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  • 1.  Fatigue Capacity of Design Pavement

    Posted 09-21-2018 07:39 AM
    Edited by Tirza Austin 09-21-2018 07:38 AM

    I'm looking for advice on flexible pavement and fatigue capacity of a design pavement section.  

    In Pakistan, we have seen many premature failures due to incapable section of bound layer of pavement structure. AASHTO empirical design does not cater for fatigue capacity. I would be very thankful to hear how others have dealt with the problem. 

     



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    Ejaz Khan S.M.ASCE
    Director
    Pakhtunkhawa Highways Authority
    Islamabad
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  • 2.  RE: Fatigue Capacity of Design Pavement

    Posted 09-22-2018 12:49 PM
    Edited by Tirza Austin 09-22-2018 12:49 PM
    Dear Mr. Kahn, great question!  And you are so right when you  said the old 1993 and previous versions of the AASHTO Guide did not even consider fatigue cracking directly, only through PSI which is roughness based.  The new AASHTO Ware Pavement ME Design considers bottom up fatigue cracking very well directly.  It also purports to consider top down fatigue cracking, but we don't believe its accurate.

    The new AASHTO ME design procedure for bottom up fatigue cracking has recently been re-calibrated by Harold Von Quintus and others at ARA.  The new calibration used hundreds of inservice HMA sections from all over the US and the results look pretty reasonable.  The problem is you need to purchase a years subscription from AASHTO.  Check their web site for details.  I highly recommend the  software as so many dedicated engineers contributed to it for over 20 years now and it is field calibrated but with a reasonable theoretical basis. 

    Best regards, 

    ------------------------------
    Michael Darter Ph.D., P.E., M.ASCE
    PRINCIPAL ENG
    Michael I. Darter
    Sandy UT
    (217) 356-4500
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  • 3.  RE: Fatigue Capacity of Design Pavement

    Posted 09-23-2018 08:35 AM
    Edited by Tirza Austin 09-23-2018 08:35 AM
    Michael,

    Thank you very much for your help. 

    If we apply asphalt institution graph for working of asphalt thickness will it work for fatigue capacity?  

    Thanks and best regards,  
    ------------------------------
    Ejaz Khan S.M.ASCE
    Director
    Pakhtunkhawa Highways Authority
    Islamabad
    ------------------------------



  • 4.  RE: Fatigue Capacity of Design Pavement

    Posted 09-24-2018 10:04 AM
    Edited by Tirza Austin 09-24-2018 10:03 AM
    Mr. Khan,

    In addition to using the ME software for appropriate structural design, in light of your statement that many pavements are experiencing failure due to fatigue, you will need to understand the type of fatigue. Bottom up cracking, top down cracking, and thermal cracking are all caused by different mechanisms and the avoidance or minimization of them is dependent on the design of the asphalt pavement mix, more than the structural design.

    There are a number of fatigue tests being used here in the United States and I would encourage you to read up on them and their applicability to the different types of fatigue. The tests that have been around the longest and are typically used as benchmarks for the others are known as the flexural fatigue test (for cracking due to loading) (https://www.pavementinteractive.org/reference-desk/testing/pavement-tests/flexural-fatigue/) and the bending beam rheometer test (for low-temperature cracking) (https://www.pavementinteractive.org/reference-desk/testing/binder-tests/bending-beam-rheometer/), but there are other tests such as the overlay test, the semi-circular bending test (there is an ASTM standard for this - D8044-16) and its variants, the disc-shaped compaction test https://mntransportationresearch.org/tag/dct-test/), etc.

    Cracking can be due to any number of factors, including volume of binder in the mix, quality and strength of the aggregates, bond strength between pavement layers, strength of the underlying soils, and pavement thickness. The appropriate solution needs to be determined depending on the cause.

    I hope this helps.

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    James Purcell P.E., M.ASCE
    Technical Director - New Jersey Asphalt Pavement Association
    Ewing, NJ
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  • 5.  RE: Fatigue Capacity of Design Pavement

    Posted 09-24-2018 01:25 PM
    Edited by Tirza Austin 09-24-2018 01:24 PM
    Thank you James for your time and a detailed response. 

    Regards, 

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    Ejaz Khan S.M.ASCE
    Director
    Pakhtunkhawa Highways Authority
    Islamabad
    ------------------------------