From micro to macro – fundamental concrete modelling considering local shear rate and microstructure inhomogeneities due to processing using a scale-bridging approach

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During processing, e.g. pumping, concrete flow behaviour is determined by the rheological properties of the bulk material and the interface layer near the wall. Hence, modelling the macroscopic flow behaviour of concrete during processing requires a clear description of interface layer forming processes and of rheological properties of bulk and interface material. Therefore, the shear induced particle migration (SIPM) has to be taken into account as a dominating mechanism, with larger particles migrating to regions with lower shear rates. Another key issue is upscaling from cement paste to concrete modelling. Today, fundamental models are largely based on particle interactions and exist for cement paste only. However, existing concrete rheology models are largely based on continuum mechanics or DEM and validated based on empirical data. The goal of the present project is to investigate and model macroscopic concrete flow behaviour with regard to both, the influence of the interface layer at the wall and the chemical-physical particle interactions acting on cement paste. These are key issues for fundamental modelling of concrete rheology. To implement particle interactions in rheological models on macro scale, the challenge of upscaling from cement paste scale to the concrete scale has to be solved. As rheology of cement paste highly depends on its microstructure, which is mainly determined by the actual shear history, fundamental models must consider effects on microstructure formation. However, shear rates acting on cement paste at concrete level are significantly higher than the global shear rate. This so called local shear rate acting on cement paste in concrete is determined by particle packing and the relative velocity of two adjacent large particles. As larger particles are subject to SIPM, leading to largely inhomogeneous particle distributions at macro scale, the local shear rates are also varying depending e.g. on the actual solid fraction. Clearly, this inhomogeneity is more pronounced in the interface layer than in the bulk material. However, also the bulk system has to be considered as inhomogeneous, if the differences in global shear rates are prevalent.Therefore, the aim of the present project is the investigation and modelling of time- and shear-dependent microstructure and rheological properties of cementitious suspensions subjected to shear in a bottom up approach. In this project, the addressed key issues are a) SIPM, b) interface layer and its formation mechanisms, c) local shear rates induced by larger particles and d) its effect on microstructure and rheology taking particle-particle interactions into account. Only a scale-bridging approach from micro scale to macro scale enables to adequately achieve this goal, which will be realized utilizing experiments and numerical simulations.