Compressive Strength of Printed Concrete
Compressive strength is the maximum compressive stress a hardened cementitious material sustains before failure, measured on standard specimens under EN 12390-3, and in printed material it depends on loading direction relative to the layers.
Updated
Compressive strength is the maximum compressive stress that a hardened cementitious material can sustain before it fails, expressed in megapascals and measured on standardised specimens loaded to destruction. For ordinary concrete it is the headline design property, the number that appears in specifications and the basis on which strength classes are assigned. For 3D printed material the same measurement is still meaningful, but it is no longer a single number: the printed material has a layer structure, so its strength depends on the direction of loading, and the specimen has to be extracted from a printed body rather than cast in a mould. Any compressive strength figure quoted for printed material without stating the loading direction, the specimen geometry and how the specimen was obtained is incomplete.
What it is / Why it matters
The compressive strength of a cementitious material is set principally by the porosity of its hardened paste. Abrams's rule, and Feret's earlier and more general formulation, both express the same physical fact: for a given binder and degree of hydration, strength falls as the water-to-binder ratio rises. Water that is not consumed by hydration leaves capillary pores behind, and those pores are where failure initiates. The load-bearing phase is calcium silicate hydrate, the C-S-H gel that forms as the silicate clinker phases react; its volume fraction relative to the capillary pore volume is what a strength test is indirectly measuring.
This is why compressive strength carries more information than its name suggests. A dense, low-porosity paste is both stronger and less permeable, so strength correlates loosely with durability. It also correlates with stiffness, although weakly: the modulus of elasticity depends heavily on the aggregate, which in a printable mortar is fine and present in a lower volume fraction than in conventional concrete, so a printed mortar can be relatively strong and comparatively less stiff. The one relationship that matters most in practice is the weakest: concrete's tensile strength is roughly an order of magnitude below its compressive strength. Printed elements crack and fail at interfaces long before the paste is crushed, which is why compressive strength is necessary but rarely sufficient information.
Printed material complicates all of this in specific ways. It is deposited without vibration and without compaction, so the air that a poker vibrator would remove from a cast element stays in the material. Layer interfaces introduce planes of reduced continuity and, where deposition geometry leaves them, interlayer voids at the flanks of the beads. The result is anisotropy: a specimen loaded perpendicular to the layer planes generally behaves differently from one loaded parallel to them, and differently again from one loaded along the print direction. Because of this, a cast control specimen of the same mix is a useful reference for the paste chemistry but is not the same material as the printed element. It tells you what the binder can do; it does not tell you what the printed body does.
Strength classes complete the picture. EN 206 defines compressive strength classes together with a conformity framework: sampling rules, specimen types, criteria for identity and production control. A printed element cannot simply be labelled with a class, because that route assumes a cast placement process and isotropic material. Assigning a class to printed material requires an agreed assessment path.
How it is measured
For hardened concrete, compressive strength is determined under EN 12390-3, with specimen shapes and tolerances given in EN 12390-1 and making and curing of specimens in EN 12390-2. The equivalent North American method is ASTM C39, on cylinders. For mortars, including many printable mixes whose aggregate is capped at a few millimetres, EN 1015-11 determines flexural and compressive strength on prisms and their broken halves.
Related properties use their own methods: EN 12390-13 for the secant modulus of elasticity in compression, EN 12390-6 for splitting tensile strength, and EN 12390-8 for water penetration under pressure.
For printed material, the standard specimen-making step does not apply. Specimens are instead sawn or cored from printed blocks after hardening, which introduces its own considerations: cutting damage, the need for plane and parallel loading faces (ground or capped), and the choice of where in the printed body the specimen is taken from, since the bottom layers of a print have been consolidated by the weight above them and the top layers have not. Each specimen must be labelled with its orientation relative to the layer planes, and results reported by direction rather than pooled.
Geometry matters as well. Cube and cylinder results are not interchangeable — cylinder strengths are conventionally lower because of platen restraint and the height-to-diameter ratio — and smaller specimens tend to report higher strengths because of the statistical size effect. Comparing a printed cube result with a cast cylinder result without acknowledging the conversion is a common source of confusion.
Age is the last variable. The 28-day result is the conventional reference, but hydration continues well beyond it, and mixes with supplementary cementitious materials such as fly ash, slag or calcined clay typically show slower early strength and continued gain at 56 and 90 days. Reporting the test age is not optional.
