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.