Bituminous Materials: Asphalt, Bitumen and Tar

 

Bituminous Materials

Bituminous materials are dark brown or black, semi-solid or liquid, thermoplastic mixtures of hydrocarbons derived from natural or synthetic processes in which hydrocarbon mixtures have lost their volatile components leaving a denser residue.

The different types of Bituminous Materials are:

1.    Asphalt

2.    Bitumen

3.    Tar

 

1. Asphalt -

The asphalt is a mechanical mixture of inert mineral matter like alumina, lime, silica, etc. and the asphaltic bitumen. It is black or brownish black in colour. It remains in solid state at low temperature and becomes liquid at a temperature of about 50°C to 100°C.

 

CLASSIFICATION OF ASPHALT -

The asphalt is classified into the following tee categories:

1. Natural asphalt

2. Residual asphalt.

 

1. Natural asphalt -

This variety of asphalt is obtained from nature. It is further subdivided into two groups, namely, lake asphalt and rock asphalt.

The lake asphalt is obtained from lakes at Trinidad and Bermudez (South America) at depths varying from 3 to 60 meters. It contains about 40% to 70% of pure bitumen. The water content is about 30%. The rest is impurities. It is refined by boiling in a tank. The water evaporates and impurities collect at the top. These impurities are removed. This refined lake asphalt is used widely for road and pavement construction.

The rock asphalt is obtained from rocks in Switzerland, France, etc. It contains about 10 to 15 per cent of pure bitumen. The rest consists of calcareous materials. The rocks are put on the road surface after being crushed and heated. The rock softens under the influence of heat to the consistency of paste, and it consolidates on cooling. The road surface is then formed by rolling. This asphalt can also be used for roofing sheets, paving tiles, etc.

2. Residual asphalt -

This variety is also known as the artificial asphalt. It is obtained by the fractional distillation of crude petroleum oils with an asphaltic base. Such process leaves a solid substance in the retort. This solid substance is the residual asphalt.

PROPERTIES OF ASPHALT -

Following are the properties of asphalt:

(i) It is a tough and durable material.

(ii) It is a water-proof material and can be easily cleaned.

(iii) It is good insulator of electricity, heat and sound.

(iv) It is non-inflammable and non-absorbent.

(v) It is not attacked by acids and is safe against vermin.

(vi) It is resilient and reasonably elastic.

 

USES OF ASPHALT -

Due to the properties mentioned above, the asphalt is widely used for various engineering purposes as follows:

(i) as damp-proof courses,

(ii) as water-proof layer for tanks, basements, swimming pools, etc.

(iii) for preparing paints and roofing felts,

(iv) for constructing roads and pavements, etc.

 

2. Bitumen -

The bitumen is the binding material which is present in asphalt. It is also sometimes called the mineral tar. It is obtained by partial distillation of crude petroleum. It is chemically a hydro-carbon. It is insoluble in water, but it completely dissolves in carbon bisulphide, chloroform, benzol, coal tar, naphtha, alkalies, alkaline carbonates, petroleum spirit and oil of turpentine. It is found on analysis to compose of 87 per cent carbon, 11 per cent hydrogen and 2 per cent oxygen by weight.

The bitumen is black or brown in colour and it is obtained in solid or semi-solid state. Its applications are same as the residual asphalt.

Bitumen is a hydrocarbon material of either natural or pyrogenous origin, found in gaseous, liquid, semi-solid or solid form.

Bitumen is not affected by light, air or water individually, but in combination they can make it brittle, porous and susceptible to oxidation forming blisters and cracks. It becomes soft at temperatures between 30°–100° C (no sharp melting point), and therefore must be protected from exposure to heat.

Classification of Bitumen -

Bitumens are classed as natural and petroleum bitumens.

Natural Bitumen - Pure natural bitumen occurs rarely. Limestones, sandstones and soils impregnated with bitumen are frequently found. It originates from the accumulation of petroleum in the top layers of earth crust through migration, filling pores and cavities of rocks, under the action of high temperature and pressure.

The natural bitumen is dark-brown in colour which on heating gradually softens and passes to liquid state and on cooling solidifies. It is insoluble in water but dissolves in carbon disulphide, chloroform, benzene and very little in gasoline. Natural bitumen may be extracted from bituminous rocks by blowing in kettles or dissolving in organic solvents (extraction).

Petroleum bitumens are product of processing crude petroleum and its resinous residues.

These are classified as residual asphaltums, oxidized, cracked and extracted bitumens.

Residual Asphaltums are black or dark-brown solid substances at normal temperatures, obtained by atmospheric-vacuum distillation of high-resin petroleum after topping of gasoline, kerosene and fractions.Oxidized Bitumen are produced by blowing air through petroleum residues. Oxygen from air combines with hydrogen of the residues to give water vapour. The petroleum residues thicken because of polymerization and condensation.

Cracked Bitumen are obtained by the cracking—high temperature decomposition—of petroleum and petroleum oils allowing high yield of gasoline. Blowing of air through residues gives oxidized cracked bitumens.

Uses -

Bitumen is used for manufacture of roofing and damp proofing felts, plastic bitumen for leak stops, waterproof packing paper, pipe asphalt, joint filler, bituminous filling compounds for cable boxes, for sealing accumulators and batteries. It is also used for fixing of roofing felts, dam proofing felts and for heat insulation materials for buildings, refrigeration and cold storage equipments.

