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Highway Engineering
ROADS CONSTRUCTION
MATERIALS & TESTING
C3010 / UNIT 4
UNIT 4
ROAD CONSTRUCTION MATERIALS AND TESTING
OBJECTIVES
General Objective
To understand basic elements of road construction materials and testing
Specific Objectives
At the end of the unit you should be able to: identify the material used in the road construction.
 describe the function of the road materials and testing.
 explain the purpose of road materials and testing.
 perform the testing material testing.
 analyze result of the test.
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ROADS CONSTRUCTION
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INPUT
ROAD CONSTRUCTION MATERIALS AND THEIR
FUNCTIONS
4.0
INTRODUCTION
Construction materials for roads may be broadly classified into four
categories:
a. Soil
b. Aggregates
c. Bituminous materials
d. Cement
ROAD
= SOIL + AGGREGATES + PAVEMENT
MATERIALS
(BITUMENS/CEMENTS)
Aggregate which has been defined as inert mineral fragments and
particles forming the main structure of a mixture such as asphalt, tar-macadam or
concrete is the basic material for road construction. This material which forms the
greater part of the body of the road, bears the main stresses occurring in the
road and resists wear from surface abrasion.
The particles are bound together using bituminous materials and cement
which act as binders and develop a strong structure forming the crust of the road.
As such properties of all these materials are of great importance to the highway
engineers. A knowledge of their characteristics is useful in the design of road
structure.
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4.1
ROADS CONSTRUCTION
MATERIALS & TESTING
C3010 / UNIT 4
SOILS AND ITS CHARACTERISTIC
The term soil has different meanings and connotations to different
professional groups. From an engineering stand-point the term soil refers to all
the unconsolidated mineral material lying above the bed rock with which and
upon which highways and other structures are constructed. It may contain air,
water, organic matter and other substances which may be dispersed throughout
the minerals particles. The mineral matter is formed by the disintegration or
decomposition of the rocks. The engineering properties and behavior are
generally influenced by changes in moisture content and density or compactness
of the soil. Wide variation in soil types is generally found in nature and hence a
study of their properties becomes essential.
4.1.1 CHARACTERISTICS OF SOIL PARTICLES
Certain characteristics of the soil particles are useful in predicting
the performance and behavior of soils ; namely the grain size, shape,
surface texture, and chemical position. These are briefly discussed here.
4.1.1.1
Grain size.
The size of the soil particles and the distribution of various
sizes throughout a soil mass are important factors that influence
soil properties and performance. A number of grain size
classifications have been proposed by different highway agencies
for convenience in expressing the size characteristics of various
fractions.
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The general characteristics of the various particle size groupings
are as follows:
1. Gravel.
It consists of rock fragments more or less rounded by water
action or abrasion. Quartz which is the hardest rock forming mineral
is usually the principal constituent. Well rounded pebbles and
boulders that have undergone long wear are almost entirely quartz.
Gravels which are angular and rough due to their high resistance to
wear, usually contain rocks such as granite, schist, basalt or
limestone.
2. Course Sand.
It is frequently rounded like gravel with which it may exist in
combination. It generally contains the same minerals.
3. Fine Sand.
Particles are more angular than coarse sand particles
because the film of water that usually surrounds the finer particles
serves as cushion to protect them from abrasion. Silt grains are
usually similar to fine sand and have the same mineral composition.
However, they may be produced by chemical decay rather than by
grinding. Occasionally, silts contain pumice, loess or other
materials foreign to the associated sand. The presence of silt in fine
soil may be detected in the field by the feeling of roughness that it
gives when rubbed between fingers.
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ROADS CONSTRUCTION
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4. Clay.
Clay results almost entirely from chemical weathering and
are often plate – like in shape. The structure of moist clays is
considered to be very complicated and has not been fully
understood yet, although attempts have been made by many
Chemists, Physicists, Geologists and Engineers to interpret it in the
various fields of the Lambe has given a very good account of the
structure of clays in his paper entitled “The Structure of Inorganic
Soil”, presented to the ASCE.
5. Colloids.
These are finer clay particles that remain suspended in
water and do not settle under the force of gravity. When observed
under microscope they are found to be in a state of disorderly
motion. For most of the highway engineering works, coarse grained
materials are preferable. Clayey soils are extremely troublesome
and require specialized treatments and methods of handlings.
