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Introduction to Pavement Design Concepts

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.

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Introduction to Pavement Design Concepts

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  1. Introduction to Pavement Design Concepts

  2. 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

  3. 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.

  4. TYPES OF PAVEMENT Flexible Pavements Rigid Pavements

  5. 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.

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

  7. Typical Load Distribution in Flexible Pavement Wheel Load Bituminous Layer Sub-grade

  8. Typical Stress Distribution in Flexible Pavement. Vertical stress Foundation stress

  9. 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.

  10. Rigid Pavement Concrete Slab Sub-grade

  11. 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.

  12. 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.

  13. 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

  14. Philosophy of Pavements • Pavements are alivestructures • 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.

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

  16. 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.

  17. 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.

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

  19. 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

  20. 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.

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

  22. Structural Performance Strength Safety Comfort Functional Performance

  23. PAVEMENT THICKNESS DESIGN 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 RUDIMENTARY DEFINITION

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

  25. Truck Asphalt Concrete Thickness ? ? Base Course Thickness ? Sub-base Course Thickness ? • Pavement Design Life = Selected • Structural/Functional Performance = Desired • Design Traffic = Predicted

  26. WHAT DO WE MEAN BY ? SELECTED DESIGN LIFE

  27. DESIGN LIFE OF CIVIL ENGINEERING STRUCTURES?

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

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

  30. WHAT DO WE MEAN BY ? PREDICTED DESIGN TRAFFIC

  31. Traffic Loads Characterization Pavement Thickness Design Are Developed To Account For The Entire Spectrum Of Traffic Loads Cars Pickups Buses Trucks Trailers

  32. 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 4.5 Tons 13.6 Tons Failure = Repetitions ? 11.3 Tons 2.3 Tons

  33. 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

  34. = 16.4 Tons Axle 8.2 Tons Axle 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,

  35. = 4.1 Tons Axle 8.2 Tons Axle 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.,

  36. AXLE LOAD & RELATIVE DAMAGE

  37. 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.

  38. EMPIRICAL PROCEDURE MECHANISTIC- EMPIRICAL PROCEDURE PAVEMENT THICKNESS DESIGN APPROACHES

  39. EMPIRICAL PROCEDURES • These procedures define the interaction A given set of paving materials and soils, geographic location and climatic conditions Pavement performance, traffic loads & pavement thickness for between • 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)

  40. 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.

  41. 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

  42. 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.

  43. North Maintenance Building Proposed FA 1 Route 80 Frontage Road 23 Loop 4 Loop 5 Utica Road Loop 6 Loop 3 2 1 US 6 ArmyBarracks US 6 71 178 Ottawa AASHO Adm’n Frontage Road 23 71 Utica Pre-stressed / Reinforced Concrete X X Test Tangent X X Flexible X X Rigid X X Test Tangent Steel I-Beam Typical Loop Layout of the AASHO Road Test.

  44. 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

  45. LOOP LANE WEIGHT IN TONS FRONT AXLE LOAD AXLE GROSS WEIGHT 0.9 0.9 1.8 LOAD LOAD 0.9 2.7 3.6 2 2 FRONT LOAD 1.8 5.5 12.7 3 FRONT LOAD LOAD 2.7 10.9 24.6 LOAD FRONT LOAD 2.7 8.2 19.1 4 1 1 1 1 1 FRONT LOAD LOAD 4.1 14.6 33.2 LOAD FRONT LOAD 2.7 10.2 23.2 5 FRONT LOAD LOAD 4.1 18.2 40.5 LOAD FRONT LOAD 4.1 13.6 31.4 6 FRONT LOAD LOAD 5.5 21.8 49.1 LOAD FRONT LOAD AXLE WEIGHTS & DISTRIBUTIONS USED ON VARIOUS LOOPS OF THE ASSHO ROAD TEST

  46. AASHO ROAD TEST RIDE QUALITY Asphalt Concrete = ? Initial Base = ? Subbase = ? Terminal Soil 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.

  47. LIMITATIONS OF THE AASHTO EMPIRICAL PROCEDURE AASHTO being an EMPIRICAL procedure is applicable to the AASHO Road TEST conditions under which it was developed.

  48. 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)

  49. 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

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