5 to 400 kNTJ-125
Direct Compression Fixture
Flat parallel platens of a hardness and flatness suited to concrete, with a spherically seated upper platen so the load finds the specimen rather than the specimen finding the platen.
SpecificationsTesting standard
ASTM C39/C39M
Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM C39 determines the compressive strength of cylindrical concrete specimens — moulded cylinders and drilled cores — for concrete with a density above 800 kg/m³. The reported strength is the maximum load over the cross-sectional area, and the fracture pattern is recorded with it.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ASTM
- Edition
- C39/C39M-23
- Material
- Wood, concrete & ceramics
- Runs on
- Series 7200 and Series 9000
What the test does
A moulded cylinder or a drilled core of concrete denser than 800 kg/m³ has its diameter measured and its ends checked for planeness and perpendicularity, being capped or ground where they fail. It is centred on the lower bearing block, the spherically seated upper block is brought into contact and allowed to seat itself, and load is applied at a specified stress rate without easing off as failure approaches. The maximum load divided by the measured average area gives the compressive strength, and the fracture pattern is identified against the types the method illustrates.
What it measures, and why it matters
The strength of the concrete as placed — the number a structure is designed around and accepted on. The most useful thing to understand about it is that the specimen shape is part of the answer. Friction between platen and specimen restrains the ends from spreading, putting them into triaxial compression, which is a stronger state than uniaxial. That restraint reaches a fixed distance into the specimen, so it dominates a squat cube and largely spares the mid-height of a slender cylinder. A cube and a cylinder of identical concrete therefore give different strengths, and the difference is geometry rather than material.
Cylinders, and why the shape matters
A cylinder and a cube of the same concrete do not give the same strength. Platen restraint is the reason.
- Specimens
- Moulded cylinders and drilled cores
- Density limit
- Concrete above 800 kg/m³ [50 lb/ft³]Lightweight concretes below that are outside the method.
- Units
- SI and inch-pound, used independentlyNot exact equivalents — a report must not mix them.
- Ends
- Plane and perpendicular, capped or ground where they are notAn unprepared end concentrates load on a high spot and the cylinder splits from there.
- Spherical seat
- On the upper bearing blockIt lets the block seat itself against the specimen. A rigid platen loads whichever end is proudest.
- Record the fracture pattern
- Cone, cone-and-split, cone-and-shear, shear or columnarThe method illustrates the types because an unusual one signals a bad end, a bad seat or a bad specimen.
Platen friction restrains the ends of the specimen and puts them into triaxial compression, which raises the measured strength. That restraint reaches further into a short specimen than a slender one, which is why a cube reads higher than a cylinder of the same concrete.
Test speed
- Rate
- A specified stress rate, held through the later part of loadingLoading faster than specified raises the apparent strength; concrete is rate-sensitive.
- Reported
- Compressive strength and the fracture type
- Cores
- Length-to-diameter ratio recorded, and a correction applied where required
- Check the spherical seat moves freely before a series
- DakA seized seat is a rigid platen wearing the right name, and it will not announce itself.
Calculations
Compressive strength—
Maximum load divided by the average cross-sectional area
- area
- from the measured diameter, averaged over the specified measurements
Measured, not nominal. A mould wears and a core is rarely exactly its nominal diameter.
Why a cube reads higher—
Platen restraint reaches further into a squat specimen than a slender one
A 150 mm cube and a 150 x 300 mm cylinder of identical concrete give different numbers. Converting between them is a convention, not a measurement, and the specification says which shape it means.
Core correction—
Applied where the length-to-diameter ratio is below the standard value
A short core is more restrained and reads high, so the raw figure needs correcting before it is comparable.
How the test runs
- 01Confirm the concrete density is above the method's lower limit.
- 02Measure the diameter as the method specifies and compute the average area.
- 03Check the ends for planeness and perpendicularity; cap or grind where they fail.
- 04For cores, record the length-to-diameter ratio.
- 05Confirm the spherical seat on the upper block moves freely.
- 06Centre the specimen on the lower bearing block.
- 07Bring the upper block into contact and allow it to seat.
- 08Load at the specified stress rate without adjustment as failure approaches.
- 09Record the maximum load.
- 10Identify and record the fracture pattern against the method's illustrated types.
- 11Calculate the strength on the measured area, applying any core correction.
