Three consecutive strength tests for a 4000 psi mix are 4100 psi, 3980 psi, and 3920 psi. Applying the ACI average-and-low-test rule, what is the result?
Correct Answer
C) Pass; the three-test average is 4000 psi and no test is below 3500 psi
The average is (4100 + 3980 + 3920)/3 = 4000 psi, meeting f'c. The lowest test is 3920 psi, which is above the 3500 psi low-test limit for a 4000 psi mix. Other choices: an individual test may be below f'c within the ACI limit, the average is not 3920 psi, and cylinder-by-cylinder 4000 psi is not the acceptance rule.
Why This Is the Correct Answer
The average is (4100 + 3980 + 3920)/3 = 4000 psi, meeting f'c. The lowest test is 3920 psi, which is above the 3500 psi low-test limit for a 4000 psi mix.
Why the Other Options Are Wrong
Option A: Fail, because one test is below 4000 psi
strength acceptance, choice a: Fail, because one test is below 4000 psi applies an individual-cylinder rule.
Option B: Fail, because the average is 3920 psi
strength acceptance, choice b: Fail, because the average is 3920 psi names the wrong average or compares it to the wrong threshold. The calculation in the steps is.
Option D: Pass only if all individual cylinders were above 4000 psi
strength acceptance, choice d: Pass only if all individual cylinders were above 4000 psi applies an individual-cylinder rule.
Memory Technique
Say the strength acceptance rule aloud as a chain; the chain ends at Pass; the three-test average is 4000 psi and no test is below 3500 psi, not an intermediate stop.
Reference Hint
Open ACI 318-19, Building Code Requirements for Structural Concrete; use ACI 318-19 26.12.3.1 to confirm that The average is (4100 + 3980 + 3920)/3 = 4000 psi, meeting f'c.
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