(6)
The ICAR rheometer [13, 14], shown in prototype form in Fig. 3, is a portable rheometer for fresh concrete. The device utilizes a four-bladed vane that is immersed into the concrete sample and rotated at a series of fixed speeds. The entire rheometer is approximately the size of a hand-drill and can be either operated by hand or secured into a fixed position above a standard container.
For each concrete mixture, the ICAR rheometer was used to perform a stress growth test and to measure a flow curve. In the stress growth test, the vane was rotated at a constant speed of 0.16 rad/s (0.025 rps) while the torque was measured. The peak torque was recorded as an approximation of the yield stress as discussed in Ref. [15, 16, 17]. After this peak torque was reached, the flow curve was then measured. The vane was first rotated at a speed of 6.3 rad/s (1.0 rps) for a breakdown period of 25 s. Torque measurements were then recorded for five speeds ranging in descending order from 6.3 rad/s to 1.3 rad/s (1.0 rps to 0.2 rps). The resulting data were analyzed based on the Bingham model, whereby a straight line was fit to the plot of torque, T(N·m), versus rotation speed, N (rad/s):
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The intercept, Y(N·m), and the slope, V(N·m·s), of this line were considered to be related to yield stress and plastic viscosity, respectively. Due to the geometry of this rheometer, it is not possible to determine the shear rate analytically in fundamental units [13].
The concrete was placed in a 410 mm diameter container and filled to a height of 390 mm, as shown in Fig. 3. The vane, which measured 130 mm in diameter and 130 mm in height, was positioned in the center of the concrete sample, resulting in a gap size of 140 mm between the vane and the sidewalls and a gap of 130 mm above and below the vane.
Fig. 3. View of the ICAR rheometer prototype, vane, and principal dimensions.