Rheological Measurement of Suspensions Without Slippage: Experimental and Model

Rheological measurements of suspensions are often performed using a rotational rheometer. In this type of rheometer, the tested fluid is sheared between two surfaces, one of which is rotating to generate a laminar flow of the fluid between the surfaces (i.e., a Couette flow). Manufacturers of commercially available rheometers generally recommend the use of a standard oil of known viscosity to verify that the rheometer is operating correctly. However, in the case of concrete rheometers, this approach would require large volumes of oil and was deemed not economically feasible by two international studies [1] [2]. The conclusion of those international studies was that the optimal approach to calibrate concrete rheometers would be to develop a non-Newtonian standard reference material (SRM) that contained inclusions similar in size to aggregates used commonly in concrete. This could be achieved by using a multi-stage approach where each stage corresponds to a different level of complexity of the fluid. The first stage would be to create a paste reference material, as was done in the SRM 2492 [3]. The second stage would be to mimic the mortar phase of a concrete, and SRM 2493 [4], with 1 mm beads added to SRM 2492, accomplishes that goal. The third, and final stage, is the creation of SRM 2497 for concrete, with larger beads added to SRM 2493, which is currently in development at National Institute of Standards and Technology (NIST). During the certification of SRM 2493, it was found that differences in rheometer geometry affect the accuracy of the rheological measurements. In order to gain fundamental insight about the impact that different rheometer geometries have on measurements of suspensions, a comprehensive analysis was conducted on three different rheometer families. The analysis included both experimental testing and computer simulation. The comparison between the model and rheological results showed that the increased viscosity due to the addition of the 1 mm beads to SRM 2492 was significantly higher in the Couette model than in the experimental data. It was also determined that some geometries, such as a double spiral, resulted in a higher viscosity than a simple serrated cylinder or vane. This finding led to the inference that slippage should also be considered. Ultimately, this report highlights that industrial rheometers experience slippage issues caused by their choice of geometry and their internal boundary conditions (free surfaces), and discusses the most accurate alternative available for calibrating rheometers. i _____________________________________________________________________________________ This puication is avilable ree of carge rom : https://rg/10.6028/N IS T.TN .946