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Fresh concrete can be considered to behave as a Bingham plastic i.e a viscoplastic material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. This can be described by the equation:
τ = τo + μγ
τ = shear stress at rate of shear γ
τo = yield stress
μ = plastic velocity
The yield stress is closely related to slump and represents the threshold value for flow to begin. The plastic velocity reflects the increase in shear stress with an increase in the rate of shear. These two values (yield stress and plastic velocity) provide a measure of the fundamental rheological properties of fresh concrete.
There are a variety of tests developed to measure these two values including the BML viscometer, the BT RHEOM and the two-point test (sometimes referred to as the Tattersall test).
Acknowledgement: Concrete Society