block 29 sp 13
TRANSCRIPT
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HMA Charaterization Fatigue Cracking 1
Fatigue Cracking
Senior/GraduateHMA Course
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HMA Charaterization Fatigue Cracking 2
Fatigue Cracking
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3HMA Charaterization Fatigue Cracking
Mechanisms• Traditionally considered to start at the
bottom and work up to the top
• Crack starts when tensile strain exceeds tensile strength of mix
• When cracks visible on top, full layer cracked
Subgrade
Base
AC Mix t
Longitudinal pavement profile
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4HMA Charaterization Fatigue Cracking
Mechanisms• Recent observations of fatigue cracking that starts
from the top at the outside edges of the wheel path
• Tensile stresses due to tire-pavement interactions at surface
Subgrade
Base
AC Mix t
Transverse pavement profile
Transverse pavement profile
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5HMA Charaterization Fatigue Cracking
Fatigue Testing
• Most commonly used
• Flexural beam
• Cantilevered beam
• Others
• Diametral fatigue
• Notched beam
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6HMA Charaterization Fatigue Cracking
General Terms• Dynamic load
• Load applied using a sinusoidal wave form
• Repeated load
• Load pulse applied then removed
• Rest period between loads
Load
Load
Time
Time
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7HMA Charaterization Fatigue Cracking
Flexural Beam Fatigue Testing
• Repeated load preferred to sinusoidal to permit stress relaxation
• Loading can be either constant stress or constant strain
• Failure = 50% loss of stiffness (controlled strain)
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8HMA Charaterization Fatigue Cracking
Determining Failure for Constant Strain
0
0.2
0.4
0.6
0.8
1
1.2
100 1,000 10,000 100,000
Numbers of Cycles
Sti
ffn
ess
Rat
io
Failure = 0.5 Stiffness Ratio
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Flexural Beam Fatigue TestingLoadingClamps
Clamps for holding beam
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Test Results
Strain,
• Results dependent upon how test run
• Constant stress means stiffer asphalt binders perform better
Stress,
Soft
Stiff
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Test Results
Strain,
• Results dependent upon how test run
• Constant strain means softer asphalt binders perform better
Stress,
Soft
Stiff
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Constant Stress vs. Constant Strain
Subgrade
Base
HMA 100 mm or less
Subgrade
Base
HMA150 mm or more
Strain at bottom of AC layer controls
Stress controls
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13HMA Charaterization Fatigue Cracking
Other Fatigue Tests
• Cantilevered beam
• Diametral
• Notched beam
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Cantilevered Beam Testing
• Trapezoid beam configuration
• Requires concrete beam be fabricated then sawn
• Fixed at bottom, loaded in a cantilever fashion at top
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Diametral Fatigue Testing
• Repeated load (usually)
• Considered less sensitive to mix properties than flexural
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16HMA Charaterization Fatigue Cracking
Example of Test Results
0
15,000
30,000
45,000
Cycles to Failure
20C
Test Temperature
Flexural
Trapezoid
Diametral
Reported in SHRP A-404, 1994
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17HMA Charaterization Fatigue Cracking
Advanced Fatigue Topics
• Notched-beam test (C* line integral)
• Dissipated Energy
• Models for Predicting Fatigue Life
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Notched Beam Testing
• C*-line integral approach
Fixed Movable
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19HMA Charaterization Fatigue Cracking
Dissipated Energy
• Dissipated energy is the amount of energy lost for each loading cycle
• Calculated from the changes in stresses and strains for each cycle of testing
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20HMA Charaterization Fatigue Cracking
Difficulties
• Research showed that dissipated energy equations are dependent on mix variables and conditions of testing
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21HMA Charaterization Fatigue Cracking
Predicting Fatigue from Binder and Mix Properties
• SHRP strain-dependent model
• Asphalt Institute’s DAMA Program
• University of Nottingham
• Shell
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22HMA Charaterization Fatigue Cracking
SHRP Strain-Dependent Model
• Low air voids and crushed, rough-textured aggregates• Increase stiffness• Increase fatigue life (constant strain)
• Indicate that asphalt binder property information not sufficient for predicting fatigue life
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23HMA Charaterization Fatigue Cracking
QUESTIONS ?