Open time in 3D concrete printing

Open time is the period during which a cementitious material remains usable. In 3D concrete printing the term covers two different windows, and confusing them is one of the more common sources of trouble on site. The first is processing open time: how long the batch stays fluid and homogeneous enough to be mixed, pumped through the hose and extruded cleanly. The second is interlayer open time: how long a layer already deposited on the object can stand exposed before the next layer arrives and still form a sound bond with it. The two are governed by related chemistry but by different limits, and the second window is usually the shorter, less forgiving and more consequential one.

What it is / Why it matters

Processing open time ends when the material can no longer be moved. Workability is lost through three overlapping mechanisms. Flocculation rebuilds the particle network as soon as shear stops, so a hose left standing stiffens faster than one in continuous flow. Hydration converts the paste irreversibly, first slowly during the dormant period and then rapidly once setting begins. And water is lost — to evaporation from exposed surfaces and, where the material contacts dry aggregate or an absorbent substrate, to absorption. A mix that has lost even a small proportion of its mixing water behaves very differently in the pump.

Interlayer open time ends earlier, and for a more specific reason. The top surface of a deposited layer dries first. A skin forms: stiffer, drier and partly carbonated, with the water that would have supported continued hydration across the future interface already gone. When the next bead is extruded onto that skin it cannot wet it fully, the contact area is reduced by trapped air at the interface, and fewer hydration products grow across the joint. The result is a plane of lower tensile and shear strength inside an otherwise sound element.

The practical link between the two is the print path. The time gap between layers is not a material property at all — it is the length of one closed loop divided by the print speed. A large footprint, a long perimeter or a slow travel speed all lengthen the gap. That is why the same mix bonds perfectly on a small test cylinder and produces cold joints on a full-size wall, and why open time must always be discussed together with the geometry that will be printed.

Open time also pulls against buildability. A material that stays open for a long time is easy to pump and bonds well, but it does not stiffen fast enough to carry layers. The conventional resolution is set-on-demand: keep open time deliberately long upstream, and accelerate at the nozzle.

How it is measured

There is no single harmonised test for open time; it is assessed by repeating a workability test over time.

• Flow table, EN 1015-3. The standard consistence test for fresh mortar. Repeating it on subsamples of one batch at fixed intervals and plotting spread against elapsed time gives a workability-loss curve. Open time is read off as the moment the flow leaves the printable band.
• Slump and slump flow, EN 12350-2 and EN 12350-8. The equivalent approach for coarser concrete-scale mixes, used the same way — repeated at intervals rather than once.
• Penetration resistance. A penetrometer tracks the stiffening of the exposed surface and is the most direct proxy for the closing of the interlayer window. It is a research and site method rather than a harmonised printing standard.
• Vicat setting time, EN 196-3. A standardised test on cement paste of standard consistence. It is related to open time and useful for comparing binders, but it measures initial and final set of a paste, not the workability retention of a printable mortar. It should not be reported as open time.
• Interlayer bond testing. Specimens printed with deliberately varied time gaps and then tested in direct tension or splitting tension quantify what a given delay costs. This is the only method that measures the consequence rather than a proxy.
• Print trials. Printing the real geometry at the real speed and inspecting the joints remains the practical check, because it captures hose residence time, ambient conditions and path length together.

Typical ranges

The values below are indicative figures reported in the 3DCP literature and in general mortar practice. They are not Concreef measurements and are not design values.

What changes it

Ambient temperature. The dominant variable. Hydration roughly accelerates with warmth, so a mix with comfortable open time at 15 °C can close its window uncomfortably fast on a hot day.

Wind and relative humidity. Both act on the exposed surface. Moving air over a fresh layer removes water far faster than still air at the same humidity, and low humidity compounds it. Wind is the reason outdoor prints lose interlayer bond in conditions that would be harmless indoors.

Substrate absorption. Printing onto a dry, porous base or against absorbent material draws water out of the first layers and shortens their window locally.

Retarders and hydration-control admixtures. These extend processing open time by delaying the onset of rapid hydration. They are the standard tool, but they delay strength development too, so they trade against buildability and demoulding or handling schedules.

Superplasticisers and viscosity-modifying admixtures. They set the starting workability and how quickly it decays; some also affect water retention at the surface.

Accelerators dosed at the nozzle. Set-on-demand decouples the two windows: long open time in mixer, pump and hose, fast stiffening immediately after deposition. It adds a second dosing system and a mixing device at the nozzle, and calls for careful control, but it is the architecture that resolves the conflict between open time and buildability.

Hose length and standing time. Residence time in the line is part of the open-time budget. Any interruption — a tool change, an operator pause, a software stall — consumes it without producing any printed height.

Path length per layer. As above: the single largest influence on the interlayer window, and one that belongs to the design of the object rather than to the mix.

Failure modes

Pump or hose blockage. The material stiffens in the line, pressure rises and flow stops. Usually the result of an interruption, an over-long hose, or a batch that has stood too long. Clearing it costs far more time than the pause that caused it.

Cold joints. Visible horizontal lines with poor continuity between layers. Caused by exceeding the interlayer window; the strength loss is in tension and shear across the joint and is not detectable by looking at the element alone.

Surface skin and poor wetting. A dried crust on the top of a layer that the next bead sits on rather than fuses with. Often accompanied by a row of voids at the interface.

Inconsistent bond around one layer. On a large footprint the start of a loop has waited much longer than the end. The joint quality then varies systematically around the object, with the weakest zone at the point where each pass begins.

Loss of extrudability late in a batch. Tearing, discontinuous beads and surface cracking as the material is pushed past the end of its processing window.

Over-retardation. The opposite error: open time extended so far that the object cannot carry its own layers, delaying the whole print and risking plastic collapse.

Concreef context

Concreef is a Bulgarian 3D concrete printing company based in Sofia. It runs material trials in its Sofia workshop on a Crane WASP printer, currently printing with a commercial premix while developing its own dry mix. Concreef has no published test data, and the values on this page come from the general literature rather than from its own measurements.

In workshop practice open time is what determines how a printing session is organised rather than a number quoted from a datasheet. Batch size is matched to how much can realistically be placed before the mix stiffens in the line; the print path is planned so the time gap between layers stays inside the bonding window for the geometry being printed; and workshop temperature and ventilation are treated as variables that shift both windows. Because the commercial premix and the in-house dry mix under development behave differently in this respect, open time is one of the properties re-checked by simple repeated flow-table observations and trial prints whenever the material, the season or the geometry changes.