EVALUATION OF THE 1986 AASHTO OVERLAY DESIGN METHOD

The 1986 AASHTO Guide for Design of Pavement Structures is currently being evaluated in Oregon for use in the design of overlays. This newly revised guide presents two nondestructive methods for determining the strength of existing pavement structures so that the remaining life of the pavement can be evaluated. In the evaluation of the AASHTO overlay design procedure, five project sites around Oregon were selected, including four flexible pavements and one rigid pavement. Deflection measurements were taken using both the Falling Weight Deflectometer and Dynaflect. Cores were also tested to aid in evaluating paving materials. Three backcalculation programs (BISDEF, ELSDEF, MODCOMP2) were used to compute the moduli of the pavements, and the results were used to perform the overlay design. These results were compared with the current ODOT procedure, which is based on tolerable maximum deflection; and considerable inconsistency was observed. Generally, the AASHTO method provided a thinner overlay than the ODOT procedure. The results of this study show that the guide has an important advantage in that pavement strength for each layer can be quantified (NDT method 1) and remaining life may be taken into account. However, further investigation is still required to verify the method of determining existing pavement strength from backcalculation programs and the remaining life of the existing pavement. Therefore, the guide should be used with caution at the present time.