Brownian Movement specially silty soils present problems in the
areas susceptible to frost action.
4.1.2 SHAPE OF PARTICLES.
Strength and toughness of the material is indicated to a large extent
by the shape of the particles. For example, gravels found in the river beds
have undergone a large process of wear and are stronger than the flat and
flaky particles.
In the construction of base courses where usually granular material
is recommended, there is little percentage of the clay particles. Here the
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shape of the bigger stone particles recommended is angular as they are
better resist the tendercy to deformation. Hence many specifications for
base-course require that the mixture of different materials used must
contain a specified percentage of crushed rock particles.
The effect of the shape of particles on the performance of mass
composed mainly of clay-size fraction is not conclusively known. Here the
presence of the free and absorbed moisture, particle ionization presence
of chemicals and density are some of the more important factors.
4.1.3 SURFACE TEXTURE
The surface texture of the large size soil particles greatly influences
their performance in granular soil mixtures. For example, the friction
developed on the grains of wind-blown and beach sands and of crushed
quartz is relatively low since the particles have slicked surfaces and as a
result thereof the mixture that contains large percentage of such materials
offer small resistance to deformation.
Some authorities assert that a rough surface of a freshly crushed
rock is more important than its angularity in developing greater strength in
soil mixture. With small size particles this effect of surface texture
becomes negligible.
4.1.4 CHEMICAL COMPOSITION
Chemical composition indirectly affects the behavior of soils since
the presence of certain chemicals influences the water-film that usually
surrounds the particle and hence the bond holding any two particles
mutually together.
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Some complex reactions might develop in the soil mass as a result
of the addition of an additive as a stabilizer in the presence of an
appropriate chemical, resulting in the improvement of structural behavior
of the compacted soil mass.
4.2
EFFECT OF MOISTURE ON THE PERFORMANCE OF SOILS
All soil engineers agree that the properties of soils composed largely of
coarse grained materials are primarily controlled by the characteristics of the
particles but for soils composed largely of clays and colloids, the properties are
primarily controlled by the characteristics of the moisture film surrounding the
particles.
The effect of the change in moisture content is reflected in
three ways :
a. The effective diameter of the soil particle
changes with change in the moisture
content. The effect is more prominent in
the clay particles than in the sand
INFO
particles.
b. The soil particles have the property of
clinging to each other due to the existence
of thin tensile forces in water films. These
forces are due to the surface tension. The
force exerted by surface tension
decreases as the moisture content
increases.
c. The volume of the soil-mass as a whole
changes with the change in the moisture
content at the particle surface. The
changes in volume are more prominent in
clays than in sands and silts.
The effect of
increase in moisture
content is to
decrease the
strength of soil
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4.3
ROADS CONSTRUCTION
MATERIALS & TESTING
C3010 / UNIT 4
EFFECT OF DENSITY ON THE BEHAVIOUR OF SOILS
Dry density of soil is the dry weight per unit volume of the material. The
soil mass consists of dry soil particles and air when in the dry state, and soil
particles, air and water when in the wet state. If the density of soil changes, say
increase, the quantity of soil solids in unit volume increases so that lesser voids
would be left for air and water.
This increase in density and decrease in moisture content would add to
the strength of the soil. The maximum density is achieved at optimum moisture
content which can be determined by any standard test. In addition to the increase
in strength, the rate of water movement through the soil and volume change due
to variation in moisture, subsequent to compaction, register a measurable
decrease. In order to secure these advantages, it has been accepted as a normal
practice to compact the sub grade soils so as to obtain maximum dry density.
4.4
ROAD AGGREGATE
The raw material which is used as road aggregate can be further classified
as :
a. Naturally occurring aggregate :
-
Crush Rock Aggregate
-
Gravels
-
Sand.
b. Artificial Aggregate
-
Blast Furnace Slag
Each of the three types mentioned under (a) above is one or the other
form of natural rock. The crushed road aggregate is prepared by quarrying
natural rock from the quarries having suitable road construction material and
crushing it to the required size. The coarse material which results from the
disintegration of natural rocks due to weathering, is carried away by water into
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the rivers and is deposited as gravel. During transit, the particles are worn out by
attrition and become more or less rounded in shape with smooth surfaces. Sand,
on the other hand, is the final residue of resistant mineral grains resulting from
rock-weathering and has often been through many cycles of deposition and
weathering.