Typical ranges
All values below are indicative and drawn from the published 3DCP and mortar literature in general terms. They are not Concreef measurements.
| Quantity | Indicative range | Basis |
|---|---|---|
| 28-day compressive strength, printable cementitious mortars | roughly 30–80 MPa | values reported across published 3DCP studies, wide because binder content varies |
| Anisotropy between loading directions | commonly a difference of a few per cent up to tens of per cent | typical spread reported in 3DCP literature for sawn printed specimens |
| Tensile strength relative to compressive | roughly one order of magnitude lower | general concrete behaviour, textbook |
| Modulus of elasticity, cementitious mortars | typically lower than a conventional concrete of the same strength | consequence of lower coarse-aggregate volume, reported in mortar literature |
| Cube versus cylinder result | cylinder values conventionally lower than cube values | specimen-geometry convention embodied in EN 206 class notation |
| Later-age gain with SCMs | continued gain measurable at 56 and 90 days | typical for fly ash, slag and calcined clay systems in published studies |
What changes it
The dominant variable is the water-to-binder ratio. Lowering it raises strength, up to the point where the mix cannot be extruded or where insufficient water limits hydration. Because printable mortars are usually designed at a low w/b with a high binder content, they tend to sit at the strong end of the mortar range — and to pay for it in autogenous shrinkage.
Binder type and content follow. Cement type under EN 197-1, the proportion and reactivity of supplementary cementitious materials, and the fineness of the powder all shift both the level and the rate of strength development. Aggregate contributes through its own strength and stiffness, its grading and its bond with the paste; a printable mortar's fine aggregate limits the stiffness contribution.
Admixtures act indirectly. Superplasticisers allow a lower water content at the same workability, which raises strength. Accelerators, often dosed at the nozzle, speed early stiffening but can reduce later strength if overdosed.
Placement and curing are where printed material separates from cast. Absence of vibration leaves entrapped air; poor extrusion control leaves voids at bead flanks; long open times leave weakly bonded interfaces. Curing conditions — temperature, humidity, and whether the surface was covered — change both the rate and the ceiling of strength development, and a printed element has far more exposed surface per unit volume than a cast one.
Failure modes
The most common practical problem is direction-dependent results reported as a single figure, producing an apparent inconsistency between test campaigns that is really an unrecorded orientation difference.
Interlayer voids produce low and highly scattered results, often with failure surfaces that follow the interface. Insufficient compaction shows up as visible entrapped air on a sawn face and a strength well below the cast control of the same mix. Specimen preparation defects — non-parallel faces, saw damage, poorly capped ends — produce premature failure and misleadingly low values, recognised by the failure pattern rather than the number.
Over-accelerated mixes can show acceptable early strength and disappointing 28-day strength. Inadequate curing, especially in dry or windy conditions, leaves the surface layers underhydrated and cracked, so a specimen sawn from the outer skin under-reports the bulk material — while one taken only from the lower, self-compacted part of a tall print over-reports it.
Concreef context
Concreef is a Bulgarian 3D concrete printing company based in Sofia. It runs material trials in its workshop on a Crane WASP printer, currently printing with a commercial premix while developing its own dry mix. It has no published test data, and nothing on this page should be read as a Concreef measurement or a performance claim.
In practical workshop terms, compressive strength anchors everything else the trials look at. A mix that prints well but has an unknown hardened strength is not a usable mix, and a figure obtained on a cast control of a premix does not describe what comes out of the nozzle. Concreef's working approach is therefore to treat printed and cast material as distinct, to plan for specimens sawn from printed blocks with the loading direction recorded, and to keep the placement variables — extrusion consistency, open time, curing of the printed surface — under observation. Any strength values published in future will state the test method, specimen geometry, extraction method, age and loading direction relative to the layers.
Frequently asked questions
- Why is compressive strength the property everyone asks about first?
- It is the cheapest and most repeatable measurement on hardened cementitious material, and it correlates loosely with most other mechanical properties, so codes have historically used it as the single design input. It also reflects the capillary porosity of the paste, which governs durability as well as strength. That makes it a useful indicator, but it is an indicator, not a complete description of the material.
- Is printed material weaker than cast material of the same mix?
- Not necessarily weaker, but it is different. Printed material is placed without vibration and contains interfaces and interlayer voids that a cast specimen does not have, so its strength is direction-dependent and its scatter is usually wider. Published 3DCP studies report printed specimens that fall below, match, or occasionally exceed their cast controls, depending on the mix and the loading direction.
- Can a printed element be assigned an EN 206 strength class?
- Not automatically. EN 206 strength classes come with a conformity framework built around sampling and testing cast specimens from a concrete production process. Printed material is placed differently and is anisotropic, so assigning a class requires an agreed conformity route, usually through a technical assessment rather than the standard concrete production rules.
- Why does the loading direction have to be stated?
- Because a printed specimen has a layer structure, and loading perpendicular to the layers, parallel to them, or along the print path gives different results on the same material. A compressive strength quoted without the loading direction relative to the layer planes is not reproducible. Reporting the direction, the specimen geometry and the extraction method is the minimum for a meaningful number.
- How does compressive strength relate to tensile strength?
- Concrete's tensile strength is roughly an order of magnitude below its compressive strength, and it does not grow proportionally: doubling compressive strength gives far less than double the tensile strength. This is why cracking, interlayer bond and reinforcement, rather than compressive capacity, usually govern the design of printed elements.