Baumert Christian, Garrecht Harald
Institute of Construction Materials, University of Stuttgart, 70569 Stuttgart, Germany.
Materials (Basel). 2020 Mar 27;13(7):1542. doi: 10.3390/ma13071542.
Determining the exact rheological properties of cementitious materials in fundamental units is a crucial step in concrete science. It is undisputed that before measuring rheological properties in concrete rheometers, it is necessary to pre-shear the fresh mortar or concrete. Due to the migration of the coarse particles into areas with lower shear stress, however, segregation takes place. An experimental set-up was developed to determine the effects on the measured values of the concrete rheometer ICAR. This allows the active homogenization (pre-shearing) of the material before each change of speed. In the tests higher raw values (macroscopic data) could be measured. This clearly influences the calculated rheological Bingham parameters and modified Bingham parameters for a self-compacting concrete (SCC) with a maximum grain size of 16 mm. Therefore, the homogeneity of the material, a main hypothesis of rheological measurements, does not seem to be fulfilled with the coaxial rheometer used. The process of the indispensable pre-shearing therefore requires more attention in the future so that measurement errors can be minimized. Especially in numerical simulation, suitable rheological models and the realistic determination of parameters are crucial. Since the shear-induced particle migration is largely dependent on the maximum grain size, an ultra-high performance concrete (UHPC) with a maximum particle size of only 0.5 mm was also investigated in the laboratory mixer KNIELE KKM-RT. The integrated rheometer enables also the active homogenization of the fresh concrete during pre-shearing but without the danger of over-mixing, as it is the case for the experimental ICAR setup. This article proves that relevant differences can also be identified for such a material.
在混凝土科学中，以基本单位确定胶凝材料的确切流变特性是关键的一步。无可争议的是，在混凝土流变仪中测量流变特性之前，有必要对新鲜砂浆或混凝土进行预剪切。然而，由于粗颗粒迁移到剪切应力较低的区域，会发生离析现象。开发了一种实验装置来确定对混凝土流变仪ICAR测量值的影响。这使得在每次速度变化之前能够对材料进行主动均匀化（预剪切）。在测试中，可以测量到更高的原始值（宏观数据）。这明显影响了最大粒径为16mm的自密实混凝土（SCC）的计算流变宾汉参数和修正宾汉参数。因此，使用的同轴流变仪似乎无法满足流变测量的主要假设——材料的均匀性。因此，未来不可或缺的预剪切过程需要更多关注，以便将测量误差降至最低。特别是在数值模拟中，合适的流变模型和参数的实际确定至关重要。由于剪切诱导的颗粒迁移在很大程度上取决于最大粒径，还在实验室搅拌机KNIELE KKM-RT中研究了最大粒径仅为0.5mm的超高性能混凝土（UHPC）。集成流变仪在预剪切过程中也能对新鲜混凝土进行主动均匀化，但不会出现过度搅拌的风险，而实验ICAR装置则存在这种风险。本文证明，对于这种材料也能识别出相关差异。
