Abstract: Based on the test results of four types of geosynthetic reinforcements (PET, PAR, PVA and Aramid) during arbitrary loading history, their elasto-viscoplastic, especially viscous properties are investigated. Some tests were performed at constant but different strain rates. To evaluate the loading-rate effects on geosynthetic reinforcements, the strain rate was changed stepwise many times with a pair of creep and load relaxation under otherwise monotonous loading on the different types of reinforcement specimens. It is found that there are three types of viscous properties for different geosynthetic reinforcements, including Isotach viscosity, TESRA (Temporary Effects of Strain Rate and Acceleration) viscosity, and combined viscosity. An elasto-viscoplastic constitutive model, described in a nonlinear three-component framework, is presented to simulate the effects of the viscous property on the load-strain behavior of geosynthetic reinforcements. The model is able to simulate the load-strain-time behavior of geosynthetic reinforcements for arbitrary loading history, which is not only the load-strain behavior during monotonic loading with and without step changes in the strain rate but also the one after creep and load relaxation. By comparing the simulated results with the experimental data of geogrids presented in this study, it is shown that the three-component elasto-viscoplastic model can simulate well the viscous properties of geosynthetic reinforcements.