Mechanistic Flexible Pavement Overlay Design Program

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 LADOTD.