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

PRESENTATIONPRESENTATION  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  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

INTRODUCTION : A NEW APPROACH RDEC 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 Material’s Ratio of Dissipated Energy Change is Constant During the Fatigue Test  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 Material’s Ratio of Dissipated Energy Change is Constant During the Fatigue Test

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.  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.

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.  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.

TRADITIONAL FATIGUE PLOT Fatigue Under Heavy Load Fatigue In Very Thick Pavement

ENERGY CONCEPTS: DISSIPATED ENERGY Ratio of Dissipated Energy Change (RDEC) STRAIN STRESS INITIAL LOAD CYCLE SECOND LOAD CYCLE DIFFERENT DISSIPATED ENERGY BETWEEN FIRST AND SECOND LOAD CYCLE

RATIO OF DISIPATED ENERGY CHANGE CALULATION

TYPICAL RDEC PLOT WITH THREE BEHAVIOR ZONES I II III Plateau Value Ratio of Dissipated Energy Change, Log Load Repetitions, Log

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  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

RDEC APPROACH Various mixture types and testing conditions under heavy aircraft loads

RDEC APPROACH Including the strain conditions (low strain/damage level) in very thick airport pavements

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.  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.

UNIQUE ENERGY LEVEL AT WHICH NO FATIGUE DAMAGE EXISTS PV=8.57E-9 Nf=1.10E+7

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  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

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

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

HEALINGHEALING  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  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

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  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

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

CONCLUSIONSCONCLUSIONS  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;  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;

CONCLUSIONSCONCLUSIONS  PV shows a unique threshold for the fatigue endurance limit (PV L ).. Current results shows such PV L 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).  PV shows a unique threshold for the fatigue endurance limit (PV L ).. Current results shows such PV L 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).

FUTURE WORK  Substantiate the relationship between PV and rest periods:  Relate PV with healing using energy concepts Healing Index Healing Rate Asphalt Type Influence  Substantiate the relationship between PV and rest periods:  Relate PV with healing using energy concepts Healing Index Healing Rate Asphalt Type Influence

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  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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