A 4000 psi normalweight concrete mix is being evaluated from standard strength tests. Which ACI 318 result set satisfies the acceptance rule for compressive strength?
Correct Answer
B) The average of any three consecutive tests is at least 4000 psi, and no test is below 3500 psi.
For specified strength of 5000 psi or less, ACI 318-19 26.12.3.1 accepts concrete when the average of any three consecutive strength tests is at least f'c and no strength test falls more than 500 psi below f'c; for 4000 psi that floor is 3500 psi. Other choices: requiring every cylinder to reach 4000 psi is too strict, a whole-project average is the wrong window, and the 10 percent branch is for strengths above 5000 psi.
Why This Is the Correct Answer
For specified strength of 5000 psi or less, ACI 318-19 26.12.3.1 accepts concrete when the average of any three consecutive strength tests is at least f'c and no strength test falls more than 500 psi below f'c; for 4000 psi that floor is 3500 psi. Other choices: requiring every cylinder to reach 4000 psi is too strict, a whole-project average is the wrong window, and the 10 percent branch is for strengths above 5000 psi.
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