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.
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