Internal dynamics in concrete and model systems of concrete: 3D particle tracking, flow and concentration profiles.

混凝土的流动在现代建筑技术中具有极其重要的意义。尽管在过去的几十年里进行了大量的研究，但关于流动过程的许多基本问题仍在争论中。胶凝材料（水泥浆、砂浆和混凝土）是颗粒的悬浮物，其尺寸分布跨越几个数量级（从纳米级添加剂到微米级水泥颗粒到毫米级沙子和砾石颗粒）。这些颗粒对流动特性有非常特殊的影响，即使是在简单的情况下，比如在水化反应的休眠期，只含有水泥和沙子颗粒的模型砂浆的流动。水泥颗粒的聚集产生屈服应力，必须克服屈服应力才能允许流动。当混凝土流动时，由于流动引起的颗粒迁移，靠近墙的砂粒浓度降低。这两种过程的结合导致形成了一个类似塞的剪切轮廓，在靠近壁面的地方有一个薄的强烈剪切的润滑层。这一过程的空间高分辨率光学研究自然受到胶凝材料不透明度的阻碍。提出的项目的目的是阐明背后形成润滑层的过程之间的相互作用。要做到这一点，必须开发一个高度透明的模型系统，并仔细地进行交叉检查，以展示真实胶凝材料的所有相关特性（颗粒相互作用和浓度、密度差、宏观流动特性和流动剖面）。这样的模型系统将允许以高空间和时间分辨率进行调查。我们将应用非破坏性和快速3D成像技术，并在两个长度尺度上使用单粒子跟踪。剪切模型水泥浆（粒径通常在10µm和50µm之间）的流动将通过高速共聚焦显微镜进行研究。这将产生一系列的3D图像，从中可以提取几乎所有粒子的轨迹。粒子轨迹的知识允许阐明粒子相互作用，流动剖面和聚集体的大小和结构之间的关系。对沙粒运动的分析也遵循类似的路线。然而，成像技术将是不同的。一个薄的激光片将被用来逐层照射和成像样品。在SPP中，我们将通过与项目中其他实验和理论小组的比较来验证我们的实验方法。通过不同小组之间的结果交流和同一样本上实验方法的结合，我们的实验方法将得到加强。我们打算提供在润滑层形成背后的不同过程之间的相互作用的见解。

The flow of concrete is of enormous importance in the modern building technology. Despite intense studies over the last decades, still many fundamental questions concerning understanding of the flow process are still under debate. Cementitious materials (cement pastes, mortars and concrete) are suspensions of particles with a size distribution that spans several orders of magnitude (ranging from nano-metric additives over micro-metric cement particles to milli-metric sand and gravel particles). These particles have very specific influences on the flow properties, even in simple situations, like the flow of a model mortar containing only cement nd sand particles during the dormant period of the hydration reaction. The aggregation of the cement particles generates a yield stress that has to be overcome to allow flow. When the concrete is flowing the concentration of the sand particles is reduced close to walls, due to flow-induced particle migration. The combination of both processes leads to the formation of a plug-like shear profile, with a thin strongly sheared lubrication layer close to the walls. Spatially highly resolved optical studies of this process are naturally hindered by the opacity of cementitious materials. The aim of the proposed project is to elucidate the interplay between the processes behind the formation of a lubrication layer. To do so, a highly transparent model system shall be developed and carefully cross-checked to exhibit all relevant properties of real cementitious material (particle interactions and concentrations, density difference, macroscopic flow properties and flow profiles). Such a model system will then allow the investigation with high spatial and temporal resolution. We will apply non-destructive and fast 3D imaging techniques and use single particle tracking at two length scales. The flow of the sheared model cement paste (particle diameter typically between 10 µm and 50 µm) will be investigated by high-speed confocal microscopy. This will result in series of 3D images from which the trajectories of almost all particles can be extracted. The knowledge of the particle trajectories allows to elucidate the relation between particle interaction, flow profile and size and structure of the aggregates. The analysis of the motion of sand particles follows similar lines. However, the imaging technique will be different. A thin laser sheet will be used to illuminate and image the sample layer by layer. Within the SPP we will validate our experimental approach through comparisons to other experimental and theoretical groups in the program. Our experimental approach will be enhanced through the exchange of results between the different groups and combination of experimental methods on the same sample. We intend to provide insight in the interplay between the different processes behind the formation of a lubrication layer.