different Properties of Aggregate

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. Aggregate can either be natural or manufactured. Natural Aggregates are generally extracted from larger rock formations through an open excavation. Extracted rock is typically reduced to usable sizes by mechanical crushing. Manufactured aggregate is often a bye product of other manufacturing industries.

➤Strength:

The aggregates used in top layers are subjected to
➝Stress action due to traffic wheel load,
➝Wear and Tear,
➝crushing. for a high quality pavements, the aggregates should posses high resistance to crushing,    and to withstand the stresses due  to traffic wheel load.

➤Hardness:

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

➤Toughness:

Resistance of the aggregate to impact is termed as toughness. aggregate 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 causes severe impact on the aggregate.

➤Durability:

The property of aggregates to withstand adverse action of weather is called soundness. The aggregates are subjected to the physical and chemical action pf rain and bottom water, impurities there-in 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.

➤Shape of aggregate:

Aggregate 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.
➝Rounded aggregates are preferred in cement concrete mix.
➝Angular aggregates are preferred in bituminous construction.

➤Adhesion with bitumen:

The aggregate 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.