Blast furnace slag is, perhaps, the only artificially prepared aggregate
used in road construction. It is obtained as a by-product in the manufacture of
steel.
4.4.1 Crushed Rock
Crushed rock aggregate is produced from different types of rocks.
All rocks are composed of grains or crystals of minerals held together in a
variety of ways. The properties of rocks, therefore are dependent upon the
properties of minerals forming the rocks and the nature of the bond.
Igneous, Sedimentary and Metamorphic are the three major classifications
of these rocks.
The Igneous rocks are further classified into Plutonic Hypabyssal
and Volcanic, according to their mode of formation, the properties of acid
and basic radicals and the chemical constituents of the mineral present. In
the Plutonic variety, the main types are the Granite and Syenite, though
Diorite and Gabbro are also frequently available. They are mainly coarsegrained varieties being Porphyry, Porphyrite, Dolorite and Diabase. These
rocks very frequently posses intergrown texture and as such are among
the best of the road stones. Basalts and Andersite fall in the Volcanic
category and are fine grained varieties of ignoeus rocks. They are usually
not considered good as road stones.
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All igneous rocks are subject to decomposition and weathering on
exposed surface or along fissures and also internally due to the chemical
instability of their minerals. In fact, these factors require time on geological
scale and so long as the quarried rock is fresh and reasonably sound, it is
unlikely to suffer from decomposition during the life of the road. A rock
which is badly decomposed, discoloured and is friable, should never be
used as road-stone.
The sedimentary rocks are classified as Calcareous, Siliceous and
Agrillaceous. Limestones, Dolomies and Chalk fall under the Calcareous
variety. Only the best varieties of limestones serve as good road-stones,
since they have adequate strength and have excellent adhension to
bituminous binders. Dolomites have the same properties as the lime
stones. Chalk is poor road-stone. Sandstones and quartzite are the main
varieties of the Silsceous sedimentary rocks. Sandstones are used as
road-aggregate though adhension to bitumen is not very good. Quartzite is
quite hard but is liable to be brittle and the adhension to bitumen is rather
poor. The Argillaceous sedimentary rocks have clay-shales and mudstones as the main varieties. They are poor roadstones.
Among the third general class of rocks, Hornfels which are formed
by the thermal metamorphism are the best from road-making point of
view. Gneiss and Granulite have the same road-making qualities as the
granites of coarse variety. Slate and Schist are quite unsuitable as road
stones.
4.4.2 Gravels
Gravel may consist entirely of one type of hard rock such as Flint or
Quartzite (silicious sedimentary variety) or may have a wider variety of
rocks of different types and hardnesses. Gravels of softer variety of rocks
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ROADS CONSTRUCTION
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C3010 / UNIT 4
are generally not available, though at places, limestone gravels also occur
and are used for the construction of roads, where other suitable material is
not available.
In general, the properties of the gravels depend upon the properties
of the rocks from which they are formed. Mixed gravels usually contain, in
addition to the hard varieties, soft particles of chalk, clay lumps, shale or
soft sandstone and, therefore, whenever gravels have to be used in road
construction, they must be tested lest any such materials should form a
high proportion and thus act as a source of weakness.
4.4.3 Sands
Natural sands consist largely of the final residue of resistant mineral
grains resulting from rock weathering and have often been through many
cycles of deposition and weathering. Quartz is the most important mineral
in sands, since this substance is hardly affected by ordinary weathering
agents. Sand occurs sometimes in combination are the larger particles of
sand in such a combination are the chips from gravel itself.
4.4.4 Slag
Slag obtained as a by-product from blast furnaces is extensively
used as road aggregate. It is composed of a number of alumino-silicates
of calcium and magnesium, minerals that are found in some basic igneous
rocks. Owing to the rapid cooling of material, slag becomes a much more
variable material than natural rock. There is tremendous difference in the
grain-size and porosity of exterior and interior of the cooling masses, the
exterior cooling rapidly so as to form an amorphous glass. Slag can
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ROADS CONSTRUCTION
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become a very good aggregate, if it is manufactured under carefullycontrolled conditions.
