7.5. Selection Criteria for Aggregates in Pavement Construction

Strength Max. Abrasion Layer Requirements Shape Value Surface Course High Angular <30–35%

In pavement construction, the surface course requires high strength to withstand significant traffic loads. The aggregates used in this layer must be angular in shape, as angular aggregates provide better interlocking and structural integrity. Furthermore, the maximum abrasion value must be less than 30-35%, indicating that the aggregates should not wear down too quickly when subjected to friction and wear from traffic.

Imagine trying to build a sturdy tower using different types of bricks. If you use soft, round pebbles, they may roll and not hold together well under pressure. However, if you use sharp-edged stones that fit tightly together, just like angular aggregates in a pavement, your tower will be much more stable and durable.

Base Course Medium–High Angular <40%

The base course of a pavement also requires aggregates that are medium to high in strength. Similar to the surface course, these aggregates should be angular, which helps in achieving necessary support for the layers above. The maximum abrasion value in this case should be less than 40%, allowing for adequate resistance to wear while ensuring the layer remains stable under load.

Think of the base course as the foundation of a house. Just like how you would want strong, solid blocks for the foundation to support the weight of the entire house, the base course aggregates need to be strong and angular to effectively support the surface layer that will be laid on top.

Sub-Base Course Moderate Sub-angular <50%

For the sub-base course, aggregates are required to have moderate strength and should preferably be sub-angular rather than fully angular. This provides a balance between workability and stability. The maximum abrasion value allowable for the sub-base is less than 50%, indicating a slightly higher tolerance for wear compared to the surface and base courses, as this layer is not in direct contact with the heaviest traffic.

Imagine the sub-base course as a mattress under the sheets of a bed. It doesn't need to be as firm as the mattress you lie on, but it still needs to provide some support. Using moderate-strength, sub-angular aggregates is similar to choosing a soft, supportive mattress that complements the comfort of the sheets on top.

Bituminous High adhesion, Clean, dry <30% layers low stripping

In a bituminous layer, aggregates must ensure high adhesion with the bitumen. This means the aggregates should be clean and dry, helping to achieve better bonding with the adhesive material. Stripping value must be low (below 30%), which ensures that the bitumen does not peel away from the aggregates when exposed to moisture, thereby reducing the risk of pavement failure.

Think of the bituminous layer as a layer of glue used to attach tiles to a surface. If the tiles (aggregates) are dirty or wet, the glue won’t stick properly, and the tiles might peel off. Just like how using clean and dry tiles ensures a strong bond with the glue, using clean and dry aggregates ensures the bituminous layer stays intact on the road.

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