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

在处理过程中，例如泵送，混凝土的流动行为是由大块材料和靠近墙的界面层的流变特性决定的。因此，对混凝土在加工过程中的宏观流动行为进行建模需要对界面层形成过程以及块体和界面材料的流变特性进行清晰的描述。因此，必须考虑剪切诱导颗粒迁移（SIPM）作为主要机制，较大的颗粒迁移到剪切速率较低的区域。另一个关键问题是从水泥膏体到混凝土模型的升级。目前，基本模型主要基于颗粒相互作用，仅适用于水泥浆体。然而，现有的混凝土流变模型大多基于连续介质力学或DEM，并基于经验数据进行验证。本项目的目标是研究和模拟宏观混凝土流动行为，包括两方面，墙界面层的影响和作用于水泥浆体的化学-物理颗粒相互作用。这些都是混凝土流变学基本建模的关键问题。为了在宏观尺度上实现流变模型中的颗粒相互作用，必须解决从水泥浆尺度升级到混凝土尺度的挑战。由于水泥浆体的流变性高度依赖于其微观结构，而微观结构主要由实际剪切历史决定，因此基本模型必须考虑对微观结构形成的影响。然而，在混凝土水平上作用于水泥浆体的剪切速率明显高于整体剪切速率。这种所谓的局部剪切率作用于混凝土中的水泥浆体是由颗粒堆积和两个相邻大颗粒的相对速度决定的。由于较大的颗粒受到SIPM的影响，导致宏观尺度上颗粒分布极不均匀，局部剪切速率也会随着实际固体分数的不同而变化。显然，这种不均匀性在界面层中比在块状材料中更为明显。然而，如果整体剪切速率的差异是普遍存在的，那么也必须认为体系是不均匀的。因此，本项目的目的是研究和模拟时间和剪切相关的胶凝悬浮物的微观结构和流变特性，以自下而上的方式受到剪切。在本项目中，解决的关键问题是a) SIPM, b)界面层及其形成机制，c)大颗粒引起的局部剪切速率，d)考虑颗粒-颗粒相互作用的微观结构和流变学影响。只有从微观尺度到宏观尺度的尺度桥接方法才能充分实现这一目标，这将通过实验和数值模拟来实现。

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.