What is Road Bitumen? : Properties, Classification, & Testing Methods

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What is Road Bitumen?

Road bitumen, also known as asphalt in some regions, is a vital construction component. It is a black, sticky, and highly viscous liquid or semi-solid form of petroleum.

It is obtained by partial distillation of crude oil.

It contains 87% carbon, 11% hydrogen, and 2% oxygen.

It is a black or dark brown viscous material composed principally of high molecular weight hydrocarbons derived from the distillation of petroleum or natural asphalt. It has adhesive properties and is soluble in carbon disulfide.

Bitumėn is known as petroleum in the fluid state, mineral tar in the semi-fluid state, and asphalt in the solid state.



Properties of Road Bitumen

The following are the properties of bitumen:

1. It is dark black and has better adhesive power than asphalt.

2. It is generally solid or semi-solid in the state.

3. It is insoluble in Water but completely soluble in carbon disulfide.

4. It has low permeability and high insulation resistance.

5. It is a binder in all types of asphalt.

6. It melts or softens on heating.

7. It resists acid and Water.

8. Its specific gravity is 1.09.



Classification of Road Bitumen


1. Natural Bitumen

Natural Bitumen

The bitumen/asphalt occurs in nature as solid or semi-solid deposits in certain parts of the world. It is just collected and used.

If it requires any purification, it is simply done by melting and boiling. Depending upon the source of occurrence, natural asphalt may be broadly divided into two classes:


i. Lake Asphalt

It is obtained from lakes at depths varying from 3 to 60 meters (South America). It contains as much as 70% pure bitumen, some water, and other impurities; it is easily refined by heating and boiling in a tank.

In doing bituminous matter bene, it is collected at the top, leaving the bituminous matter beneath. Lake asphalt is used widely for road and pavement construction.


ii. Rock Asphalt

It is a naturally occurring rock formation, usually limestone or sandstone, intimately impregnated throughout its mass with 6%-14% bitumen. It can be used for road making, roofing sheets, paving tiles, etc.



2.  Petroleum Bitumen

Petroleum bitumen

This type of bitumen is also known as artificial asphalt. This is the solid or semi-solid bi-product material left behind after distilling crude petroleum. Following are some common types of petroleum bitumen:


i. Penetration Grade Bitumen

The penetration grade bitumen is refinery bitumen manufactured at different viscosities. The penetration test is carried out to characterize the bitumen based on the hardness. Thus, it has the name penetration bitumen.

The penetration bitumen grades range from 15 to 450 for road bitumen. But the most commonly used range is 25 to 200.

This is acquired by controlling the test, i.e., the distillation process. The partial control of fluxing the residual bitumen with the oils can help bring the required hardness.



ii. Liquid Bitumen

There are following two types of liquid bitumen:


a) Cutback Bitumen

This is the type of bitumen whose Viscosity can be reduced temporarily by using volatile solvents such as naphtha, kerosene, diesel oil, and furnace oil. After the application, the volatile material is evaporated, and the bitumen will bind the aggregate.

Normal practice is to heat bitumen to reduce its Viscosity. Still, in some situations, preference is given to use liquid binders such as cutback bitumen, which can be used without heating.

From the environmental point of view also, cutback bitumen is preferred. Cutback bitumen is used for cold weather bituminous road construction and maintenance. There are different types of cutback bitumen:


i. Rapid curing (RC)

Obtained by mixing bitumen with highly volatile agents such as naphtha or gasoline.

RC is recommended for surface dressing and patchwork.


ii) Medium curing (MC)

Obtained by mixing bitumen with medium volatile agents such as kerosene.

MC is recommended for premix with more than 20% fine aggregates.

Used in bituminous soil stabilization.


iii) Slow curing (SC)

Obtained by mixing bitumen with low volatile and nonvolatile oils.

It is used with aggregate having more than 20% fine aggregates.


b) Bitumen Emulsion

Bitumen emulsion is a liquid product in which bitumen is suspended in a finely divided condition in an aqueous medium and stabilized by suitable material. The bitumen content in the emulsion is around 60%, and the remaining is Water.

