Thiedeitz Mareike, Habib Nasime, Kränkel Thomas, Gehlen Christoph
Centre for building materials, Technical University of Munich, 81245 Munich, Germany.
Materials (Basel). 2020 Apr 9;13(7):1760. doi: 10.3390/ma13071760.
Rheological properties of cementitious pastes and mortar affect the casting, placement, and setting properties of fresh concrete. Fundamental rheological knowledge thus helps in predicting concrete flowability and workability. Empirical equations correlate actual rheological parameters based on physical material characteristics to workability tests. Still, these equations generally only take the dynamic yield stress of the material into account. This is not sufficient for thixotropic cementitious pastes or mortars, which possess structural buildup at rest. Workability predictions regarding the flow of concrete are thus more complicated with thixotropic materials. During form-filling in L-shaped formworks, the flow velocity of concrete slows down, wherefore rheological parameters change with time. At initial fast flow, high shear rates without structural buildup can be assumed. Dynamic yield stress and a steady state viscosity thus are proper parameters for empirical equations describing concrete flowability. During low shear rates, partial structural buildup takes place. Viscosity and yield stress increase due to agglomeration and affect the flowability of concrete tremendously. Rheological parameters of various cementitious pastes and mortars varying in their solid volume fraction and flowability were investigated in a vane-in-cup rheometer. The workability of the same mixtures was investigated by measuring the flow length in an L-shaped formwork. The effect of yield stress, viscosity, and thixotropic structural buildup on flow length was investigated. Subsequently correlations and discrepancies between flowability parameters and workability equations were analyzed.
胶凝材料浆体和砂浆的流变特性会影响新拌混凝土的浇筑、摊铺和凝结性能。因此，基本的流变学知识有助于预测混凝土的流动性和工作性。经验方程基于材料的物理特性将实际流变参数与工作性试验关联起来。然而，这些方程通常只考虑材料的动态屈服应力。这对于触变胶凝材料浆体或砂浆来说是不够的，因为它们在静止时会形成结构。因此，对于触变材料，关于混凝土流动的工作性预测更为复杂。在L形模板的填充过程中，混凝土的流速会减慢，因此流变参数会随时间变化。在初始快速流动阶段，可以假设剪切速率较高且无结构形成。因此，动态屈服应力和稳态粘度是描述混凝土流动性的经验方程的合适参数。在低剪切速率下，会发生部分结构形成。由于团聚，粘度和屈服应力会增加，并极大地影响混凝土的流动性。在杯式叶片流变仪中研究了各种胶凝材料浆体和砂浆在固体体积分数和流动性方面不同的流变参数。通过测量L形模板中的流动长度来研究相同混合物的工作性。研究了屈服应力、粘度和触变结构形成对流动长度的影响。随后分析了流动性参数与工作性方程之间的相关性和差异。
