This research study focuses on the actual performance of the flexible pavements and its relationship with the in-situ mechanistic and volumetric properties. The data required for the study were obtained using the Long Term Pavement Performance database. Approximately, 116 flexible pavement sections throughout United States were analyzed and discussed. The results indicated that the temperature has a significant affect on the backcalculated modulus of the hot mix asphalt layer. However, no strong relationship was observed between the hot mix asphalt backcalculated modulus and in situ air voids. It was found that fatigue life was a function of tensile strain at the bottom of hot mix asphalt layer, peak surface deflection, hot mix asphalt air voids and maximum specific gravity, and ambient air temperature. Similar relationships between the rut life, mechanistic and volumetric properties were established for wet-freeze and wet-no-freeze climatic zones. The sensitivity analysis revealed that the rut performance in wet-no-freeze sections is mainly affected by higher base and roadbed compressive stresses and strains. On the other hand, the performances in wet-freeze sections are highly depended on roadbed compressive strain and modulus ratio of subbase to roadbed. 1. Introduction This study focuses on the actual performance of the flexible pavements and its relationship with the in situ, mechanistic and volumetric properties. In general, pavements are subjected to various kinds of loading and different environmental conditions, over time that manifest various distresses and affects the pavement performance. These distresses include rutting, fatigue cracking, temperature cracking, transverse cracking, and age-related block cracking [1]. Under a set of loading and environmental conditions the performance of the flexible pavements is function of properties of asphalt concrete mixture, volumetric properties (air voids, volume in mineral aggregate (VMA), specific gravity, and asphalt content) and mechanistic properties of HMA mixture, and underlying base and subbase and roadbed materials [2]. The temperature also has detrimental effects on the performance of the pavement; if the temperature is too high it causes rutting and low temperature will cause thermal cracking. By providing appropriate VMA, it is believed that rutting may be minimized, and mixture durability can be enhanced [3]. It is found that tender mixtures compacted to lower air void content (AV) will undergo less permanent deformation than if they are compacted to higher AV content. Lateral distortion
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