JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year BUILDING MATERIALS, CONSTRUCTION AND PLANNING Previous Question Paper (July/August - 2021 - R16)

The selection of building stones for civil engineering works is critical for ensuring the durability, structural integrity, and aesthetic appeal of a project. The primary characteristics to consider are:

1. Strength: Stones must possess high compressive strength to withstand the loads imposed by the structure. For heavy structures, a minimum crushing strength of 100 N/mm² is typically required.

2. Durability: The stone must resist weathering agents such as rain, frost, wind, and chemical pollutants. It should not contain minerals that decompose or expand upon exposure to the atmosphere.

3. Hardness: For flooring and paving, stones must exhibit high resistance to abrasion and wear caused by friction or impact. This is often measured using the Mohs scale of hardness.

4. Specific Gravity: A higher specific gravity generally indicates a denser and stronger stone. For most structural works, a specific gravity greater than 2.4 is preferred.

5. Porosity and Absorption: Stones with low porosity are less prone to water absorption. High absorption leads to the entry of moisture and salts, which can cause internal stresses and disintegration during freeze-thaw cycles.

6. Appearance: For facing works and architectural elements, the stone should have a uniform color, texture, and the ability to take a fine polish without showing cracks or veins.

7. Workability: The stone should be easy to quarry, dress, and carve into desired shapes. Excessive hardness can increase labor costs, while extreme softness may limit its use in load-bearing applications.

8. Seasoning: Stones should be free from quarry sap when used in construction. Proper seasoning allows the stone to harden and reach its maximum strength before being placed in a structure.

9. Cost and Availability: The proximity of the quarry to the construction site is essential to minimize transportation costs. The stone should be available in sufficient quantities to ensure uniformity throughout the project.

Brick earth is composed of several constituents, each serving a specific function in the manufacturing process and the final quality of the brick:

• Silica (50-60%): Prevents cracking, shrinking, and warping of raw bricks. It provides durability and ensures the brick retains its shape; however, an excess makes the brick brittle.
• Alumina (20-30%): Acts as the binding agent. It imparts plasticity to the earth, allowing it to be molded into shape. Excess alumina causes the brick to shrink and crack upon drying.
• Iron Oxide (5-6%): Acts as a flux to lower the fusion point of silica and provides the characteristic red color to the bricks upon burning.
• Magnesia (less than 1%): Influences the color (often giving a yellow tint) and acts as a flux, reducing shrinkage during burning.
• Lime (less than 5%): Acts as a flux, promoting the fusion of silica. It also prevents shrinkage. If present in excess, it causes the brick to melt and lose shape.

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

• Lime nodules: If present in lumps, they cause the brick to disintegrate when moisture enters, as the lime slakes and expands.
• Iron pyrites: These cause the brick to crystallize and split during the burning process, often leading to the release of sulfurous gases.
• Alkalies: These act as flux and cause the brick to melt, warp, and twist during burning. After construction, they may effloresce, leading to white patches and surface decay.
• Pebbles, gravel, and organic matter: These prevent the formation of a homogeneous mass, resulting in weak, porous bricks that are prone to structural failure.
• Sulfates: These can cause the brick to swell and disintegrate, particularly when exposed to damp conditions.