A concrete testing lab collects samples from a footing pour where the specified strength is f'c = 3,000 psi. Three consecutive 28-day compressive strength test results are 3,200 psi, 2,900 psi, and 3,100 psi. Under standard ACI acceptance criteria, how should this concrete be evaluated?
- AAccepted, because the average of the three tests meets f'c and no single test is more than 500 psi below f'c
- BRejected, because all three tests must individually exceed 3,000 psi
- CRejected, because one individual test result fell below 3,000 psi
- DAccepted only if a fourth confirming test is performed at 56 days
Show answer & explanationAnswer & explanation
Correct answer: A. Accepted, because the average of the three tests meets f'c and no single test is more than 500 psi below f'c
ACI 318 acceptance criteria require that the average of any three consecutive strength tests equal or exceed f'c, and no single test fall more than 500 psi below f'c. The average here is (3,200+2,900+3,100)/3 = 3,066.7 psi, which exceeds 3,000 psi, and the lowest test (2,900 psi) is only 100 psi below f'c, well within the 500 psi allowance, so the concrete is acceptable.
Why the other options are wrong
- B. ACI does not require every individual test to exceed f'c, only the average and a maximum deviation.
- C. A single low reading does not automatically fail the concrete under ACI criteria.
- D. No additional test is required once the two acceptance conditions are already met.
Concrete Strength Acceptance Criteria
ACI 318 rule stating concrete is acceptable if the average of three consecutive strength tests meets or exceeds f'c and no single test is more than 500 psi below f'c.
- Based on 28-day cylinder breaks
- Average of 3 tests must equal or exceed f'c
- No single test may fall more than 500 psi below f'c
Memory trick: 'Average wins, but no big loser allowed' — check both the mean and the worst test.