Three cores are drilled after low cylinder results for a 5000 psi member. Which ACI core result is acceptable for the questioned concrete area?
Correct Answer
D) Average of three cores at least 85 percent f'c, and no single core below 75 percent f'c
ACI 318-19 26.12.6 accepts core strength when the average of three cores is at least 85 percent of f'c and no individual core is less than 75 percent of f'c. Other choices: 75/65 is too weak, requiring every core to meet f'c is too strict, and one good core does not satisfy the three-core average rule.
Why This Is the Correct Answer
ACI 318-19 26.12.6 accepts core strength when the average of three cores is at least 85 percent of f'c and no individual core is less than 75 percent of f'c.
Why the Other Options Are Wrong
Option A: Average of three cores at least 75 percent f'c, with no core below 65 percent
Choice A gives Average of three cores at least 75 percent f'c, with no core below 65 percent. Its specific error: Average of three cores at least 75 percent f'c, with no core below 65 percent is a different published number than the cited Core strength evaluation value; ACI 318-19 26.12.6 supports Average of three cores at least 85 percent f'c, and no single core below 75 percent f'c.
Option B: Every individual core at least 5000 psi
Choice B (Every individual core at least 5000 psi) is rejected because Every individual core at least 5000 psi is a different published number than the cited Core strength evaluation value; ACI 318-19 26.12.6 supports Average of three cores at least 85 percent f'c, and no single core below 75 percent f'c.
Option C: Any one core at least 85 percent f'c
In concrete work, Any one core at least 85 percent f'c is the wrong selection for Core strength evaluation: Any one core at least 85 percent f'c is a different published number than the cited Core strength evaluation value; ACI 318-19 26.12.6 supports Average of three cores at least 85 percent f'c, and no single core below 75 percent f'c.
Memory Technique
Use a switch memory cue for core strength evaluation: the stem's condition unlocks Average of three cores at least 85 percent f'c, and no single core below 75 percent f'c.
Reference Hint
Open ACI 318-19, Building Code Requirements for Structural Concrete; use ACI 318-19 26.12.6 to confirm that ACI 318-19 26.12.6 accepts core strength when the average of three cores is at least 85 percent of f'c and no individual core is less than 75 percent of f'c.
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