| Asphalt concrete is one of the most widely used paving materials in service today. Consisting of the heaviest and perhaps most variable of petroleum fractions and specifically graded aggregate, asphalt concrete can be used to produce a high-quality road at minimal cost. However, despite its widespread use, accurate prediction of service life has eluded researchers for decades. Fortunately, asphalt concretes have lasted longer than predicted through laboratory trials. As we optimize our societal and private purchases, the need to better track our resources has us investigating why these mixtures outperform our expectations.;A common way for an asphalt concrete road to fail is through widespread cracking associated with repeated use, also called fatigue cracking . Fatigue cracking has been shown to be a complicated process in which many very small cracks come together to form visible ones. This process has nearly always occurred more rapidly in accelerated laboratory conditions as opposed to field service. To rectify this situation, a shift factor, or multiple, has repeatedly been applied to laboratory predictions of service life. Much recent research has centered on the cause of this shift, through which Microcrack or Microdamage Healing has been identified as a primary cause. Healing is the re-closing of the very small cracks which precede larger, more visible ones. The re-closing of cracks is associated with a stiffening of asphalt concretes, just as cracking is associated with softening. Fatigue cracking therefore is the result of two competing processes, healing and fracture.;Healing and its relationship to the chemical properties of asphalts is explored in this study. An experiment was designed to eliminate several mechanical variables to which healing has been attributed. Five well-documented asphalt binders were selected and numerous specimens tested in tensile-compressive fatigue. Through statistical analysis of the healing data using a stress/pseudostrain transformation and also measured surface free energies of the binders, it is found that high aromatic content is beneficial to healing, whereas high amphoteric content is not. Furthermore, it appears healing may occur in two components simultaneously, with the surface properties of the crack faces playing an important role. |