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ACTIVITY 4A
“WANT TO IMPROVE YOUR UNDERSTANDING? TRY THESE
ACTIVITIES AND GOOD LUCK!!!”
4.1
Name four categories of road construction materials.
a.____________________________________
b.____________________________________
c.____________________________________
d.____________________________________
4.2
Write down the short information of soil characteristics in the chart
below.
Grain Size - _____________________
_______________________________
_______________________________
Soil
Characteristics
Shape Of Particles - _______________
_______________________________
_______________________________
Surface Texture -_________________
_______________________________
_______________________________
Chemical Composition - ___________
_______________________________
_______________________________
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4.3
C3010 / UNIT 4
ROADS CONSTRUCTION
MATERIALS & TESTING
There are 10 types of crushed rock aggregate given below and
hidden within the puzzle. You are required to identify the
aggregate and then write down into their categories given in the
table.
GRANITE
HORNFELS
CALCAREOUS
SCHIST
GRANULITE
SILICEOUS
SYENITE
DOLORITE
AGRILLACEOUS
PORPHYRITE
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ROADS CONSTRUCTION
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C3010 / UNIT 4
FEEDBACK ON
ACTIVITY 4A
Let’s Check The Answers!
4.1
a.
Soil
b.
Aggregates
c.
Bituminous materials
d.
Cement
4.2
Grain Size – The size of the soil particles and the distribution of
varies sizes throughout a soil mass are important factors that
influence soil properties and performances.
Shape of Particle – Strength and toughness of the material is
indicated to a large extend by shape of the particles.
Surface Texture – The surface texture of the large size soil
particles greatly influences their performance in granular soil
mixtures.
Chemical Composition – Chemical composition indirectly affects
the behavior of soil since the presence of certain chemicals
influences the water-film that usually surrounds the particles and
hence the bond holding any two particles mutually together
Highway Engineering
C3010 / UNIT 4
ROADS CONSTRUCTION
MATERIALS & TESTING
4.3
G
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The categories are as follows :
IGNEOUS
SEDIMENTARY
METAMORPHIC
Granite
Calcareous
Hornfels
Syenite
Siliceous
Granulite
Dolorite
Agrillaceous
Schist
Porphyrite
Well, how’s your performance so far?
Let’s continue the adventures of unit 4….
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ROADS CONSTRUCTION
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Question
1.
The effect of the change in moisture content is reflected in three ways.
Describe two from the three ways that listed.
2.
Describe the following aspects: a. Gravel.
b. Slag.
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Answer
1. The effect of the change in moisture content is reflected in three ways :
a. The effective diameter of the soil particle changes with change
in the moisture content. The effect is more prominent in the clay
particles than in the sand particles.
b. The soil particles have the property of clinging to each other due
to the existence of thin tensile forces in water films. These
forces are due to the surface tension. The force exerted by
surface tension decreases as the moisture content increases.
c. The volume of the soil-mass as a whole changes with the
change in the moisture content at the particle surface. The
changes in volume are more prominent in clays than in sands
and silts.
2.
i.
Gravels
Gravel may consist entirely of one type of hard rock such as
Flint or Quartzite (silicious sedimentary variety) or may have a
wider variety of rocks of different types and hardnesses. Gravels of
softer variety of rocks are generally not available, though at places,
limestone gravels also occur and are used for the construction of
roads, where other suitable material is not available.
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In general, the properties of the gravels depend upon the
properties of the rocks from which they are formed. Mixed gravels
usually contain, in addition to the hard varieties, soft particles of
chalk, clay lumps, shale or soft sandstone and, therefore, whenever
gravels have to be used in road construction, they must be tested
lest any such materials should form a high proportion and thus act
as a source of weakness.
ii. Slag
Slag obtained as a by-product from blast furnaces is
extensively used as road aggregate. It is composed of a number of
alumino-silicates of calcium and magnesium, minerals that are
found in some basic igneous rocks. Owing to the rapid cooling of
material, slag becomes a much more variable material than natural
rock. There is tremendous difference in the grain-size and porosity
of exterior and interior of the cooling masses, the exterior cooling
rapidly so as to form an amorphous glass. Slag can become a very
good aggregate, if it is manufactured under carefully controlled
conditions.