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To get a representative rebound number from a Schmidt hammer on a concrete area, the unit recommends taking how many readings spread evenly over the surface?
Answer & Solution
Correct answer: D.
1. The local rebound number is sensitive to voids and aggregate just under the plunger.
2. A single reading is unreliable for assessing the concrete mass.
3. The unit advises taking 15 to 20 readings spread over the test area.
4. Their mean value is reported as the concrete's rebound number.
5. The other counts are too few or excessive.
_Source: IGNOU ET-522 Concrete Technology, Block 2 Unit 9 "Testing of Concrete"._
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