CONSIDERING HOT-MIX-ASPHALT FATIGUE ENDURANCE LIMIT IN FULL-DEPTH MECHANISTIC-EMPIRICAL PAVEMENT DESIGN AUTHORS:

Recent studies (including comprehensive lab testing efforts) have confirmed the existence of a “Fatigue Endurance Limit” (FEL) for HMA. The FEL concept (there is a repeated HMA flexural strain level below which HMA damage is not cumulative) is the basis of “PERPETUAL PAVEMENT” design. The FEL principle is not considered in the NCHRP 1-37A Mechanistic Empirical Pavement Design Guide. At the 2004 ISAP Conference on Design and Construction of Long Lasting Asphalt Pavements (Auburn, AL), the authors proposed M-E design principles (based on the ILLIPAVE structural model) for “Long Lasting HMA Pavements.” The proposed critical condition is the HMA flexural strain for the “hottest month” of the year. If the critical HMA flexural strain is < the FEL, the pavement is considered to be PERPETUAL. For pavements that do not meet the PERPETUAL criterion, there are times during the year (lower HMA temperatures than for the hottest month) when the HMA flexural strains are also less than the FEL. Previous studies by Carpenter suggest that strain repetitions greater than the FEL can be sustained and the HMA will retain FEL behavior. It is not clear how the FEL concept applies in these circumstances. Limited studies by Thompson for the IL DOT suggest that for pavements that do not meet the PERPETUAL criterion, consideration of the FEL has a limited impact on the predicted HMA fatigue life. The selection of the HMA fatigue algorithm (for considering HMA strains > FEL) appeared to be a more important factor. In this paper, M-E HMA fatigue concepts are reviewed and summarized and various approaches for considering the HMA FEL in practical M-E HMA FULL-DEPTH thickness design are considered. Procedures are recommended.