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Compared with the direct tensile strength of the same concrete, the modulus of rupture obtained from a flexure beam test is generally:
Answer & Solution
Correct answer: A.
1. In direct tension the whole specimen volume is subjected to the maximum stress simultaneously.
2. In flexure only the extreme fibre reaches maximum stress, while less stressed material adjacent to the neutral axis impedes crack propagation.
3. This restraint allows the extreme fibre to develop a higher peak stress before failure.
4. Together with the linear-stress assumption built into M/Z, this inflates the computed modulus of rupture above the true direct tensile strength.
5. So flexure consistently reports a higher tensile strength than the direct tension test, not equal or lower.
_Source: IGNOU ET-522 Concrete Technology, Block 2 Unit 9 "Testing of Concrete"._
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