Early-age buildability-rheological properties relationship in additively manufactured cement paste hollow cylinders

Abstract

Ability to extrude and to achieve shape stability of layer-wise additively manufactured cement-based elements at large scale depends upon the early-age rheological properties (shear moduli, yield stress, viscosity) of the deposited materials. Upon successful extrusion, buildability challenges can manifest themselves through two common failure mechanisms: yielding of lower layers or buckling of the element. However, it is yet unclear which of the various rheological properties control the early-age materials' deformation, specifically buckling, during the printing processes and thus influence the resulting buildability of the elements. This paper focuses on how buildability is dependent upon rheological properties as well as on predicting the buildability using a buckling theoretical framework. Specifically, the relationship between early-age rheological properties of various cement pastes and the buildability of hollow cylinders with failure dominated by buckling mechanism was investigated. It was found that certain types of shear moduli of the fresh pastes (G, G*, and G′) obtained from oscillatory shear stress sweep tests, performed within the first 30 min of hydration, correlate better with the buildability of hollow elements than some other rheological properties (loss modulus G″, yield stress σyield, yield strain γyield, and complex viscosity η*). Measured values of shear modulus (G) were used to calculate elastic modulus (E) of the pastes using the assumed value of 0.5 for Poisson's ratio (ν) of fresh cement paste. Euler's buckling theory was used to predict buildability (height of the element) of hollow cylinders. It was found that Euler's theory overestimates the buildability by 93%–194%, mainly due to assumption of ideal geometry (i.e., absence initial or printing imperfection), rate-independent behavior and linear elasticity.