 

Properties of Bitumen -

Following are the properties of Bitumen :

1. Adhesion

2. Resistance to Water

3. Hardness

4. Viscosity

5. Ductility

6. Softening Point

7. Specific Gravity

8. Durability

9. Versatility

10. Economical

11. Strength

12. Resilience

 

 

 

Adhesion - The adhesive property of bitumen binds together all the components without bringing about any positive or negative changes in their properties. Bitumen has the ability to adhere to a solid surface in a fluid state depending on the nature of the surface. The presence of water on the surface will prevent adhesion.

Resistance to Water - Bitumen is insoluble in water and can serve as an effective sealant Bitumen is water resistant. Under some conditions water may be absorbed by minute quantities of inorganic salts in the bitumen or filler in it.

Hardness - To measure the hardness of bitumen, the penetration test is conducted, which measures the depth of penetration in tenths of mm. of a weighted needle in bitumen after a given time, at a known temperature. Commonly a weight of 100 gm is applied for 5 sec at a temperature of 77 °F. The penetration is a measure of hardness. Typical results are 10 for hard coating asphalt, 15 to 40 for roofing asphalt and up to 100 or more for water proofing bitumen.

Viscosity - It depends greatly on temperature. At lower temperature, bitumen has great viscosity and acquires the properties of a solid body, while with increase in temperature the viscosity of bitumen decreases and it passes into liquid state.

Ductility - Presence of ductility means the formation of the film and coating would be proper. It depends upon temperature, group composition and nature of structure. Viscous bitumens, containing solid paraffins at low temperatures are very ductile.

Softening Point - It is related to viscosity. Bitumen needs sufficient fluidity before specific application.

Specific Gravity - Specific gravity of a binder does not influence its behaviour. But all the same, its value is needed in mix design. The property is determined at 27º C.

Durability - Bitumen durability refers to the long-term resistance to oxidative hardening of the Material in the field. Although, in-service, all bitumen harden with time through reaction. With oxygen in the air, excessive rates of hardening (poor durability) can lead to premature binder embrittlement and surfacing failure resulting in cracking and chip loss. Bitumen lives upto twenty years if maintained properly throughout the pavement life.

Versatility - Due to versatility property of Bitumen it is relatively easy to use it in many applications because of its thermoplastic property. It can be   -spread easily along the underlying pavement layers as it liquefies when heated making the job easier and hardens in a solid mass when cooled.

Economical - It is available in cheaper rates almost all over the world which makes it feasible and affordable in many applications.

Strength - Though the coarse aggregates are the main load bearing component in a pavement, bitumen or asphalt also play a vital role in distributing the traffic loads to the layers beneath.

Resilience - Bitumen is resilient, non-rigid and as such it is capable of absorbing shocks and accommodate itself to the movement in structure due to temperature, settlement or shrinkage.

 

General Properties of Bitumen -

·       Most bitumen are colloidal in nature.

·       Bitumens are thermoplastics.

·       They have no specific melting, boiling or freezing point.

·       Bitumens are insoluble in water.

·       They are highly impermeable to the passage of water.

·       They are generally hydrophobic. They are chemically inert.

·       Bitumen oxidises slowly.

 

3. Tar -

Tar is a high viscous liquid which contains high amount of carbon content. It is a dark (deep black) viscous liquid produced by destructive distillation of organic material such as coal, oil, lignite and wool. Depending upon the source of origin it is classified as coal tar, wood tar and mineral tar. Tar is restraint to petroleum-based solvents. It has very low bitumen content.

 

Forms of Tar -

General forms of tar are as follows.

·       Coal tar

·       Wood tar

·       Mineral tar

 

Coal Tar -

Coal tar is the by-product obtained during the production of coal gas. It is used for preserving timber, laying macadam roads etc.

The process consists of coal heated in closed iron vessels and the evaporated gases are collected in tubes. These tubes are circulated with cooling water. So, some matter is deposited in these tubes and it is nothing but coal tar. It is dense and strong-smelling liquid. It is in black color.

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.

These generally have high specific gravities and viscosities, and good adhesive properties. On the further distillation of coal tar (from coal gas) coal naphtha, creosote oil, dyes, etc. are obtained.

 

Wood Tar -

Wood tar is obtained by the distillation of resinous woods like pines etc. The distillation of resinous wood provides the creosote oil which has very good preservative property. So, wood tar is well suitable for preserving wood.

 

Mineral Tar -

Mineral tar is produced by the exploitation of kerogens. Kerogens are generally obtained from bituminous shale’s which are nothing but rocks. Volatile content of mineral tar is very less.

 

Grades of road tar -

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 top course and light carpets.

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

RT-5: Used for grouting. 

Comparison Between Asphalt, Bitumen and Tar Properties -

Property	Asphalt	Bitumen	Tar
Color	brownish-black	black or brown	dark (deep black)
State	Solid state	solid or semi-solid state	Viscous Liquid
Carbon content	Low	Medium	High
Water Resistance	More	More	Less
Acid resistance	More	More	Less
Adhesive power	Less	More	High
Setting time	Less	Less	More