Download CE 303-121- lecture 17

LECTURE NO. 17 (Handout)
PAVEMENTS AND BITUMINOUS MATERIALS
Objectives:
• To introduce types of pavements
• To
explain
various
bituminous
emphasizing asphalts
• To explain properties of asphalts
materials,
PAVEMENT: Definition
Pavement is defined as a road or highway with hard, smooth, and
leveled surface made using a suitable material such as Portland
cement concrete or asphalt concrete
Asphalt concrete pavement
Portland cement concrete pavement
PAVEMENT: Classification
• Pavements are classified as “flexible” or “rigid” depending on how they
distribute surface loads
• Flexible pavements are those which are surfaced (i.e., paved) with bituminous
materials such as asphalt concrete
• Asphalt concrete possess a lower stiffness (EI) than Portland cement concrete
due to lower modulus of elasticity of asphalt concrete as compared to Portland
cement concrete
• Due to lower stiffness of asphalt concrete the total structure of a flexible
pavement "bends" or "deflects" under traffic loads. This is the logic behind
calling such a pavement as "flexible pavement”.
•
Flexible pavements distribute the wheel load over a cone-shaped area under the
wheel, reducing the imposed unit stresses as depth increases, as shown below:
Load distribution under a
flexible pavement
Stress at lower depth is higher than
the stress at higher depth
• Flexible pavements comprise about 93 percent of U.S. paved roads
PAVEMENT: Classification----contd.
• Rigid pavements are those which are surfaced (i.e., paved) with Portland
cement concrete
• Portland cement concrete possess a substantially higher stiffness (EI)
than asphalt concrete due to higher modulus of elasticity of Portland
cement concrete as compared to asphalt concrete
• Due to high stiffness of Portland cement concrete the total structure of a
rigid pavement "bends" or "deflects" very little under traffic loads. This is
the logic behind calling such a pavement as “rigid pavement”.
• Rigid pavements act as flexural members and distribute the wheel load
fairly uniformly over the area under the pavement slab, as shown below:
Load distribution under a
rigid pavement
• Rigid pavements comprise only 7 percent of U.S. paved roads
PAVEMENT: Elements of flexible pavement
• A flexible pavement consists of four elements, as follows:
– Subgrade: Prepared roadbed consisting of natural or imported soil
– Subbase course: A layer between the subgrade and the base course, made
from materials superior to that of subgrade. In case of a good quality of
subgrade, the subbase course is omitted.
– Base course: This is the layer directly below the asphalt concrete layer and
generally consists of aggregate (either stabilized or unstabilized).
– Surface course: This is the top layer and the layer that comes in contact with
traffic. It may be composed of one or several different asphalt concrete
sublayers.
• The above four elements of flexible pavement are shown below:
PAVEMENT: Elements of rigid pavement
• A flexible pavement consists of four elements, as follows:
– Subgrade: Prepared roadbed consisting of natural or imported soil
– Subbase course: This is the layer (or layers) under the base layer. A subbase
is not always needed and therefore may often be omitted .
– Base course: This is the layer directly below the Portland cement concrete
layer and generally consists of aggregate or stabilized subgrade.
– Surface course: This is the top layer and the layer that comes in contact with
traffic. It consists of the Portland cement concrete slab.
• The above four elements of rigid pavement are shown below:
Typical rigid pavement cross-section
BITUMINOUS MATERIALS: Introduction
• Bituminous material (or bitumen) is a solid, semisolid, or viscous
cementitious material (i.e., binder) natural or manufactured, and
composed of “hydrocarbons”
• Bitumen are usually fairly hard at normal temperatures. When
heated, they soften and flow.
• Bitumens possess a number of properties that make them useful
in the construction industry
• When mixed with aggregates in their hot fluid state, and then
allowed to cool, they solidify and bind the aggregates together,
forming a pavement surface.
• The most common bituminous materials are, as follows:
– Asphalts (available as natural deposits or are produced from
petroleum processing)
– Tars (obtained through the destructive distillation of materials such
as wood, coal, and shale, i.e., by heating wood or coal or shale in
absence of air)
– Pitches (obtained through further processing of tars)
BITUMINOUS MATERIALS: Classification
• Classification of bituminous materials is shown below:
ASPHALTS
• Asphalts are the most common and most widely used bituminous
materials
• The term “asphalt” refers to a black cementitious material which
varies widely in consistency from solid to semisolid at normal
temperatures
• Asphalts are found as natural deposits or are produced from
petroleum crude
• The classification of asphalts is already presented in the previous
slide
• Natural Rock asphalts are rock deposits containing bituminous
materials which have been used for road surfaces in localities
where they occur
• Native asphalts are obtained from asphalt lakes in Trinidad and
other Caribbean areas, these were used in some of the earliest
pavements in North America
• Petroleum asphalts are products of the distillation of crude oil.
