Workability or placeability is usually defined by concrete slump, or, in some parts of Europe, as the spread of the concrete after a kind of slump cone test DIN 1045 [4]. On the other hand, workability is often intuitively defined as the ease of handling, placing and finishing. The premature loss of workability of HPC is often reported as a major problem. Operators quantify this phenomenon as a slump loss. Survey responses indicated that complying with a target slump range was difficult to achieve. Most of the producers from the survey controlled the workability by either delayed addition of a specified amount of high-range water-reducing admixture (HRWRA) or by the adjustment of the admixtures dosage at the job site. The literature reports the same tendency in other countries [ 5, 6, 7], while small changes in the water content can be also a means of controlling the workability of HPC [ 8].
Survey responses indicated that more than half considered the slump test to be adequate to characterize workability but a better test was needed. In the literature, the same problems and needs are reported [ 5, 9, 10,11 ]. It is generally recognized that the slump test does not fully quantify the workability of HPC, but no other currently available test is as easy to conduct.
The reason for the discrepancy often encountered between the slump test results and the perceived workability or pumpability of a given concrete is that the concrete behaves like Bingham fluid (Figure 1). Concrete rheology is characterized by two parameters: the yield stress and the plastic viscosity. The yield stress is the stress needed to initiate movement and the plastic viscosity is the slope of the stress versus the shear rate after the stress exceeds the yield stress.
FIGURE 1. - Definition of a Bingham fluid.
The yield stress can be related to the measurement of the slump. Viscosity measurements are more sophisticated, because the measurements must be done while the concrete is moving. Only a few instruments exist that can measure either the viscosity directly or a value related to the viscosity [12].
The importance of distinguishing between the two factors, yield stress and viscosity, is illustrated in Figure 2. The difference in the rheological characteristics of two concrete mixtures can be differentiated only if both parameters are measured. A study by Punkki et al. [13] showed that, depending on time of addition and dosage of the chemical and mineral admixtures, yield stress and viscosity are modified in different ways. More precisely, the slump loss phenomenon generally corresponds to an increase of the yield stress, while the plastic viscosity only displays minor changes [14].
Figure 2 - Concrete Rheology.
Another problem related to the rheological properties of concrete is segregation. While HPC is generally cohesive and holds together well, slight changes in the material characteristics can result in segregation, such as aggregate grading or moisture content. The survey indicated that segregation could be avoided if the concrete slump was maintained at a 76 to 127 mm (3 to 5 inch) range. Vibration can increase the segregation if improperly performed. On the other hand, adjusting the viscosity of cement paste can prevent the problem. Unfortunately, as only the slump is measured, the difference in viscosity is only perceived by comparison between mixes, not by actual measurement. Therefore, the prediction of segregation is not possible from the tests routinely performed at the job site. The laboratory mixing method is not identical to the one used in the field. Therefore, laboratory trial batches can be quite different from production batches.