Characterization of Fatigue Damage in Asphalt Mixtures Using Pseudostrain Energy

Fatigue cracking and permanent deformation develop simultaneously in asphalt mixtures under repeated destructive loading regardless of temperature. Because these two distinctively different damage mechanisms always occur together, it is not possible to obtain pure cracking damage or pure permanent deformation damage in any asphalt mixture by varying the temperature or loading level. This paper develops an energy-based method to separate fatigue cracking from permanent deformation in terms of energy consumption. Specifically, the dissipated pseudostrain energy (DPSE) expended on cracking is separated from that expended on permanent deformation. Controlled-strain repeated direct tension (RDT) tests are performed on asphalt mixture specimens to determine their nondestructive properties, critical nonlinear viscoelastic properties, and destructive properties. The total DPSE, which is the amount of energy dissipated for the fatigue damage, including both fatigue cracking and permanent deformation, is calculated based on the critical nonlinear viscoelastic properties in order to separate both linear and nonlinear viscoelastic effects from the strain energy that is available to cause damage. The energy distribution in an asphalt mixture under destructive loading is investigated at the microscopic level. Balance equations are established between the true stress/strain/energy of the intact material and the apparent stress/strain/energy of the bulk specimen. These balance equations are utilized to completely formulate the DPSE for cracking and the DPSE for permanent deformation, respectively. As a result, the DPSE for cracking is separated from the DPSE for permanent deformation. The formulations of the two types of DPSE and the RDT data will be used to calculate the amount of each type of DPSE. The DPSE for cracking will be used to predict the crack growth, average crack size, and number of cracks in the asphalt mixture under repeated destructive loading. The DPSE for permanent deformation will also be used to predict the growth of permanent deformation. This will be introduced in following papers. This paper is one of a series of companion papers that characterize nondestructive and destructive behaviors of an asphalt mixture under tensile loading.