These asphalts are used as the most common bituminous paving
materials
PETROLEUM ASPHALTS
• Petroleum asphalts are produced
from the residual matter (i.e.,
impurities) present in the crude oil
• Higher-grade crude oil (i.e., crude
oil with less residual matter) may
contain as little as 10% asphalt ,
whereas lower-grade crude oil (i.e.,
crude oil with more residual
matter) may contain as much as
90% asphalt
• A flow chart showing production
of different types of asphalts are
shown in the adjoining figure.
TYPES OF PETROLEUM ASPHALTS
• Following types of petroleum asphalts are used in pavement
construction:
– Asphalt cement
– Emulsified asphalt
– Liquid or cutback asphalt
TYPES OF PETROLEUM ASPHALTS:
Asphalt cement
• Asphalt cement (also called paving asphalt) are the primary
asphalt products produced by the distillation of crude oil.
• At ambient temperatures asphalt cement is a black, sticky,
semisolid and a highly viscous material
• It is strong and durable cement with excellent adhesive and
waterproofing characteristics. It is also highly resistant to the
action of most acids, alkalis and salts
• The largest use of asphalt cement is in the production of asphalt
concrete, which is primarily used in the construction of flexible
pavements throughout the world
• The asphalt cement can readily be liquefied by applying heat for
mixing with mineral aggregates to produce asphalt concrete
TYPES OF PETROLEUM ASPHALTS:
Asphalt cement---contd.
•
Several standard grades of asphalt
consistency, are available commercially.
•
Two methods, viscosity and penetration are used to classify
asphalt cements into different grades, as follows:
cement,
based
on
– The viscosity grades based on original asphalt cements (AC), as
specified in ASTM D3381 are: AC– 5; AC– 10; AC– 20; AC– 30; and
AC– 40 (The numerical values indicate viscosity at 140 ºF in hundreds
of poise)
– The penetration grades, as specified in ASTM D946 are: 200-300; 120150; 85-100; 60-70; and 40-50 (higher the penetration, the softer the
asphalt cement, therefore, 40-50 is the hardest grade and 200-300 is
the softest grade
TYPES OF PETROLEUM ASPHALTS:
Asphalt cement---contd.
TYPES OF PETROLEUM ASPHALTS:
Emulsified asphalt
• Emulsified asphalt (also simply called emulsion) is a mixture of
asphalt cement, water, and emulsifying agent
• Because the asphalt cement will not dissolve in water, asphalt
cement and water exist in separate phases as shown in the
following figure:
• To mix the asphalt cement with water, an emulsifying agent (usually
a type of soap) is added
TYPES OF PETROLEUM ASPHALTS:
Emulsified asphalt-----contd.
• Process of manufacture of emulsified asphalt consists passing the
hot asphalt cement and water containing the emulsifying agent
under pressure through a colloid mill, as shown in the following
figure:
• The colloid mill breaks up the
asphalt cement and disperses
it, in the form of very fine
droplets, in the water carrier
• The emulsified asphalt when
used, the emulsion sets as
the water evaporates
• The
emulsion
usually
contains
55-75%
asphalt
cement and up to 3%
emulsifying
agent,
with
balance being water
TYPES OF PETROLEUM ASPHALTS:
Emulsified asphalt-----contd.
• Two most commonly used types of emulsified asphalts are
specified in ASTM D977 and ASTM D2397:
– Anionic – electro-negatively charged asphalt droplets
– Cationic – electro-positively charged asphalt droplets
• Anionic emulsions adhere better to aggregate particles with
positive surface charges (e.g., limestone)
• Cationic emulsions adhere better to aggregate particles with
negative surface charges (e.g., sandstone, quartz, siliceous gravel).
Cationic emulsions also work better with wet aggregates and in
colder weather
• The anionic emulsified asphalts include rapid setting (RS), medium
setting (MS), and slow setting (SS), as specified in ASTM D977
• The cationic emulsified asphalts include rapid setting (CRS),
medium setting (CMS), and slow setting (CSS), as specified in
ASTM D2397
TYPES OF PETROLEUM ASPHALTS:
Emulsified asphalt-----contd.
• Selection and uses of emulsified asphalts are given in ASTM D3628.
