Flow-induced particle migration in cement-based materials at high shear rates

泵送引起的高剪切速率下的非均质水泥悬浮液的流动是一个复杂的过程，受材料成分及其产生润滑层（LL）的能力的影响很大，润滑层（LL）可降低管壁与混凝土界面处的摩擦。泵送性能取决于这种润滑层的流变特性和厚度以及混凝土块体的流变特性。润滑层的形成主要与高剪切速率下的流动诱导颗粒迁移（FIPM）有关。到目前为止，还没有深入的见解FIPM在混凝土在高剪切速率下，其对润滑层形成的影响，现有的调查方法是不够的。该项目的目的是使人们能够清楚地了解混凝土中FIPM的因果关系和潜在机制，并最终将这些知识应用于泵送过程的现实描述和预测。FIPM对LL的形成和由此产生的材料流动行为的后果，重点是可泵性，将被建立作为有目的地分析混凝土泵送过程及其在数值模拟中的实施的基础。这些主要目标将逐步实现，首先发展适当的方法进行实验覆盖，基本分析和高剪切速率下混凝土中颗粒迁移的描述，然后进行悬浮介质流变特性影响的实验研究，即，水泥浆和细粉，在FIPM上。其次，研究了颗粒形状、粒径分布和堆积密度对FIPM的影响，并分析了颗粒迁移、LL形成、流型和压力-流量函数之间的因果关系。所获得的数据和见解将用于开发用于CFD分析的FIPM的数值表示，该数值表示将用于预测管道中混凝土的泵送行为。为了验证的结果，在早期的研究项目中进行的全尺寸泵送实验将被使用。因此，该工作计划包括适当的实验方法，新的测量技术和新的方法，充分模拟粒子迁移的全面结合，通过与数值模拟的紧密联系而丰富。它由三个部分组成：1）实验研究，着重于FIPM的定性和定量分析，LL的形成，LL和混凝土体积的流变特性; 2）数值模拟，基于FIPM到现有CFD模型中的实现，以计算LL的厚度和流变特性，从而计算泵送行为;（3）考虑泵送过程，其中混凝土流动的预测可以与全尺寸泵送实验的可用结果进行比较。

The flow of heterogeneous, cementitious suspensions at high shear rates induced by pumping is a complex process greatly influenced by material composition and its ability to generate a lubricating layer (LL) which reduces the friction at the pipe wall-to-concrete interface. Pumping behaviour depends both on the rheological properties and the thickness of such lubrication layer and on the rheological properties of the concrete bulk. The formation of the lubricating layer is mostly related to flow-induced particle migration (FIPM) under high shear rates. To date there are no in-depth insights on FIPM in concrete under high shear rates and its effect on lubricating layer formation; existing investigation approaches are insufficient. The project at hand is to enable the attaining of a clear understanding of causality and underlying mechanisms of FIPM in concrete and eventually to apply this knowledge to realistic description and prediction of pumping process. The consequences of FIPM on the formation of LL and on the resulting material flow behaviour, with the focus on pumpability, are to be established as the basis for purposeful analysis of the concrete pumping process and its implementation in numerical simulation. These main objectives will be accomplished stepwise by first developing adequate approaches for experimental coverage, fundamental analysis and description of particle migration in concrete at high shear rates followed by experimental investigation of the effects of rheological properties of the suspending medium, i.e., paste of cement and fines, on FIPM. Next, the effects of particle size shape, size distribution and packing density on FIPM will investigated and the analysis of cause-effect relationships between particle migration, formation of LL, flow pattern and pressure-flow rate function will be performed. The obtained data and insights will be then used for developing a numerical representation of FIPM for CFD analysis, which will be applied in predicting the pumping behaviour of concrete in the pipeline. For validation the results of full-scale pumping experiments performed in an earlier research project will be used. Accordingly, the work program is comprised of a comprehensive combination of appropriate experimental methods, novel measuring techniques and new approaches to mimic particle migration adequately, enriched by a strong link to numerical simulation. It consists of three segments: 1) experimental investigations focusing on the qualitative and quantitative analysis of FIPM, formation of LL, and the rheological properties of LL and concrete bulk; 2) numerical simulation based on implementation of the FIPM into existing CFD-model to calculate the thickness and rheological properties of LL and, consequently, of pumping behaviour; 3) consideration of the pumping process, in which the prediction of concrete flow can be compared with the available results of full-scale pumping experiments.