About the Influence of Shear-Induced Particle Migration and Sedimentation on the Measurement Results of Concrete Rheometers

Concrete chemistry is an integral component in the development of eco-concretes, easily compactible concretes, high-strength and very durable concretes. It is always necessary to adapt the fresh and hardened concrete properties to the respective application. In order to be able to develop a specific mix design, two essential requirements have to be taken into account. Firstly, mixing technology must also be reproduced on a laboratory scale. Secondly, the fresh concrete properties must ideally be determined in absolute units. A so-called intensive mixer is preferred in order to map the various mixing technologies used in practice. The shear intensity and thus the machine’s Froude number can be changed over a wide range. A rheometer, which uses so-called absolute measuring tools, enables the calculation of the fresh concrete properties in absolute units. The KNIELE KKM-RT 22.5/15 integrates a rheometer into an intensive mixer. This unique combination is extended by a device for pre-shearing and homogenization of the mixture. In this article it will be shown that the mix is segregated during classic pre-shearing and that the subsequent measurements deliver too low raw data. The rheological parameters calculated from these raw data are too low and do not describe the concrete correctly. Pre-shearing with the newly developed device eliminates this fundamental problem of other concrete rheometers.