This Discussion Paper includes an elasto-plastic stress-strain constitutive model developed at the University of Wollongong, Australia, for coarse granular aggregates such as ballast, incorporating the degradation of particles as a particular feature. An attempt has also been made to extend the monotonic loading model to a cyclic stress-strain relationship. Most available constitutive models do not consider the breakage of particles during shearing, and the effects of cyclic loading on particle degradation and plastic deformation are rarely found in the literature. The developed model is based on the critical state framework and is able to predict the stress-strain and volume change behavior of coarse granular aggregates accurately. The model well captures the plastic dilation and contraction features under various confining pressures, and the strain-hardening and post-peak strain-softening behavior is adequately simulated.