The BML viscometer (Gjorv 1998; Ferraris and Brower 2001; Bartos, Sonebi, and Tamimi 2002) is a coaxial cylinders rheometer that is based on both the Power and Wiler plastometer and the Tattersall two-point device. The BML viscometer was developed in Norway in 1987.
Like conventional coaxial cylinders viscometers, the outer cylinder of the BML viscometer rotates while the stationary inner cylinder measures torque. Both cylinders have vertical ribs to reduce slip. The inner and outer cylinder sizes can be changed based on the size of the aggregate in the concrete being tested. At a constant angular velocity, the shear rate in the concrete is nonuniform at the bottom of the outer cylinder. Therefore, in order to more accurately measure torque, the inner cylinder is split into three parts so that only the middle section of the inner cylinder measures torque. The concrete near the middle section of the inner cylinder is subject to two-dimensional shearing, whereas the concrete in the bottom of the outer cylinder is subject to complex three-dimensional shearing. The device is operated at various fixed speeds. The cylinders are mounted on a much larger unit that houses the mechanical equipment. A computer software package converts the output data to values for flow resistance, G, and relative viscosity, H, which can then be related to yield stress and plastic viscosity, respectively.
The BML viscometer is intended for flowable concretes with slumps greater than 120 mm and can be used for self-compacting concretes. The device has also been used successfully for low slump concretes with slumps of 50-60 mm. For lower slump concretes the inner cylinder can be replaced with a blade impeller system, similar to the one used in the Tattersall two-point device. The device must be calibrated with external weights or calibration oils.
The BML viscometer has been commercially available since 1992 as the ConTec viscometer. From 1992 to February 2001, approximately thirty devices were sold (Ferraris and Brower 2001).
Advantages:
The device measures yield stress and plastic viscosity.
The operation of the device is automated.
The device is appropriate for a wide range of concrete workability, although some accuracy is lost in measurements of highly flowable concrete mixtures and low workability concrete mixtures.
Disadvantages:
Unlike the BTRHEOM, the device is too large to be used outside of a lab.
The device is complex and expensive.
Concrete in the shearing zone between the inner and outer cylinders has a tendency to dilate, resulting in artificially low measurements of torque.
The device must be calibrated.
