Feasibility of using a concrete mixing truck as a rheometer

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Fresh concrete properties dictate the performance of the concrete in its hardened state. The measurement of workability is usually done using a simple slump test. This test, although widely used for almost 100 years, does not provide a full picture of the workability or flow properties of fresh concrete. Therefore, many tests [1] were designed to fully characterize concrete. Few are consistently used in the field. The most fundamental are the tests based on rheology, such as the rotational rheometers [2, 3]. There is a need for a relatively simple test that could be used in the field but that will fully characterize fresh concrete flow properties. The mission of this study was to evaluate the feasibility of using a concrete truck mixer as a rotational rheometer. To succeed, it is necessary that the methodology measure fundamental rheological parameters (plastic viscosity and yield stress) during the mixing process. This report summarizes the state of the art on concrete truck mixers used as tools to characterize fresh concrete. Then, a short proof of the concept is attempted by using a concrete truck and a rotational rheometer to characterize nine concrete mixtures [4]. The results are presented and discussed. The methodology developed is based on using the truck drum rotation combined with the measurement of the torque induced by the concrete load. This is similar to the typical method of determining the flow behavior in a traditional rotational rheometer, i.e., the Bingham model: the shear rate (or drum rotational speed) versus the shear stress (or torque) was plotted. The slope of the linear relationship is the plastic viscosity and the intercept at zero shear rate is the yield stress. The results of these experiments are discussed and compared with data provided by the ICAR [5] rheometer, a portable rheometer designed for measuring concrete rheology in the field. The test results indicate that the mixing truck is sufficiently sensitive to detect differences in yield stress, slump, and plastic viscosity. However, the plastic viscosity determined by the truck measurement is not correlated with plastic viscosity results obtained from the ICAR rheometer, while the yield stress determined by the truck measurement correlates with the slump and the ICAR rheometer results. Suggestions are given on how to improve the mixing truck for use as a rheometer.