Several European companies sell a hand-held impeller-type device (Steiner 1996; Wong et al. 2000), marketed under various trade names, to measure the consistency of concrete in place. Wong et al. (2000) evaluated the feasibility of using a Colebrand tester. At least two UK companies sell the Fresh Concrete Tester, FCT 101.
The test device resembles a hand-held version of the Tattersall two-point test or the IBB rheometer. The device, which is battery operated, is approximately the size of a drill and includes an impeller with two small hemispheres that is inserted into fresh concrete. An electronic interface records the torque required to turn the impeller. The device can also be fitted with a temperature probe. The device must be calibrated for each particular concrete mixture in order to correlate torque readings to slump, water/cement ratio, and predicted compressive strength. According to product literature, the FCT 101 test can be performed in 2 minutes. This rapid testing speed allows concrete essentially to be tested continuously in order to monitor the stiffening of concrete and the loss of water over time. The readings made by the device are logged and can be downloaded to a computer for documentation.
Although the Tattersall two-point test and the IBB rheometer measure parameters directly related to both yield stress and plastic viscosity, the FCT 101 only measures a value of consistency that is correlated to slump. According to Wong et al. (2000), the device appears to use signal averaging to minimize the variability caused by large aggregates. The device does not operate at multiple shear rates. The rotating hemispheres tend to create channels within low and moderate slump concretes, making it difficult to measure the plastic viscosity by using multiple shear rates at one location in the concrete sample. The H-shaped impellers on the Tattersall two-point device and on the IBB rheometer move in a planetary motion to help avoid this problem.
Advantages:
The device allows for fast and continuous measurement of workability.
The embedded electronic interface allows for the instant correlation of test readings to slump and water content and for the prediction of compressive strength. The results can be logged for documentation purposes.
Disadvantages:
Although the impeller design of the device resembles more advanced rheometers, the device only measures consistency and not plastic viscosity.
The device must be calibrated for each concrete mix. Each calibration is only valid for one particular concrete mix.
The impeller could create a continuous channel in stiff concretes. The resulting torque measurements would suggest an artificially high consistency reading.
