The use of highly fluid concretes, the rheology of which cannot be suitably characterized with the slump test (ASTM C 143 [1]) alone, has resulted in the emergence of many new methods for characterizing the flow of freshly mixed concrete. A synthesis of more than 61 existing test methods [2] for workability characterization found that available devices vary widely in their geometry, cost, method of operation, and suitability for field use.
To describe concrete flow behavior, both yield stress and plastic viscosity, as defined by the Bingham model, are key properties that should be determined. The measurement of these parameters is currently possible only by using a rheometer adapted to concrete [3, 4, 5]. Unfortunately, the current cost of these devices (even if very low compared to the cost of the produced concrete) and their complexities, as compared to the slump test, has restricted the use of rheological measurements mainly to research laboratories.
The idea of measuring rheological properties during mixing is not new [6]. In fact, some existing concrete rheometers are based on this idea. These devices operate by measuring the torque induced on a mixing blade rotated at a range of different speeds. Another possibility for measuring concrete flow properties is based on relating the energy data recorded during mixing, as investigated by de Larrard et al. [7, 8]. These researchers compared the curve of electric power versus mixing time of the concrete with the measurements obtained from their rheometer and were able to provide a correlation curve between the two instruments [7, 8].
The scatter in data from one rheometer to another can reduce the trust an operator has in a given measurement [3, 4, 5]. Indeed, the slump test has remained the standard tool throughout the world for characterizing the workability of freshly mixed concrete because of the device's simple calibration, which creates little ambiguity or confusion. The slump test, however, only measures a value related to yield stress, which is insufficient for fully describing the flow properties of concrete. An attempt to modify the slump test to measure plastic viscosity showed the limits of such an approach [9]. A simple and reliable method of rheological characterization adapted to the needs of industry is needed. In particular, few studies exist on the prospect of determining the flow properties of concrete in a mixing truck in transit to a jobsite. To the authors' knowledge, only one study has been published on this topic [10].
An analysis of the concrete production process shows that the transport phase in a mixing truck−particularly just before discharging concrete from the truck−is the most suitable time to measure rheological properties. In order to make rheological measurements during the mixing process, the mixing truck must be able to mix the concrete at different speeds to generate a range of shear rates. A mixer in a central plant−despite being considered more efficient than a truck mixer−is not typically capable of operating at different speeds [6, 11]. Given this background, an investigation of the feasibility of determining rheological properties by using the available data from a mixing truck was conducted.