6.2. Types of Road Aggregates

Aggregates are classified based on their source:

6.2.1 Based on Source

(a) Natural Aggregates

• Definition: Aggregates obtained directly from natural rock sources.
• Examples:
  • Gravel (river deposits)
  • Sand (natural sand)
  • Crushed Stone (granite, basalt, limestone)
• Advantages: Economical, widely available
• Limitations: May require washing and grading

(b) Artificial Aggregates

• Definition: Aggregates produced by industrial processes or modified from natural sources.
• Examples:
  • Crushed bricks
  • Slag aggregates (from steel industries)
  • Fly ash aggregates
• Applications: Low-traffic roads, secondary pavements

(c) Recycled Aggregates

• Definition: Aggregates obtained from demolished concrete, asphalt, or building waste.
• Use: Sustainable construction, sub-base, and base layers

Aggregates can be classified based on their source into three main categories. First, natural aggregates come directly from the earth, such as gravel from rivers or crushed stones from quarries. They are generally cost-effective and abundant but sometimes require additional processing like washing. Second, artificial aggregates are man-made, often from recycled materials like crushed bricks or industrial byproducts, such as slag from steel manufacturing. They are typically used in less trafficked road scenarios. Lastly, recycled aggregates are sourced from deconstructed buildings and can significantly promote sustainable construction practices.

Imagine building a sandcastle at the beach (natural aggregates) versus using building blocks (artificial aggregates) or upcycled materials from an old playground (recycled aggregates). Each material serves a different purpose based on where it comes from and its qualities.

6.2.2 Based on Size

(a) Coarse Aggregates

• Size: Retained on 4.75 mm sieve
• Use: Base course, bituminous concrete, cement concrete pavements
• Common Sizes: 10 mm, 20 mm, 40 mm

(b) Fine Aggregates

• Size: Passing through 4.75 mm sieve and retained on 75 µm sieve
• Use: Filler, mortar, fine matrix in bituminous mixes

(c) Filler Materials

• Size: Passing 75 µm sieve
• Examples: Stone dust, cement, lime
• Function: Improves gradation, fills voids, improves cohesion

Aggregates are also classified by their size. Coarse aggregates are larger particles, such as gravel, that are retained on a 4.75 mm sieve and are used in structural layers of roads. Fine aggregates are smaller and pass through this sieve, filling gaps between larger particles in mixtures. Filler materials are the smallest-sized aggregates, like stone dust, that help improve the mix’s properties by filling voids and enhancing cohesion.

Think of a jigsaw puzzle where coarse pieces are the large corner and edge pieces, fine pieces are smaller, and filler pieces are the tiny bits that fit in the gaps. Each piece is crucial to completing the puzzle (the pavement) effectively.

6.2.3 Based on Shape

• Rounded Aggregates: Found in riverbeds; less friction, used in concrete.
• Angular Aggregates: Crushed rocks; excellent interlock, used in road construction.
• Flaky Aggregates: Thickness is small compared to length and breadth.
• Elongated Aggregates: Length is considerably higher than other dimensions.
• Cubical Aggregates: Preferred for superior interlocking and stability.

The shape of aggregates significantly influences their performance in construction. Rounded aggregates, typically from river beds, lead to smoother surfaces but may not interlock well. Angular aggregates are rough and provide better interlocking, which is essential for the stability of roads. Flaky and elongated aggregates, while less desirable, can still be used in specific applications, though they're not preferred compared to cubical aggregates, which provide balanced interlocking and strength.

Consider how differently a soccer ball rolls on the ground versus a box—a round (rounded aggregate) ball rolls smoothly, while a block (cubical aggregate) can stack well and be stable, important for teamwork and strategy in the game.

6.2.4 Based on Geological Origin

(a) Igneous Rocks

• Examples: Granite, basalt
• Properties: Hard, strong, durable
• Use: High-load pavements

(b) Sedimentary Rocks

• Examples: Limestone, sandstone
• Properties: Moderate strength and durability
• Use: Low to medium traffic roads

(c) Metamorphic Rocks

• Examples: Quartzite, gneiss
• Properties: Very hard and dense
• Use: Suitable for heavy traffic roads

Aggregates can also be categorized based on their geological origin into three types: igneous, sedimentary, and metamorphic. Igneous aggregates like granite are highly durable and ideal for heavy-duty applications. Sedimentary aggregates, such as limestone, are often used for medium-traffic roads due to their lower strength. Metamorphic aggregates like quartzite are very dense and hard, making them suitable for roads that handle substantial loads.

Think of different types of rocks as tools in a toolbox: granite is like a hammer—strong and reliable for heavy work; limestone is like a screwdriver—useful but not as robust for demanding tasks; and quartzite is a specialized tool for specific, high-load needs, much like a torque wrench.

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