A slab placement has enough volume to require regular strength testing. Which sampling frequency statement matches ACI 318 for each concrete mixture placed on a day?
Correct Answer
C) At least once per day, once for each 150 yd3, and once for each 5000 ft2 of slab or wall surface area
ACI 318-19 26.12.2.1 uses the most restrictive of the listed frequency checks: at least once per day, at least once for each 150 yd3, and at least once for each 5000 ft2 of slab or wall surface area. Other choices: 500 yd3 is too loose, start/end sampling alone misses the volume and area triggers, and slump does not replace strength-test frequency.
Why This Is the Correct Answer
ACI 318-19 26.12.2.1 uses the most restrictive of the listed frequency checks: at least once per day, at least once for each 150 yd3, and at least once for each 5000 ft2 of slab or wall surface area.
Why the Other Options Are Wrong
Option A: One sample for every 500 yd3
The trap in A is One sample for every 500 yd3 only. That selection is wrong because One sample for every 500 yd3 only is a different published number than the cited Concrete sampling frequency value; ACI 318-19 26.12.2.1 supports At least once per day, once for each 150 yd3.
Option B: One sample at the start and one sample at the end of the pour
Choice B gives One sample at the start and one sample at the end of the pour. Its specific error: the option does not produce the required Concrete sampling frequency outcome, At least once per day, once for each 150 yd3.
Option D: One sample only when slump is outside the specified range
In concrete work, One sample only when slump is outside the specified range is the wrong selection for Concrete sampling frequency: the action skips the timing, documentation, or field-control step needed for Concrete sampling frequency.
Memory Technique
Link concrete sampling frequency to its book condition; a different condition means a different answer than At least once per day, once for each 150 yd3, and once for each 5000 ft2 of slab or wall surface area.
Reference Hint
Open ACI 318-19, Building Code Requirements for Structural Concrete; use ACI 318-19 26.12.2.1 to confirm that ACI 318-19 26.12.2.1 uses the most restrictive of the listed frequency checks: at least once per day, at least once for each 150 yd3, and at least once for each 5000 ft2 of slab or wall surface area.
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