Implementing the excess paste concept for a systematic mix design of printable concrete

The properties of concrete depend on a number of interrelated factors and for this reason, the mix design process is, to a significant extent, an educat-ed guess. Nevertheless, the choice of factors and implementation of multifactorial analysis can provide new insights into complex correlations. A factor, rarely adopted in the design of printable concrete, is the quantity of excess paste.
Concrete can be considered a two-phase system comprising the aggregate and the cementitious paste. The paste required to fill the voids between the aggregate grains is the requisite paste. However, to coat the grains and form a cohe-sive mix the excess paste must be added which, in turn, increases the distances between aggregate particles. The volume fraction of excess paste is thus the principal factor in the design process. The second factor is the paste composi-tion. The paste consists of reactive and inert powders, water and soluble admix-tures. Although chemically reactive materials are necessary for binding, cohe-siveness can be achieved by inert particles while improved packing of particles also enhances the mechanical characteristics.
The excess paste mix design is discussed on two modifications of a concrete mixture suitable for extrusion-based 3D printing. The necessary input data and the associated tests on aggregate and powders are presented. Additionally, a computerized, multi-factorial approach to mix design optimization is proposed.