Bitumen is a naturally occurring semi-solid form of petroleum that is formed from the remains of ancient algae and other organisms. It is refined from crude oil through processes like vacuum steam distillation. Tar is a similar black, thermoplastic material produced from the destructive distillation of coal. Both were widely used in the early 20th century as binders for road construction, leading to the term "tarmac". Bitumen has properties that make it a strong adhesive and durable sealant for engineering projects like roads. Various tests are conducted to evaluate properties like viscosity and durability.

1. Bitumen and Tar

2. Origin
• Naturally occurring deposits of bitumen are
formed from the remains of ancient, microscopic
algae and other once-living things.
• When these organisms die, their remains were
deposited in the mud on the bottom of the ocean
or lake where they lived.
• Under the heat and pressure of burial deep in the
earth, the remains were transformed into
materials such as bitumen, kerosene, or
petroleum.

3. • Asphalt or bitumen can sometimes be confused
with "tar", which is a similar black, thermoplastic
material produced by the destructive distillation
of coal.
• During the early and mid-20th century when
town gas was produced.
• Tar was a readily available product and
extensively used as the binder for road
aggregates.
• The addition of tar to macadam roads led to the
word tarmac, which is now used in common
parlance to refer to road-making materials.

4. • Preparation of Bitumen:
– Bitumen is the residue or by-product when the
crude petroleum is refined.
– A wide variety of refinery processes, such as the
straight distillation process, solvent extraction
process etc. may be used to produce bitumen of
different consistency and other desirable
properties.
– Depending on the sources and characteristics of
the crude oils and on the properties of bitumen
required, more than one processing method may
be employed.

5. • Vacuum steam distillation of petroleum oils:
– In the vacuum-steam distillation process the crude
oil is heated and is introduced into a large
cylindrical still.
– Steam is introduced into the still to aid in the
vapourisation of the more volatile constituents of
the petroleum and to minimise decomposition of
the distillates and residues.
– The volatile constituents are collected, condensed,
and the various fractions stored for further
refining, if needed.

7. – The bottom fraction from this unit is the vacuum-
steam-refined asphalt cement.
– The consistency of asphalt cement from this
process can be controlled by the amount of heavy
gas oil removed.
– Normally, asphalt produced by this process is
softer.
– As the asphalt cools down to room temperature, it
becomes a semi solid viscous material.

8. Types of bitumen used in flexible
pavement construction are:
(a) Paving grade bitumen
(b) Modified bituminous binders
(c) Cut-back bitumen
(d) Bitumen emulsion

9. Paving grade bitumen
• Paving grade bitumen which is obtained from the
distillation process of petroleum crude is extensively used
in the construction of flexible pavement layers, particularly
the surface and binder courses.
• At normal range of atmospheric temperature, bitumen is in
semi-solid state and remains highly viscous and sticky.
• When the paving grade is heated, it softens at rapid rate
and attains fluid consistency and the viscosity decreases
with further increases in temperature.
• Heated to temp of 130 ⁰ C – 175⁰ C.
• Mixing of the bitumen with the aggregate is done in a hot
mix plant to obtain ‘hot bituminous mix’

10. Modified bituminous binders
• The viscosity of ordinary paving grade bitumen varies considerably
with temperature, resulting in bituminous pavement surface course
being susceptible temperature changes.
• During hot weather, the bituminous pavement surface course
becomes soft and during cold weather it becomes too stiff and
brittle with the possibility of early cracking.
• Bitumen modifiers reduces the temperature susceptibility of the
bituminous binders and that of the bituminous mix.
• Bituminous mixes prepared using suitable type of modified binders
offer better resistance to deformation oat higher temperatures and
remains relatively more flexible and elastic at low temperatures.

11. Cut-back bitumen
• In order to achieve fluid consistency of the bitumen at
relatively low temperatures with nominal heating, ‘cut-
back bitumen’ has been developed.
• Cut-back biutmen is prepared by diluting a paving
grade bitumen with a volatile solvent such as light fuel
oil or kerosene.
• The consistency of the cut-back and the rate at which it
hardens after application depends on the grade of the
bitumen selected and the characteristics and
proportion of the light oil/diluent used.

12. Bitumen emulsion
• Another entirely different approach of achieving fluid consistency of
bitumen for use in road works without the need to heat the binder
is the bitumen emulsion.
• Bitumen emulsion or emulsified bitumen is prepared by dispersing
bitumen in the form of fine globules suspended in water with the
help of suitable emulsifier.
• The properties of the bituminous emulsions vary depending upon
the properties of the bituminous binder, its proportion with repect
to water and the properties of emulsifier.
• Appropriate type and gate od bitumen emulsion may be selected
for being directly sprayed as prime coat or tack coat and for being
mixed with aggregates to prepare ‘cold bituminous mix.

13. • Properties:
– A very strong and durable adhesive that binds
together a very wide variety of other materials
without affecting their properties.
– Its durability is essential to major engineering projects
such as roads and waterways where it must do its job
for 20 years or more.
– Bitumen is insoluble in water but is soluble in
numerous organic solvents. As it is highly waterproof,
it can act as an effective sealant.
– It also resists action by most acids, alkalis and salts.
– It does not contaminate water so it can be used to
line watercourses.

14. – It is a thermoplastic material: it softens and
becomes liquid with the application of heat and
hardens as it cools.
– Bitumen can be spread relatively easily in the
areas where it is required because it can readily be
liquefied by one of three methods:
• Applying heat
• Dissolving it in petroleum solvents
• Dispersing with water (emulsification).

15. • Requirements of Bitumen:
– Viscosity- at time of mixing and compaction
should be adequate.
– This is achieved by heating the bitumen and
aggregate prior to mixing.
– Bitumen materials should not be temperature
susceptible
• During hottest weather of the region the bituminous
mix should not become too soft or unstable
• During cold weather, it should not become too hard or
brittle, causes cracking of surface
• Bituminous material should be durable

16. – In presence of water the bitumen should not strip
off from the aggregates.
– There should be adequate affinity and adhesion
between the bitumen and aggregates used in the
mix.

17. • Tests on bitumen:
There are a number of tests to assess the properties of
bituminous materials. The following tests are usually
conducted to evaluate different properties of bituminous
materials.
Penetration test
– Ductility test
– Softening point test
– Specific gravity test
– Viscosity test
– Flash and Fire point test
– Float test
– Water content test
– Loss on heating test

18. • Tar:
– When coal is heated without air, it does not burn
but produces many by-products.
– The process of heating coal in the absence of air is
called destructive distillation of coal.
– The main by products are:
a. coke
b. coal tar
c. amino acid liquor
d. coal gas