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6 been termed fatigue endurance limit. If a pavement is designed Objectives and constructed so that under repeated traffic loads no dam- age occurs, then that pavement should last indefinitely without The objectives of this study are to a structural failure. This pavement will still require overlays on some regular basis to maintain the surface in good operating 1. Test the hypothesis that there is an endurance limit in the condition, but the pavement structure should provide a very fatigue behavior of HMA mixtures and measure its value long life. for a representative range of HMA mixtures. 2. Recommend a procedure to incorporate the effects of the endurance limit into mechanistic pavement design Research Problem Statement methods. Work is needed to determine if there is an endurance limit for HMA and, if so, how this information should be used in Scope pavement design. One would not want to design all pave- ments, regardless of traffic level, so that the endurance limit This study included a literature review, laboratory testing, is not exceeded since this would result in the same design and analysis of field data. A mix design, used to construct struc- thickness regardless of traffic level. For very high traffic lev- tural sections at the 2003 National Center for Asphalt Technol- els it might be desirable to design the pavement so that the ogy (NCAT) Test Track, was replicated. Four versions of the endurance limit is not exceeded, but for low traffic levels it mix design were evaluated encompassing the effect of increased is likely that this approach would be too expensive. How- asphalt binder content and polymer modification. The mix- ever, a similar concept could be employed for pavements tures with higher asphalt binder contents were prepared at subject to very low traffic levels to ensure a long life. As always, higher densities to simulate the improvement in compaction the pavement should be designed to provide the lowest life- expected in the field. Beam fatigue and uniaxial tension test- cycle costs. ing were conducted to determine fatigue life. Beam fatigue Hence, there is a need first of all to determine if there is testing was conducted at progressively lower strain levels until an endurance limit for HMA. Once the endurance limit is a fatigue life in excess of 50 million cycles was achieved. Data determined it is important to determine how this endurance from the Long-Term Pavement Performance (LTPP) study limit would fit into the new mechanistic pavement design were analyzed to determine if they supported the endurance procedures. limit concept.