ROAD MATERIALS:
[1] SOIL: - Sub grade soil is an
integral part of the road pavement structure as it provides the support to the
pavement from beneath. The sub grade soil and its properties are important in
the design of pavement structures.
Function: - The main of sub grade
is to give adequate support to the pavement and for this the sub grade should
possess sufficient stability under adverse climate and loading conditions.
Different types of soil:-
[1] Clay Soil: - Clay is a fine-grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides
and organic matter. Clays are plastic due to their water content and become
hard, brittle and non–plastic upon drying or firing.
[2] Silt Soil: - Silt is granular
material of a size somewhere between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspensionwith water (also known as a suspended load) in a body
of water such as a river. It may also exist as soil deposited at the
bottom of a water body.
[3] Sandy Soil: - Sandy soils are granular soils that contain
small rock and mineral particles. The texture of sandy soils is
usually gritty. This soil feels
course whether it is dry or wet.
[4] Black Cotton Soil: - Black cotton soil is one of the major soil deposits of India. They exhibit high
swelling and shrinking when exposed to changes in moisture content and hence
have been found to be most troublesome from engineering considerations.
[5] Loam Soil: - Loam is soil composed mostly of sand (particle
size > 63 µm), silt (particle size >
2 µm), and a smaller amount of clay (particle size <
2 µm). Its composition is about 40%-40%-20% concentration of
sand-silt-clay, respectively. These proportions can vary to a degree, however,
and result in different types of loam soils: sandy loam, silty loam, clay loam,
sandy clay loam, silty clay loam, and loam
AGGREGATES: - Aggregates form the
major portion of pavement structure and they form the prime materials used in
pavement construction. Aggregates have to bear stresses occurring due to the
wheel loads on the pavement and on the surface course they also have to resist
wear due to abrasive action of traffic.
Properties of Road Aggregates
(i)
Strength: -The aggregates which
are to be used in top layers of the pavements, particularly in the wearing
course have to be capable of withstanding high stresses in addition to wear and
tear; hence they should possess sufficient strength resistance to crushing.
(ii)
Hardness:
-The
aggregates used in the surface course are subjected to constant rubbing or
abrasion due to moving traffic. Abrasive action may be increased due to the
presence of abrasing material like sand between the tyres of vehicle and the
aggregates exposed to the top surface. Thus, they should be hard enough to
resist the wear due to abrasive action of traffic.
(iii)
Toughness: -Aggregates
in the pavement are also subjected to impact due to moving wheel loads. The
magnitude of impact increase with roughness of road and speed of vehicle.
Severe impact is common when heavily loaded steel tyred vehicles move on WBM.
The resistance to impact or toughness is thus another desirable property of
aggregates.
(iv)
Durability: -The
aggregates are subjected to physical and chemical actions of rains and ground
water, the impurities in them and that of atmosphere. Thus it is desirable that
the road stones used in the construction should be sound enough to withstand
the weathering action. The property of aggregates to withstand the adverse
actions of weather may be called soundness.
(v)
Shape of Aggregate:-Aggregates
may be rounded, angular, flaky or elongated. Flaky and elongated particles have
less strength than rounded and cubical particles. Thus, too flaky and too much
elongated particles should be avoided.
(vi)
Adhesion with Bitumen:-The
aggregates in bituminous pavements should have less affinity with water when
compared with bitumen; otherwise the bituminous coating on the aggregates will
be stripped off in presence of water.
