JNTU B.Tech II BUILDING MATERIALS CONSTRUCTION AND PLANNING Previous Question Paper (July/August - 2021 - R13)

The selection of stones for civil engineering works is based on a balance between aesthetic requirements, structural performance, and economic viability. The following characteristics are critical for evaluating the suitability of stone:

1. Crushing Strength: The stone must possess sufficient compressive strength to support the intended loads. For structural applications, a minimum crushing strength of 100 N/mm² is typically required.
2. Durability: The material must withstand environmental factors such as temperature variations, frost, and chemical action. Stones should be free from minerals that decompose or expand when exposed to moisture.
3. Hardness: This is the resistance to abrasion or wear. For flooring, paving, or heavy traffic areas, a high degree of hardness is essential to prevent surface degradation.
4. Appearance and Color: For facing work, the stone should have a uniform color, be free from streaks or spots, and be capable of retaining its luster after polishing.
5. Specific Gravity: A higher specific gravity (typically above 2.4) generally indicates a denser and more compact stone, which is preferred for heavy engineering works like dams, retaining walls, and bridge piers.
6. Porosity and Absorption: Stones with high porosity absorb water, which leads to weathering, frost damage, and the growth of vegetation. A good building stone should have a water absorption percentage of less than 5% by weight.
7. Workability: The stone should be easy to dress, cut, and carve into required shapes. While harder stones are durable, they increase labor costs significantly due to the difficulty of shaping.
8. Seasoning: Freshly quarried stone contains 'quarry sap' which makes it soft. Stones must be properly seasoned to allow this moisture to evaporate, increasing the stone's hardness and strength before use.
9. Resistance to Fire: Stones should be able to withstand high temperatures without cracking or disintegrating. Siliceous stones generally offer better fire resistance than calcareous stones.
10. Cost and Availability: The stone should be readily available in the vicinity of the construction site to minimize transportation costs, which often constitute a significant portion of the total project budget.

Brick earth is composed of several key constituents, each serving a specific function in the manufacturing and quality of bricks:

• Silica (50-60%): Provides the structural skeleton of the brick. It prevents cracking, shrinking, and warping during drying. Excess silica makes the brick brittle and weak.
• Alumina (20-30%): Acts as the binding agent. It imparts plasticity to the earth, allowing it to be molded into shapes. Excess alumina causes the brick to shrink and crack upon drying.
• Iron Oxide (5-6%): Acts as a flux during burning, lowering the fusion temperature. It is responsible for the characteristic red color of bricks and improves durability.
• Magnesia (less than 1%): Acts as a flux and helps in reducing shrinkage. It imparts a yellow tint to the brick.
• Lime (less than 5%): Acts as a flux and prevents shrinkage. It must be finely divided; if present in lumps, it causes the brick to disintegrate upon slaking after burning.

Harmful substances in brick earth that must be avoided or minimized include:

• Lime lumps: Cause the brick to crack or crumble when moisture causes the lime to slake and expand.
• Iron Pyrites: Cause the brick to crystallize and split during burning, resulting in a porous and weak structure.
• Alkalies: Act as a flux, causing the brick to melt, warp, and lose its shape during high-temperature firing. They also form white efflorescence on the surface after the brick dries.
• Pebbles, Gravel, and Organic Matter: Prevent the formation of a homogeneous mass, leading to weak spots and uneven burning. Organic matter may burn away, leaving voids that reduce the strength of the brick.
• Sulphates: Can lead to the formation of efflorescence and may cause the brick to disintegrate over time due to chemical reactions.