Permanent deformation in asphalt concrete

Permanent deformation in asphalt concrete pavements manifests itself as depressions along the wheel paths, ruts. The prediction of rut depth requires a knowledge of materials characteristics that relate the asphalt concrete permanent strains to stress and temperature. This paper analyzes the character of permanent strains as inferred from a creep/recovery test. It is shown that the separation of the strains into the time-independent elastic and plastic strains and the time-dependent viscoelastic strains requires tests with various times to unloading. The analysis is illustrated with the results of a series of uniaxial compression creep/recovery tests on one asphalt concrete mixture. The tested material exhibits temperature-dependent elastic and plastic strains that are proportional to the level of stress. The time- and temperature-dependent viscoelastic strains are nonlinearly related to the stress level and stress history. An approximate constitutive equation that disregards this nonlinearity is presented.