22. Pavement materials: Aggregates

Aggregate is a collective term for the mineral materials such as sand, gravel, and crushed stone that are used with a binding medium (such as water, bitumen, Portland cement, lime, etc.) to form compound materials (such as bituminous concrete and Portland cement concrete). By volume, aggregate generally accounts for 92 to 96 percent of Bituminous concrete and about 70 to 80 percent of Portland cement concrete. Aggregate is also used for base and sub-base courses for both flexible and rigid pavements. Aggregates can either be natural or manufactured. Natural aggregates are generally extracted from larger rock formations through an open excavation (quarry). Extracted rock is typically reduced to usable sizes by mechanical crushing. Manufactured aggregate is often a byproduct of other manufacturing industries. The requirements of the aggregates in pavement are also discussed in this chapter.

This section defines what aggregates are and their significance in pavement materials. Aggregates are essentially raw materials like sand and gravel that are combined with binding agents to create concrete. They make up a large portion of concrete mixtures, providing strength and stability. There are two types of aggregates: natural aggregates, which come from quarries, and manufactured aggregates, which are byproducts from other processes. Finally, this section emphasizes the importance of aggregates in both flexible and rigid pavement construction.

Think of aggregates like building blocks in a structure. Just like a house needs bricks or blocks to stand strong, pavements require aggregates to ensure they hold up under traffic. Without these key elements, the pavement would crumble and become unusable.

The aggregates used in top layers are subjected to (i) Stress action due to traffic wheel load, (ii) Wear and tear, (iii) Crushing. For a high-quality pavement, the aggregates should possess high resistance to crushing, and to withstand the stresses due to traffic wheel load.

Aggregates need to be strong to cope with the pressures exerted by vehicles driving over them. The section outlines how aggregates face stress from the weight of traffic, wear from movement, and crushing from pressure. Therefore, choosing aggregates with high strength helps ensure the durability and longevity of pavements.

Imagine the wear and tear on the ground where you park your car. Over time, the constant weight can cause the surface to crack or degrade. If you use strong and durable materials, like concrete made with tough aggregates, the surface will last much longer, just like using high-quality tires on a vehicle ensures their longevity.

Aggregates which happen to fall in a particular size range may have rounded, cubical, angular, flaky or elongated particles. It is evident that the flaky and elongated particles will have less strength and durability when compared with cubical, angular, or rounded particles of the same aggregate. Hence too flaky and too much elongated aggregates should be avoided as far as possible.

The shape of aggregates plays a crucial role in determining the strength and durability of the pavement. Aggregates that are flaky or elongated can create weaker pavements compared to those that are cubical or rounded, which interlock better to resist movement and deterioration. Therefore, it is essential to select appropriate shapes for optimal pavement performance.

Picture trying to build a stable structure with playing blocks. If you use irregularly shaped ones, like long and thin pieces, they might not stack well or hold together. However, if you use blocks that fit together nicely, like cubes, the structure will be much more stable and stronger—just like quality aggregates help build durable pavements.

The aggregates used in bituminous pavements should have less affinity with water when compared with bituminous materials, otherwise, the bituminous coating on the aggregate will be stripped off in presence of water.

In pavements where bitumen is used as a binder, aggregates must be chosen carefully based on their interaction with water. If aggregates attract too much water, the bitumen won't adhere properly, leading to degradation and failure of the pavement. Thus, selecting aggregates with appropriate affinity for water ensures that the bituminous coating remains intact.

Think of it like spreading butter on toast. If the toast is wet, the butter won’t stick well, right? Similarly, if aggregates are too water-friendly, they won't hold onto the bitumen well, creating weak points in the pavement.

In order to decide the suitability of the aggregate for use in pavement construction, following tests are carried out: Crushing test, Abrasion test, Impact test, Soundness test, Shape test, Specific gravity and water absorption test, Bitumen adhesion test.

Before using aggregates in pavement construction, several standardized tests are performed to ensure their quality. Each test evaluates different properties such as strength, hardness, and adhesion to bitumen. This testing process helps guarantee that only the best materials are used in pavements, ultimately affecting their durability and performance.

Just as a chef tastes ingredients before cooking to ensure they are fresh and suitable for the dish, engineers test aggregates to confirm they meet the necessary standards for pavement construction.

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