When the emulsion is applied on the road, it breaks down, releasing Water, and the mix starts to set. The time of setting depends upon the grade of bitumen. This type of bitumen forms a two-phase system with two immiscible liquids.

One of them is dispersed as fine globules within the other liquid. A bitumen emulsion is formed when discrete bitumen globules are dispersed in a continuous form of Water.


Depending upon the stability of the protective coating of emulsifiers, it may be classified as:

1. Rapid setting (RS): suitable for surface dressing and patch repair

2. Medium setting (MS): Used in premix with lesser fines.

3. Slow setting (SS): Used in a premix with a more significant percentage of fines.


Depending on charge

1. Anionic: Anionic emulsion particles are negatively charged.

2. Cationic: Cationic emulsion particles are positively charged.

3. Neutral: It has no charge and is rarely used in road construction.


Application of emulsion

1. Surface treatment: Used for scaling, micro surfacing, etc., for the surface treatment of pavement.

2. Recycling: Cold-in-place recycling

3. It can be used in soil stabilization.

4. It can be used in patch repair works on bituminous roads.

5. It also can be used in wet conditions.

6. During pavement construction, It can be used for track coats and prime coats.


Advantages of emulsion

1. Do not require petroleum solvent to make it liquid.

2. Asphalt emulsions can usually be used without additional heat.

3. It contributes to energy conversion.

4.  It can be used in wet conditions.

5. Environmentally friendly than cutbacks.

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This is a dark (deep black) viscous liquid produced by the destructive distillation of organic materials such as coal and wood. The following are the properties of tar:


1. It consists of 75% to 95% bituminous substance.

2. It contains a higher percentage of carbon than asphalt and bitumen.

3. It hardens much quicker than asphalt and bitumen.

4. It is more adhesive than asphalt.

5. It is highly toxic compared to asphalt and bitumen.

6. It has low resistance to acid and Water.

7. It becomes fluid when heated.


Types of tar

Depending upon the source of origin, it is classified as coal tar, wood tar, and mineral tar.

1. Coal Tar

Coal tar

It is obtained as a byproduct in destructive coal distillation or coal gas manufacture.

It is heavy, strong-smelling, and black. These generally have high specific gravities and viscosities and good adhesive properties. It is used in roads, preserving timber, etc.


2. Wood Tar

Wood tar

It is obtained by the destructive distillation of resinous wood (pine, etc.). It contains creosote and, as such, is a powerful preservative.

On further distillation, wood tar produces wood creosote. Compared to coal tar creosote, it is an inferior preservative for wood.


Uses of tar

The following are the uses of tar:

1. It is generally used for roofing and road-making work.

2. It is used as a preservative for timber to elongate its life.

3. It is used for making bituminous paints and waterproofing compounds.

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Tests on Bituminous Binder

Following are the various tests that are carried out on bituminous binder:

1. Penetration test

2. Ductility test

3. Viscosity test

4. Float test

5. Softening point test

6. Specific gravity test

7. Flash and fire point test

8. Solubility test

9. Spot test

10. Loss on heating test

11. Water content test



1. Penetration Test

Penetration Test is carried out to determine the consistency of bituminous material.

It measures the hardness or softness of bitumen at a particular temperature by measuring the depth in terms of millimeters to which a standard loaded needle will penetrate vertically in 5 seconds.



A penetrometer consisting of a needle assembly with a total weight of 100 gm.

Time measuring device with accuracy ± 0.1 sec.

Dial gauge to read penetration value.

Penetration Test



Softened the bitumen to pouring consistency, stirred thoroughly, and poured into containers at a depth of at least 15 mm over expected penetration.

Placed the sample containers in a water bath and cooled them at 25°C for one hour.

Placed the container on the testing table of the penetrometer.

Bring the needle in contact with the sample’s surface and adjust the dial reading to zero.

The needle is released to penetrate the sample under the full load.

The penetration at the end of 5 sec is noted; now, the difference in initial and final readings gives penetration value.

At least three penetration tests are made on this sample by testing at distances at least 10 mm apart, and the mean of this measurement is reported as the penetration value.

The bitumen grade is specified in terms of penetration value 80-100
or 80/100-grade bitumen means that the penetration value of the bitumen is in the range of 80 to 100 at standard test conditions.

