Structural Overlay Design of Flexible Pavement by Nondestructive Test Methods in Louisiana

The Louisiana Department of Transportation and Development (DOTD) currently uses empirical book values in the overlay thickness design. This can lead to errors since the values do not necessarily represent actual field conditions. The objective of this study was to establish an overlay thickness design method for flexible pavement in Louisiana based on in-situ pavement conditions and non-destructive test methods. Four overlay rehabilitation projects with different soil subgrade types and traffic levels were selected for this study. On each pavement section, nondestructive deflection tests (NDT) including Falling Weight Deflectometer (FWD) and Dynaflect were performed at a 0.1-mile interval, and a detail condition survey data including cracking, rut depth, International Roughness Index (IRI), mid-depth temperature, and pavement thickness was also collected. Four NDT-based overlay design methods, including the 1993 AASHTO procedure, were investigated and used in the design of required overlay thicknesses. Results indicated that the 1993 AASHTO procedure method was generally over-estimated the effective structural number for existing pavements, which would result in an under-designed overlay thickness. On the other hand, other NDT methods, such as Asphalt Institute MS-17, Arkansas ROADHOG and ELMOD5, were found not directly applicable to Louisiana pavement condition. Therefore, a modified NDT overlay design method based on the 1993 AASHTO pavement design guide was proposed in this study. After comparing with the current DOTD overlay design and new AASHTO Mechanistic-Empirical (M-E) design methods, the proposed NDT-based overlay design method was found to better reflect the in-situ pavement structural condition and be more effective in a routine use.