Measurement of yield stress of cement pastes using the direct shear test

The direct shear test is widely used in soil mechanics to determine the cohesion (C) and angle of internal friction (ϕ). This paper aims to assess the suitability of this test to evaluate yield stress (τ0) of cement pastes having different flowability levels. Special emphasis was taken to eliminate friction between shear boxes, thus allowing the measurement of C ranging from several kPa to just a few Pa. Tests have shown that the maximum shearing stress prior to failure is not a material constant, but rather varies with the normal stress as per the Mohr-Coulomb law. Good correlations between C and τ0 determined using the vane method were established. Nevertheless, the vane method was found to over-estimate τ0 when the blades are positioned inside the specimen, particularly for cohesive materials.