22.2. Desirable properties

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

This chunk explains that the aggregates (the materials like sand, gravel, etc.) that are used in the surface layers of pavements need to be strong. They face pressures from heavy vehicles driving over them which can crush them. Therefore, the materials must be high-quality and strong enough to endure this crushing and the weight of traffic without breaking down.

Think of aggregates like the bones in our body. Just as our bones need to be strong to support our weight and allow us to move without breaking, aggregates must be strong to support the heavy vehicles that drive over them.

The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic. The aggregates should be hard enough to resist the abrasive action caused by the movements of traffic. The abrasive action is severe when steel-tyred vehicles move over the aggregates exposed at the top surface.

This section emphasizes the need for hardness in aggregates. The surface aggregates are continually worn down by the friction caused by vehicles. If the aggregates are not hard enough, they can be easily worn away, leading to a rough and damaged surface. Stronger aggregates can resist this rubbing and last longer under heavy use.

Imagine a sidewalk made of soft clay versus one made of hard concrete. The clay will wear away quickly under the feet of pedestrians, while the concrete remains intact for many years. Similarly, hard aggregates help ensure a durable pavement.

Resistance of the aggregates to impact is termed as toughness. Aggregates used in the pavement should be able to resist the effect caused by the jumping of the steel-tyred wheels from one particle to another at different levels, which causes severe impact on the aggregates.

Here, toughness refers to the ability of aggregates to withstand impacts. When vehicles travel uneven surfaces, their steel wheels can hit the aggregates hard, which may cause fractures in weaker aggregates. Therefore, aggregates need to be tough, meaning they should be able to absorb and withstand impacts without splitting or breaking.

Consider a rubber ball versus a glass ball. The glass ball is rigid but breaks easily on impact, while the rubber ball can take a lot of bouncing around without breaking. Tough aggregates are like the rubber ball, built to endure impact.

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 significantly affects their performance in pavement. Rounded aggregates can pack tightly together, providing strength and stability. In contrast, flaky or elongated shapes may lead to poor interlock and weaker surfaces, making the pavement less durable. Therefore, it is preferred to use aggregates that have more angular or rounded shapes for better stability.

Think of a jigsaw puzzle. Pieces that have more edges fit together better and create a stronger image compared to oddly shaped pieces that don't fit well. Similarly, well-shaped aggregates contribute to a stronger and more durable pavement.

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 this part, the relationship between aggregates and bitumen (the binding agent) is discussed. For the pavement to be effective, the aggregates should repel water. If they attract water, it can wash away the bitumen that holds them together, therefore weakening the pavement structure.

Imagine trying to get glue to stick on wet paper. The water prevents the glue from adhering properly, leading to a weak bond. Likewise, aggregates need to stay dry to maintain a strong bond with bitumen.

The property of aggregates to withstand adverse action of weather is called soundness. The aggregates are subjected to the physical and chemical action of rain, bottom water, impurities therein, and that of atmosphere; hence it is desirable that the road aggregates used in the construction should be sound enough to withstand the weathering action.

This section conveys the importance of durability. Aggregates should not only withstand traffic but should also resist weather conditions such as rain or extreme temperatures. If they are not durable, they can break down over time due to these environmental factors, compromising the stability of the pavement.

Think about how certain materials like wood can decay when exposed to the rain. Durable aggregates are like resistant materials that don't break down easily, ensuring the pavement lasts in different weather conditions.

Specifications for aggregates used in bituminous mixes usually require the aggregates to be clean, tough, and durable in nature and free from excess amounts of flaky or elongated pieces, dust, clay balls, and other objectionable material. Similarly, aggregates used in Portland cement concrete mixes must be clean and free from deleterious substances such as clay lumps, chert, silt, and other organic impurities.

For concrete and bituminous mixes to perform well, the aggregates must be free of unwanted elements known as deleterious particles. These contaminants can weaken the overall strength of the pavement and lead to premature failure. Thus, aggregates must be thoroughly cleaned and inspected before use.

Just like how you want to make cookies with clean, quality ingredients, we need quality aggregates in pavement construction. Any odd bits (like dirt or bad ingredients) can ruin the final product's taste or strength.

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