Up to now, many constitutive models were proposed for semi-solid deformation. However, some aspects of the deformation mechanisms are confusing. For example, what is the main breakdown mechanism of the solid skeleton (at relatively high solid fractions)? is it the fracture of the solid bonds or the local dilatancy (as stated experimentally with the in-situ X-ray tomography)? The former one is related to the thixotropic models (e.g. the micromechanical model proposed by Favier and Atkinson), while the latter is related to the granular models. This problem is important to analyse the deformation pattern of semi-solid materials such as the formation of segregation bands, shear banding and plastic deformation of part of solid grains.