In the Vicat apparatus test for setting time of cement, the initial set is said to have occurred when
- Athe Vicat needle penetration is 0 mm from the bottom of the mould
- Bthe Vicat needle penetration is 5 to 7 mm from the bottom of the mouldCorrect
- Cthe Vicat needle penetration is 25 to 35 mm from the bottom of the mould
- Dthe needle does not leave an impression
Solution
Concept
Vicat initial setting time penetration depth
Formula
Penetration = 5 to 7 mm from base
Step-by-step solution
1. The setting time of standard cement paste is determined using the Vicat apparatus conforming to IS 4031 (Part 5). 2. The initial setting time test utilizes a standard 1 mm square needle released into a paste mould of 40 mm depth. 3. Initial setting occurs when the needle fails to pierce completely and penetrates to a point 5 to 7 mm from the bottom of the mould (or 33 to 35 mm from the top). 4. Therefore, option B correctly specifies the standardized penetration depth. Answer: B
Explanation
In accordance with IS 4031 (Part 5), the initial setting time of cement paste is the duration between initial gauging with water and the moment when the standard 1 mm square Vicat needle penetrates to a depth of 5 to 7 mm from the bottom of the Vicat mould. At this transition point, the paste loses sufficient plasticity to resist complete needle penetration.
Why the other options are wrong
A) 0 mm penetration indicates that the needle pierces completely to the glass plate, representing fresh uncemented paste. C) 25 to 35 mm from bottom is an incorrect depth reading. D) No impression is the criterion for final setting time using the annular attachment needle, not initial setting time.
Shortcut
Vicat initial set = Needle penetrates to 5 to 7 mm from bottom of mould (or 33 to 35 mm from top).
Common mistake
Confusing depth from bottom (5 to 7 mm) with depth from top surface (33 to 35 mm).
Correct answer: B) the Vicat needle penetration is 5 to 7 mm from the bottom of the mould