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According to the ACI Building Code 318, selection of mix proportions should be based on the...

According to the ACI Building Code 318, selection of mix proportions should be based on the average strength, not the specified (design) strength. Is this justified? Given a specified strength value, what procedures are used to determine the average strength?

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Answer #1

Ans. The average strength of the mix design must always exceed the specified value of the 28 day compressive strength, f'c. ACI 318 indicates the amount by which the average strength of a concrete mix, f'cr should exceed f'c. The amount of excess strength depends on the expected variability of test results as expressed by a standard deviation s, for a particular concrete mix. ACI criteria are based on probabilistic concepts and are intended to ensure that adequate strength will be developed in the structure

Procedure to determine average strength from specified strength value-

Since f'cr= f'c + s

i.e. Average strength = Specified strength value + standard deviation

For standard deviation values refer ACI 318 table 5.3.2.2

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