Highway Materials & Pavement
Week # 6
History of Asphalt Materials, Bituminous Material
Types, & Use of Asphalt Cement
Lecturer: Asst. Prof. Dr. Hardy Kamal Karim
Ph.D in Highway Engineering
University of Sulaimani 1
History of Asphalt
• Asphalt is one of the oldest materials used in construction.
• It is used since 6000 BC as a waterproofing and binder material
of great quality.
• A prominent example of bitumen use is cited in the Old
Testament, since it was used as coating for Noah’s Ark.
• The Sumerians used to use it in the prosperous shipbuilding
industry, whereas the Babylonians used it as a binder in the
mixture production for castle construction (Babel Tower).
• Asphalt was also used by the Egyptians both to mummify the
dead bodies and to waterproof tanks.
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• Around 3000 BC, the Persians also used bitumen for road
construction.
• Before the mid-1850s, asphalt came from natural pools found in
various locations throughout the world, such as the Trinidad Lake
asphalt.
• Natural asphalt also exists in the form of rocks, which is mainly
limestones or sandstones enriched with bitumen.
• However, with the discovery and refining of petroleum in
Pennsylvania, use of asphalt cement became widespread.
• By 1907, more asphalt cement came from refineries than came
from natural deposits.
• Today, practically all asphalt cement is from refined petroleum.
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Bituminous Materials
• Bituminous materials are classified as asphalts and tars.
• Asphalt is used mostly in pavement construction, but is also used
as sealing and waterproofing agents.
• Tars are produced by the destructive distillation of bituminous coal
or by cracking petroleum vapors.
• Tar is used primarily for waterproofing membranes, such as roofs.
It may also be used for pavement treatments, particularly where
fuel spills may dissolve asphalt cement, such as on parking lots and
airport aprons.
• Tar differs in chemical composition and odor to bitumen.
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• Over the last years, the usage of tar has been minimized mainly
for environmental and health reasons.
• Nowadays, tar is used almost exclusively for the production of
specific bituminous mixtures, not affected by petrol or oils,
known as fuel-resistant mixtures.
• It may also mix with petroleum bitumen to produce better-quality
binder for surface dressing works.
• Better coating of aggregates, as well as better adhesion of
bitumen to aggregate, is achieved with the use of tar/bitumen
binder.
• On the contrary, it is more sensitive to temperature variations, and
as a result, it softens easier and it hardens and becomes brittle
quicker than bitumen of similar viscosity.
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The fractional distillation process of crude petroleum
• Different products are separated at
different temperatures.
• Figure 9.2 shows the main
products, such as gasoline,
kerosene, diesel oil, and asphalt
residue (asphalt cement).
• The quantity and quality of the
asphalt depends on the crude
petroleum source and the refining
method. Some crude sources, such
as the Nigerian oils, produce little
asphalt, while others, such as many
of the Middle Eastern oils, have a
high asphalt content.
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Photo showing asphalt cement, aggregate, and asphalt concrete. 8
Types of Asphalt Products
• Asphalt used in pavements is produced in three forms:
1) asphalt cement,
2) asphalt cutback, and
3) asphalt emulsion.
• Asphalt cement is a blend of hydrocarbons of different molecular
weights.
• The characteristics of the asphalt depend on the chemical composition
and the distribution of the molecular weight hydrocarbons. As the
distribution shifts toward heavier molecular weights, the asphalt
becomes harder and more viscous.
• At room temperatures, asphalt cement is a semisolid material that
cannot be applied readily as a binder without being heated.
• Liquid asphalt products, cutbacks and emulsions can be used without
heating. 9
• Asphalt cement has excellent adhesive characteristics, which make it a
superior binder for pavement applications.
• A cutback is produced by dissolving asphalt cement in a lighter
molecular weight hydrocarbon solvent.
• When the cutback is sprayed on a pavement or mixed with aggregates,
the solvent evaporates, leaving the asphalt residue as the binder.
• Three disadvantages have severely limited the use of cutbacks for
highway construction.
1) as petroleum costs have escalated, the use of these expensive solvents
as a carrying agent for the asphalt cement is no longer cost effective.
2) cutbacks are hazardous materials due to the volatility of the solvents.
3) application of the cutback releases environmentally unacceptable
hydrocarbons into the atmosphere.
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• In fact, many regions with air pollution problems have outlawed the use
of any cutback material.
• An alternative to dissolving the asphalt in a solvent is dispersing the
asphalt in water as emulsion as shown in figure below.
Photo of magnified asphalt emulsion showing minute droplets of asphalt cement
dispersed in a water medium. 11
• In this process the asphalt cement is physically broken down into
micron-sized globules that are mixed into water containing an
emulsifying agent.
• Emulsified asphalts typically consist of about:
1) 60% to 70% asphalt cement,
2) 30% to 40% water, and
3) a fraction of a percent of emulsifying agent.
• There are many types of emulsifying agents; basically they are a soap
material.
• Since the specific gravity of asphalt is very near that of water, the
globules have a neutral buoyancy and, therefore, do not tend to float or
sink.
