New Insights on the Vane Method to Evaluate Yield Stress of Cementitious Mortars

The yield stress (τ₀) of cementitious materials has a direct influence on concrete casting and postcasting processes, such as stability, formwork pressure, interfacial bond strength between successive lifts, and bond to embedded steel bars. This article provides new insights for reliable τ₀ measurements using the four-bladed vane method; it compares the effect of vane configuration (i.e., slotted versus nonslotted) and positioning (i.e., immersed in specimen versus placed flush with upper material surface) on τ₀ responses. Test results have shown that the elastic-to-plastic transition region in τ₀ profiles occurs progressively with the nonslotted vane, whereas it becomes more abrupt with the slotted vane. The presence of slots is believed to reduce the end effects and strengthen the assumption of shearing that takes place within the suspension along uniform stresses distributed at the cylindrical vane surfaces. For the given configuration, positioning the vane flush with the upper mortar surface minimized the effect of normal stresses on torque measurements, which resulted in decreased τ₀ values. Useful equations with good correlation coefficients are established between τ₀ measurements with respect to mortar composition, flow, and vane positioning.