Highway Construction: Desirable properties of aggregates

Desirable properties of aggregates

Strength

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 posses high resistance to crushing, and to withstand the stresses due to traffic wheel load.

Hardness

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 moves over the aggregates exposed at the top surface.

Toughness

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 causes severe impact on the aggregates.

Shape of aggregates

Aggregates which happen to fall in a particular size range may have rounded, cubical,

angular, aky 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.

Adhesion with bitumen

The aggregates used in bituminous pavements should have less affinity with water when

compared with bitu-minous materials, otherwise the bituminous coating on the aggregate

will be stripped o in presence of water.

Durability

The property of aggregates to withstand adverse action of weather is called soundness.

The aggregates are sub-jected to the physical and chemical action of 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 Freedom from deleterious particles

Specifications for aggregates used in bituminous mixes usually require the aggregates to

be clean, tough and durable in nature and free from excess amount of at 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.