Lower penetration grade bitumen is used in warmer regions, and higher penetration grade bitumen is used in cold regions.



2. Ductility Test

Ductility is the property of bitumen that permits it to undergo great deformation or elongation. Ductility is the distance in cm to which a standard sample or briquette of the material will be elongated without aking.

The ductility test measures the adhesive properties of bitumen and its ability to stretch. In flexible pavement design, forming a thin, pliable film around aggregates is necessary to improve the aggregate’s physical interlocking.



Briquette of standard dimension

Pulling device with distance measuring dial.

Water bath arrangement


Ductility Test



Heat the bitumen sample to bring it into a fluid state at 75°C to 100°C.

Assembled the mold on a brass plate and wet the mold with a mixture of equal parts of dextrin and glycerin.

Poured the fluid bitumen in the briquette assembly and allowed it to cool in the air and then in Water, both maintained at 27°C.

The briquette sample is taken out from the water bath. Excess bitumen material is cut, and the surface is leveled using a hot knife.

The briquette sample is replaced in a ductility testing machine water bath for 85 to 95 minutes.

The sides of the mold are removed, the clips are hooked on the machine without causing any initial strain, and the pointer is adjusted to read zero.

Then, the clips were pulled horizontally at 50 mm/min, and the distance was noted at breaking the thread. The distance in cm gives the value of ductility of bitumen.

The ductility value should not be less than 50 cm for satisfactory performance of bitumen. However, according to ISI, the minimum ductility value is 75 cm.



3. Viscosity Test

Viscosity is the property by which it offers resistance to flow. The higher the viscosity, the slower the movement of the liquid will be.



i) Orifice of 10 mm size (orifice viscometer)

ii) Sample collector

iii) Thermometer

Viscosity Test



Heat the bitumen sample to a higher temperature than the specified test temperature.

Cool the sample by continuously stirring, and when the temperature becomes slightly above the test temperature, it is poured into a tar cap.

The quantity of bitumen 50 ml can flow from a cup through a specified orifice under standard conditions and a specified temperature.

The time 50 ml of the material takes to flow is noted in seconds, giving the bitumen viscosity value.



4. Float Test

Usually, the consistency of bituminous material can be measured by penetration or viscosity tests. However, these tests are not applicable for a specific range of consistencies, and the Float test is used.

Float Test

The float test is a modified viscosity test for small quantities of very viscous bituminous materials.

The apparatus consists of an aluminum float and a brass collar filled with bitumen to be tested.

The specimen in the mold is cooled to a temperature of 5°C and screwed in to float.

The total test assembly is floated in the water bath at 50°C, and the time required for Water to pass through the specimen plug is noted in seconds and is expressed as the float value.



5. Softening Point Test

The softening point of bitumen or tar is the temperature at which the substance attains a particular degree of softening. The softening point of bitumen is usually determined by ring and ball test.



i) Two brass rings having a depth of 6.4 mm

ii) Two steel balls of 9.5 mm diameter and 3.5 gm wt

iii) Thermometer

iv) Heating Arrangement and beaker

Softening Point Test



The bitumen test sample is placed in the brass ring, and the ring is then suspended in Water if S. P≤ 80°C and in glycerin if S. P≥ 80°C.

Steel balls are placed upon the bitumen sample, and the liquid (Water) in the beaker is heated at five °C per minute.

The temperature at which the softened bitumen touches the metal plate placed at a specified distance below the ring (usually 2.5 cm) is recorded as a softening point of bitumen.

The softening point of various bitumen grades used in passing jobs varies between 35°C and 70°.



6. Specific Gravity Test

The specific gravity of bitumen is defined as the ratio of the mass of a given volume of bitumen of known content to the mass of an equal volume of Water at 27°C.

The specific gravity can be measured using a pycnometer or preparing a cube specimen of bitumen in a solid or solid state.

The specific gravity of bitumen varies from 0.97 to 1.02.




Weighing balance

Specific Gravity Test



The specific gravity bottle is cleaned, dried, and weighed.

Water is filled in it, and the weight of the bottle, along with the Water, is taken.

