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Cracks and voids inside concrete affect ultrasonic pulse velocity only when their plane is oriented in a particular way relative to the pulse. The strongest reduction in velocity occurs when the crack plane is:
Answer & Solution
Correct answer: C.
1. A pulse travels at the highest speed through sound concrete.
2. If a crack lies in the path so its plane is perpendicular to the propagation, the pulse must travel around it.
3. This added path length increases the time of travel, so the apparent pulse velocity drops noticeably.
4. If the crack plane is parallel to the pulse path, the pulse passes alongside it and the velocity is unaffected.
5. Pulse direction therefore matters; the perpendicular orientation produces the largest decrease.
_Source: IGNOU ET-522 Concrete Technology, Block 2 Unit 9 "Testing of Concrete"._
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