Introduction to Pavement Design Concepts

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.

Natural Soil (Subgrade) Aggregate Subbase Course Aggregate Base Course Asphalt Concrete

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.

Rigid Pavement 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 sub-grade to an acceptable level, and the upper layers must in a similar manner protect the layers below.

PRINCIPLES OF 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.

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 in accordance with LR 833.

Failure Mechanism (Fatigue and Rut) Bitumen Layer Unbound Layer Nearside Wheel Track Fatigue Crack Rut Depth

Typical Strains in Three Layered System Elastic Modulus ’E1’ Poison’s Ratio ‘ v1’ Thickness ‘H1’ Bituminous bound Material Er Maximum Tensile Strain at Bituminous Layer Elastic Modulus ’E2’ Poison’s Ratio ‘ v2’ Thickness ’H2’ Granular base/Sub-base Ez Maximum Compressive on the top of the sub-grade Elastic Modulus ’E3’ Poison’s Ratio ‘ v3’ 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 20C; log N = -9.38 - 4.16 logr (Fatigue, bottom of bituminous layer) log N = - 7.21 - 3.95 logz (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.

Can sustain Traffic Load ASPECTS OF DESIGN Functional Structural Safety Riding Quality Can sustain Traffic Load

Structural Performance Functional Performance Strength Functional Performance Safety Comfort

RUDIMENTARY DEFINITION PAVEMENT THICKNESS DESIGN 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 3 Psi Given In Situ Soil Conditions Asphalt Concrete Thickness? Base Course Thickness? Subbase Course Thickness?

PAVEMENT DESIGN PROCESS Climate/Environment Load Magnitude Volume Traffic Material Properties Asphalt Concrete Roadbed Soil (Subgrade) Base Subase

Asphalt Concrete Thickness ? Truck ? 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?

DESIRED STRUCTURAL AND FUNCTIONAL PERFORMANCE WHAT DO WE MEAN BY ? DESIRED STRUCTURAL AND FUNCTIONAL PERFORMANCE

FUNCTIONAL PERFORMANCE CURVE STRUCTURAL PERFORMANCE CURVE Ride Quality Perfect Traffic/ Age Rehabilitation Unacceptable limit STRUCTURAL PERFORMANCE CURVE Rehabilitation Structural Failure Structural Capacity Perfect Traffic/ Age

PREDICTED DESIGN TRAFFIC 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

Failure = 10,000 Repetitions Failure = 100,000 Repetitions 13.6 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 Tons Failure = Repetitions ? 13.6 Tons 4.5 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: EXAMPLE 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: EXAMPLE 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 works, 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

AXLE LOAD & RELATIVE DAMAGE

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 MECHANISTIC-

Pavement performance, traffic loads & pavement thickness EMPIRICAL PROCEDURES These procedures are derived from experience (observed field performance) of in-service pavements and or “Test Sections” 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 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)

EMPIRICAL PROCEDURES These methods or models are generally used to determine the required pavement thickness, the umber 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.

Layout of the AASHO Road Test. 178 Utica Utica Road 23 71 US 6 North Ottawa Loop 4 Loop 5 Loop 6 Loop 3 Frontage Road Maintenance Building AASHO Adm’n 1 2 Proposed FA 1 Route 80 Army Barracks Pre-stressed / Reinforced Concrete Typical Loop X 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 1 FRONT LOAD 2 WEIGHT IN TONS 0.9 FRONT AXLE LOAD AXLE GROSS WEIGHT 1.8 2.7 3.6 4 3 6 5 5.5 12.7 10.9 24.6 8.2 19.1 4.1 14.6 33.2 10.2 23.2 18.2 40.5 13.6 31.4 21.8 49.1

AASHO ROAD TEST RIDE QUALITY Initial Terminal Subbase = ? ESALs “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 RIDE QUALITY 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 out put 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 Methods Kerkhoven and Dormon (1953) first suggested the use of vertical compressive strain on the surface of sub-grade as a failure criterion to reduce permanent deformation, while Saal and Pell(1960) recommended the use of horizontal tensile strain at the bottom of asphalt layer to minimize fatigue cracking. The use of above concepts for pavement design was first presented in the United States by Dormon and Metcalf (1965)

Mechanistic- Empirical Methods By limiting the elastic strains on the sub-grade, the elastic strains in other components above the sub-grade will also be controlled; hence, the magnitude of permanent deformation on the pavement surface will be controlled as well. These two criteria have since been adopted by Shell Petroleum International (Claussen et al., 1977) and the Asphalt Institute (Shook et al., 1982) in their mechanistic-empirical methods of design, the ability to predict the types of distress, and the feasibility to extrapolate from limited field and laboratory data.

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. The modulus of resilience, creep compliance, dynamic modulus, Poisson's ratio, etc., have replaced arbitrary terms for sub-grade and material strength used in earlier empirical 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.

SOURCES OF PREMATURE PAVEMENT FAILURE Thickness Design Construction Practices & Quality Control Material Design Thickness Design Material Design Construction Practices & Quality Control Material Design Thickness Design 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 PROBLEM STATEMENT

COMPARISON OF TRUCK DAMAGE PAKISTAN Vs USA 1 7 13 19 2 8 14 20 3 9 15 4 10 16 21 5 11 17 6 12 18 22

Plastic Flow Rutting

Rutting in Asphalt Layer

Rutting in Sub-grade or Base

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