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Per §7.2 ('Age of Concrete'), in normal working conditions the 7-day compressive strength of OPC concrete is taken to be at least:
A{'text': 'About one-quarter of the 28-day strength', 'label': 'A'}
B{'text': 'Two-thirds of the 28-day strength', 'label': 'B'}
C{'text': 'Equal to the 28-day strength', 'label': 'C'}
D{'text': 'Five times the 28-day strength', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'Two-thirds of the 28-day strength', 'label': 'B'}
1. §7.2 (Age of Concrete): 'in normal working range of concrete 7 days strength may be taken as not less than 2/3 of 28 days strength.'
2. For OPC the 28-day to 7-day ratio is between 1.3 and 1.7, with most results above 1.5.
_Source: IGNOU ET-522 Block-2 Unit-7 Strength of Concrete, §7.2, p. 49_
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