How do you know the concrete poured on your job is as strong as the plans call for? NRMCA says compressive strength results are primarily used to check that the concrete as delivered meets the specified strength. That check starts with a cylinder of fresh concrete, and this guide follows that cylinder from the truck to the testing machine.
A concrete cylinder test breaks a 6 x 12 inch or 4 x 8 inch sample of the delivered concrete in a compression machine, usually at 28 days. The sample must be molded, cured and handled by standard steps, because NRMCA says most mistakes lower the measured strength. The engineer judges the result against the specification.
What Does a Concrete Cylinder Test Show?
It shows the compressive strength of the concrete as delivered, found by breaking a cylindrical specimen in a compression-testing machine, according to NRMCA CIP 35 (hosted copy). The strength is the failure load divided by the cross-sectional area, reported in psi. CIP 35 says requirements run from 2,500 psi for residential concrete to 4,000 psi and higher in commercial structures.
The number to beat is the specified strength, written f’c in the job specification. CIP 35 says the mixture is designed to average higher than f’c, which keeps the risk of missing the specification small.
Once the lab gives you a break load, the concrete compressive strength calculator handles the arithmetic. This guide covers what happens before that: how the sample is made, cured and broken.
How Are Test Cylinders Sampled and Molded?
The sample comes from the middle of the load. NRMCA CIP 34 says the first and last 10% of the discharge will not be representative and should not be used.
Slump, density and air tests begin within 5 minutes, and molding starts within 15 minutes after the sample is taken.
Acceptance cylinders are 6 x 12 inch, or 4 x 8 inch when specified (CIP 35). The diameter should be at least 3 times the nominal maximum size of the coarse aggregate.
| Step | What the source says | Source |
|---|---|---|
| Layers | 3 equal layers for 6 x 12 inch, 2 equal layers for 4 x 8 inch; vibrated samples go in 2 equal layers | CIP 34; Oklahoma DOT checklist (rodded sizes) |
| Rodding | 25 strokes per layer, evenly spread; bottom layer through its depth, upper layers 1 inch into the layer below | CIP 34; Oklahoma DOT checklist |
| Tapping | Tap the sides of the mold 10 to 15 times with a mallet to close insertion holes | CIP 34 |
| Finishing | Strike off flat and level with the mold edge, then cover with a plastic lid or bag | CIP 34 |
| Labeling | Mark the outside of the mold, not the lid or top; Oklahoma DOT says not on removable caps | CIP 34; Oklahoma DOT checklist |
How Are Cylinders Cured Before Testing?
Cylinders get two kinds of care: initial curing at the site, then standard curing at the lab. For initial curing, CIP 34 says to hold the temperature at 60 to 80 F. The Pennsylvania Aggregates and Concrete Association (PACA) adds that cylinders must sit where moisture cannot escape and stay free from disturbance.
For concrete of about 6,000 psi and above, both sources narrow the range to 68 to 78 F. CIP 34 says above 6,000 psi and PACA says 6,000 psi or greater, so this guide uses the looser wording. CIP 34 calls complete immersion in water acceptable and preferred, and says to shield cylinders from direct sun, radiant heat and freezing.
Cylinders should not be moved until at least 8 hours after final set, and they must reach the lab within 48 hours of casting. Transport should take no more than 4 hours.
At the lab, PACA says cylinders stay in a lime bath or moist room at about 73 F (plus or minus 3.5 F) until tested. PACA prints the figure as 73.5 F in one place and 73 F in another, which is why this guide says about 73 F. For curing a finished slab rather than a sample, see curing new concrete.
Standard-Cured or Field-Cured: What Is Each For?
Standard-cured cylinders judge the concrete, and field-cured cylinders judge the curing on the job. CIP 34 says standard-cured results serve acceptance testing, mixture checks and quality control. Field-cured results help decide when a structure can go into service and when to remove forms or shoring.
NRMCA says the two should not be confused. PACA adds that field-cured cylinders are not used to accept concrete strength and are kept as near as possible to the placed concrete.
How Is a Cylinder Broken and Reported?
Each cylinder is capped, measured, centered in the machine and loaded until it fails completely (CIP 35). Caps are generally sulfur mortar or neoprene pads, which spread the load evenly.
The technician measures the diameter in two places at right angles and averages them. On a hydraulic machine the loading rate should stay between 20 and 50 psi per second during the latter half of loading, and the type of break is recorded.
Strength is the maximum load divided by the average cross-sectional area. A test result is the average of at least two standard-cured cylinders from one sample, tested at the same age, reported to the nearest 10 psi. NRMCA CIP 9 counts two 6 x 12 inch cylinders, or three 4 x 8 inch cylinders, as one test.
CIP 9 says cylinders are usually tested at 28 days. CIP 35 says a 3 or 7-day break may help spot concrete or lab problems but is not a basis for rejecting concrete.
How Are Cylinder Results Judged?
NRMCA summarizes the ACI 318 acceptance criteria as two conditions that both apply (CIP 35 and CIP 9). The average of three consecutive tests must equal or exceed f’c, and no single test may fall below f’c by more than 500 psi. For f’c above 5,000 psi, CIP 9 gives the single-test limit as 0.9 times f’c.
