Particle migration during concrete rheometry: How bad is it?

The development and use of concrete rheometers has undergone a shift from complicated to more simple geometries over the last 50 years, mainly due to the introduction of flowable concrete mixtures. One of the reasons for the complex rheometer designs in the 1970s and 1980s was to avoid or minimize particle migration. Literature has revealed that particle migration can become an important factor disturbing the accuracy of rheological measurements in concentrated suspensions. In this paper, the rheological properties of concrete and mortar mixtures with different consistencies and different aggregate contents are investigated. These properties are obtained with three different coaxial cylinder rheometers configurations. Two of the configurations are identical, except the size of the outer cylinder. Mortar results appear not to be affected by particle migration, but measurements on concrete are affected. The difference in rheological properties between the different configurations increases with increasing aggregate content and increasing yield stress-to-plastic viscosity ratio, which is related to plug flow. Particle migration appears to be a minor factor affecting the measurements on flowable concrete mixtures, but for conventional concrete mixtures, the measurements are less reliable and can even become invalid.