Material Testing for Printable Mixes
Material testing for a 3D printable mix is a three-stage protocol — fresh-state rheology and workability, green-state build-up during the first hours, and hardened mechanical and durability testing on specimens cut from printed elements rather than cast.
Updated
Testing a 3D printable mix means characterising the same material three times, in three different conditions: fresh, while it is being pumped and extruded; green, during the first minutes and hours when it must carry the layers above it; and hardened, once the mechanical and durability properties are stable. Conventional concrete testing addresses the first and third stages and largely ignores the second, which is precisely the stage that decides whether a print stands up. A useful protocol therefore combines established standard methods with a small set of research methods that have no standard equivalent — and records, for every result, which of the two it is.
What it is / Why it matters
A printable mortar has to satisfy requirements that pull against each other. It must flow through a hose and a nozzle without blocking or segregating, then stiffen fast enough to support the next layers, then bond to a surface that is already partly stiffened, and finally reach the strength and durability the element was designed for. No single test covers that. What matters is the sequence of properties over time, so the protocol is organised by material state rather than by instrument.
The second reason testing matters more here than in cast concrete is anisotropy. A printed element contains a layer structure, and the interface between layers is usually the weakest plane. A compressive strength measured perpendicular to the layers and one measured parallel to them are different numbers for the same material. A result without a stated loading direction is incomplete, and comparing two published values without that information is meaningless.
Finally, the field has no acceptance standard. Until one exists, every claim about a printable mix rests on a documented test programme rather than on a certificate, which puts the weight on the quality of the documentation itself.
How it is measured
Fresh state. Consistence is measured with the flow table to EN 1015-3, or ASTM C1437 for the equivalent mortar method, and with the slump and slump-flow tests of EN 12350-2 and EN 12350-8 where the mix is fluid enough for them to be meaningful. These give a quick, repeatable index but not the underlying physics. For that, rotational and vane rheometry is used to separate the static and dynamic yield stress from the plastic viscosity — the two parameters that respectively govern whether a bead holds its shape and how much pressure the pump must develop. A hand penetrometer gives a cheap field proxy for yield stress. Fresh density to EN 12350-6 and air content complete the batch record, and repeating the flow measurement at intervals on a single batch maps the loss of workability over time, which is the practical definition of open time.
Green or transitional state. This is where the standard toolkit thins out. Green strength and structural build-up are characterised by uniaxial unconfined compression on fresh cylindrical specimens tested at increasing rest times, which traces how quickly the material gains stiffness at rest; the vane shear test gives the same information as a yield stress. The most representative test remains the print-until-failure hollow cylinder: printing a continuous cylinder at a fixed speed and layer height and recording the height and layer count at collapse. It is a research method, not a standard, and its result is only comparable when geometry, print speed and layer height are reported with it. Setting is bracketed by the Vicat needle to EN 196-3 and by penetration resistance to ASTM C403, while isothermal calorimetry follows the heat of hydration continuously and shows how an accelerator or retarder has shifted the whole reaction.
Hardened state. Compressive strength is determined to EN 12390-3, flexural strength to EN 12390-5, splitting tensile strength to EN 12390-6 and modulus of elasticity to EN 12390-13; EN 1015-11 covers flexural and compressive strength of mortar specimens where the mortar route is more appropriate. Fibre-reinforced mixes are characterised for residual post-cracking strength by the three-point notched beam test of EN 14651. Interlayer bond is assessed by direct pull-off tension applied across the interface, and by splitting or shear tests oriented on the interface plane; these are adaptations of standard methods rather than standardised interface tests. Durability is covered by water penetration under pressure to EN 12390-8, capillary absorption, carbonation depth to EN 12390-12, freeze-thaw resistance following CEN/TS 12390-9, and density and open porosity. Where the internal structure itself is the question, X-ray CT scanning and optical or electron microscopy resolve the voids at the interface that mechanical tests can only infer.
Specimen preparation is the step most often done wrong. Printed material should be sawn or cored from printed blocks, not cast into moulds, because casting removes the interfaces and the deposition history that make the material what it is. Every specimen is labelled with its loading direction relative to the layers and its position in the printed element. Cast control specimens from the same batch are still worth making, but as a batch reference for consistency, not as an equivalent to the printed material. Alongside this go batch records, mixing and rest times, and logged ambient temperature and relative humidity, because early-age results are not interpretable without them. Repeatability requires a stated number of specimens per condition, and results from small laboratory batches regularly differ from full print-scale production, so lab-scale numbers are treated as an indication of the direction of an effect rather than as a value that transfers.
