Mix design concepts for 3D printable concrete: A review

Abstract Over the past decade, 3D concrete printing has shown enormous application potential for the industry. The new technology poses specific requirements on 3D printable concrete (3DPC), especially on its rheological properties in the fresh state. The current codes and standards for conventional concrete construction are not applicable for guiding the mix design of 3DPC. Developing robust mix-design guidelines of printable concrete while considering the principles of sustainability, use of indigenous materials and available printer characteristics contributes to a broader practical application of 3DPC. This article presents state-of-the-art on mix design concepts for 3DPC. In this, various aspects of mixture compositions and their effects on properties of 3DPC are highlighted and mix design approaches are described. While currently and out of necessity, a trial and error approach remains the norm, for finding the appropriate mix composition for 3DPC, it would clearly be beneficial to develop rational and scientific guidelines for a particular set of parameters. Thus, the use of models providing a quantitative relationship between the rheological parameters of fresh concrete and its composition is instrumental to guide the mix design of printable concrete. Some such models do exist already, but considerable research is still needed to develop them into reliable mix design tools. Furthermore, the contemporary mix design for 3DPC mainly focuses on the printability of concrete in fresh state, while the characteristics of hardened concrete such as strength or durability are often not targeted explicitly. Given the engineering application of 3DPC, further research is needed to develop mix design concepts involving the targeted properties of hardened concrete under consideration of process-induced anisotropy.