Generally, they are used as follows:
Type of emulsified
asphalts
Uses
Rapid-setting
Surface treatments and penetration
macadam's
Medium-setting
Open-graded
cold
aggregate mixtures
Slow-setting
Track coat, fog seal, dense-graded
cold asphalt-aggregate mixtures,
and slurry seals
asphalt-
TYPES OF PETROLEUM ASPHALTS:
Liquid or cutback asphalt
• Liquid asphalts or cutback asphalts are asphalt cements mixed with a
solvent to reduce their viscosity to make them easier to use at ordinary
temperatures
• They are commonly heated and then sprayed on aggregates
• Upon evaporation of the solvent, they cure or harden and cement the
aggregate particles together
• Types and grades, as given below, are based on the type of solvent, which
governs viscosity and the rates of evaporation and curing
– Rapid-Curing (RC). Produced by
adding a light diluent of high volatility
(generally gasoline or naphtha) to asphalt cement. These are used primarily
for tack coat and surface treatments. Specifications for RC type given in
ASTM D2028.
– Medium-Curing (MC). Produced by adding a medium diluent of intermediate
volatility (generally kerosene) to asphalt cement. These are generally used for
prime coat, stockpile patching mixtures, and road-mixing operations.
Specifications for MC type given in ASTM D2027.
– Slow-Curing (SC). Produced by adding oils of low volatility (generally diesel or
other gas oils) to asphalt cement. They are also called road oils. They are
generally used for prime coat, stockpile-patching mixtures, and as dust
palliatives. Specifications for SC type given in ASTM D2026.
TYPES OF PETROLEUM ASPHALTS:
Liquid or cutback asphalt------contd.
• Cutback asphalts are commercially available in different grades, as
shown in the following Fig.:
• The thinnest and most fluid grade is designated by the suffix
number 30, which is available in MC type only
TYPES OF PETROLEUM ASPHALTS:
Liquid or cutback asphalt------contd.
Emulsified asphalts are increasingly being used in lieu of cutback
asphalts for the following reasons:
– Environmental regulations: Emulsions are relatively pollution
free
– Loss of high-energy products: When cutback asphalts cure, the
diluents which are high energy, high price products are wasted
into atmosphere
– Safety: Emulsions are safe to use
– Lower application temperature: Emulsions can be applied at
relatively low temperatures saving the fuel costs. Emulsions can
also be applied effectively to a damp pavement, whereas dry
conditions are required for cutback asphalts
PROPERTIES OF ASPHALTS
• Following are the properties of asphalt that are of great
importance in pavement design and construction:
–
–
–
–
–
–
–
–
–
Consistency
Specific gravity
Ductility
Purity
Flash point
Rate of curing
Aging or hardening
Resistance to reaction with water
Durability
PROPERTIES OF ASPHALTS
Consistency
• Consistency describes the degree of fluidity of asphalt cement at
any particular temperature
• Consistency of asphalt cement is measured in terms of viscosity
and penetration values
• Softening point is also measured to determine the temperature at
which an asphalt changes its phase from solid to liquid
PROPERTIES OF ASPHALTS
Consistency: Viscosity
• Viscosity is a measure of the resistance to flow and is the
fundamental consistency measurement in absolute units
• There are many methods to measure the “viscosity”
– Absolute viscosity (in Pa-s or poise, 1 poise = 0.1 Pa-s) measured at
140 ºF, as per the ASTM D2171 specifications
– Kinematic viscosity (in cm2/s or stoke, 1 stoke = 100 centistokes = 1
cm2/s) measured as per the ASTM D2170 specifications
– kinematic viscosity = absolute viscosity/density
• Viscosity decreases (i.e. materials become more fluid) as
temperature increases
• A very viscous fluid is very thick
PROPERTIES OF ASPHALTS
Consistency: Viscosity----contd.
•
Following figure illustrates typical temperature-viscosity relationships for asphalts
• Grades of asphalt materials and
temperatures at which they are used
depend to a great extent on their
viscosity
• Plant temperatures for mixing
asphalt-paving materials are usually
specified in terms of viscosity
• Temperature limits corresponding to
viscosities of 1.5 to 3.0 cm2/s (150 to
300 centistokes) are sometimes used
For example, the plant temperatures
for heating the asphalts A, B, and C
(as shown in the figure) will
respectively be in the following
ranges corresponding to viscosities
of 150 to 300 centistokes: 153 ºC to
167 ºC; 148 to 163 ºC; and 137 ºC to
149 ºC.
PROPERTIES OF ASPHALTS
Consistency: Penetration
• Penetration test (ASTM D5) is used to measure the consistency of asphalt
empirically (i.e., indirectly)
• Penetration is the consistency of asphalt expressed as the distance in
tenths of a millimeter that a standard needle penetrates a sample of
asphalt vertically under standard conditions of loading (100 g), time (5
seconds), and temperature (77 °F), as shown in figure below:
Asphalt cements have an upper
limit on penetration value of
300 (i.e., 30 mm)
PROPERTIES OF ASPHALTS
Consistency: Softening point
• Softening point is measured by ring and ball (R & B) method in
accordance with ASTM D36, as shown in the following figure:
Softening point can simply be
defined as the temperature at
which asphalt cement cannot
support the weight of a steel
ball and starts flowing.
