
Philosophy of Pavements •
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.
HOW MUCH DESIGN LIFE? For roads in Britain the currently recommended design is 20 years for flexible pavements.
PERFORMANCE AND FAILURE CRITERIA 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 CRITERIA 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)
Nearside Wheel Track
Rut Depth Bitumen Layer Fatigue Crack Unbound Layer
Elastic Modulus ’E1’ Poison’s Ratio ‘ v1’
Bituminous bound Material
Er
Thickness ‘H1’ Maximum Tensile Strain at Bituminous Layer
Elastic Modulus ’E2’ Poison’s Ratio ‘ v2’
Granular base/Sub-base Ez
Thickness ’H2’ Maximum Compressive on the top of the sub-grade
Sub-grade
Elastic Modulus ’E3’ Poison’s Ratio ‘ v3’
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 20 °C; log N = -9.38 - 4.16 logεr (Fatigue, bottom of bituminous layer) log N = - 7.21 - 3.95 logεz (Deformation, top of the sub-grade) εr
= is the permissible tensile strain at the bottom of the bituminous layer
εz
= is the permissible Compressive strain at the top of the sub-grade.
ASPECTS OF DESIGN
Functional
Safety
Riding Quality
Structural
Can sustain Traffic Load
Structural Performance
Strength Functional Performance Safety
Comfort
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. Given Wheel Load 150 Psi Asphalt Concrete Thickness? Base Course Thickness? Subbase Course Thickness?
3 Psi
Given In Situ Soil Conditions
PAVEMENT DESIGN PROCESS
Climate/Environment
Load Magnitude Volume
Traffic Asphalt Concrete
Material Properties
Base Subase Roadbed Soil (Subgrade)
Truck
Asphalt Concrete Thickness ?
Base Course ? Thickness ? Sub-base Course Thickness ?
• Pavement Design Life
= Selected
• Structural/Functional Performance
= Desired
• Design Traffic
= Predicted
WHAT DO WE MEAN BY ?
SELECTED DESIGN LIFE
DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?
WHAT DO WE MEAN BY ?
DESIRED STRUCTURAL AND FUNCTIONAL PERFORMANCE
FUNCTIONAL PERFORMANCE CURVE Perfect
Ride Quality
Rehabilitation
Unacceptable limit
Traffic/ Age
STRUCTURAL PERFORMANCE CURVE Structural Capacity
Rehabilitation
Perfect
Traffic/ Age
Structural Failure
WHAT DO WE MEAN BY ?
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
13.6 Tons
Failure = 10,000 Repetitions 11.3 Tons
Failure = 100,000 Repetitions 4.5 Tons
Failure = 1,000,000 Repetitions 2.3 Tons
Failure = 10,000,000 Repetitions 13.6 Tons
4.5 Tons
Failure = Repetitions ? 11.3 Tons
2.3 Tons
RELATIVE DAMAGE CONCEPT
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
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,
= 16.4 Tons Axle
8.2 Tons Axle
Consider two single axles A and B where: 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.,
= 4.1 Tons Axle
8.2 Tons Axle
DAMAGE PER PASS
80 70 60 50 40 30 20 10 0
1.0 1.1 2.3 3.3 4.7 6.5 8.7 11.5 14.9 18.9 23.8 29.5 36.3 44.1 53.1 63.4 75.2
AXLE LOAD & RELATIVE DAMAGE
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SINGLE AXLE LOAD (Tons)
PAVEMENT THICKNESS DESIGN Comprehensive 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
EMPIRICAL PROCEDURE
MECHANISTICEMPIRICAL PROCEDURE
EMPIRICAL PROCEDURES • These procedures are derived from experience
(observed field performance) of in-service pavements and or “Test Sections” These procedures define the interaction A given set of paving materials and soils, for between geographic location and climatic conditions • 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. Pavement performance , traffic loads & pavement thickness
• EXAMPLE
•AASHTO Procedure (USA) •Road Note Procedure (UK)
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.
EMPIRICAL PROCEDURES 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
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.
Utica Road
North
Frontage Road
Maintenance Building
Proposed FA 1 Route 80 Loop 4
Loop 5 Loop 6
Loop 3
178
2
US
1 Army Barracks
6
AASHO Adm’n
Frontage Road
Test Tangent Flexible
Steel I-Beam
Rigid Test Tangent
Typical Loop
US 6
Ottawa 71
Utica
X X X X
23
71
23
Pre-stressed / Reinforced Concrete X X X X
AASHO ROAD TEST CONDITIONS ENVIRONMENT •Climate -4 to 24oC •Average Annual Precipitation 34 Inches (864 mm) •Average Frost Penetration Depth 28 Inches Soil •Classification •Drainage •Strength Pavement Layer Materials •Asphalt Concrete •Base Course •Subbase Course
A-6/A-7-6 (Silty-Clayey) Poorly Drained 2-4 % CBR (Poor)
AC Crushed Stone Sandy Gravel
a1 = 0.44 a2 = 0.14 a3 = 0.11
AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST LOOP LANE
WEIGHT IN TONS
1
FRONT AXLE
LOAD LOAD
2 2
FRONT
LOAD
1 3
FRONT
LOAD
FRONT
LOAD
LOAD LOAD
1 4
FRONT
LOAD
FRONT
LOAD
LOAD LOAD
1 5
FRONT
LOAD
FRONT
LOAD
LOAD LOAD
1 6
FRONT
FRONT
LOAD
LOAD
LOAD AXLE
GROSS WEIGHT
0.9 0.9
0.9 2.7
1.8 3.6
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
LOAD LOAD
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
RIDE QUALITY
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.
Initial Asphalt Concrete = ? Base = ?
Terminal
Subbase = ? ESALs
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
Mechanistic - Empirical Design Approach
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:
Benefits of 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.
Benefits of Mechanistic - Empirical Design Approach ❧ 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.
Draw Back of Mechanistic - Empirical Design Procedures 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.
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
Construction Practices & Quality Control
Inadequately Designed Pavements Will Fail Prematurely Inspite Of Best Quality Control & Construction Practices
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 DAMAGE PAKISTAN Vs USA
1
7
13
2
8
14
3
9
15
4
10
16
5
11
17
6
12
18
19
20
21
22
Plastic Flow Rutting
Rutting in Sub-grade or Base
