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Why is the top surface of a cylindrical specimen capped before compression testing?
Answer & Solution
Correct answer: A.
1. The top surface, finished by float, is rougher and less plane than the cast sides.
2. A lack of planeness as small as 0.25 mm can reduce apparent strength by one-third.
3. Capping with a thin layer (1.5–3 mm) of material such as sulphur or gypsum plaster restores a flat, parallel surface.
4. With a uniform contact, the platen load distributes evenly across the section.
5. Area, moisture and hydration are not the purpose of capping.
_Source: IGNOU ET-522 Concrete Technology, Block 2 Unit 9 "Testing of Concrete"._
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