DISSIPATED ENERGY STUDY OF FATIGUE IN AIRPORT PAVEMENTS PHD Candidate: Shihui Shen Advisor: Prof. S. H. Carpenter FAA Project Review Nov. 9, 2005

INTRODUCTION Dissipated Energy (RDEC)– Unifying Concept Load Modes, Gear Configurations, Load Pulse Duration, Load Levels Fatigue Endurance Limit Thick airport pavements Thickness below which no damage occurs Healing is the Recovery from Damage Impacts total loads to failure Damage Concept is Amenable to a Thickness Design Procedure

DISSIPATED ENERGY CONCEPT REVIEW Ratio of Dissipated Energy Change (RDEC) STRAIN STRESS Initial Load Cycle Second Load Cycle Different Dissipated Energy Between First And Second Load Cycle

RDEC REVIEW I II III Plateau Value Ratio of Dissipated Energy Change, Log Load Repetitions, Log

PV CALCULATION

UNIQUE PV-Nf RELATIONSHIP Data Points: 546 Mixture types: 98 Loading modes: 2 Frequency: Hz Rest period: sec.

UNIQUE PV-Nf RELATIONSHIP Nf=1.10E+7 PV L =6.74E-9 Fatigue Endurance Limit Unique energy level at which no fatigue damage exists

APPLY RDEC IN HEALING STUDY Researchers suggest healing can be better understood by carefully considering energy behavior and viscoelastic properties of an HMA PV, energy based, can provide a unique indication of the impact of a load pulse followed by a rest period Research hypothesis: Rest periods promote healing effect and make it quantifiable through lab test Healing reduces the damage Reduced damage produces a lower PV, which translates into a longer fatigue life

FATIGUE-HEALING TEST Four Point Bending Beam (SHRP T321-03) Mode of Loading: Constant 500 microstrain Wave Shape: Haversine Load Pulse Width: 100ms (10 Hz) Rest Period: 0~9 second Temperature: 20 ℃

FATIGUE-HEALING TEST ε Loading time, second No rest period, 0.1 second load pulse 1 second 3 second 9 second Haversine Load Pulse Sequence A short rest period after each load pulse ~ to simulate rest between loads; An intermediate strain level, 500 microstrain, is used ~ relatively high damage level and shorter tests. The whole nature of energy change during a test is still continuous, thus the dissipated energy and the PV can be obtained for each test to perform the RDEC analysis,

PV-Nf FOR HEALING TEST Unique PV-Nf relationship for healing test With 9 second rest period, the fatigue life is extended 5 times for neat binder mix and 10 times for polymer binder mix neat polymer

HEALING TEST ~ PV vs. (RP+1) FOR NEAT BINDER

HEALING TEST ~ PV vs. (RP+1) FOR POLYMER BINDER

HEALING AND FEL Healing study using RDEC approach helps to understand the existence of FEL

PV-IDE AT NORMAL STRAIN (DAMAGE) At normal strain/damage condition and continuous loading, the PV is found has good correlation with Initial Dissipated Energy (IDE). IDE, initial energy capacity PV, total effect of fatigue behavior (material fatigue resistance and load effect) Under normal condition, load effect is dominant, while healing is negligible

PV-IDE FOR WHOLE LOADING RANGE With decreased damage level, the role of healing becomes more significant: healing starts to dominate the performance comparing to damage At low damage level when PV reaches threshold PV L, the PV-IDE curve start to flatten, leading to extended fatigue life PV, combined healing effect, can no longer be represented by initial loading status, IDE, starting from the endurance limit, PV L

PAVEMENT DESIGN Requires Relation to Standard Parameters Load Level Speed Repetition Interval Mix Variables Asphalt Grade

PRELIMINARY PV PREDICTION MODEL Based on 19 IDOT mix Including neat binder and polymer binder mix; various gradations; 2 air voids levels; Rich bottom binder mixes are included Where: ε: controlled tensile strain S : the flexural stiffness of HMA mix, MPa AV : the air voids of mixture Vb : the asphalt content by volume C : the aggregate gradation parameter, C=(P NMS -P PCS )/P #200 P NMS : the percent of agg. passing nominal maximum size sieve P PCS : the percent of agg. passing primary control sieve (PCS = NMS * 0.22) P #200 : the percent of agg. passing #200 sieve

PV PREDICTION Line of equality R 2 = 0.902

BINDER FATIGUE TEST - DSR DSR Testing Geometry Schematic Gemini 200 Dynamic Shear Rheometer

TYPICAL BINDER DE-Nf CURVE

TYPICAL BINDER RDCE CURVE Plateau Stage, II III

COMPARE PV-Nf CURVES FOR BINDER AND MIXTURE

CONCLUSIONS RDEC provides a successful way to study HMA, fatigue, healing, and its role on fatigue endurance limit Healing does exist and its effect on fatigue behavior can be observed through lab accelerated fatigue-healing test:

CONCLUSIONS (cont.) The occurrence of healing is highly related to the level of damage; healing effect can be dominant under low load damage condition. Healing under the low damage condition is a key to understand the existence of a fatigue endurance limit (FEL). Healing potential exceeds damage potential for any one load cycle.

RECOMMENDATIONS PV can be predicted based on material properties, which can be integrated into pavement structural design. Combine this with PV L, the energy based FEL established before, provides a simple way to estimate the strain level that can reach “unlimited” fatigue life. The fatigue-healing test can be extended to different temperature, load levels, and more diversity of mix types. Asphalt binders’ energy recovery rates and healing capacity can be studied using the RDEC approach. It should provide an insight of the healing kinetics derived from the existing fatigue-healing study for mixture.

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