Interlayers in asphalt pavements are potential regions for damage initiation and their shear strength is heavily influenced by, among other things, particle interlock and binder conditions. In this presentation a discrete element study is presented of the interlayer shear strength of a model asphalt material allowing for isolating the effect of particle interlock on the shear strength. The model material consists of spherical steel balls blended with a stiff bituminous binder. A shear box experiment is simulated where two layers of asphalt material is sheared against each other with and without a confining stress. During the test, the force-displacement response of the sample is recorded. Tests with and without binder are simulated and contact models for these cases are developed including a combined tensile and shear fracture law of the binder. The simulated force-displacement responses of the tests are compared with previous experimental results in the literature [1] and a good qualitative and quantitative agreement is found. With this experimentally validated numerical tool, the optimal particle size ratio between the asphalt layers is determined to obtain as high shear strength as possible showing that the shear strength can be roughly doubled by choosing an appropriate aggregate size ratio. Finally, a simple model is also proposed linking the shear strength with the applied confining stress. The results are useful from a practical point of view when choosing stone size distributions when maintaining and repairing paved roads.