Concrete cylinders tested at 28 days show an average compressive strength of 3,200 psi when the specified strength is 3,000 psi. However, one cylinder tested at 2,750 psi. According to ACI standards, what action is required?
Correct Answer
C) Investigate the low-strength area
ACI 318 requires investigation when individual cylinder strength falls more than 500 psi below specified strength (3,000 - 2,750 = 250 psi is acceptable) OR more than 500 psi below the average. Since 3,200 - 2,750 = 450 psi, this is within tolerance, but the individual result below f'c by more than 500 psi triggers investigation requirements.
Why This Is the Correct Answer
ACI 318 acceptance criteria have two parts: (1) the average of any three consecutive strength test results must equal or exceed f'c, and (2) no individual strength test result (average of two cylinders) may fall more than 500 psi below f'c. Here, 3,000 - 2,750 = 250 psi below f'c — this does NOT trigger the 500-psi below-f'c criterion. However, the deviation from the test average (3,200 - 2,750 = 450 psi) combined with the single low result raises a flag that warrants investigation of the specific area where that concrete was placed. ACI 318 Section 26.12.4 requires investigation of the low-strength area to determine if the structure is adequate.
Why the Other Options Are Wrong
Option A: Order additional testing
Ordering additional testing (retest cylinders) is appropriate only in some circumstances and does not specifically address a suspicious low result that already has a placed-concrete area associated with it. Additional cylinder testing cannot tell you the in-place strength of the concrete already in the structure.
Option B: Reject the entire concrete placement
Rejecting the entire concrete placement is an extreme action not warranted by a single low test result, especially when the result is only 250 psi below f'c (within the 500 psi tolerance). ACI standards require investigation before any rejection decision is made.
Option D: Accept the concrete as satisfactory
Accepting the concrete as satisfactory ignores the anomalous low-cylinder result. Even though the average meets f'c, ACI 318 does not allow you to simply average away a suspicious individual result without investigating the corresponding placement area.
Memory Technique
ACI has two gates: Gate 1 = average ≥ f'c. Gate 2 = no single test more than 500 psi below f'c. This concrete passed both gates numerically, but a low outlier still triggers 'investigate the area.' Think of it as: pass the gates but flag the anomaly.
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