Civil MDC

Concrete Thixotropy (ACI 238.2T-14) 2

Concrete Thixotropy (ACI 238.2T-14)

Description

Fresh concrete can exhibit different rheological behavior when at rest than when flowing. This difference is due to thixotropy, which can have important consequences for formwork pressure, multi-lift casting, slip-form paving, pumping, and segregation resistance. This TechNote defines thixotropy and distinguishes it from other changes in rheological properties; discusses the origins of, test methods for measuring, and factors affecting thixotropy; and concludes with its applications.

Description of thixotropyRheology is the science dealing with the deformation and flow of materials and is typically described based on the relationship between shear stress and shear rate, as shown in Fig. 1. Viscosity (η) is the ratio of shear stress to shear rate at a given shear rate.Concrete rheology is most commonly characterized in terms of the Bingham model (Fig. 1), which is defined in terms of yield stress (t0) and plastic viscosity (mpl) (ACI 238.1R).

Two measures of yield stress are typically reported. Static yield stress (t0-static) is the minimum shear stress to initiate flow from rest. Dynamic yield stress (t0-dyanmic) is the minimum shear stress to maintain flow.Plastic viscosity is the change in shear stress divided by the change in shear rate, for the shear stress greater than the dynamic yield stress. For Bingham materials, plastic viscosity is independent of shear rate, but the viscosity depends on the shear rate and is typically referred to as apparent viscosity (ηapparent).

Thixotropy is a reversible, isothermal, time-dependent decrease in viscosity when a fluid is subjected to increased shear stress or shear rate (Mewis 1979). The change in viscosity is considered reversible because the viscosity will increase to its original value when the shear stress or shear rate is decreased to its original value. The change in viscosity due to thixotropy is considered to be isothermal to distinguish changes in viscosity due to thixotropy from changes in viscosity due to changes in temperature. The change in viscosity is considered to be time-dependent because the change in viscosity occurs over a period of time, rather than instantaneously when the shear is applied or removed


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