Interpreting dissipated energy from complex modulus data
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An asphalt mixture's ability to absorb energy without fracturing is directly related to cracking performance of asphalt pavement. The dissipated energy per load cycle is commonly determined as the area of the hysteresis loop developed during cyclic loading of asphalt mixture. However, it is unclear whether all dissipated energy determined in this manner is irreversible and associated with damage, or whether it is at least partially reversible and not fully associated with damage. Therefore, this study was undertaken to evaluate the nature of the dissipated energy determined from the area of the hysteresis loop developed during cyclic loading of asphalt mixture. Dissipated energies determined experimentally from cyclic load tests were compared to dissipated energies predicted using rheological parameters determined from static creep tests. For a range of asphalt mixtures, the area of the hysteresis loop appeared to be strongly affected by the delayed elastic behavior of the mixture, even when cyclic response had reached steady-state conditions. Therefore, it appears that the area of the hysteresis loop is not fully associated with damage and very probably overestimates the rate of dissipated energy and damage development during cyclic load testing of asphalt mixture. Furthermore, it is generally not possible to reliably separate reversible from irreversible dissipated energy in the hysteresis loop using conventional complex modulus data. Consequently, it is recommended that irreversible dissipated energy be determined using rheological parameters obtained from static creep test data. The key is to have parameters in the rheological model that properly separate the elastic (immediate and delayed) from the viscous response, since only the viscous response is irreversible and contributes to damage.