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Examination of Washington State Department of Transportation Transfer Functions for Mechanistic-Empirical Asphalt Concrete Overlay Design

A review of transfer functions for mechanistic-empirical design procedures is addressed. Specific emphasis is placed on those transfer functions currently used by the Washington State Department of Transportation (WSDOT) and shift factors that relate estimates of laboratory to field fatigue cracking. To achieve this goal, brief discussions about how the WSDOT transfer functions were developed or chosen are presented. A comparison of WSDOT with South African transfer functions is presented. This comparison is of special interest because the South African transfer functions have been updated recently and are in part based on extensive accelerated pavement testing. Finally, mechanistic-empirical overlay designs have been performed by WSDOT for more than 10 years, and a selection of prior overlay projects is reviewed to examine fatigue cracking shift factors. Only projects exhibiting fatigue cracking or its early manifestation are used. The annual visual distress surveys contained in the WSDOT Pavement Management System make this review a bit easier because all pavement sections on the WSDOT route system have been systematically monitored for the preceding 26 years. The conclusion is that the laboratory-based tensile strain relationship currently used by WSDOT must be shifted to predict field fatigue cracking. Such shift factors appear to fall most commonly into a range between 4 and 10.