Workability loss of high-strength concrete

In the present paper, the results of an experimental study on workability loss of high-strength concrete are presented. Loss of workability is often expressed in terms of slump loss, which does not necessarily reflect the changed workability properties. Even for a small slump loss, the concrete may have a substantial loss of workability. Therefore, changed workability should be expressed in terms of more basic rheological parameters such as yield stress and plastic viscosity. These parameters express the resistance of the concrete to start a flow and to increase the rate of flow, respectively. While a short delay in the addition of the superplasticizer both increased the yield stress and the plastic viscosity over a period of 60 min, a longer delay mainly increased the plastic viscosity. While a small dosage of superplasticizer mainly increased the yield stress over the same period of time, a higher dosage caused a smaller change in yield stress but a distinct increase of plastic viscosity. An increasing content of silica fume reduced the initial plastic viscosity, while the yield strength was not much affected. Over a period of 60 min, however, the presence of silica fume reduced the change in plastic viscosity and distinctly increased the yield stress.