Bituminous material is heated to pouring temperature and poured in a bottle to its one-fourth.

The sample is cooled for 30 minutes in the air and is weighed.

Water is filled in it, and weight is taken.



Sp. gravity = (W2-W1) / {(W3-W₁)-(W4-W2)}


W₁ = Wight of empty specific gravity bottle

W₂ = Wight of bottle + bitumen

W3 = Wight of bottle + water

W = Wight of bottle + water + bitumen



7. Flash and Fire Point Test

At high temperatures, depending upon the grades of bitumen materials, leave out volatiles. These volatiles catch fire, which is very hazardous, so it is essential to qualify this temperature for each bitumen grade.

Flash point: the lowest temperature at which the test flame application causes the bitumen vapor to momentarily catch fire in the form of a flash under specified test conditions.

Fire point: The fire point is the lowest temperature under specified test conditions at which the bituminous material gets ignited and burns for at least 5 seconds.



Pensky-martens apparatus


Flash and Fire Point Test



A. Flash Point

Soften the bitumen between 75°C and 100°C. Stir it thoroughly to remove air bubbles and Water.

Fill the cup with the material to be tested upto the filling mark.

Place it in the bath. Fix the open clip. Insert the thermometer of the high or low range as per requirement.

Light the test flame and adjust it. Supply heat at such a rate that the temperature increase, recorded by the thermometer, is neither less than five °C nor more than six °C per minute.

Open flash point is taken as the temperature when a flash first appears at any point on the surface of the material in the cup. Discontinue the stirring during the application of the test flame.

When the flash occurs, the flash point should be taken as the temperature read on the thermometer.


B. Fire Point

After the flash point, heating should be continued at such a rate that the increase in the temperature recorded by the thermometer is neither less than five °C nor more than six °C per minute.

The test flame should be lighted and adjusted to be the size of a bead 4 min in diameter.

Fire point should be taken as the temperature read on the thermometer when the test flame is applied, ignited, and burns for at least 5 seconds.



8. Solubility Test

This test determines the presence of inert materials, carbons, salts, etc., in bitumen.

Pure bitumen is completely soluble in carbon disulfide (C₂S) and carbon tetrachloride (CCL4).

Hence, any impurity in the bitumen can be determined by dissolving the bitumen in any of the above solvents.

Solubility Test


Procedure for solubility test

A sample of about 2 grams of bitumen is dissolved in about 100 C.C. of solvent.

The solution is filtered, and the residue is washed, dried, and weighted.

Solubility is expressed as a percentage of the original sample. The insoluble 4. matter should preferably be less than 1% in carbon disulfide.

If carbon tetrachloride is used as solvent, and the black carbonaceous residue is more than 0.5% then the bitumen is called to be cracked.



9. Spot Test

This test is used to determine the overheated or cracked bitumen. This test is considered more sensitive than the solubility test for detection cracking.

Spot Test



2 gm of bitumen is mixed with 10 ml of Naphthalene.

From the solution, the first drop is taken after one hr. of solution preparation and placed in filter paper, and the same thing happens after 24 hrs

If the spot strain has a uniform color, it means uncracked bitumen, but the strain forms an annular ring of thick brown or black at the center and faint at the periphery; it’s over-heated or cracked.



10. Loss of Heating Test

This test determines the amount of volatile materials present in the bitumen.




Aluminium rotating self



Weighing balance

Loss of Heating Test



About 50-gram sample of bitumen is taken and weighted accurately,

The bitumen sample is heated at 163°C for 5 hours in a specified oven designed for this test as per IS: 1212-1958.

The specimen is weighed again after heating, and loss in weight is determined.

The loss is expressed as a percentage of the original weight; this should not be more than 1% for bitumen.



11. Water Content Test

This test is conducted to determine the water content of bitumen. Water present in the bitumen forms foams of bitumen when heated above 100°C.

Thus, water content in bitumen should be minimal. Maximum water content is not more significant than 0.2% by weight.



The known weight of bitumen is mixed in a pure petroleum distillate free from Water.

The solution is heated and distilled.

The weight of distilled Water is found and expressed as a percentage of the original weight of the sample.

Maximum water content is not more significant than 0.2% by weight.


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