ASTM C39 is the test behind almost every concrete strength number a contractor or engineer ever sees. A cylinder goes into the compression machine, it is loaded until it fails, and the maximum load is turned into a stress in pounds per square inch. The idea is simple. The details are where small labs lose time and credibility: a diameter measured once instead of twice, a short specimen with no correction, an average built from rounded values, a fracture type nobody wrote down. This guide walks through the test in plain terms and shows the arithmetic the way a reviewer should be able to retrace it.
What is the ASTM C39 test for?
The compressive strength of molded cylinders is the number most projects use to judge whether the concrete delivered matches what was specified. Results at early ages tell the contractor how the mix is gaining strength, which can matter for form stripping or loading decisions. Results at the acceptance age, usually 28 days, are the ones compared with the specified strength f'c under the project's acceptance rules.
It is worth being clear about what a cylinder break is not. It measures the strength of a specimen that was made, cured and handled in a particular way. It is not a direct measurement of the concrete in the structure. That is why the field procedures in ASTM C31 matter so much: a perfect break on a badly handled cylinder still produces a misleading number.
What equipment does a concrete break test need?
You need a compression testing machine that is calibrated and verified on the schedule your quality system and the standard require, bearing blocks in good condition, a way to prepare the ends so the load is applied evenly (capping or unbonded pads with retainers, as the standard allows for your strength range), and calipers or another measuring device suitable for reading diameters and length.
Keep the machine identifier with every result. When a reviewer or an assessor asks which machine broke a set, the answer should be on the record, not in someone's memory.
How is a concrete cylinder break performed?
Take the cylinders out of curing at the scheduled age and keep them moist until testing, as the standard describes. Check the ends for planeness and perpendicularity and prepare them by the method your lab uses. Measure two diameters at right angles to each other near mid-height. Measure the length if the specimen might be short, which is common with cores, trimmed specimens or cylinders with damaged ends.
Center the specimen on the lower bearing block, seat the upper block, and load continuously at the rate the current standard specifies. Do not adjust the rate as the specimen approaches failure. Record the maximum load the machine reached, then look at the broken specimen and record the fracture pattern. The standard numbers the patterns; record the number, labeled Type 1 to Type 6, and add a note on anything unusual such as a defect in the concrete or a cap that failed.
How do you calculate concrete compressive strength?
Average the two diameters to get D. The cross-sectional area is A = π × D² ÷ 4 in square inches. Strength is the maximum load in pounds-force divided by A, reported to the nearest 10 psi.
When the length-to-diameter ratio L/D is 1.75 or less, multiply the strength by a correction factor: 0.98 at 1.75, 0.96 at 1.50, 0.93 at 1.25 and 0.87 at 1.00, with straight-line interpolation between those values. Above 1.75 there is no correction. Below 1.00 the specimen is outside the correction range, and the honest output is a flag, not a number.
Two rounding habits keep results defensible. Keep every cylinder at full precision and average those values, then round the average once for display. And make the pass or fail decision on the number you actually report, so the decision and the printed value never disagree.
What formulas are used?
- 01D = (D1 + D2) ÷ 2
- 02A = π × D² ÷ 4 (in²)
- 03Strength = max load (lbf) ÷ A, to the nearest 10 psi
- 04If L/D ≤ 1.75: × correction (1.75 → 0.98, 1.50 → 0.96, 1.25 → 0.93, 1.00 → 0.87, linear between); L/D > 1.75: none; L/D < 1.00: outside the correction range
What does a worked example look like?
Inputs
Cylinder A, 6 × 12 in: D1 6.00 in, D2 6.02 in, length 12.00 in, maximum load 120,000 lbf. Cylinder B, 4 × 8 in: D1 4.00 in, D2 4.00 in, length 8.00 in, maximum load 50,000 lbf. Separately, correction factors at L/D 1.50, 1.375 and 1.80.
Result
Cylinder A: average diameter 6.01 in, area 28.37 in², L/D 2.00, no correction, strength 4,230 psi. Cylinder B: area 12.57 in², L/D 2.00, strength 3,980 psi. Correction factors: L/D 1.50 gives 0.960, L/D 1.375 gives 0.945 by interpolation, and L/D 1.80 gives 1.00, meaning no correction.
Calculated by the same tested functions as the ProofGrit free calculators.
How do you read a cylinder strength result?
A single cylinder strength is one data point, not an acceptance decision. For acceptance, cylinders of the same size broken at the same age are averaged into a strength test: at least two 6 × 12 in cylinders or at least three 4 × 8 in cylinders. That strength test is then judged against f'c using the two ACI 318 criteria, which the ACI 318 acceptance calculator shows step by step.
