Summary
Highlights
The rheometer measures the relationship between shear stress and shear rate in concrete, which is often described by the Bingham model. This model defines yield stress (minimum stress for flow initiation) and plastic viscosity (resistance to flow once yield stress is exceeded). Yield stress is related to slump, and high plastic viscosity indicates sticky, cohesive concrete. The video also introduces static yield stress (flow from rest) and dynamic yield stress (flow after thixotropic breakdown).
The ICAR rheometer uses a vane geometry, measuring torque as the vane rotates at different speeds. A standard 125mm diameter and height vane is positioned in the center of a container with ribbed sides to prevent slippage. Container sizes vary based on aggregate size; the minimum gap between the vane and container should be at least four times the maximum aggregate size. Assembly involves inserting the vane into the rheometer's chuck, tightening it, ensuring proper height, and securing the frame over the container.
The rheometer connects to a computer via USB and requires a power supply. The software setup begins by choosing a directory and naming the file. Users can opt to save a raw data file for diagnostics, in addition to the automatic summary file. Geometry settings, particularly the container radius, should be adjusted based on the maximum aggregate size. It's crucial to zero out the torque at the start of each day and between tests by pressing the 'reset' button when no material is touching the vane.
The stress growth test measures static yield stress. After carefully inserting the vane into the concrete, ensuring it's not too stiff, a consistent shear history is important for comparable results. The software is preset to 0.025 revolutions per second. Users press 'start' and monitor the increasing stress until it reaches a maximum, then press 'finish'. The software then displays the maximum torque and static yield stress.
After the stress growth test, a flow curve test can be run on the same sample. This test measures dynamic yield stress and plastic viscosity. The software's pre-populated parameters are generally suitable, but customizable. The test has two phases: a breakdown phase to eliminate thixotropy (operating at a breakdown speed for a set time), followed by measuring at different speeds (e.g., seven points for 5 seconds each). The data is plotted in real-time, and the software fits relative and Bingham parameters, providing yield stress, plastic viscosity, and mean squared error (MSE).
The software automatically generates summary files. For a stress growth test, the file includes the file name, date, time, test type, speed, geometry, peak torque, and calculated static yield stress. For a flow curve test, it includes test inputs (breakdown time/speed, initial/final speeds, points/time), geometry, calculated relative parameters, Bingham parameters (yield stress, plastic viscosity), and flow curve points (speeds, average torque).
Thixotropy can be measured in two ways. The first is by comparing the static yield stress (from a stress growth test) to the dynamic yield stress (from a flow curve test); a greater difference indicates higher thixotropy. The second method uses a hysteresis curve from two consecutive flow curve tests. The first test is run with minimal breakdown time (e.g., 1 second) and a low initial breakdown speed to capture fresh concrete behavior. The second test is immediately run after, with a long breakdown period at high speed to eliminate thixotropic effects. The difference in area between these two curves indicates thixotropy.