State of Knowledge of Interface Rheometers and Tribometers

The present chapter deals with the opportunities to investigate the shear behavior of concrete in the interface zone between concrete and a solid wall. The knowledge about this behavior helps to understand more deeply some aspects which are relevant for different placing technologies for concrete, such as the pumping process or the realistic prediction of formwork pressure. Several measuring devices to study interface between concrete and a surface have been developed within the last decades. An overview of the devices is given in tabular form, including the field of study, principles, test parameters and references. It is followed by a more detailed description of the test procedures. Regarding the measurement of interface properties in the field of fresh concrete technology, two device categories should be distinguished: tribometers and interface rheometers. Interface rheometers are able to cover high shear rates and continuous shearing at the concrete/wall interface which is common in concrete pipes during pumping. Interface rheometers are often based on conventional rheometers with coaxial cylinders. They allow especially a better understanding of the lubricating layer between the wall and the actual concrete by providing magnitudes for the interface yield stress and viscosity. Tribometers with a plane shear zone are very useful tools to measure the concrete/wall friction for very small shear rates and under relatively high pressure. Based on the measurements, the frictional shear stress or the Coulomb friction coefficient of the concrete/wall interface can be determined. These results are especially important for the realistic prediction of formwork pressure.