Testing standard
Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 50 mm [2 in.] Cube Specimens)
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM C109 crushes 50 mm mortar cubes made to a prescribed 1:2.75 cement-to-standard-sand mix, normally at 3, 7 and 28 days. Because everything except the cement is held constant, the result is a property of the cement — which is why cement is bought, sold and certified on this test. The current edition is C109/C109M-24.
Mortar cubes 50 mm on a side are cast from a standard mix, cured, and crushed between the platens of a compression machine. The maximum load divided by the bearing area is the compressive strength.
The mix is prescribed, not chosen: one part cement to 2.75 parts of graded standard sand by mass, with a water content fixed by the type of cement. That is the point of the method. It is not testing a concrete mix; it is testing the cement, with everything else held constant so that differences in the result are differences in the cement.
Strength is normally determined at 3, 7 and 28 days, with the 28-day figure the standard reference.
This is the test on which cement is bought, sold and certified worldwide. A cement specification quotes minimum compressive strengths at ages, and those figures come from this method or its close national equivalents. A mill that misses them has produced material it cannot sell as that grade.
It is also the routine control test in a cement works, run on every production interval, and the early-age results are what tell a plant whether the clinker, the grinding fineness and the gypsum addition are right while there is still time to change them.
What it does not do is predict the strength of a concrete made with that cement. The mortar mix is fixed and the water content is fixed; a concrete has aggregate, admixtures, a different water-cement ratio and a different placing history. C109 characterises one input to that system.
Loading rate is specified as a relative platen movement giving 900 to 1800 N/s, equivalently 200 to 400 lbf/s. The rate is established during the first half of the anticipated maximum load and is not altered in the second half.
Nothing about this mix is a design decision. The proportions are fixed so that the only variable left is the cement.
Maximum load divided by the cube bearing area
Every variable except the cement is fixed
Sand grading, proportion, water content, mixing, tamping and curing are all specified. What is left in the result is the cement.
The strength of a concrete made with the same cement
A concrete has aggregate, admixtures, a different water-cement ratio and a placing history. This method characterises one input to that system.
Hard, plane bearing blocks larger than the 50 mm cube, with the upper one spherically seated so it aligns with a face that is never perfectly parallel to the lower. The seat must move freely at the start of loading and not wander during it — a seized seat loads one edge first and the cube reads low.
SpecificationsA 50 mm cube at 40 to 60 MPa fails at roughly 100 to 150 kN, well inside an ordinary cement-works frame. The demanding part is control: 900 to 1800 N/s is a narrow band and needs a machine that regulates well at a low rate.
SpecificationsA 50 mm cube at a typical mortar strength of 40 to 60 MPa fails at roughly 100 to 150 kN. A compression frame of 250 kN upwards covers cement work comfortably, with force accuracy to ASTM E4.
Platen requirements are strict. The upper platen must be spherically seated so that it aligns with a cube face that is not perfectly parallel to the lower one, and the seat must be free enough to move at the start of loading and stiff enough not to wander during it. Bearing blocks should be hard, plane and larger than the cube.
The load rate is a real control requirement rather than a formality: 900 to 1800 N/s is a narrow band and a frame that cannot hold a steady low rate will fail the method rather than the cement.
Cube faces out of plane are the most common mechanical fault and always read low. A seized spherical seat does the same thing by loading one edge first. Loading too fast raises the apparent strength — a well-known bias, and the reason the rate band is specified so narrowly. And any departure from the mixing and tamping procedure changes the specimen before the machine sees it.
Three documents, three specimens, three questions. They are routinely confused with one another.
| ASTM C109 | EN 196-1 | ASTM C39 | |
|---|---|---|---|
| Tests | The cement | The cement | The concrete |
| Specimen | 50 mm mortar cube | 40 by 40 by 160 mm prism | A concrete cylinder |
| Procedure | Compression only | Flexure, then compression on the halves | Compression |
| Mix | Prescribed 1:2.75 | Prescribed, different proportions | The concrete as supplied |
| Numbers interchangeable | No | No | No |
A cement certified to a strength class under EN 196-1 will not give the same figure under C109, because the specimen and the mix differ. Converting between them by a fixed factor is a rule of thumb, not a substitution.
ASTM C109, published as C109/C109M, determines the compressive strength of hydraulic cement mortars using 50 mm cube specimens. Cubes are cast from a prescribed mix of one part cement to 2.75 parts graded standard sand, cured, and crushed at set ages — normally 3, 7 and 28 days. The current edition is C109/C109M-24, which retitled the document to lead with the metric cube size.
Because the cement is the subject, not the mortar. Sand grading, proportion, water content, mixing, tamping and curing are all fixed by the method, so anything different in the result is something different about the cement. A test on a mortar of the laboratory's own devising would measure the mortar, and two laboratories would never agree.
No, and treating it as though it does causes real disputes. A concrete has coarse aggregate, a different water-cement ratio, admixtures, a placing and compaction history and a curing regime that is nothing like a cube in a laboratory. C109 characterises one input to that system. It is excellent at telling you whether the cement is what it claims to be, and it makes no claim about the concrete.
Because sand grading changes mortar strength substantially, and if the sand varies then the test is measuring the sand as well as the cement. Graded standard sand is a controlled, certified material produced specifically so that every laboratory using the method is mixing against the same aggregate. Substituting local sand produces numbers that cannot be compared with anyone else's, including the specification.
Because concrete and mortar strength are rate-sensitive: load faster and the apparent strength rises. On a certification test that decides whether a grade can be sold, a rate that drifts is a systematic bias, so the method fixes it at 900 to 1800 N/s, requires it to be established during the first half of the expected load, and forbids altering it in the second half. A frame that cannot hold that band fails the method rather than the cement.
It lets the upper bearing block tilt to match a cube face that is not exactly parallel to the lower one. Without it the cube is loaded on one edge first and fails early. The seat has to be free enough to find its position as load begins, and stiff enough not to keep moving once it has — a seized seat and an over-loose one both produce low results, and a seized one is the commoner of the two.
They test the same thing by different means. EN 196-1 casts 40 by 40 by 160 mm prisms, breaks them in flexure, and then crushes the two halves in compression; C109 casts 50 mm cubes and crushes them. The specimen geometry, the mix proportions and the sand all differ, so the numbers are not interchangeable and a cement certified to a strength class under one will not reproduce that figure under the other.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Roughly 100 to 150 kN for a 50 mm cube at 40 to 60 MPa | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Hard plane bearing blocks larger than the cube, with a freely moving spherically seated upper platen | Our compression anvils, built to the specimen |
| Environment | Prescribed mixing, moulding and curing conditions; the mix and water content are fixed by the method | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
