GATE CE 2017 Set 2 — Question 15
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Structural Engineering → Construction Materials → Concrete Mix Design
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Question
Let the characteristic strength be defined as that value, below which not more than 50% of the results are expected to fall. Assuming a standard deviation of 4 MPa, the target mean strength (in MPa) to be considered in the mix design of a M25 concrete would be
Correct answer
(C) 25.00
Solution
The standard definition of characteristic strength () is the value below which not more than 5% of results fall, given by . However, the question redefines characteristic strength as the value below which not more than 50% of results fall. In a normal distribution, the 50% probability level corresponds exactly to the mean (). Therefore, under this new definition, . For a M25 concrete, where the characteristic strength is 25 MPa, the target mean strength is also 25 MPa.
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