As stated earlier, workability and other flow properties are related to the rheology of concrete which requires at least two parameters, such as the Bingham parameters, for adequate description. What are the principal factors that influence the rheological parameters of concrete? The first factors are the composition of the concrete, including the chemical and mineral admixture dosage and type, the gradation, shape and type of the aggregates, the water content and the cement characteristics. The same mixture design can result in different flow properties if secondary factors are not taken into account. These are:
To determine the rheological characteristics needed for a specific application, the following items need to be considered:
In considering the application, some of these items will be automatically selected. For instance, if a structure with a very high amount of reinforcement is built, the concrete needs to be self-consolidating because it will be impossible for a vibrator to reach all the concrete.
Another variable that should be addressed is the time dependence of the rheological parameters. This phenomenon is often described as "slump loss" or excessive retardation. The placement of the concrete becomes either difficult (slump loss) or the demolding is retarded and strength development is delayed [48].
A detailed review of how the various factors mentioned above will influence the flow, and specifically the Bingham parameters, will not be attempted here but a good review is given by Khayat et al. [49]. We will examine in more depth some special cases that are of interest for high-performance concrete usage: pumpable concrete and self-consolidating concrete.