Validating the Fatigue Endurance Limit for Hot Mix Asphalt (2010) / Chapter Skim
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Pages 59-72
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From page 59... ...
The introduction of the endurance limit design premise into those design procedures that use the equivalent temperature and equivalent axle load concepts is straight forward. Stated simply, the maximum tensile strain is calculated at the equivalent temperature and axle load and C H A P T E R 6 Examination of LTPP Database for Indications of an Endurance Limit
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From page 60... ...
The maximum tensile strain is calculated at the equivalent temperature for each axle load within the axle load distribution. The axle load distribution for each axle type is used to determine the probability of the tensile strain exceeding the endurance limit.
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From page 61... ...
Increasing maximum tensile strains for varying singleaxle loads for different HMA thicknesses (HMA dynamic modulus equals 450 ksi; equivalent annual modulus)
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From page 62... ...
A survival analysis also can be completed using a specific level of distress and pavement response value. In other words, the survival curves can be used to define the probability that a specific area of alligator cracking will be less than some specified amount for different HMA thicknesses or tensile strains at the bottom of the HMA layer.
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From page 63... ...
A constant failure rate represents the time period when chance failures occur, or the failure occurs at random with pavement age. In some survival methods, this area is referred to as the useful life of a pavement.
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From page 64... ...
cracking. As shown in Figure 6.8, many of the LTPP test sections have no alligator cracking.
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From page 65... ...
Histogram of the number of test sections with different levels of alligator cracking (69)
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From page 66... ...
In summary, the endurance limit was determined to be 65 ms at a 95% confidence level for an 18-kip single-axle load applied to the pavement at the equivalent annual temperature for each LTPP site included in the analysis. Preliminary Definition of the Endurance Limit as an HMA Mixture Property The AAMAS project sponsored by NCHRP recommended use of the indirect tensile strength and modulus tests to estimate the fatigue strength/life of specific HMA mixtures (59)
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From page 67... ...
Distribution of HMA thickness for the test sections used in the updated survivability analysis. 0 10 20 30 40 50 60 70 <11 11 12 13 14 15 16 17 18 19 20+ N um be r o f T es t S ec tio ns Age, years HMA Thickness > 10 inches Figure 6.14.
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From page 68... ...
Figure 6.18 compares the maximum tensile strain at the bottom of the HMA layer and the amount of fatigue cracking observed on the LTPP test sections from the most recent distress survey included in the LTPP database. As shown and expected, the test sections with the lower tensile strains have less fatigue or alligator cracking.
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From page 69... ...
Figure 6.18. Comparison of the area fatigue cracking and maximum tensile strain computed at the bottom of the HMA layer.
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From page 70... ...
Figure 6.20 graphically presents the change in percentage of survival sections as a function for varying alligator cracking levels for different tensile strains at the bottom of 70 50.00 55.00 60.00 65.00 70.00 75.00 80.00 85.00 90.00 95.00 100.00 0 50 100 150 200 250 300 350 400 Su rv iv al , % Tensile Strain, micro-strains Fatigue <1% Fatigue <2% Fatigue <4% Fatigue <8% Figure 6.19. Survival curves based on the maximum tensile strain at the bottom of the HMA layers of flexible pavements included in the LTPP program.
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From page 71... ...
In summary, it is still believed that the endurance limit is an HMA mixture property. Based on the results from the updated survival analysis, however, forensic investigations of the test sections with the thicker HMA layers are needed to confirm the location of crack initiation and other assumptions used noted above in the survivability analysis.
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From page 72... ...
Comparison between the fracture damage index and measured fatigue cracking for neat HMA mixtures and PMA mixtures (71)
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