Aggregate is a collective term for the mineral materials such as gravel, sand and crushed stone, which are used along with a binding medium such as water, bitumen, cement, lime, etc to form compound materials. Some properties of aggregates are toughness, hardness, strength, etc.
Aggregate should be :
1)Strong and hard so that the stronger, harder aggregate will give a stronger final concrete. Never use a crumble or flakey rock like sandstone.
2)Durable to stand up to wear and tear and weathering.
3)Chemically inactive so the aggregate does not react with the cement.
Properties of aggregates
Road aggregates should process the following:
Note: Generally elongated and flaky aggregates are avoided because of it’s less strength.
The aggregate to be used in road construction should be sufficiently strong to withstand wear and tear crossing and spaces due to traffic wheel load. The aggregate to be used in the top layer particularly in wearing course should withstand high stress and should withstand high stress and should pass high resistance to crushing, in addition, to wear and tear.
The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic so the aggregate should be hard enough to resist wear and tear due to abrasive action of traffic.
Resistance to impact is termed as toughness. Aggregates used in the pavement are also subjected to impact due to moving wheel loads and hammering action due to jumping of tyres. Hence, toughness is another desirable property of road aggregate.
The property to withstand adverse action of weather or ability to remain strong over a long period of time is called durability or soundness. Aggregates are subjected to the physical and chemical action of rain and groundwater. The impurities therein and that of the atmosphere. Hence, it is desirable that the aggregates used in road construction should be durable to withstand weathering action.
e.Shape of aggregate
Aggregates may be of different shapes such as cubical, rounded, flaky, elongated, etc. Flaky and Elongated aggregates have lower strength. So, generally round and angular are preferred.
fig: the shape of aggregate
f.Adhesion with bitumen
Adhesion simply means a binding property with other materials. So, aggregate should have proper or good binding properties with bitumen.
g.A free from foreign particles
Foreign materials like dust, chemicals, etc reduce the binding property of aggregate i.e. adhesion between aggregate and bitumen is reduced. So, the material should be free from foreign materials.
( For properties of good building stones – Click here)