APPLICATIONS OF ENERGY CONCEPTS FOR FATIGUE ANALYSIS OF AIRPORT PAVEMENTS

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APPLICATIONS OF ENERGY CONCEPTS FOR FATIGUE
ANALYSIS OF AIRPORT PAVEMENTS

• FAA Fatigue Project Briefing
• October 7th, 2004 Urbana, IL
• Samuel H. Carpenter, Professor
• Shihui Shen, Graduate Research Assistant
• University of Illinois at Urbana-Champaign

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PRESENTATION

• Introduction
• Objectives to the New Approach
• Problems in Traditional Fatigue Analysis Approach
• Energy Concepts RDEC Approach
• Findings Results
• Implications for Airport Pavements
• Conclusions Future Work

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INTRODUCTION A NEW APPROACHRDEC APPROACH

• Fatigue is a Damage Phenomenon Described by
  Energy Principles
• All Fatigue Behavior can be Described by a Single
  Parameter
• Ratio of Dissipated Energy Change (RDEC)
  Percent of Load Cycle Input Energy Producing
  Damage
• Plateau Value (PV) the Value When Materials
  Ratio of Dissipated Energy Change is Constant
  During the Fatigue Test

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OBJECTIVES OF THE NEW APPROACH

• Overcome the problems of traditional fatigue
  analysis approach
• Examine energy concepts and the ratio of
  dissipated energy change (RDEC) approach
• Apply RDEC approach to airport pavement, and
  develop design considerations suitable for
  airfield conditions
• Heavy aircraft loads
• Very thick pavements
• Low load frequency.

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PROBLEMS IN TRADITIONAL FATIGUE ANALYSIS APPROACH

• Cannot explain fatigue behavior under heavy loads
  on pavements of varying thicknesses
• Non-unique strain-Nf relationship. Such
  relationship is not fundamentally material based.
• A distinctly different fatigue behavior appears
  when airport pavement is built very thick (low
  strain/damage levels)
• Low load frequency in thick pavement amplifies
  the healing effect which cannot be observed and
  described by traditional approach.

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TRADITIONAL FATIGUE PLOT
Fatigue Under Heavy Load
Fatigue In Very Thick Pavement
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ENERGY CONCEPTS DISSIPATED ENERGY
INITIAL LOAD CYCLE

• Ratio of Dissipated Energy Change (RDEC)

STRESS
SECOND LOAD CYCLE
STRAIN
DIFFERENT DISSIPATED ENERGY BETWEEN FIRST AND
SECOND LOAD CYCLE
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RATIO OF DISIPATED ENERGY CHANGE CALULATION
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TYPICAL RDEC PLOT WITH THREE BEHAVIOR ZONES
Ratio of Dissipated Energy Change, Log
I
III
II
Load Repetitions, Log
Plateau Value
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CURRENT FINDINGS IN RDEC APPROACH

• Unique relationship between PV and Nf
• Different mixtures, load levels, loading modes
  and testing conditions
• PV is a comprehensive energy based parameter.
• A lower (higher) PV is always associated with a
  lower (higher) damage, producing a longer
  (shorter) fatigue life

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RDEC APPROACH
Various mixture types and testing conditions
under heavy aircraft loads
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RDEC APPROACH
Including the strain conditions (low
strain/damage level) in very thick airport
pavements
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IMPLICATIONS FOR AIRPORT PAVEMENTS

• A fatigue endurance limit exists, and is an
  important consideration for the
  design/performance of thick pavements (low
  strain/damage)
• Healing effect is significant in pavements with
  low load occurrence.

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UNIQUE ENERGY LEVEL AT WHICH NO FATIGUE
DAMAGE EXISTS
PV8.57E-9
Nf1.10E7
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FATIGUE ENDURANCE LIMIT

• Crucial for Design and Performance of Thick
  Pavements
• Limit to HMA Thickness
• Unique fatigue curves
• Independent of Traffic Level
• Significant element for structural design
• Minimizes Effect of Overloads

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USE OF PV TO SHORTEN FATIGUE TESTING FOR THICK
PAVEMENTS
Compare PV projected from shortened testing with
extended testing
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USE OF PV TO SHORTEN FATIGUE TESTING FOR THICK
PAVEMENTS

• Plateau Value period can be reached much earlier
  than Nf _at_ 50 stiffness reduction point
• Reasonable projection can be obtained through
  greatly shortened low strain/damage testing.
• 500,000 Load repetitions

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HEALING

• Accepted Description
• Between loads the damage is reversed as the
  asphalt-aggregate interface reattaches, removing
  micro-cracks
• Actual Occurrence
• A continuous physical-chemical reaction that
  occurs even during continuous loadings at low
  strain levels

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HEALING IN AIRPORT PAVEMENTS

• A Material Property Constant
• The HMA has the potential to recover a relative
  amount of damage
• When Load Damage Falls Below Healing Potential,
  Damage Accumulation is Minimal or Non-Existent
• Fatigue Endurance Limit exists
• Field Fatigue Life is Increased Over Lab Testing

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CURRENT HEALING STUDY
PV An energy level related to fatigue life, Nf
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CONCLUSIONS

• Ratio of Dissipated Energy Change (RDEC) provides
  a unique way to study fatigue behavior of HMA
• Plateau Value, PV, is a function of material
  properties and pavement response
• PV-Nf relationship is unique for mixture type,
  loading mode, and all testing conditions

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CONCLUSIONS

• PV shows a unique threshold for the fatigue
  endurance limit (PVL). . Current results shows
  such PVL is around 8.57E-9
• PV-Nf uniqueness can be used to predict long
  fatigue life without running test to failure
• Healing can be observed with the PV.
• Represents an energy level of damage, which
  decreases with an increase in rest periods
  (healing effect).

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FUTURE WORK

• Substantiate the relationship between PV and rest
  periods
• Relate PV with healing using energy concepts
• Healing Index
• Healing Rate
• Asphalt Type Influence

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FUTURE WORK

• Integrate the energy based healing effects, rest
  periods, into an improved rational design
  procedure suitable for airfield conditions
• Heavy aircraft load
• Thick pavement layers
• Low loading frequency and extended rest periods

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THANK YOU