A 1st class brick immersed in water for 24 hours, should not absorb water (by weight) more than
💧 Understanding Water Absorption Limits
Water absorption is a measure of a brick's porosity and is a key indicator of its quality and durability. The test, specified in IS 3495 (Part 2), involves immersing a dry brick in water for 24 hours. The amount of water absorbed is then expressed as a percentage of the brick's original dry weight (referred to as "by weight" in the question). A lower water absorption percentage signifies a denser, stronger brick that is more resistant to weathering.
🔬 Detailed Analysis of the Options
C. 20%
This is the correct answer. According to Indian Standards, the water absorption of a first-class brick should not be more than 20% of its dry weight. This limit ensures the brick has low porosity and high durability, making it suitable for high-quality, load-bearing construction.
D. 22%
This is incorrect. A maximum water absorption of 22% is the specified limit for a second-class brick.
A. 25%
This is incorrect. A maximum water absorption of 25% is the specified limit for a third-class brick.
B. 18%
This is incorrect. While a brick with 18% water absorption would certainly qualify as a first-class brick (since 18% is less than 20%), the question asks for the maximum permissible limit as per the standard, which is 20%.
📊 Summary: Water Absorption Limits by Brick Class (IS 3495)
| Brick Class | Maximum Water Absorption (% of Dry Weight) |
|---|---|
| First Class | 20% |
| Second Class | 22% |
| Third Class | 25% |
💡 Study Tips
- Memorize the "20-22-25" Rule: This is a fundamental set of numbers in building materials. Associate 20% with 1st class, 22% with 2nd class, and 25% with 3rd class bricks.
- "By Weight" = Dry Weight: In the context of material testing, "by weight" or "self weight" refers to the dry weight of the sample before testing.
- Lower is Better: For both water absorption and many other tests, a lower value generally indicates a higher quality material.
- Know the Code: Remember that the method for testing water absorption is detailed in IS 3495.