The roads having their
surface consisting of bituminous materials are known as bituminous roads or
black-top roads. The bituminous binders used in the construction of bituminous
roads are either straight-run bitumen, road tar, cut back or emulsion. The choice
of particular binder depends upon the type of construction, availability of
material, equipments, climatic conditions etc. The bitumen and tar require
heating to bring them to proper viscosity before their use. The construction
technique using these materials is known as hot mix technique. The construction
technique using cut-back and emulsion is called cold mix technique, since the
bituminous materials are applied cold. These types of roads are considered as
high cost roads.
Components
of bituminous surface and their functions
The bituminous surface
rests on a base course. A layer known as levelling course may be provided to
make the base course a level surface. The various components of bituminous
surface road as follows.
Blinder
Course
It is a layer of
bituminous material provided in between the base course and surface course. It serves
as stress distribution medium and provides bed for surface course.
Surface
Course
It is a layer of
bituminous material provided on the top of binder course. It is also known as
wearing course. This course is directly exposed to traffic. This course
provides a good riding surface and bears safely the wheel pressure due to
traffic.
Seal
Coat
It is a final course of
bituminous material provided on the top of surface course for scaling the voids
with the purpose to provide water tightness, desirable texture, to reduce slipperiness
and to enliven an existing surface.
In addition to the
components of a bituminous surface the following two coats are also recommended
to achieve proper bond.
Prime
Coat
It is provided to an
existing untreated road surface for the following purpose wise to provide bond
between existing surface and wearing surface, to bind any loose aggregate, to
plug the rise of moisture and to provide a temporary seal. This is done with a
low viscosity bituminous material.
Tack
Coat
This is provided with a
purpose to ensure adequate bond between the existing impervious base and
wearing surface. This is done by providing low viscosity bituminous binder to
an existing treated base.
Types
of Bituminous Roads
These are classified on
the basis of their methods of construction.
Inverted penetration
type of construction. Ex- Surface dressing
Penetration type of
construction. Ex- Premix macadam
Premix type of
construction. Ex- Premix macadam, premix carpet, asphaltic concrete, sheet
asphalt or rolled asphalt and mastic asphalt.