Nondestructive Deflection Testing based Mechanistic-empirical Overlay Thickness Design Approach for Low Volume Roads: Case Studies

Illinois Department of Transportation (IDOT) uses an empirical design approach to conduct thickness designs of structural overlays for low volume roads. Modified layer coefficients for a limited number of material types are utilized to assign the structural capacity of in-service pavements. Despite the ease in use and simplicity, such an empirical approach is outdated and lacking in many aspects to characterize recycled and/or nontraditional construction materials nowadays more commonly used in pavements. As far as the rehabilitation of low volume roads is concerned, the lack of testing for evaluating the structural condition of existing, in-service pavements often results in uneconomical and unreliable practices. This paper presents a mechanistic-empirical approach for overlay thickness designs of low volume pavements through a combination of nondestructive deflection testing and pre-established pavement damage models. Twenty different pavement sections were selected from six counties in Illinois with varying structural and traffic characteristics. Falling Weight Deflectometer (FWD) tests were conducted on these road segments and FWD data were analyzed with appropriate temperature correction procedures to determine and monitor the structural conditions of existing, in-service pavement sections. Then, the corresponding required overlay thicknesses were determined for these twenty case sections using three different methods commonly used by local agencies such as, AASHTO 1993 NDT method, IDOT modified layer coefficient method, and Asphalt Institute deflection approach. The M-E Overlay Design method successfully identified structural deficiencies in the original pavement configurations through FWD tests and subsequently led to more economical or safer and more reliable overlay solutions for low volume roads.