NTS STUDY

🏗️ Criteria for Selection of Building Stones

The selection of stones must be based on their intended use, durability, and freedom from defects. Whether it is for a temple's decorative carving or a harbor's heavy foundation, each stone must meet specific engineering standards.

1. Application-Based Selection

• Masonry Work: For rubble masonry, almost any durable stone can be used. However, for Ashlar work (fine finish), softer stones like Limestone and Sandstone are preferred as they are easier to dress compared to Granite.

• Heavy Engineering: For docks, harbors, and bridge piers, high-strength Granite is the top choice.

• Flooring: * Heavy Duty: Granite slabs are used for high-traffic areas.

  • Residential: Marble is preferred for bathrooms and interiors due to its ability to take a high polish and its pleasing colors.

  • Kitchens & Shelves: Kadappa slabs and Kota stone are popular choices in India.

• Concrete Aggregates: Hard igneous rocks (Granite/Basalt) are used for High-Strength/Pre-stressed concrete, while Limestone is suitable for medium-strength concrete.

2. Key Engineering Characteristics (Technical Properties)

A. Crushing Strength (Compressive Strength)

The ultimate strength of the stone must be significantly higher than the load it will carry.

Note: For comparison, a standard hand-made brick in India has a strength of only 2 to 10 N/mm^2.

B. Durability & Weathering

Stones used in exposed conditions must resist environmental decay. A specific gravity of more than 2.7 and water absorption of less than 0.6% are indicators of a high-quality, dense stone.

C. Fire Resistance

Surprisingly, softer stones like Limestone resist fire better (up to 800°C) than Granite. Granite contains quartz, which can burst or crack at temperatures above 600°C.

D. Seasoning (Quarry Sap)

Many stones contain "Quarry Sap" (natural moisture) when freshly extracted.

• Laterite is a unique example: it is soft when quarried but hardens only after exposure to the atmosphere (oxidation of iron compounds).

• Stones must be seasoned (dried) before use to ensure maximum strength.

E. Toughness & Hardness

• Impact Test: A value of 19 is excellent; below 13 indicates poor quality.

• Hardness: A value above 17 is good for road works; below 14 is considered poor.

💡 NTS Study Pro-Tips for Site Engineers

1. Visual Check: Always check for a "ringing sound" when struck with a hammer; a dull thud often indicates internal cracks or high porosity.

2. Texture: Fine-grained, compact textures generally indicate better durability than coarse-grained structures.