One low test is not automatically a failure. CIP 35 says that when job averages sit at the required average strength, about 10% of individual tests fall below f’c, and the criteria account for that. CIP 9 separately says normal variability makes results miss the acceptance criteria about one time in 100 tests.
What Does CIP 9’s 4,000 psi Example Look Like Worked Out?
CIP 9 shows both rules on a specified strength of 4,000 psi, where each test is the average of two cylinders. The sums below are arithmetic on CIP 9’s numbers.
- Acceptable set. Tests 1 to 5 are 4,185, 3,960, 4,435, 3,745 and 4,595 psi. (4,185 + 3,960 + 4,435) / 3 = 12,580 / 3 = 4,193.3. (3,960 + 4,435 + 3,745) / 3 = 12,140 / 3 = 4,046.7. (4,435 + 3,745 + 4,595) / 3 = 12,775 / 3 = 4,258.3. All three averages are at least 4,000, and the lowest single test, 3,745, is above 4,000 – 500 = 3,500.
- Low set. Tests 1 to 5 are 3,585, 4,015, 4,040, 4,780 and 3,250 psi. (3,585 + 4,015 + 4,040) / 3 = 11,640 / 3 = 3,880, which is below 4,000. (4,015 + 4,040 + 4,780) / 3 = 12,835 / 3 = 4,278.3. (4,040 + 4,780 + 3,250) / 3 = 12,070 / 3 = 4,023.3.
- The single-test limit. Test 5 in the low set is 3,250 psi, and 4,000 – 3,250 = 750, which is more than 500 below the specified strength. The first average and this one test both miss the rules.
CIP 9 says that failing either condition means steps must be taken to increase the strength of the concrete. What happens next belongs to the engineer and the specification, not a calculator.
Why Do Cylinders Break Low?
A low break can be a testing problem, not a concrete problem. CIP 35 says the failure may be in the testing, especially when making, handling, curing and testing do not follow standard procedures, and that most deviations lower the measured strength. PACA calls improper making, curing and handling of test cylinders the most common field problem, and many times the most costly.
CIP 9 lists four major reasons for low results. They are improper cylinder handling, curing and testing, which it calls typically the reason in most cases; excess water added at the jobsite; high air content; and production or delivery errors. For the water side, see how much water per bag.
CIP 9 calls these handling factors more significant: improperly finished surfaces, initial curing over 80 F, frozen cylinders, extra days in the field, transport damage, improper capping and careless breaking. CIP 35 says to hold tested cylinders from a low set for later examination.
Enter it in the strength calculator for test cylinders to see psi. For footings, columns and walls, browse the structural concrete calculators.
FAQs About the Concrete Cylinder Test
What is a concrete cylinder test?
It is a strength test in which a sample of fresh concrete is molded into a cylinder, cured under set conditions and broken in a compression machine. The maximum load divided by the area gives the strength in psi.
What size are concrete test cylinders?
NRMCA says acceptance cylinders are 6 x 12 inches, or 4 x 8 inches when specified. The diameter should be at least 3 times the nominal maximum size of the coarse aggregate in the concrete.
Why are concrete cylinders broken at 28 days?
Because most strength requirements are written for that age. NRMCA says that in most cases the requirement is at 28 days, and cylinders are tested at the age the specification gives, usually 28 days.
Is a 7-day cylinder break a pass or fail?
Neither. NRMCA says a 7-day break gives an early indication of potential strength, but it is not used to decide whether concrete is acceptable or as a basis for rejecting it against a 28-day requirement.
Why are test cylinders cured on site first?
The fresh cylinders must stay put while the concrete hardens. CIP 34 and PACA say to hold them at 60 to 80 F without moisture loss or disturbance, and not to move them until 8 hours after final set.
What is the difference between field-cured and standard-cured cylinders?
Standard-cured cylinders are used to accept the concrete against the specified strength. Field-cured cylinders are kept near the placed concrete to show whether it is cured and strong enough for steps like removing forms.
What causes low concrete cylinder results?
NRMCA CIP 9 says improper cylinder handling, curing and testing is typically the reason in most cases. Other causes it names are extra jobsite water, high air content, and production or delivery errors.
Sources and Further Reading
References Used in This Article
- NRMCA CIP 35, Testing Compressive Strength of Concrete (hosted copy), accessed 2026-10-21.
- NRMCA CIP 9, Low Concrete Cylinder Strength, accessed 2026-10-21.
- NRMCA CIP 34, Making and Curing Cylinders in the Field, accessed 2026-10-21.
- Pennsylvania Aggregates and Concrete Association, Making & Curing Concrete Test Cylinders, accessed 2026-10-21.
- Oklahoma DOT, Independent Assurance Checklist IACL005, Making and Curing Concrete Test Specimens in the Field, accessed 2026-10-21.
These steps come from NRMCA, PACA and Oklahoma DOT pages, not the standards. How a cylinder is made and cured changes its strength, so judge a result only against the specification and with your engineer. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-21.
Shakeel Muzaffar is the Founder and Editor-in-Chief, with over 15 years of experience in digital publishing and online tool development. He oversees editorial quality and formula verification to ensure accurate, practical, and easy-to-use calculators.