Set on demand

Set on demand is a 3D concrete printing strategy in which the change from pumpable to stiff is triggered at or near the nozzle, usually by dosing an accelerator into the flowing mix.

Updated

Set on demand is a 3D concrete printing strategy in which the transition from pumpable to stiff is triggered at or near the print head, usually by dosing an accelerator or a reactive second component into the flowing material. Its purpose is to decouple pumpability from buildability. Without it, one mix has to be fluid enough for the delivery line and stiff enough to stand as soon as it leaves the nozzle, and those requirements pull in opposite directions.

In practice

The base mix is formulated to stay workable for a long time, so the line can be filled, paused and restarted without risk of blockage. An admixture, commonly an alkali-free or aluminium-based accelerator of the type used in sprayed concrete, is metered by a separate pump into a static or dynamic mixer in the head, with the dosing rate slaved to the material flow rate. Systems using a fast-reacting second binder such as a calcium aluminate work on the same principle. The reported gain is a higher vertical build rate and taller unsupported sections. The costs are added equipment and calibration, sensitivity to dosing error, more cleaning, and the risk that a surface which stiffens very quickly bonds less well to the next layer.

Related terms

  • Accelerator — the admixture that shortens setting time, dosed at the nozzle in this method.
  • Buildability — the property this strategy raises without sacrificing pumpability.
  • Open time — the workable window in the line, which set on demand deliberately keeps long.

Related

AcceleratorAn accelerator is an admixture that shortens setting time or raises early strength, so a printed layer stiffens fast enough to carry the next one.BuildabilityBuildability is the ability of a printed mix to carry the layers deposited above it without deforming, buckling or collapsing during the print.ThixotropyThixotropy is the reversible loss of viscosity in a material under shear and its recovery at rest, the behaviour printable mortars depend on.Open timeOpen time is the window after mixing in which a printable mortar stays workable, and the interval in which a layer still bonds to the next.Material Science of Printable ConcretePrintable concrete explained: rheology, thixotropy, yield stress, open time, interlayer bond, admixtures, fibres and why printed concrete is anisotropic.