What f'c means and when it is tested
The specified compressive strength, f'c, is the strength the design is built on. ACI 318-19 §19.2.1.3 says f'c shall be based on the 28-day test results unless the construction documents specify otherwise. If the documents name another age, such as 56 days, that age applies; 56 days is never the default.
3-day and 7-day cylinders are early-strength checks. They track strength gain so the crew can see how the mix is developing, but they are not the acceptance basis.
The 7-day trap
On a 4000 psi job, a 7-day break of 2900 psi does not decide anything. Acceptance is judged on the 28-day results.
- Do not reject because a 7-day break is more than 500 psi below f'c. The §26.12.3.1 limit applies to acceptance tests, not early breaks.
- Do not accept because a 7-day break reached half of f'c. ACI 318 uses no such criterion for acceptance.
- Do not core the slab on the strength of 7-day results. They are not the acceptance tests.
The low-test limit, ACI 318-19 §26.12.3.1
No individual strength test may fall below f'c by more than a set amount, and the amount depends on f'c:
- f'c of 5000 psi or less: no more than 500 psi below f'c.
- f'c above 5000 psi: no more than 10% of f'c below f'c.
An individual test does not have to reach f'c. The code gives each test this margin.
Working the numbers
- f'c = 4000 psi: 4000 − 500 = 3500 psi floor. Using 10% here (3600 psi) is wrong, because the 10% rule governs only above 5000 psi.
- f'c = 6000 psi: 10% is 600 psi, so the floor is 5400 psi. Using 500 psi (5500 psi) is the common high-strength error.
- f'c = 8000 psi: 10% is 800 psi, so the floor is 7200 psi. A 7300 psi test is only 700 psi low and meets the limit, even though it is more than 500 psi below f'c.