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Guide for Mechanistic-Empirical Design of New and Rehabilitation Pavement Structures

Guide for Mechanistic-Empirical Design of New and Rehabilitation Pavement Structures. By Matt Mason. Overview. Objective Current Use Justification Flexible Pavement Design Process Input Levels and Selection Required Inputs Design Procedure Pavement Response Models Current Status.

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Guide for Mechanistic-Empirical Design of New and Rehabilitation Pavement Structures

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  1. Guide for Mechanistic-Empirical Design of New and Rehabilitation Pavement Structures By Matt Mason

  2. Overview • Objective • Current Use • Justification • Flexible Pavement Design Process • Input Levels and Selection • Required Inputs • Design Procedure • Pavement Response Models • Current Status

  3. Objective • “The overall objective of the Guide for the Mechanistic-Empirical Design for New and Rehabilitated Pavement Structures is to provide the highway community with a state-of-the-practice tool for the design and rehabilitated pavement structures, based on mechanistic-empirical principles.” • Dictionary Definitions • Mechanistic- Anything explained in terms of physical forces • Empirical- Depending upon experience or observation alone

  4. Current Use • 80% of States use the 1972, 1986, or 1993 AASHTO guides • Iowa Department of Transportation currently uses the 1993 Guide • Development from empirical performance equations from the 1950s AASHO Road test

  5. AASHO Road Test • Location • Ottuwa, Illinois • Time • 1958-1960 • Test Facilities • 6 two-lane test loops • Loop 1 control • Soils • A-6 and CBR approximately 2-4

  6. Justification for a New Design Guide • Rehabilitation • Climate • Soils • Surface Material – HMA & PCC • Truck Characteristics • Design Life – short duration of road test • Performance • Reliability – AASHTO 1986 guide

  7. Design of New and Reconstructed Flexible Pavements • Flexible Pavements • Conventional • Deep Strength • Full Depth • “Semi-Rigid”

  8. Design Process

  9. Design Input Levels • Level 1 • Site and/or material specific inputs • Level 2 • Use of correlations to establish or determine the required inputs • Level 3 • Use of national or regional default values to define the inputs

  10. Input Level Selection • Sensitivity of the pavement performance to a given input • The criticality of the project • Information available at the time of design • Resources & time available to the designer to obtain the inputs

  11. Inputs • General information • Site/Project Identification • Analysis Parameters • Traffic • Climate • Drainage and Surface Properties • Pavement Structure

  12. General Information • Design Life • Base/subgrade construction month • Pavement construction month • Traffic opening month

  13. Site/Project Identification • Project Location • Project Identification • Functional Class • Principal Arterial – Interstate & Defense routes • Principal Arterial – Other • Minor Arterial • Major Collector • Minor Collector • Local Routes & Streets

  14. Analysis Parameters • Initial IRI • Performance Criteria • Surface-down fatigue cracking • Bottom-up fatigue cracking • Thermal cracking • Fatigue fracture of chemically stabilized layers • Total permanent deformation • Smoothness

  15. Traffic • ESAL Approach • Data required • Base year truck traffic volume • Vehicle operation speed • Truck traffic directional and lane distribution factors • Vehicle class distribution • Axle load distribution factors • Axle and wheel base configurations • Tire characteristics & Inflation pressures • Truck lateral distribution factor • Truck growth factors

  16. Traffic Collection • Agency’s typically collect • Weigh in Motion (WIM) • Automatic Vehicle Classification (AVC) • Vehicle counts • Iowa Department of Transportation • Two-way AADT • Percentage of trucks in design lane • Operational speed • Monthly adjustment factors • Vehicle class distribution information • Weigh in Motion • Reasonable assumptions about number of axles & axle spacing for each truck class • Tire inflation pressure & wheel wander data is not collected

  17. Traffic Input Levels • Level 1 • Site specific vehicle classification & axle weight data • Level 2 • Site specific vehicle classification & regional axle weight data • Level 3 • Regional vehicle classification & axle weight data • Level 4 • Default vehicle classification & axle load distribution

  18. Traffic Module Data Analysis • AVC – Vehicle Classification • WIM – Axle Load Distribution • AADTT • AADT • Percent Trucks • Truck Traffic Classification (TTC)

  19. Truck Traffic Classification • Functional Classification • Distribution of trucks

  20. Loading details Tire pressures Tire & axle load Axle & tire spacing Average number of axles per vehicle classification Traffic factors Time distribution factors Weekday & Weekend truck factors Directional Distribution Lane Distribution Lateral Distribution Traffic growth function Traffic Module Data Analysis

  21. Lateral Distribution • Accounts for the wander of trucks across one lane of traffic

  22. Climate • Weather information • Hourly air temperature • Hourly precipitation • Hourly wind speed • Hourly percentage of sunshine • Hourly ambient relative humidity • Seasonal or constant water table depth

  23. Drainage & Surface Properties • Pavement shortwave absorptivity • Ratio of the amount of solar energy absorbed by the pavement surface • Typical values • 0.80-0.90 for weathered • 0.90-0.98 for new pavement • Potential for Infiltration • At this time does not allow • Recommend subdrainage • Pavement cross slope • Length of drainage path

  24. Layer Properties defined by user 4-6 layers Maximum of 10 is recommended subdivide layers 12-15 sub layers Maximum of 19 Pavement Structure

  25. Pavement Structure • Asphalt Concrete & Asphalt Stabilized layers • Specify maximum of 3 HMA layers • Asphalt aging modeled only for top sub layer • Layer inputs • Layer thickness • Poisson’s ratio • Thermal conductivity • Heat capacity • Total unit weight

  26. Dynamic Modulus, E* Stiffness property Function of Temperature rate of loading Age binder stiffness aggregate gradation binder content air voids Inputs Asphalt mixture properties Asphalt binder Air voids Pavement Structure

  27. Pavement Structure • Bedrock • Presence within 10 feet of the pavement surface influences the structural response of the pavement layers • Inputs • Layer thickness (infinite) • Unit weight • Poisson’s ratio • Layer modulus

  28. Design Procedure • Predict a variety of distress types • Asphalt fatigue facture • Permanent deformation • HMA thermal fracture • Chemically stabilized load associated fatigue fracture

  29. Design Procedure • Select an initial trial pavement structure • Identify pavement cross section • Specify layer material types & thickness • Is Seasonal Analysis required? • NO – constant values for modulus & Poisson's ratio • YES – Two options • EICM • Monthly Seasonal values

  30. Pavement Response Models • Determines structural responses of the pavement system • Outputs are the stresses, strains and displacements within the pavement layers • Tensile horizontal strain at the bottom of the HMA layer • Compressive vertical stresses/strains within the HMA layer • Compressive vertical stresses/strains within the base/subbase layers • Compressive vertical stresses/strains at the top of the subgrade

  31. Pavement Response Models • Permanent Deformation • 3 distinct stages of rutting behavior

  32. Pavement Response Models • Fatigue Cracking • Both top-down & bottom-up • Based on calculating the fatigue damage at the surface & at the bottom of each asphalt layer • Based upon Miner’s Law

  33. Pavement Response Models • Thermal Fracture • Amount of transverse cracking expected is predicted by relating the crack depth to the amount of cracking present • Enhanced version of model developed by the SHRP A-005 research contract

  34. Current Status • My involvement • Current Activity • Gyratory compactor

  35. Questions • http://www.trb.org/mepdg/guide.htm

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