4.4 Stone Aggregates|Types|Requirements

  

Stone aggregates are fundamental materials extensively utilized in the construction and maintenance of highways. These aggregates, sourced from natural rock formations or manufactured through crushing processes, play a crucial role in providing structural integrity and stability to the highway. Their use contributes significantly to the load-bearing capacity of the highway, ensuring that it can withstand the pressure exerted by heavy traffic and environmental factors.

Types of stone aggregate

A. Based on shape

1. Round Aggregate

The rounded aggregates are completely shaped by natural weathering and are available in the form of river bed gravel. They cannot provide high strength road pavement as well as in case of concrete, because of poor interlocking behavior and weak bond strength.

2. Angular Aggregate

The angular aggregates consist well defined edges and these are obtained by crushing the rocks. They give 10 – 20% more strength than that of rounded aggregates, due to development of stronger bond between aggregates and fine particles.

3. Flaky Aggregate

When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate or in the other words, when the least dimension of aggregate is less than the 60% of its mean dimension, then it is said to be flaky aggregate

4. Elongated Aggregate

When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension.

B. Based on size

a. Fine Aggregate

When the aggregate is sieved through 4.75 mm sieve, the aggregate passed through it called as fine aggregate. It includes material such as natural sand, crushed stone sand, crushed gravel sand, etc.  The purpose of the fine aggregate is to fill the voids in the coarse aggregate and facilitate the transfer of loads from one grain to another grain of coarse aggregates.

b. Coarse Aggregate

When the aggregate is sieved through 4.75 mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under coarse aggregate category. The coarse aggregates are responsible for taking and transfer of load applied from the top course.

Requirement of Stone Aggregates

• They must be crushed aggregate.
• They shall be clean, hard, durable and angular in shape.
• They must be free from the dust, organic substances and other deleterious matter.
• They not be flakey or elongated.
• They must not consist of injurious or harmful materials which reduce the strength of structure.
• They should resist wear and tissue due to abrasive action of traffic.

Requirements/Properties Stone Aggregates:

1. Strength: The aggregates which are to be used in top layer of the pavements, particularly in the wearing course shall have to be capable of withstanding high stresses in addition to wear and tear, because of this they should possess sufficient strength to crushing.
2. Hardness: The aggregates used in the surface course are subjected to be constant rubbing or abrasion due to moving vehicle. Stone should be hard enough to resist the wear due to the abrasive action of traffic. Heavy wheel loads can also cause deformations of pavement resulting in relative movement of aggregates and rubbing of aggregates with each other in the pavement.
3. Toughness: Resistance of the aggregates to impact loads called toughness. Severe impact like hammering is quite common when heavily loaded steel tyred vehicles move on water road surface, where stones break out.
4. Durability: The ability of stone aggregates to withstand the applied load as well as various environmental impacts on it is called durability of materials. The stone used in the pavement construction should be strong enough and durable and should resist disintegration due to the action of weather.
5. Adhesion with Bitumen: The aggregates used in bituminous pavements should have less tendency with water when compared with bituminous materials, otherwise the bituminous coating on the aggregate will be stripped off in presence of water.
6. Free from deleterious materials: Specifications for aggregates used in bituminous mixes should be clean, tough and durable in nature and free from excess amount of flat or elongated stone, dust, clay balls.