In the recent years, there has been a push towards designing roadways which will sustain 50 years of trafficking without having any structural fatigue damage. These "perpetual pavements" are designed using mechanistic-empirical design methodologies which limit the strain imposed on the pavement at the bottom of the bituminous layer of a pavement structure to a specific value which negates bottom-up fatigue damage. Previously, only laboratory data had been developed to identify this strain threshold, and it has proven difficult to make correlations between laboratory and field data in the area of fatigue cracking. Therefore, the objectives of this research included developing field-based strain thresholds for perpetual pavement design and establishing a relationship between laboratory fatigue thresholds and field-measured strain. Accomplishing these objectives would ultimately lead to more efficient perpetual pavement design.;To accomplish these objectives, strain distributions were developed for twenty-one test sections at the National Center for Asphalt Technology (NCAT) Pavement Test Track. The sections represented three cycles of research and trafficking using both theoretical and measured strains. A unique method of characterizing the entire cumulative probability distribution of strains experienced by each test section was developed.;Strain distributions of those sections that experienced fatigue cracking were compared with the distributions of those that did not experience cracking. Interestingly, it was found that sections with bottom-up fatigue cracking experienced higher strain levels above the 55th percentile compared to those that did not crack. New strain criteria, in the form of a recommended maximum strain distribution, for flexible perpetual pavements design were developed using an average of the four sections which experienced the most traffic without fatigue cracking. This strain distribution can be used provisionally for the design of perpetual pavements to prevent fatigue cracking.;Laboratory fatigue thresholds were related to strain distributions through the concept of a fatigue ratio. The fatigue ratio accurately quantified the entire distribution of strain by comparing it to the laboratory-developed 95th percentile lower bound of the confidence interval fatigue threshold. Attempts to compare the laboratory fatigue thresholds to one point on the cumulative strain distribution proved difficult for relationship development.;Though this study developed a provisional recommendation for both a strain distribution and fatigue ratio, it is recommended that more investigations take place to validate this work. Materials representing a wider range of fatigue thresholds should be field-tested to validate these concepts.
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