Rutting in flexible pavements is often associated with permanent deformation of the unbound granular layer. The current permanent deformation models are only applicable to a single stress path in repeated load triaxial (RLT) tests. In this paper, a simple approach using the time hardening concept was introduced to continuously model the permanent deformation of unbound granular materials (UGM) in multi-stage (MS) repeated load triaxial (RLT) tests. Three existing permanent deformation models were reconstructed to be implemented with this method and one model was slightly modified to better suit this approach. This approach was validated by calibrating these reconstructed models using data from MS RLT tests on three different UGM used in base layers of flexible pavement structures. The calibrations were done by optimizing the material parameters of the models using least square curve fitting method. The shakedown ranges were also calculated for each stress path of the MS RLT tests to compare these models in simulating these ranges. Generally, very good fits were obtained for these models where the modified model showed the best agreement.