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.
Why the Other Options Are Wrong
Option A: Every individual cylinder must be at least 4000 psi
In concrete work, Every individual cylinder must be at least 4000 psi. is the wrong selection for Strength acceptance: Every individual cylinder must be at least 4000 psi.
Option C: The project average is at least 4000 psi, even if one test is 3200 psi
Choice C gives The project average is at least 4000 psi, even if one test is 3200 psi. Its specific error: The project average is at least 4000 psi, even if one test is 3200 psi.
Option D: Assume a single test may fall more than 10 percent below 4000 psi
Choice D (No test may be more than 10 percent below 4000 psi.) is rejected because No test may be more than 10 percent below 4000 psi. is a different published number than the cited Strength acceptance value; ACI 318-19 26.12.3.1 supports The average of any three consecutive tests is at least 4000 psi, and no test is below 3500 psi.
Memory Technique
Link strength acceptance to its book condition; a different condition means a different answer than The average of any three consecutive tests is at least 4000 psi, and no test is below 3500 psi.
Reference Hint
Open ACI 318-19, Building Code Requirements for Structural Concrete; use ACI 318-19 26.12.3.1 to confirm that 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 40...
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