A lab switches to 4 by 8 in. cylinders for field strength tests. Under the ACI definition, how many cylinders from the same sample are averaged for one strength test?
Correct Answer
A) At least three 4 by 8 in. cylinders
ACI 318-19 26.12.1 defines a strength test using 4 by 8 in. cylinders as the average of at least three cylinders made from the same sample and tested at the same age. Other choices: two cylinders is the 6 by 12 in. rule, one cylinder is not a strength test under this definition, and discarding the lowest is not the ACI rule.
Why This Is the Correct Answer
ACI 318-19 26.12.1 defines a strength test using 4 by 8 in. cylinders as the average of at least three cylinders made from the same sample and tested at the same age.
Why the Other Options Are Wrong
Option B: At least two 4 by 8 in. cylinders
In concrete work, At least two 4 by 8 in. cylinders is the wrong selection for Strength testing: At least two 4 by 8 in. cylinders is a different published number than the cited Strength testing value; ACI 318-19 26.12.1 supports At least three 4 by 8 in.
Option C: One cylinder if it is capped
The trap in C is One cylinder if it is capped. That selection is wrong because it leaves out the controlling Strength testing result: At least three 4 by 8 in. cylinders.
Option D: Four cylinders, discarding the lowest
Choice D gives Four cylinders, discarding the lowest. Its specific error: the choice would not get the project to At least three 4 by 8 in. cylinders, which is the Strength testing result verified by the source.
Memory Technique
File strength testing under a tab: condition first, number or duty second, answer At least three 4 by 8 in. cylinders.
Reference Hint
Open ACI 318-19, Building Code Requirements for Structural Concrete; use ACI 318-19 26.12.1 to confirm that ACI 318-19 26.12.1 defines a strength test using 4 by 8 in.
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