The Bertta apparatus (Leivo 1990; Ferraris 1999) is a coaxial cylinders rheometer that measures the fundamental rheological parameters of concrete and the compactability of concrete under shear compaction.
The Bertta apparatus features traditional coaxial cylinders geometry with outer and inner cylinders that are 480 mm and 330 mm in diameter, respectively, and 400 mm in height. Vanes attached to the outer and inner cylinders act to prevent slip. Unlike other coaxial cylinders rheometers used for concrete, the outer cylinder operates in an oscillatory mode, with a set frequency and amplitude. The stationary inner cylinder measures torque. Yield stress and plastic viscosity are calculated as a function of frequency. Generally, the shear stress is linearly related to frequency. During the test, a vertical pressure is applied to the concrete sample. The change in height of the concrete sample during the test is recorded as a measure of the compactability of concrete when subjected to shear compaction.
The geometry of the device presents problems with accurately measuring rheological parameters. Specifically, the gap between the cylinders limits the maximum aggregate size to 13 mm based on the maximum aggregate size being 1/5 of the gap size. Further, the ratio of the outer cylinder radius to the inner cylinder radius is too large for a linear flow gradient. The device was developed at the Technical Research Centre of Finland and is not commercially available.
The Bertta apparatus measures yield stress and plastic viscosity as a function of frequency while also giving an indication of compactability.
The operation of the device is automated.
The device can be used to measure a wide range of concrete workability, including zeroslump concretes and highly workable concretes.
The device is likely too large for field use and requires a computer for operation.
The geometry of the device limits the range of concretes that can be tested and reduces the accuracy of the device.