Brick strength classification: According to common building materials standards, what is the minimum compressive strength specified for second-class (II class) burnt clay bricks?

Introduction / Context:
Bricks are graded by compressive strength, dimensional tolerances, and appearance. Knowing the minimum strength helps designers specify appropriate classes for load-bearing or non-load-bearing masonry.

Given Data / Assumptions:

• Question refers to typical textbook/MCQ values for second-class bricks.
• Units are in kg/cm² (1 MPa ≈ 10.197 kg/cm²).

Concept / Approach:
Common exam references state that first-class bricks have a compressive strength around 100–105 kg/cm² or higher, while second-class bricks are typically specified at about 70–75 kg/cm² minimum. Hence among the given choices, 75 kg/cm² is the accepted minimum for II class bricks.

Step-by-Step Solution:

Verification / Alternative check:
Many handbooks and solved question banks align II class bricks near 7 MPa (≈ 71–75 kg/cm²), confirming the choice.

Why Other Options Are Wrong:

• 90/100/120/150 kg/cm²: these values correspond to first-class or special bricks; too high for the minimum of II class.

Common Pitfalls:

• Mixing up MPa and kg/cm² leading to numerical mis-selection.
• Assuming the same minimum for both I and II class bricks.

Final Answer:
75 kg/cm²