Extrudability in 3D Concrete Printing

Extrudability is the ability of a fresh cementitious mortar to be pushed continuously through a hose and a nozzle and laid down as a coherent filament of the intended width and height, without blocking, separating or tearing. It is one of the two fresh-state requirements that define a printable material, the other being buildability. The two pull in opposite directions: everything that makes a mix easier to push through a pipe tends to make it less able to stand up afterwards. Extrudability is therefore not a single property but the outcome of a chain running from the pump, through the line, to the nozzle geometry and the motion of the machine.

What it is / Why it matters

Pumpability is the upstream condition. A mortar in a pipe does not flow as a sheared continuum. Fine particles and water migrate slightly away from the wall and form a thin lubrication layer, and most of the transport is slip along that layer rather than shear through the bulk. Pressure loss per metre depends far more on this slip layer than on bulk viscosity, which is why two mixes with similar rheometer results can behave very differently in a line. Hose diameter, length and the number of bends set the pressure the pump must supply.

Pump type matters for the same reason. Progressive cavity (screw) pumps deliver an almost continuous flow and are the common choice for printing, because steady output makes flow control straightforward; they impose high shear and wear the rotor and stator. Piston pumps deliver higher pressure and handle stiffer, coarser mixes, but their flow pulses between strokes unless damped, and pulsation is visible in the filament.

The dominant pumping failure is phase separation. Under pressure, water is driven out of the mix through the solid skeleton — filter pressing — leaving a stiffened plug that jams the line. The same physics appears as bleeding and as segregation of the coarse fraction. Adequate fines, a continuous grading and a viscosity-modifying admixture all work by making the paste hold its water under pressure.

Nozzle geometry does the shaping. A round nozzle gives a rounded cross-section and tolerates direction changes; a rectangular one gives flat faces and a more predictable contact area with the layer below, usually improving dimensional accuracy and interlayer contact. The contraction ratio between hose and outlet controls how hard the material is re-sheared as it accelerates into the opening: a gradual contraction is easier on the mix, an abrupt one raises extrusion pressure and encourages surface defects. Nozzle standoff — the gap between outlet and previous layer — decides whether material is simply deposited or pressed and troweled; a standoff smaller than the layer height turns the nozzle mouth into a shaping tool that flattens the bead, at the cost of higher back-pressure.

Filament geometry follows from flow and motion. The deposition rate must equal the filament cross-section times the nozzle travel speed. Layer width is set chiefly by the opening and standoff, layer height by the vertical step — commonly a fraction of the width, so the bead is compressed rather than merely stacked. When the two drift apart the filament degrades: too much material gives over-extrusion and bulging, too little gives thinning and eventually a break.

Corners and acceleration are where matching fails. A printer decelerates into a corner and accelerates out of it; unless flow is modulated with it, material piles up inside the turn and starves on the way out. Small radii, sharp direction changes and short segments amplify the effect, which is why path geometry is a material question and not only a toolpath question. The other geometric constraint is hose length and dwell time: a long line holds a large volume that sits and stiffens, consuming open time before the mortar reaches the nozzle.

How it is measured

There is no harmonised standard test for extrudability. What exists is a set of methods with different status.

The ram extruder is the standard research instrument: a known volume of mortar is pushed from a cylindrical barrel through a die at a controlled rate while force and displacement are recorded. The resulting curve separates shaping resistance at the die entry from friction along the barrel and is repeatable enough to compare mixes. It is a laboratory method, not a code-defined test.

Filament-quality criteria assess a printed test line directly: continuity, absence of tearing, edge definition, width and height against target, and the cross-section after a fixed number of layers. Several rating schemes appear in the literature; none is a standard, and all depend on the nozzle and machine.

Flow table to EN 1015-3 is used as a practical proxy. Spread diameter correlates with yield stress and therefore with ease of extrusion, and it is quick enough for batch-to-batch control. It says nothing about viscosity, cohesion under pressure or behaviour at the die, so it is a consistency check rather than a measure of extrudability. Slump to EN 12350-2 serves the same purpose for concretes but rarely discriminates between printable mixes, which are too stiff.

Pressure monitoring at the pump is the most useful in-process signal: a stable trace means steady flow, a slow rise means stiffening or the onset of filter pressing, sharp spikes mean partial blockage. Where segregation and bleeding matter, sieve stability and bleeding tests from the EN 12350 fresh-concrete family serve as indicators rather than extrudability tests.

Typical ranges

The values below are indicative figures reported in the 3DCP literature for extrusion-based systems. They depend on machine, mix and nozzle, and are not measurements of any specific material.

These are windows for orientation only: a system outside one is not necessarily failing, and one inside all of them is not necessarily printing well.

What changes it

On the material side, water content and superplasticiser dosage dominate: both lower yield stress and ease extrusion, at the direct expense of shape retention. Viscosity-modifying admixtures and higher fines content — limestone filler, fly ash, slag or calcined clay — improve cohesion and resistance to filter pressing, usually a better route to reliable pumping than adding water. Aggregate grading and maximum particle size set the blockage risk. Fibres, especially longer polypropylene or steel fibres, raise extrusion pressure sharply and can bridge at the nozzle. Accelerators and high ambient temperature shorten the time the material stays extrudable.

On the system side, hose diameter, length and bends set the pressure demand; contraction ratio and standoff set resistance and shaping at the nozzle; and the match between flow rate and the machine's real accelerations decides whether the filament stays consistent.

Failure modes

Blockage is the most disruptive, usually tracing back to filter pressing, an oversized particle, insufficient fines, or material left standing past its open time. Bleeding and segregation show as free water at the surface and a separating coarse fraction, producing an irregular bead. Pulsation from a piston pump or unsteady feed appears as periodic thickening of the filament. Over-extrusion gives bulging, loss of dimensional accuracy and material squeezing out sideways; under-extrusion gives a thin, weak bead and, at the limit, an interrupted line. Tearing, or shark-skin, is a rough, cracked surface caused by shearing the material too hard at the die exit — an abrupt contraction, a mix that is too stiff, or an excessive extrusion rate. Corner defects appear as accumulation inside a turn and starvation on the exit. Layer deformation under the nozzle is a buildability problem presenting as an extrusion problem: a sagging filament usually means the mortar is too fluid, not that the flow is wrong. Finally, loss of open time in the line makes the material arriving at the nozzle stiffer late in a run than at the start.

Concreef context

Concreef is a Bulgarian 3D concrete printing company based in Sofia. It runs material trials in its own workshop on a Crane WASP printer, currently prints with a commercial premix while developing its own dry mix, and has no published test data.

In workshop practice, extrudability decides whether a print run completes. The practical levers are the usual ones: consistent water dosing and mixing, a short and repeatable interval between mixing and printing, hose runs no longer than needed, and flow matched to the speed the machine actually runs at rather than a nominal figure. A printed test line before a job is the ordinary way to confirm the filament comes out continuous and dimensionally correct. Nothing here describes a Concreef mix, formulation or measured result; the figures above come from the published literature and the standards named, and no Concreef material is claimed to be certified or tested to a standard.