Typical ranges
The values below are indicative, reflecting what is reported in the 3DCP literature and the ranges the cited standards are written around. They are not Concreef measurements.
| Test | Method | Indicative range or role |
|---|---|---|
| Flow table spread | EN 1015-3 / ASTM C1437 | Printable mortars commonly reported in a narrow band of roughly 150–200 mm |
| Static yield stress, fresh | Vane or rotational rheometer | Reported in the low kPa range for extrudable mixes |
| Structural build-up rate | Compression at increasing rest times | Typically a few hundred Pa to a few kPa per minute in published studies |
| Open time | Repeated flow over time | Commonly reported from several minutes to a few tens of minutes |
| Initial setting time | Vicat, EN 196-3 | Strongly admixture-dependent; often shortened relative to a cast mix |
| Compressive strength, hardened | EN 12390-3 | Printable mortars in the literature broadly comparable to conventional structural grades |
| Interlayer bond tensile strength | Pull-off across the interface | Usually a fraction of the bulk tensile strength; falls with longer interlayer interval |
| Modulus of elasticity | EN 12390-13 | Typical of a high-binder mortar rather than of a coarse-aggregate concrete |
| Number of specimens per condition | Protocol choice | Three or more is normal practice for a meaningful mean |
What changes it
Time between mixing and test dominates the fresh and green results; a difference of a few minutes changes a yield stress measurement more than a modest change in mix design.
Temperature and humidity change hydration rate and surface drying, and therefore open time, setting time, bond and early strength. They belong in every record.
Specimen origin and orientation change the hardened result for the same material — cut versus cast, loaded across the layers versus along them, taken from the base of a print versus its top.
Interlayer interval changes bond strength directly, and is often the single most influential variable in a bond test programme.
Curing regime changes both strength and shrinkage. A printed surface is exposed on all sides with no formwork, so it dries faster than a cast specimen and reacts more strongly to how it was protected.
Sample size and batch variation change the confidence in the number rather than the number itself, which is why a single specimen result is not reported as a property.
Failure modes
The common problems are procedural rather than material. Comparing incomparable results is the most frequent: two compressive strengths from different loading directions, or a lab-batch value quoted against a print-scale one. Testing at an undefined rest time makes green-state data irreproducible. Casting instead of cutting silently removes the interfaces and flatters the material. Missing ambient data makes early-age results uninterpretable afterwards. Too few specimens produces a mean with no usable scatter. Presenting a research method as a standard — the buildability cylinder is the usual case — overstates the authority of the number. And an unrecorded specimen position hides systematic variation between the bottom and the top of a printed element.
Concreef context
Concreef is a Bulgarian 3D concrete printing company based in Sofia and runs material trials in its Sofia workshop on a Crane WASP printer. It currently prints with a commercial premix while developing its own dry mix, and it has no published test data. Nothing in this page reports a Concreef result, and no claim is made that any Concreef mix has been certified or tested to a standard.
What the protocol above describes is how such a trial programme is organised in general: fresh-state measurements taken on the batch that is actually printed, rest-time and ambient conditions logged rather than reconstructed, buildability assessed on the machine that will do the work, and hardened specimens cut from printed material with the loading direction recorded. Because no harmonised European standard yet covers acceptance of printed concrete, RILEM TC 276-DFC and ACI 564 are the reference frameworks against which a workshop protocol is written, and the documentation is what makes a result mean anything later.
Frequently asked questions
- Can printed concrete be tested with the normal concrete standards?
- Partly. The hardened tests of the EN 12390 series and EN 1015-11 are used routinely, but they assume a cast, isotropic, mould-shaped specimen. A printed element is layered and directional, so the same test gives different results depending on how the specimen was cut and loaded. The standards are applied with the loading direction relative to the layers recorded alongside every result.
- Why are cast control specimens not enough?
- A cast specimen from the same batch tells you what the mix can do without the printing process. It omits the layer interfaces, the deposition-induced porosity, the different compaction, and the thermal and drying history of a thin printed wall. Cast specimens remain useful as a batch reference and for mix development, but they are a reference, not an equivalent.
- What is the simplest buildability test?
- Printing a hollow cylinder continuously until it fails, and recording the number of layers and the total height reached. It is a crude but genuinely representative test because it uses the real machine, the real mix and the real deposition rate. It is a research method rather than a standardised one, so the geometry, speed and layer height must be reported with the result.
- Is there a European standard for accepting printed concrete?
- No harmonised European standard yet covers acceptance of 3D printed concrete. The current reference frameworks are the work of RILEM TC 276-DFC on digital fabrication with cement-based materials and the ACI 564 committee on 3D printing with cementitious materials. Until a standard exists, acceptance is project-specific and rests on documented test programmes agreed with the designer.
- How much does ambient temperature affect the results?
- Enough that it has to be logged with every test. Temperature changes hydration rate, which shifts open time, setting time and early strength, and humidity and air movement change the drying of an exposed printed surface. A mix characterised in a cool workshop can behave differently on a warm day, which is why ambient conditions are part of the record rather than a footnote.
Sources
- RILEM — Technical Committee 276-DFC, Digital Fabrication with Cement-based Materials
- ACI — American Concrete Institute (Committee 564, 3D Printing with Cementitious Materials)
- CEN — European Committee for Standardization (EN 12350, EN 12390, EN 1015, EN 196 series)
- ASTM International
- Cement and Concrete Research