Block 29 SP 13


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Block 29 SP 13

  1. 1. HMA Charaterization Fatigue Cracking 1Fatigue CrackingSenior/GraduateHMA Course
  2. 2. HMA Charaterization Fatigue Cracking 2Fatigue Cracking
  3. 3. 3HMA Charaterization Fatigue CrackingMechanisms• Traditionally considered to start at thebottom and work up to the top• Crack starts when tensile strain exceedstensile strength of mix• When cracks visible on top, full layercrackedSubgradeBaseAC Mix εtLongitudinal pavement profile
  4. 4. 4HMA Charaterization Fatigue CrackingMechanisms• Recent observations of fatiguecracking that starts from the top at theoutside edges of the wheel path• Tensile stresses due to tire-pavementinteractions at surfaceSubgradeBaseAC MixεtTransverse pavement profileTransverse pavement profile
  5. 5. 5HMA Charaterization Fatigue CrackingFatigue Testing• Most commonly used• Flexural beam• Cantilevered beam• Others• Diametral fatigue• Notched beam
  6. 6. 6HMA Charaterization Fatigue CrackingGeneral Terms• Dynamic load• Load applied using a sinusoidal wave form• Repeated load• Load pulse applied then removed• Rest period between loadsLoadLoadTimeTime
  7. 7. 7HMA Charaterization Fatigue CrackingFlexural Beam Fatigue Testing• Repeated load preferred to sinusoidal topermit stress relaxation• Loading can be either constant stress orconstant strain• Failure = 50% loss of stiffness (controlledstrain)
  8. 8. 8HMA Charaterization Fatigue CrackingDetermining Failure for Constant Strain00. 1,000 10,000 100,000Numbers of CyclesStiffnessRatioFailure = 0.5 Stiffness Ratio
  9. 9. Flexural Beam Fatigue TestingLoadingClampsClamps forholdingbeam
  10. 10. Test ResultsStrain, ε• Results dependent upon how test run• Constant stress means stiffer asphaltbinders perform betterStress, σSoftStiff
  11. 11. Test ResultsStrain, ε• Results dependent upon how test run• Constant strain means softer asphaltbinders perform betterStress, σSoftStiff
  12. 12. Constant Stress vs. Constant StrainSubgradeBaseHMA 100 mm or lessSubgradeBaseHMA150 mm or moreStrain at bottom ofAC layer controlsStress controls
  13. 13. 13HMA Charaterization Fatigue CrackingOther Fatigue Tests• Cantilevered beam• Diametral• Notched beam
  14. 14. Cantilevered Beam Testing• Trapezoid beamconfiguration• Requires concrete beambe fabricated then sawn• Fixed at bottom, loaded ina cantilever fashion at top
  15. 15. Diametral Fatigue Testing• Repeated load (usually)• Considered less sensitive to mixproperties than flexural
  16. 16. 16HMA Charaterization Fatigue CrackingExample of Test Results015,00030,00045,000Cycles toFailure20CTest TemperatureFlexuralTrapezoidDiametralReported in SHRP A-404, 1994
  17. 17. 17HMA Charaterization Fatigue CrackingAdvanced Fatigue Topics• Notched-beam test (C* line integral)• Dissipated Energy• Models for Predicting Fatigue Life
  18. 18. Notched Beam Testing• C*-line integral approachFixed Movable
  19. 19. 19HMA Charaterization Fatigue CrackingDissipated Energy• Dissipated energy is the amount of energylost for each loading cycle• Calculated from the changes in stressesand strains for each cycle of testing
  20. 20. 20HMA Charaterization Fatigue CrackingDifficulties• Research showed that dissipated energyequations are dependent on mix variablesand conditions of testing
  21. 21. 21HMA Charaterization Fatigue CrackingPredicting Fatigue fromBinder and Mix Properties• SHRP strain-dependent model• Asphalt Institute’s DAMA Program• University of Nottingham• Shell
  22. 22. 22HMA Charaterization Fatigue CrackingSHRP Strain-Dependent Model• Low air voids and crushed, rough-texturedaggregates• Increase stiffness• Increase fatigue life (constant strain)• Indicate that asphalt binder propertyinformation not sufficient for predictingfatigue life
  23. 23. 23HMA Charaterization Fatigue CrackingQUESTIONS ?