Key takeaways
- NYC Building Code §1905.6 (ACI 318 §26.12) requires at least one set of cylinders per class of concrete per day, and not less than one set per 150 cubic yards or per 5,000 square feet of slab or wall. The larger number governs.
- A set is typically four or five cylinders: one broken at 7 days, two at 28 days, and one or two held for 56 days or a retest.
- The sampling is done at the truck by the special inspection agency, not by the contractor, and the cylinders are tested in a DOB-licensed laboratory.
- Plan the sets before the pour: the number of cylinders decides how many molds, how much curing space on site, and how many pickups the lab makes.
In this article
The question comes up the day before every pour, usually from the superintendent and usually at 4 pm. The rule is short, but the details — what a set is, who takes it, what happens on a weekend — are where projects lose time. Here is the whole answer, with a worked example at the end.
The rule
NYC Building Code 2022 §1905.6, which adopts ACI 318-19 §26.12.2, sets the minimum frequency of strength tests for each class of concrete placed:
| Requirement | Minimum |
|---|---|
| Per day of placement | At least one set, for each mix design placed that day |
| Per volume | Not less than one set per 150 cubic yards |
| Per area (slabs and walls) | Not less than one set per 5,000 square feet of surface placed |
| Per project | At least five sets for each class of concrete, unless the total is under 50 cubic yards and the building official waives it |
Each "set" is one strength test. The number of sets is the largest of the three calculations, not the smallest. A 400 cubic yard mat slab poured in one day needs three sets by volume (400 ÷ 150 = 2.67, rounded up); a 40 cubic yard footing pour still needs one.
What a set contains
A strength test under ASTM C39 is the average of at least two cylinders broken at the same age. Agencies therefore cast more than two per set:
- 4-cylinder set — one at 7 days, two at 28 days, one spare (held for a retest or a 56-day break).
- 5-cylinder set — one at 7 days, two at 28 days, one at 56 days, one spare. Common on jobs with high fly-ash or slag mixes, or where the engineer wants a later confirmation.
- 6 or more — when the specification calls for additional ages (3 days for early stripping, 14 days) or for extra holds.
The standard cylinder in New York is 4 × 8 inches; 6 × 12 inches is used when the specification or the aggregate size requires it. Your agency will tell you which it uses and the project specification governs.
The 7-day break is informational: it shows whether the mix is on track (roughly 60–75% of the 28-day strength for a normal mix). The 28-day break is the one the code compares with the specified strength f′c.
Who takes the sample
The sample is taken by the special inspection agency's field technician, from the truck, in accordance with ASTM C172 and C31. The technician also records slump, air content, concrete temperature and unit weight, and the truck ticket. This is the "concrete sampling and testing" category on the TR1 (the TR2 form). The contractor does not cast its own cylinders for code purposes; a sample without an independent chain of custody does not satisfy the DOB.
After casting, cylinders stay on site for initial curing — in a cure box or a protected area at 60–80 °F — for 24 to 48 hours, then the laboratory picks them up and moves them to lime-saturated water or a moist room until the break date.
The break schedule
Count from the day of the pour:
| Age | Day | Purpose |
|---|---|---|
| 7 days | pour + 7 | Early check that the mix is gaining strength |
| 28 days | pour + 28 | Code acceptance: compared with f′c |
| 56 days | pour + 56 | Where the specification allows later strength gain |
ASTM C39 allows a testing tolerance: ±6 hours at 7 days and ±20 hours at 28 days. A 28-day break that falls on a weekend is usually tested Monday morning, within tolerance for most pour times; a 7-day break that falls on a weekend cannot, so the laboratory either tests on Saturday or tells you in advance that the 7-day result will be an off-age indicator — ask which. The 28-day break is the acceptance test, and that is the age to protect.
A worked example
A six-story residential building in Brooklyn pours its foundation mat on a Tuesday: 220 cubic yards, one mix design, one day.
- By volume: 220 ÷ 150 = 1.47 → 2 sets.
- By day and mix: 1 day × 1 mix = 1 set.
- The larger governs: 2 sets. With five cylinders per set, 10 cylinders to cast and store on site.
- Trucks: about 22 loads of 10 cubic yards, so the technician samples from two trucks spread through the pour — not the first and not the last.
- Breaks: 7-day on the following Tuesday; 28-day four weeks from the pour, a Tuesday; 56-day eight weeks out.
Two weeks later, the same project pours ground-floor columns and walls from two trucks: 18 cubic yards, same mix. By volume one set; by day one set; 1 set, 5 cylinders.
What goes wrong
- Not enough molds on site. The technician brings molds, but on large pours confirm the count the day before.
- Cylinders left in the sun or frozen. Initial curing outside 60–80 °F lowers the break results and the mix gets blamed.
- Pour split over two days without telling the agency. Day two needs its own set.
- Two mix designs, one set. Columns at 5,000 psi and slab at 4,000 psi on the same day are two classes, two sets.
- Nobody tracks the dates. Results arrive and nobody matches them to the pour. Keep the pour log with the ticket numbers and the set numbers.
What happens to those cylinders afterwards is in from the truck to the lab report, and what the early break tells you is in 7-day versus 28-day strength.
Frequently asked questions
Can we cast fewer cylinders if the pour is small?
The minimum is one set per class of concrete per day, whatever the volume, unless the building official waives it for very small quantities. One set still needs at least two cylinders for the 28-day test, plus whatever the specification requires.
Who decides 4 or 5 cylinders per set?
The project specification, then the engineer of record. If neither says, the agency's standard practice applies; ask before the first pour.
What if a 28-day break is below f′c?
ACI 318 accepts the concrete if every average of three consecutive tests meets f′c and no single test falls more than 500 psi below it (for f′c up to 5,000 psi). If those conditions fail, the engineer orders further evaluation — often cores under ASTM C42. Read our article on what happens when a cylinder fails.
Do slab-on-grade pours count toward the 5,000 square foot rule?
Yes. For slabs and walls the surface area rule applies alongside the volume rule; the larger number of sets governs.
Conclusion
One set per 150 cubic yards, per day, per mix — and take the largest of the three. Four or five cylinders in a set, broken at 7 and 28 days with a hold for 56 or a retest. Sampled at the truck by the agency, cured properly on site, tested in a licensed lab. Plan it the day before, and the question stops arriving at 4 pm.
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.