Extrudability

Extrudability is the ability of a fresh cement-based mix to be conveyed through a hose and a nozzle and to leave the nozzle as a continuous filament of constant cross-section, without blockage, tearing or segregation. It describes one half of the fresh-state balance in 3D concrete printing. The other half is buildability, the ability of the deposited filament to carry the layers placed on top of it. A mix that extrudes cleanly has to be fluid enough to be sheared through the delivery line, yet cohesive enough not to bleed water or lose its aggregate to the pump.

In practice

Extrudability is judged mostly by printing. Operators watch for tearing of the bead surface, pulsation in the flow, and variation in filament width along a straight path. Laboratory proxies exist, including ram extrusion tests and rheometer measurements of yield stress and plastic viscosity, but no single test is generally accepted as a standard. Aggregate size sets a practical limit: common guidance is that the largest particles should stay well below the nozzle opening, often taken as no more than about a third of it, otherwise particles bridge and the line blocks. The usual levers are the fines content, superplasticiser and viscosity-modifying admixture dosage, and water content, and a small change in water can move a mix from extrudable to unprintable within one batch.

Related terms

• Buildability — the ability of a deposited layer to carry the layers above it without deforming.
• Yield stress — the shear stress a fresh mix must exceed before it starts to flow.
• Printable mortar — a fine-aggregate cement-based mix formulated to be pumped and extruded.