Verification of the DOT Overlay Design Procedure: Final Report

1995Executive SummaryPDFBriefFHWA/OH-96/005Design methodsOverlays (Pavements)Rigid pavementsFlexible pavementsThicknessNondestructive testsDesignHighwaysPavementsI22: Design of Pavements,OhioUnited StatesOhio. Dept. of TransportationChou, Y. J. E.University of Toledo. Dept. of Civil EngineeringOhio. Dept. of TransportationUS Transportation CollectionThe current ODOT overlay design procedure sometimes indicates additional pavement thickness is needed right after the overlay construction. Evaluation of the current procedure reveals that using spreadabiity to back calculate existing pavement modulus for both flexible, rigid, and composite pavements could lead to substantial errors. Spreadability value may decrease instead of increase after asphalt overlay construction. Therefore, the calculated effective thickness of the pavement is not accurate, especially for composite pavements. A modified procedure for designing AC overlay on rigid or composite pavements has been developed, where effective modulus of the pavement and modulus of subgrade reaction are back calculated from Dynaflect deflection measurements based on slab on grade theory. The old composite pavement is then compared with a new composite pavement with identical thicknesses to determine the proportional relationship between the old and new pavements. The effective PCC thickness of the existing pavement is determined based on equal-rigidity concept and an empirical ratio between new AC and PCC thicknesses. The required overlay thickness is then calculated for each deflection data point based on the AASHTO design equations. The design AC overlay thickness is calculated based on the mean and standard deviation of required AC thickness at each point and the specified reliability level. A separate procedure for designing AC overlay on flexible pavements is also included. The verification study shows that the results of these new procedures are better than or as good as that of the existing ODOT procedures.75