BITUMINOUS MATERIALS | WHAT IS BITUMEN | WHAT IS TAR | CONSTRUCTION MATERIALS

BITUMINOUS MATERIALS

Bituminous materials may be broadly classified as bitumen and tar.

 

BITUMEN

Bitumen is a hydrocarbon material of either natural or pyrogenous origin, found in gaseous, liquid, semi-solid or solid form. It is a complex organic material that is completely soluble in carbon disulphide. It occurs either naturally or may be obtained during the distillation of petroleum. Bitumen consists of colloidal hydrocarbon materials of asphalte nets, resins and oils. Bitumen may be further divided as native asphalt or petroleum asphalt (generally called bitumen).

 

1. Native Asphalt

Native asphalts are those which occur in a pure or nearly pure state in nature. These are further designated as (i) Lake asphalt, (ii) Asphaltites and (iii) Rock asphalt based on their origin of occurrence.

 

(i) Lake Asphalt

These are found in depressions on the earth’s surface which have accumulated in lakes. Most of the lake asphalts possess moisture in some form. The refined asphalt has very low penetration. This material can be softened by fluxing with a petroleum flux. This may be blended with asphalts produced from petroleum to produce blended asphalts.

 

(ii) Asphalites

This is an asphalt-like substance presumably derived from the metamorphosis of petroleum. It is hard, brittle and practically a pure bitumen. These types of asphalts are further divided into three classes, viz., gilsonite, glance pitch and grahamite. It is obtained from crevices and seams in the rock formation.

 

(iii) Rock Asphalts

These are deposits of limestone or sandstone naturally impregnated with asphalt. This is extensively available in many parts of the world. The asphalt content in the rocks is about a maximum of 20% and varies from soft to hard.

 

2. Petroleum Asphalts

Petroleum asphalts are derived from petroleum crude oil which is taken from oil wells. Crude oil is transported to refineries where it is separated into various components. The character of asphalt depends on the nature of crude oil. Crude petroleum is divided into three classes, viz., asphaltic base, semi-asphaltic base and paraffin base.

 

(i) Cutback Bitumen

When the viscosity of bitumen is reduced by a volatile dilutant, the bitumen is called cutback bitumen. The viscosity of the cutback and the rate at which it hardens depends on the characteristics and quantity of both the bitumen and volatile oil.

 

(ii) Bituminous Emulsion

When the bitumen is suspended in a finely divided condition in an aqueous medium and an emulsifier is used to stabilise, the end product is known as a bituminous emulsion.

Bituminous materials are widely used in highway construction because of their dual qualities, viz., binding property and water proofing property.

The use of bitumen for road construction depends on the type of mix and the type of construction.

The following are the desirable properties bitumen should have in paving mixes:

(i) Easy to get mixed,

(ii) Attainment of desired stability,

(iii) Should maintain stability even under adverse weather condition,

(iv) Adequate flexibility to avoid cracking,

(v) Sufficient adhesion with aggregates.

 

TAR

Tar is the viscous liquid produced when natural organic materials such as coal, petroleum, wood, etc., are carbonised. Depending on the material from which tar is obtained, it is called wood tar or coal tar. Coal tar is used mostly for road work because of its superior quality. Road tar is produced by undergoing three stages, viz., carbonisation of coal to produce coal tar, refining or distillation of crude tar and blending of distillation residue with distillate oil fraction.

There are five grades of road tar, viz., RT-1 to RT-5 based on their viscosity and other properties. Uses of different grades of road tar are given below.

RT-1: Used for surface painting under adverse cold conditions.

RT-2: Used for standard surface painting under normal weather conditions.

RT-3: Used for surface painting, renewal coats, pre-mixing chips for the top course and light carpets.

RT-4: Used for pre-mixing tar macadam in base course. RT-5: Used for grouting.

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