In IS 456 Limit State Method, the characteristic compressive strength of concrete (fck) is defined as:
Choose the correct answer
Average cube strength at 7 days
The 5th percentile 28-day cube strength (5% of results may fall below it)
Minimum cylinder strength
Average cube strength at 28 days
Correct Answer
B. The 5th percentile 28-day cube strength (5% of results may fall below it)
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fck = compressive strength of 150 mm cubes at 28 days below which NOT MORE than 5% of test results are expected to fall. It is the 5th percentile strength. M20 means fck = 20 MPa. Design strength = fck/γm = fck/1.5 = 0.67fck.
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