The viscoelastic response of cement paste to three-dimensional loading

The majority of the viscoelastic constitutive data for cement paste or concrete found in the literature deal exclusively with uniaxial loading. To predict the isotropic response of concrete or cement paste under multiaxial loading or multiaxial prescribed deformation, it is necessary to have knowledge of at least two viscoelastic constitutive properties. In the past, the typical treatment of three-dimensional modeling of concrete viscoelasticity has involved the assumption of a time-independent viscoelastic Poisson ratio. However, the experimental evidence supporting this simplification is inconclusive. In this study, experiments were performed on hardened cement paste that allowed the simultaneous measurement of both the dilatational and shear compliances, allowing the full three-dimensional characterization of the constitutive response. It was found that the dilatational compliance leveled off after several days for three of four mixtures tested. In these three materials, the Poisson's ratio was found to be an increasing function of time. Prediction of the measured uniaxial compliance using the measured bulk and shear compliances indicated that the confined compressive test used in this research may cause changes in the material which affect the measured dilatational compliance, and therefore the calculated viscoelastic Poisson ratio.