Yielding of granular aggregates is associated with particle breakage. Breakage depends on the rock toughness, particle geometry, density of aggregate, current grain size distribution, stress intensity and relative humidity. Most of these factors were defined or monitored in a series of suction controlled large diameter triaxial tests performed on two widely different parent rock materials: a quartzitic fissured shale and a very hard limestone. Shale particles had a high aspect ratio and planar shapes. Limestone gravels exhibit low aspect ratios and sharp edges. The paper reviews the experimental technique used and compares the triaxial behaviour of a well graded mixture of both types of parent rock materials. Critical state conditions are reasonably well established for the softer aggregate (shale) but it is absent in the hard limestone. Yielding loci and its dependence on current relative humidity are also given. Dilatancy is shown to depend on a number of factors: stress ratio, strength envelope, accumulated plastic work and suction. Some dilatancy rules are compared and new ones, better adapted to the mechanisms of particle breakage, are proposed. The paper is a contribution towards the development of general constitutive models for granular aggregates.