Early-age breaks, such as 7-day results, describe how the mix is gaining strength. They are useful for decisions about forms or loading, but they are not compared with f'c unless the specification says so. When one cylinder in a set is far from its companions, look at the fracture type, the end preparation and the field curing record before drawing conclusions about the concrete.
What are the most common C39 mistakes?
Using the nominal diameter. A cylinder sold as 6 in is rarely exactly 6.00 in, and a small diameter difference moves the strength by tens of psi. Measure two diameters, every time.
Ignoring short specimens. Without a length reading, nobody can tell whether a correction was needed. If length is not measured, the record should say the correction was not evaluated.
Rounding before averaging. Averaging values already rounded to 10 psi can shift the set average enough to change a borderline acceptance result.
Leaving the fracture type blank. An unusual pattern combined with a low result is often the first clue that the problem was the specimen ends or the cap, not the concrete.
Which standards relate to ASTM C39?
ASTM C31 covers making and curing the field cylinders that C39 breaks, and ASTM C172 covers the sample they are made from. ACI 318 contains the strength acceptance criteria used with the results. The full list of standards explained on this site is on the standards index.
See the standards index for every standard covered on this site.
| Item | 4 × 8 in | 6 × 12 in |
|---|---|---|
| Cylinders per strength test (ACI 318) | At least 3 | At least 2 |
| Strength formula | Load ÷ (π × D² ÷ 4) | Load ÷ (π × D² ÷ 4) |
| Typical L/D for a molded cylinder | About 2.0, no correction | About 2.0, no correction |
| Reporting precision | Nearest 10 psi | Nearest 10 psi |
Frequently asked questions
Why is the strength reported to the nearest 10 psi?
That is the reporting precision for cylinder strength. Calculations keep full precision underneath; only the reported value is rounded.
Do I need to measure length on every cylinder?
Molded cylinders with good ends usually sit well above an L/D of 1.75, but measuring length is the only way to show that. It matters most for cores and specimens trimmed after end damage.
What should happen to a specimen with L/D below 1.00?
It is outside the correction range. Flag it and leave it out of averages rather than reporting a number that has no valid correction.
Does the fracture type change the strength?
No. It is recorded alongside the strength. It helps a reviewer judge whether a low result reflects the concrete or the specimen preparation.
Can 4 × 8 and 6 × 12 cylinders be averaged together?
The strength math is the same for both, but a strength test for acceptance is built from cylinders of one size at one age. Keep sets separate.
How ProofGrit helps
The free ProofGrit concrete compressive strength calculator takes two diameters, an optional length and the maximum load for several cylinders at once. It shows area, L/D, the correction factor and each strength, then averages the set from full-precision values. The ACI 318 acceptance calculator takes those strengths into the acceptance check. Both run in your browser and save nothing, so copy or print the results into your own records.
Related guides and tools
- ASTM C31: making and curing concrete cylinders in the fieldA walk-through of making and curing field cylinders, the initial curing temperature ranges, and how handling errors show up in acceptance results.
- Fresh concrete tests: slump, air, temperature and unit weightA plain-English tour of fresh concrete tests: sampling, slump, air content, temperature and unit weight, and why the field sheet matters later.
- CMT lab accreditation: AASHTO R18, ISO/IEC 17025 and recordsWhat AASHTO re:source accreditation, AASHTO R18 and ISO/IEC 17025 mean for a small CMT lab, and the records your lab and its software need to keep.
- Construction materials testing: a practical lab guideA practical overview of construction materials testing: how field observations, laboratory methods, calculations and reviewed reports fit together.
- Concrete strength calculatorCylinder strength from two diameters and the maximum load, with L/D correction and the set average.
- ACI 318 acceptance calculatorCheck strength tests against both ACI 318 acceptance criteria: running average of three and the single-test limit.
- Standards indexEvery ASTM, AASHTO and ACI standard explained on this site.
- All concrete testing guidesBrowse the full guide library by topic.
Sources and further reading
- ASTM C39/C39M, compressive strength of cylindrical concrete specimensASTM International
- ASTM C31/C31M, making and curing concrete test specimens in the fieldASTM International
- American Concrete InstituteACI
- AASHTO re:sourceAASHTO re:source
Standards are summarized in our own words. Read the current edition from the publisher before you test.
This guide is a plain-English summary. Always follow the current standard and your project specifications.