Brick quality — permissible water absorption for first-class bricks A first-class brick, after immersion in water for 24 hours, should not absorb more than what percentage (by weight) of water?

Introduction / Context:
Water absorption is a key quality parameter for burnt clay bricks. Low absorption correlates with good firing, dense structure, and durability. Standards classify bricks (first-class, second-class, etc.) using absorption limits.

Given Data / Assumptions:

• Test: immerse dried brick in water for 24 hours, then measure weight gain.
• Category: first-class building bricks.

Concept / Approach:
First-class bricks are high-quality units with sharp edges and uniform color. Their absorption is limited to control dampness, efflorescence risk, and strength loss. The typical specification caps absorption at 20% by weight for first-class bricks.

Step-by-Step Solution:

Verification / Alternative check:
Construction manuals list ≤ 20% for first-class bricks, with more stringent values for special engineering bricks.

Why Other Options Are Wrong:

• 10% or 15% (A/B): more stringent than typical first-class requirements; may apply to engineering bricks.
• 25% (D): too high for first-class; suggests under-firing or high porosity.
• 5% (E): unrealistically low for common burnt clay bricks.

Common Pitfalls:
Not surface-drying the specimen before weighing; water film inflates results. Also, testing only one brick can misrepresent a batch—always test a set and average.

Final Answer: