Additive manufacturing (AM) of concrete has a large potential to limit the carbon footprint of concrete construction through optimized production processes and the amount of used material. To unlock this potential, existing challenges must be overcome to allow for upscaling not only of the technology but also the assisting design and specific material know-how. One of the current challenges is the design of concrete mix to satisfy the conflicting requirements of sufficient flowability for pumping and deposition while subsequently low enough for stability and buildability. Modifications of the concrete mix that aim at increasing strength or sustainability, such as the addition of fibre reinforcement or replacement of cement with substitutes, affect rheology and mechanical behaviour in time. To properly predict the behaviour of the new specific mix, a method based on the finite element method mesoscale simulation of the representative volume element of concrete mix is proposed. Mesoscale simulation has proven helpful in accounting for the heterogeneous structure of common concrete and simulating mechanical behaviour, e.g. fracture \cite{THILAKARATHNA2020}. This work studies the rheological behaviour of AM concrete mix for different sizes of aggregate, i.e. sand, fibre presence and fraction, and the interface between the interfacial transition zone (ITZ) model. The optimal size of the representative volume element and boundary conditions (BC) are investigated. Plane strain finite element (FE) implementation in the commercial software, Abaqus, is scripted parametrically to automate the mesoscale geometry, load and BC definition. Validation against the laboratory rheology, buildability and strength test is performed. The framework for mesoscale modelling for AM concrete is systematically described.