Fresh concrete can be considered as a fluid, which means that it will flow under the action of shear stresses. The flow behavior of concrete can be represented by the following two-parameter relationship:
Icar_2which is known as the Bingham model: The parameter τo is the yield stress, and it represents the shear stress required to initiate flow. The slope of the line is the plastic viscosity, µ, and it affects the resistance to flow after the yield stress has been surpassed. These two parameters, which define the flow curve, provide a complete description of the flow behavior of a fluid.
Icar_3Concrete, however, is not a simple fluid because it displays thixotropic behavior, which means that the shear stress required to initiate flow is high if the concrete has been in an “at rest” condition, but a lower shear stress is needed to maintain flow once it has begun. This type of behavior is summarized in the schematic plot shown above, which shows the variation in shear stress with time for the case of a low applied shear strain rate. At the start, the shear stress increases gradually with time but there is no flow. When the stress reaches the static yield stress, the concrete begins to flow and the stress required to maintain flow is reduced to the dynamic yield stress. If the applied shear strain rate is reduced to zero and the concrete is allowed to rest, inter-particle forces create a weak framework that restores the static yield stress. With time, the static and dynamic yield stresses increase as the effectiveness of water-reducing admixtures diminish and hydration proceeds, which is commonly referred to as “slump loss.”