PROPERTIES OF ASPHALTS
Specific gravity
• Specific gravity and density of asphalt can be determined using
the procedure described in ASTM D70.
• Since the volume of asphalt changes with temperature (with the
change from semisolid to liquid state), the specific gravity of
asphalt is expressed at a given temperature (usually at 77 °F or 60
°F)
• Petroleum asphalts have specific gravity values close to unity
(0.95 – 1.05)
• Specific gravity of asphalt decreases with increasing temperature.
For example, asphalt cement has a specific gravity of 1.0176 at a
temperature of 10 °F, 1.0 at 60 °F, and 0.9187 at 300 °F.
• A knowledge of specific gravity is essential to determine the
percentage of voids in a compacted material. Also, its
measurement is required to convert the volume measurement of
asphalt to the units of mass
PROPERTIES OF ASPHALTS
Ductility
• Ductility test is performed on the asphalt cement samples in
accordance with ASTM D113.
• Ductility test measures the distance in centimeters that a standard
briquette of asphalt cement will stretch (@ 5 cm/min at 77 °F)
before breaking, as shown in the following figure:
• Ductility is sometimes
used as an indirect
gage of adhesion and
cohesion of asphalt
• Adhesion is the ability
to stick to aggregate
particles in the asphalt
concrete
• Cohesion is the ability
to hold the aggregate
particles firmly in place
PROPERTIES OF ASPHALTS
Purity
• Refined asphalt cements consist of almost pure
bitumen, which by definition is entirely soluble in
carbon disulfide. Only very little amounts of impurities
are generally present in refined asphalt cements
• To determine the purity of asphalt cement, a
“solubility” test is conducted in accordance with ASTM
D2042
PROPERTIES OF ASPHALTS
Flash point
• If asphalt cement is heated to a high enough temperature, it gives
off enough vapor to flash (ignite) in presence of spark or open
flame
• Flash point is the temperature below that of the fire point, which is
the temperature where a material combusts.
• Flash point test is conducted in accordance with ASTM D92 to
indicate the temperature to which asphalt cement may be safely
heated without the danger of instantaneous flash in the presence
of an open flame
PROPERTIES OF ASPHALTS
Rate of curing
• The process of evaporation of solvents from cutback asphalts,
and the attendant thickening of the material, is called curing.
• Curing can also be described as the change in consistency of an
asphalt due to the progressive loss of diluents by evaporation
• The rate of curing, or the time required for a cutback asphalt to
harden (from its original liquid consistency) and develop a
consistency that is satisfactory for the function as a binder in
pavements is an important property of cutback asphalts
• The rate of curing is influenced by the following factors:
–
–
–
–
–
–
Volatility or evaporation rate of the solvent
The amount of solvent
Penetration (or viscosity) of the asphalt base
Temperature and humidity of the environment
Wind velocity
Surface area of the pavement
PROPERTIES OF ASPHALTS
Aging or hardening
• Aging or hardening is the process under which asphalt becomes harder
and brittle due to increase in the viscosity of asphalt caused by:
– the evaporation and oxidation of the lighter, oily constituents during mixing at
high temperatures, called “volatilization” (short-term aging or hardening)
– the oxidation of the oils to resins and resins to asphaltenes when used over a
period of years (long-term or in-service aging or hardening)
Durability of pavement
is adversely affected by
aging
PROPERTIES OF ASPHALTS
Resistance to action of water
• Asphalt materials designed for pavements should be able to
withstand the effects of water
• The durability of the pavement is greatly affected by the ability of
asphalt to adhere to aggregate particles in the presence of water
• Loss of bond in the presence of water may lead to pavement
deterioration
PROPERTIES OF ASPHALTS
Durability
• Durability can be defined as the property that permits a pavement
material to withstand the detrimental effects of moisture, air, and
temperature.
• The performance and durability of an asphaltic pavement are
affected by a number of factors, including mix design, properties
of aggregates, workmanship, and the properties of the asphalt.
• Asphalt pavements fail or crack due to three main causes:
– Permanent deformation or rutting occurring at high
temperatures as the asphalts softens and the mix loses
elasticity
– Fatigue cracking due to high volume of load applications and
aging of the asphalt
– Low-temperature cracking as asphalt becomes brittle and the
pavement shrinks in cold weather