SP-1 instructions - boco · Title: SP-1_instructions Created Date: 3/29/2018 12:58:28 PM
Block 29 SP 13
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Transcript of Block 29 SP 13
HMA Charaterization Fatigue Cracking 1
Fatigue Cracking
Senior/GraduateHMA Course
HMA Charaterization Fatigue Cracking 2
Fatigue Cracking
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
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
5HMA Charaterization Fatigue Cracking
Fatigue Testing
• Most commonly used
• Flexural beam
• Cantilevered beam
• Others
• Diametral fatigue
• Notched beam
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
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)
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
Flexural Beam Fatigue TestingLoadingClamps
Clamps for holding beam
Test Results
Strain,
• Results dependent upon how test run
• Constant stress means stiffer asphalt binders perform better
Stress,
Soft
Stiff
Test Results
Strain,
• Results dependent upon how test run
• Constant strain means softer asphalt binders perform better
Stress,
Soft
Stiff
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
13HMA Charaterization Fatigue Cracking
Other Fatigue Tests
• Cantilevered beam
• Diametral
• Notched beam
Cantilevered Beam Testing
• Trapezoid beam configuration
• Requires concrete beam be fabricated then sawn
• Fixed at bottom, loaded in a cantilever fashion at top
Diametral Fatigue Testing
• Repeated load (usually)
• Considered less sensitive to mix properties than flexural
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
17HMA Charaterization Fatigue Cracking
Advanced Fatigue Topics
• Notched-beam test (C* line integral)
• Dissipated Energy
• Models for Predicting Fatigue Life
Notched Beam Testing
• C*-line integral approach
Fixed Movable
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
20HMA Charaterization Fatigue Cracking
Difficulties
• Research showed that dissipated energy equations are dependent on mix variables and conditions of testing
21HMA Charaterization Fatigue Cracking
Predicting Fatigue from Binder and Mix Properties
• SHRP strain-dependent model
• Asphalt Institute’s DAMA Program
• University of Nottingham
• Shell
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
23HMA Charaterization Fatigue Cracking
QUESTIONS ?