The fixture this method needs
What the report has to contain
- Reference to ASTM C39/C39M and the edition
- Specimen type — moulded cylinder or drilled core
- Measured diameter and length, and the L/D ratio for cores
- How the ends were prepared
- Age at test and curing history
- Rate of loading
- Maximum load and compressive strength
- Fracture pattern
- Any core correction applied
- Which unit system was used
What the machine must be capable of
Very high force and a well-made load train. A 150 by 300 mm cylinder at 60 MPa needs above a thousand kilonewtons and higher grades go further, so this is a dedicated compression frame rather than a general-purpose one. The upper bearing block must be spherically seated and must actually move — a seized seat behaves as a rigid platen while still being called a spherical one, and it will not announce itself. Bearing blocks need the specified hardness and flatness, and the frame enough stiffness to absorb an abrupt failure.
What goes wrong in practice
Loading faster than specified, or easing off as failure approaches; concrete is rate-sensitive and both bias the answer, in opposite directions. Untested spherical seats. Ends left unprepared. Calculating on a nominal diameter when moulds wear and cores are rarely exactly nominal. And the one that travels furthest: comparing a cube result with a cylinder result as though they measured the same thing, when the conversion between them is a convention rather than a measurement. Skipping the core correction sits alongside it: a short core is more restrained than a full-height cylinder and reads high, so an uncorrected figure flatters the concrete it came from, and the flattery grows as the core gets shorter.
ASTM C39 or IS 516
| ASTM C39 | IS 516 (Part 1/Sec 1) | |
|---|---|---|
| Specimen | 150 x 300 mm cylinder, or a core | 150 mm cube |
| Restraint | Less — the mid-height is far from the platens | More — the platens are closer together |
| Reads | Lower for the same concrete | Higher for the same concrete |
| Interchangeable | No — the specification states which shape it means | No |
The single most useful thing to know about concrete strength testing: a cube and a cylinder of the same mix give different numbers, and the difference is geometry rather than material. Never compare across shapes without saying so.
Questions we are asked about this test
What is ASTM C39?
It is the ASTM test for the compressive strength of cylindrical concrete specimens, covering both moulded cylinders and drilled cores. It applies to concrete with a density above 800 kg/m³. The specimen is crushed between bearing blocks at a specified stress rate and the maximum load divided by the measured area gives the strength.
Why does a cube give a higher strength than a cylinder?
Because of platen restraint. Friction between the bearing block and the specimen stops the ends spreading sideways, putting them into triaxial compression, which is a stronger state than uniaxial. That restraint reaches a fixed distance into the specimen, so in a squat cube it influences most of the height while in a slender cylinder the mid-height escapes it. The difference is geometry, not concrete, and it is why a specification always states which shape it means.
Why does the upper platen have a spherical seat?
So the block can rotate a little and meet the specimen's end squarely. Concrete ends are rarely perfectly plane and perpendicular, and a rigid platen would bear on whichever point is proudest, concentrating the load and splitting the cylinder from there. The seat lets contact spread over the whole face. If it has seized — which happens, and quietly — the machine has a rigid platen wearing the right name.
Why record the fracture pattern?
Because an unusual one is a warning. The method illustrates the expected types — cone, cone-and-split, cone-and-shear, shear and columnar — and a fracture that does not resemble any of them usually points at a badly prepared end, a seized spherical seat, or a specimen that was not what it should have been. The strength on its own cannot tell you that; the fracture can.
Does loading rate affect the result?
Yes, and that is why the rate is specified rather than left to the operator. Concrete is rate-sensitive: load it faster and it reads stronger. The temptation is to slow down or ease off as failure approaches, which biases the result the other way. The rate is held through the later part of loading precisely because that is where the error would enter.
Why do cores need a correction?
Because a drilled core is often shorter relative to its diameter than a moulded cylinder, and a squatter specimen is more restrained by the platens and therefore reads high. Recording the length-to-diameter ratio and applying the correction where it falls below the standard value is what makes a core result comparable with a cylinder one.
What machine capacity does this need?
More than most laboratory frames. A 150 by 300 mm cylinder at 60 MPa needs above a thousand kilonewtons, and higher-grade concretes go further. The frame also needs bearing blocks of the specified hardness and flatness, and enough stiffness that the sudden release at failure does not damage it — concrete fails abruptly and completely.
Running ASTM C39/C39M on the Series 7200 and Series 9000
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 | Very high — a 150 x 300 mm cylinder at 60 MPa needs above 1000 kN | 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 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | A compression frame with a spherically seated upper platen and bearing blocks to the specified flatness | Our compression anvils, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 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.