Mechanistic flexible pavement overlay design program.
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Mechanistic flexible pavement overlay design program.

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    Final report; Mar. 2006-Dec. 2008.
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  • Abstract:
    The current Louisiana Department of Transportation and Development (LADOTD) overlay thickness design method follows the “Component

    Analysis” procedure provided in the 1993 AASHTO pavement design guide. Since neither field nor laboratory tests are required by LADOTD for

    this method, pavement engineers usually rely on a pre-assigned parish-based typical subgrade resilient modulus value and a set of assumed layer

    coefficients for determining the effective structural number of an existing pavement in an overlay thickness design. This may lead to significant errors

    in the designed overlay thickness results because the selected design parameters do not represent actual field conditions.

    The objective of this research was to develop an overlay design method/procedure that is used for a structural overlay thickness design of flexible

    pavement in Louisiana based upon (1) in-situ pavement conditions and (2) non destructive test (NDT) methods, specifically the falling weight

    deflectometer (FWD) and/or Dynaflect.

    Fifteen overlay rehabilitation projects were selected for this study. These projects were strategically located throughout Louisiana with different

    traffic levels. At each selected project, NDT deflection tests including the falling weight deflectometer (FWD) and Dynaflect were performed at a

    0.1-mile interval. For some of the selected projects, detailed condition survey data including cracking, rut depth, International Roughness Index (IRI),

    mid-depth temperature, and pavement thickness was also collected. Six NDT-based overlay design methods were selected and used in the overlay

    thickness design analysis. Results indicated that the 1993 AASHTO NDT procedure generally over estimated the effective structural number for the

    existing asphalt pavements in Louisiana, which would result in an under-designed overlay thickness. On the other hand, other NDT methods (i.e.,

    ROADHOG, Asphalt Institute MS-17, Louisiana 1980 Deflection method, ELMOD5, and EVERPAVE) were found inapplicable to the Louisiana

    pavement conditions because all those methods rely on locally calibrated design parameters. Since further calibration of those NDT methods requires

    additional testing resources and is also considered very time-consuming, a modified FWD deflection based overlay thickness design method was

    proposed in this study. This method, based upon the Louisiana Pavement Evaluation Chart (a relation between Dynaflect deflections and the

    structural number of existing pavements) and in-situ subgrade modulus, is deemed able to directly represent Louisiana’s pavement condition. The

    cost/benefit analysis revealed that, as compared to the current LADOTD component analysis method, the proposed NDT-based overlay design

    method would potentially save millions of dollars in the flexible pavement rehabilitation in Louisiana. Therefore, before full implementation of the

    new Mechanistic-Empirical (M-E) pavement design method, the proposed NDT-based overlay design method is recommended for implementation by


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