Concrete Compressive Strength Test: Cubes, Cylinders and ACI 318 Acceptance
How concrete cubes and cylinders are tested, why results fail and how ACI 318 decides whether concrete is accepted.
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Quick answer: A concrete compressive strength test crushes standard cylinders (ASTM C39) or cubes (BS EN 12390-3) at set ages, usually 7 and 28 days. Under ACI 318, the engineer accepts the concrete when every average of three consecutive tests meets the specified strength f′c and no single test falls more than 3.5 MPa (500 psi) below it.
On most projects, the 28-day cube or cylinder result is the number everyone waits for. If it passes, nobody thinks about it again. If it fails, the questions come quickly: was the concrete bad, or was the sample? We see both, and in a surprising number of cases the problem started on site, long before the specimen reached the lab.
Here is what project engineers, QA/QC teams and contractors should know about compressive strength testing, from sampling to acceptance.
Cylinders or cubes: which test applies?
It depends on the specification. Projects designed to American standards use cylinders tested to ASTM C39; projects following British or European standards use cubes tested to BS EN 12390-3. Both measure the same property, but the results are not interchangeable. A cube of the same concrete typically reads higher than a cylinder, which is why a C30/37 mix carries two numbers.
| Cylinders (ASTM) | Cubes (BS EN) | |
|---|---|---|
| Usual size | 150 × 300 mm or 100 × 200 mm | 150 mm (100 mm also used) |
| Making and curing | ASTM C31 (field), standard moist curing at 23 ± 2 °C | BS EN 12390-2, water curing at 20 ± 2 °C |
| Loading rate | 0.25 ± 0.05 MPa/s | 0.6 ± 0.2 MPa/s |
| Common test ages | 7 and 28 days | 7 and 28 days |
Where results go wrong: sampling and curing
A specimen only tells you about the concrete if the technician made and cured it properly. The most common causes of low results we investigate are not the mix at all:
- Specimens left in direct sun on site for a day or two, especially in GCC summers
- Moulds moved or knocked while the concrete is still setting
- Poor compaction, leaving voids that become weak points
- Late transport to the laboratory, or rough handling on the way
- Uneven or unground ends that concentrate the load
ASTM C31 requires initial curing at 16 to 27 °C for up to 48 hours. On a site where air temperatures pass 45 °C, that means a shaded curing box or a temperature-controlled room, not a corner of the batching plant. ACI Concrete Field Testing Technician – Grade I training teaches technicians to get these details right.
How acceptance works under ACI 318
Under ACI 318, a “strength test” is the average of at least two 150 × 300 mm cylinders or three 100 × 200 mm cylinders from the same sample, tested at 28 days or the age stated in the specification. The concrete is acceptable when it meets both conditions:
- Every average of three consecutive strength tests equals or exceeds f′c.
- No single strength test is below f′c by more than 3.5 MPa (500 psi) when f′c is 35 MPa or less, or by more than 0.10 f′c when f′c is above 35 MPa.
The first rule controls the average quality of the supply; the second catches individual bad batches. These are long-standing ACI 318 rules, but always check which edition your project specification names. ACI 318-25, released in 2025, is the current edition, and many projects are still designed to ACI 318-19.
What about 7-day results?
Seven-day results are an early warning, not an acceptance criterion. Ordinary Portland cement concrete often reaches around 65 to 75 percent of its 28-day strength at seven days, but blended cements with fly ash or slag gain strength more slowly. Do not reject concrete on a 7-day result alone; use it to decide whether to investigate.
What to do when a result fails
Start with the paperwork before the structure. Check the sampling records, curing conditions, transport times, specimen dimensions and the fracture pattern. An abnormal fracture type often points to a specimen problem rather than weak concrete.
If the low result stands, ACI 318 allows the engineer to take cores from the structure. ACI 318 considers the concrete in the area structurally adequate when the average of three cores is at least 85 percent of f′c and no single core is below 75 percent of f′c. If cores also fall short, the engineer of record decides whether the structure needs load testing or strengthening.
Choosing a testing laboratory
Look for a laboratory accredited to ISO/IEC 17025 for the exact test method you need. Accreditation covers specific methods, so ask to see the scope rather than the certificate alone. Check that the compression machine has a current calibration, that the laboratory measures cube or cylinder dimensions and that the report states fracture type and specimen condition.
Key takeaways
- Cylinders (ASTM C39) and cubes (BS EN 12390-3) are not interchangeable.
- Most low results start with poor sampling, curing or handling on site.
- ACI 318 accepts concrete on averages of three tests plus a minimum for single tests.
- Seven-day results are a warning signal, not an acceptance test.
- Cores are the next step when a result fails: 85% average, 75% minimum.
Frequently asked questions
How many cylinders make up one strength test?
At least two 150 × 300 mm cylinders or three 100 × 200 mm cylinders from the same sample, tested at the same age.
Why are cube results higher than cylinder results?
The shape and the friction between the platens and the specimen confine cubes more, so they crush at a higher stress than taller cylinders of the same concrete.
Can concrete be accepted if one test fails?
Yes, if the single test is within the permitted margin below f′c and the running average of three tests still meets f′c.
Who should make the test specimens?
A trained technician, ideally ACI Grade I certified, following ASTM C31 or BS EN 12390-2.
Need reliable results for your project? See SRA’s civil material testing services or train your site team through our ACI training and certification programmes. Reference documents are available from the ACI 318 Building Code Portal.