• When the emulsion is mixed with aggregates or used on a pavement, the
water evaporates, allowing the asphalt globs to come together, forming
the binder. 12
• The phenomenon of separation between the asphalt residue and water
is referred to as breaking or setting.
• The rate of emulsion setting can be controlled by varying the type and
amount of the emulsifying agent.
• Since most aggregates bear either positive surface charges (such as
limestone) or negative surface charges (such as siliceous aggregates),
they tend to be compatible with anionic or cationic emulsions,
respectively.
• Although emulsions and cutbacks can be used for the same
applications, the use of emulsions is increasing because they do not
include hazardous and costly solvents.
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Uses of Asphalt
• The main use of asphalt is in pavement construction and maintenance,
sealing and waterproofing various structural components, such as
roofs and underground foundations.
• The selection of the type and grade of asphalt depends on the type of
construction and the climate of the area.
• Asphalt cements, also called asphalt binders, are used typically to
make hot-mix asphalt concrete for the surface layer of asphalt
pavements.
• Asphalt concrete is also used in patching and repairing both asphalt
and Portland cement concrete pavements.
• Liquid asphalts (emulsions and cutbacks) are used for pavement
maintenance applications, such as fog seals, chip seals, slurry seals,
and micro-surfacing 14
Placing hot mix asphalt used for the surface layer of asphalt pavement 15
Compaction of hot mix asphalt 16
Applying fog seal (diluted emulsion) for preserving existing pavement 17
Spraying tack coat (emulsion) on existing asphalt pavement before
placing an asphalt overlay. 18
Applying chip seal (emulsion followed by aggregates) for preserving existing pavement
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Applying micro-surfacing for preserving existing pavement 20
• Liquid asphalts may also be used to seal the cracks in pavements.
• Liquid asphalts are mixed with aggregates to produce cold mixes, as
well.
• Cold mixtures are normally used for patching (when hot-mix asphalt
concrete is not available), base and subbase stabilization, and surfacing
of low-volume roads.
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Table below shows common paving applications for asphalts
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Temperature Susceptibility of Asphalt
• The consistency of asphalt is greatly affected by temperature. Asphalt
gets hard and brittle at low temperatures and soft at high temperatures.
• Figure below shows a conceptual relation between temperature and
logarithm of viscosity.
• The viscosity of the asphalt decreases when the temperature increases.
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• Asphalt’s temperature susceptibility can be represented by the slope of
the line shown in the Figure; the steeper the slope the higher the
temperature susceptibility of the asphalt.
• Additives can be used to reduce this susceptibility.
• When asphalt is mixed with aggregates, the mixture will perform
properly only if the asphalt viscosity is within an optimum range.
• If the viscosity of asphalt is higher than the optimum range, the
mixture will be too brittle and susceptible to low-temperature
cracking.
• On the other hand, if the viscosity is below the optimum range, the
mixture will flow readily, resulting in permanent deformation
(rutting).
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Thermal cracking resulting from the use of too-stiff asphalt in a cold
climate area 25
Rutting that could result from the use of too-soft asphalt 26
• Due to temperature susceptibility, the grade of the asphalt cement
should be selected according to the climate of the area.
• The viscosity of the asphalt should be mostly within the optimum range
for the area’s annual temperature range; soft grade asphalts are used for
cold climates and hard-grade asphalts for hot climates.
Selecting the proper grade of asphalt binder to match the climate 27
Absolute and Kinematic Viscosity Tests
• Viscosity is defined as a measure of fluid’s resistance to flow. It describes
the internal friction of a moving fluid.
• Broadly speaking, it could be said that viscosity is an expression of the
coherence or fluidity that decreases when the temperature increases and
increases when the temperature decreases.
•Similar to the penetration test, the viscosity test is used to measure asphalt
consistency. Two types of viscosity are commonly measured: absolute and
kinematic.
• The absolute viscosity procedure (ASTM D2171) requires heating the
asphalt cement and pouring it into a viscometer placed in a water or oil bath
at a temperature of 60°C.
• The viscometer is a U-tube, with a reservoir where the asphalt is
introduced and a section with a calibrated diameter and timing marks. 28
• For absolute viscosity tests vacuum is applied at one end.
• The time during which the asphalt flows between two timing marks on the
viscometer is measured using a stopwatch. The flow time, measured in seconds,
is multiplied by the viscometer calibration factor to obtain the absolute
viscosity in units of poises.
• Different-sized viscometers are used for different asphalt grades to meet
minimum and maximum flow time requirements of the test procedure.
• The kinematic viscosity test procedure (ASTM D2170) is similar to that of the
absolute viscosity test, except that the test temperature is 135°C.
• Since the viscosity of the asphalt at 135°C is fairly low, vacuum is not used.
• The time it takes the asphalt to flow between the two timing marks is
multiplied by the calibration factor to obtain the kinematic viscosity in units of
cenistokes (cSt).
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Absolute viscosity test apparatus
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