The critical state of a granular material made of rounded tetrahedral particles is studied through DEM simulations of triaxial compressions. A minimum number of 7,500 particles is first obtained as the representative volume element (RVE) for the present triaxial simulations. Then, the macroscopic critical state line (CSL) is shown to be increasing in the (stress, density) space for low confining pressures. Such an unexpected behaviour is explained by the existence of a significant proportion of rattlers. Considering rattlers as voids indeed reinstates a classically decreasing CSL.