Key takeaways
- A concrete cylinder is a sample of the exact concrete that went into your structure, cured and crushed under controlled conditions to prove its strength.
- Sampling, casting, curing and testing each follow an ASTM standard; skipping any of them makes the result meaningless to the DOB.
- The 7-day break is an early warning; the 28-day break is the number that must meet the design strength on the drawings.
- Most low results trace back to field handling in the first 48 hours, not to the concrete itself.
In this article
Every time a ready-mix truck discharges on a New York City site with special inspections, a technician is standing nearby with a wheelbarrow, a slump cone and a row of plastic molds. What happens in the next twenty minutes decides whether, four weeks later, the lab report says the concrete met its design strength. Here is what the technician is doing, and why.
Sampling at the truck
The technician takes the sample from the middle portion of the load, never the first or last part, following ASTM C172. From that one sample come all the fresh-concrete tests and all the cylinders, so it has to represent the batch.
Before any cylinder is filled, the fresh concrete is tested on the spot:
| Test | Standard | What it tells you |
|---|---|---|
| Slump | ASTM C143 | Workability; too high often means extra water was added |
| Air content | ASTM C231 or C173 | Freeze-thaw durability; matters for exterior and exposed concrete |
| Temperature | ASTM C1064 | Hot or cold concrete sets and gains strength differently |
| Unit weight | ASTM C138 | A check on the mix proportions |
If the slump or air content is outside the specification, the technician reports it immediately. The contractor and the engineer decide whether the load is placed or rejected. That decision is far cheaper at the truck than after the pour.
Casting the cylinders
Cylinders are cast following ASTM C31, in 4 by 8 inch or 6 by 12 inch molds depending on the specification and the aggregate size. The concrete goes in in layers, each rodded or vibrated a set number of times, and the top is struck off level. A set normally has enough cylinders for a 7-day break, two or three 28-day breaks, and spares in case a result needs confirmation.
How many sets? The Building Code and the project specification set the frequency, typically at least one set per day per class of concrete and one for every set volume of concrete placed or area of slab. The special inspection agency tracks this so that no pour goes unsampled.
The first 48 hours
This is where most problems start. ASTM C31 requires initial curing on site at 60 to 80 degrees Fahrenheit, protected from sun, wind, vibration and freezing, for up to 48 hours. A cylinder left in a pickup truck bed in January, or on a roof deck in July, will test low no matter how good the concrete was.
The technician either places the cylinders in an insulated curing box on site or picks them up within the window. At the laboratory they go into a moist room or a lime-water tank at 73.5 degrees, give or take 3.5, until the day of the test. From that point the result reflects the concrete, not the weather.
Breaking the cylinders
On the test day the cylinder is measured, capped or fitted with pads so the load is applied evenly, and placed in a compression machine. The machine loads it at a controlled rate until it fails, following ASTM C39. The maximum load divided by the cross-section area is the compressive strength, reported in psi.
- The 7-day break is an early indicator. For normal mixes it typically reaches 60 to 75 percent of the 28-day strength. A very low 7-day result lets the engineer react before the structure is loaded.
- The 28-day break is the acceptance test. The average of the cylinders in the set must meet or exceed the specified strength, and no single cylinder may fall more than a set margin below it.
When a result is low
A low 28-day result does not automatically mean the structure is deficient. The engineer of record looks at the whole picture: the fresh-concrete tests, the curing record, the other sets from the same pour. Common causes, roughly in order of frequency:
- Field curing failure: cylinders frozen, overheated or disturbed in the first two days.
- Water added at the truck after the slump test, which raises the slump and lowers the strength.
- Sampling from the first or last part of the load.
- Poor consolidation of the cylinder, leaving voids.
- An actual batching problem at the plant, which usually shows up across several loads.
If the question remains open, the code allows further investigation: testing the spare cylinders, taking cores from the structure, or load testing. The special inspection agency's records make that investigation possible; without them, the engineer has nothing to work from.
Two companions: how many cylinders a pour needs, and why a DOB-licensed laboratory matters.
Frequently asked questions
Can the contractor cast and cure the cylinders?
No. On a project with special inspections, sampling, casting and testing must be done by the registered special inspection agency and its licensed laboratory. That independence is what makes the result acceptable to the DOB.
Why both 7-day and 28-day tests?
The 28-day result is the one the design is based on. The 7-day result exists so that a problem is caught three weeks earlier, while the formwork is still in place and the structure is not yet loaded.
What does a laboratory need in order to test cylinders in NYC?
A DOB concrete testing laboratory license, with the accreditation and qualified personnel that license requires. Results from a lab without the license are not accepted.
How fast do I get the report?
The fresh-concrete results are available the same day. The 7-day and 28-day strengths are reported within a day or two of each break. Ask the agency how it delivers reports and who at your company receives them.
Conclusion
A cylinder test is only as good as the twenty minutes at the truck and the forty-eight hours that follow. The standards exist because every step changes the number. When the sampling, casting, curing and testing are done by a qualified technician and a licensed lab, the 28-day report becomes what it is meant to be: proof that the concrete in your building is the concrete the engineer designed.
Written by the engineers and laboratory technicians of Metric Consulting and Inspection, a Class 1 special inspection agency with a DOB-licensed concrete testing laboratory in Hicksville, New York, serving NYC construction since 2003.