REQUIREMENTS OF A GOOD ROAD AGGREGATE

REQUIREMENTS OF A GOOD ROAD AGGREGATE 

Following are the desirable properties or requirements of a good road aggregate: 

(1) Adhesion with bitumen: The aggregates which are to be used for the bituminous pavements should have less affinity with water as compared with the bituminous material. If this quality is absent in the aggregate, it will lead to the separation of bituminous coating in the presence of water.

(2) Cementation: The binding quality of the road aggregate depends on its ability to form its own binding material under traffic so as to make the rough broken stone pieces grip together to resist displacement by traffic. This quality is especially required in the case of water-bound macadam construction where the binding action of the surface layer is imparted by the stone dust and the film of water.

(3) Durability: The durability of an aggregate indicates its resistance to the action of weather and is largely dependent upon its petrological composition. The road metal is subjected to the oxidizing influence of air and rain water, it is therefore desirable that the road aggregate should possess sufficient soundness to resist the action of weather and age so that the life of road made
 with it may be prolonged.

(4) Hardness: The road aggregates should be reasonably hard to offer resistance to the actions of abrasion and attrition. The aggregates are always subjected to the constant rubbing action with the moving traffic. It is known as abrasion and it will be increased due to the presence of abrasive material like sand between the exposed top road surface and the tyres of moving vehicles.

(5) Shape: The shapes of aggregates may be rounded, cubical, angular, flaky or elongated. The flaky and elongated particles possess less strength and durability and their use in the road construction should be avoided as far as possible. The rounded particles are preferred in cement Concrete roads. But they are unsuitable in W.B.M. construction, bituminous construction and in granular base course because their stability due to interlocking is less. The angular particles are preferred in such types of construction.

(6) Strength: The road aggregates should be sufficiently strong to withstand the stresses developed due to the wheel loads of the traffic. This property is especially desirable for the road aggregates which are to be used in top layers of the pavements. Thus, the wearing course of road should be composed of aggregates which possess enough strength in addition to enough resistance to crushing.

(7) Toughness: The toughness of an aggregate is that property which enables the aggregate to resist fracture when struck with a hammer and it is necessary in a road metal to withstand the impact blows caused by traffic. It is desirable that the road aggregate is reasonably tough.