According to CBC Section 1904.4, concrete testing in California requires that the average of any three consecutive strength tests equals or exceeds the specified compressive strength (f'c), and no individual test falls below what percentage of f'c?
Correct Answer
C) 85%
CBC Section 1904.4 requires that no individual concrete strength test result falls below 85% of the specified compressive strength f'c. This quality control measure ensures consistent concrete performance in California's seismic environment where structural reliability is critical.
Why This Is the Correct Answer
CBC Section 1904.4 requires that no individual concrete strength test result fall below 85% of the specified compressive strength f'c. This threshold allows for minor statistical variation while still ensuring structural adequacy. Combined with the three-test average requirement, it creates a two-part quality control standard.
Why the Other Options Are Wrong
Option A: 95%
95% is not the CBC threshold for individual test results. This figure may seem reasonable but is more stringent than what the code requires. Applying a 95% minimum would reject acceptable concrete that statistically falls within the code's allowable variation.
Option B: 90%
90% is not the correct threshold under CBC 1904.4. While a plausible-seeming number, the actual code value is 85%. Using 90% would be an incorrect interpretation of the quality control requirement.
Option D: 100%
100% would mean no individual test could fall below f'c, which is statistically unrealistic given normal concrete variability. The CBC acknowledges that individual tests can be slightly below f'c as long as the running average meets requirements and no single test falls below 85%.
Memory Technique
85% = 'B-minus concrete is acceptable' — just like a B-minus grade (85%) passes in school, an individual concrete test at 85% of f'c passes the CBC minimum. The class average (three consecutive tests) still must be at least 100%.
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