Bituminous road constructions steps.docx

8/12/2019 Bituminous road constructions steps.docx

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Bituminous road constructions steps:

1. Preparation of the existing base course layer

The existing surface is prepared by removing the pot holes or rust if any. The irregularities are filled in with premix chippings at least a week

before laying surface course. If the existing pavement is extremely way, a bituminous leveling course of adequate thickness is provided to lay

a bituminous concrete surface course on a binder course instead of directly laying it on a WBM.

2. Application of Tuck Coat

It is desirable to lay AC layer over a bituminous base or binder course. A tack coat of bitumen is applied at 6.0 to 7.5 kg per 10 sq.m area,

this quantity may be increased to 7.5 to 10 kg for non-bituminous base.

3. Preparation and placing of Premix

The premix is prepared in a hot mix plant of a required capacity with the desired quality control. The bitumen may be heated upto 150 177

deg C and the aggregate temperature should not differ by over 14 deg C from the binder temperature. The hot mixed material is collected

from the mixture by the transporters, carried to the location is spread by a mechanical paver at a temperature of 121 to 163 deg C. the

camber and the thickness of the layer are accurately verified. The control of the temperatures during the mixing and the compaction are of

great significance in the strength of the resulting pavement structure.

4. Rolling

A mix after it is placed on the base course is thoroughly compacted by rolling at a speed not more than 5km per hour.

The initial or break down rolling is done by 8 to 12 tonnes roller and the intermediate rolling is done with a fixed wheel pneumatic roller of 15

to 30 tonnes having a tyre pressure of 7kg per sq.cm. the wheels of the roller are kept damp with water.

The number of passes required depends on the thickness of the layer. In warm weather rolling on the next day, helps to increase the density

if the initial rolling was not adequate. The final rolling or finishing is done by 8 to 10 tonne tandem roller.


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Fig: Tandem Roller

5. Quality control of bituminous concrete construction

The routine checks are carried out at site to ensure the quality of the resulting pavement mixture and the pavement surface.

Periodical checks are made for

a) Aggregate grading

b) Grade of bitumen

c) Temperature of aggregate

d) Temperature of paving mix during mixing and compaction.

At least one sample for every 100 tonnes of the mix discharged by the hot mix plant is collected and tested for above requirements.Marshall

testsare also conducted. For every 100 sq.m of the compacted surface, one test of the field density is conducted to check whether it is atleast

95% of the density obtained in the laboratory. The variation in the thickness allowed is 6mm per 4.5m length of construction.

6. Finished surface:

The AC surface should be checked by a 3.0 m straight edge. The longitudinal undulations should not exceed 8.0 mm and the number of

undulations higher than 6.0 mm should not exceed 10 in a length of 300 m. The cross-traffic profile should not have undulations exceeding

4.0mm.

COMPOSITION AND STRUCTURE OF FLEXIBLE PAVEMENT

Flexible pavements support loads through bearing rather than flexural action. They comprise several

layers of carefully selected materials designed to gradually distribute loads from the pavement surface

to the layers underneath. The design ensures the load transmitted to each successive layer does not

exceed the layers load-bearing capacity. A typical flexible pavement section is shown in Figure 1. Figure

2 depicts the distribution of the imposed load to the subgrade. The various
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layers composing a flexible pavement and the functions they perform are described below:

a) Bituminous Surface (Wearing Course). The bituminous surface, or wearing course, is made up of a

mixture of various selected aggregates bound together with asphalt cement or other bituminous

binders. This surface prevents the penetration of surface water to the base course; provides a smooth,

well-bonded surface free from loose particles, which might endanger aircraft or people; resists thestresses caused by aircraft loads; and supplies a skid-resistant surface without causing undue wear on

tires.

b) Base Course. The base course serves as the principal structural component of the flexible pavement.

It distributes the imposed wheel load to the pavement foundation, the subbase, and/or the subgrade.

The base course must have sufficient quality and thickness to prevent failure in the subgrade and/or

subbase, withstand the stresses produced in the base itself, resist vertical pressures that tend to

produce consolidation and result in distortion of the surface course, and resist volume changes caused

by fluctuations in its moisture content. The materials composing the base course are select hard and

durable aggregates, which generally fall into two main classes: stabilized and granular. The stabilized

bases normally consist of crushed or uncrushed aggregate bound with a stabilizer, such as Portland

cement or bitumen. The quality of the base course is a function of its composition, physical properties,

and compaction of the material.

c) Subbase. This layer is used in areas where frost action is severe or the subgrade soil is extremely

weak. The subbase course functions like the base course. The material requirements for the subbase are

not as strict as those for the base course since the subbase is subjected to lower load stresses. The

subbase consists of stabilized or properly compacted granular material.

d) Frost Protection Layer. Some flexible pavements require a frost protection layer. This layer functions

the same way in either a flexible or a rigid pavement.

e) Subgrade. The subgrade is the compacted soil layer that forms the foundation of the pavement

system. Subgrade soils are subjected to lower stresses than the surface, base, and subbase courses.

Since load stresses decrease with depth, the controlling subgrade stress usually lies at the top of the

subgrade. The combined thickness of subbase, base, and wearing surface must be great enough to

reduce the stresses occurring in the subgrade to values that will not cause excessive distortion or

displacement of the subgrade soil layer.


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Fig 2: Distribution of load stress in flexible pavement

Compaction means pressing of soil particles close to each other by mechanical methods. Air during

compaction is expelled from the void space in the soil mass and therefore the mass density is increased.

Compaction is done to improve the engineering properties of the soil. Compaction of soil is required for

the construction of earth dams, canal embankments, highways, runways and many other structures.

STANDARD PROCTOR TEST

To assess the amount of compaction and water content required in the field, compaction tests are done

on the same soil in the laboratory. The test provides a relationship between the water content and the

dry density. The water content at which the maximum dry density is attained is obtained from the

relationship provided by the tests. Proctor used a standard mould of 4 inches internal diameter and an

effective height of 4.6 inches with a capacity of 1/30 cubic foot. The mould had a detachable base plate

and a removable collar of 2 inches height at its top. The soil is compacted in the mould in 3 layers, eachlayer was given 25 blows of 5.5 pounds rammer filling through a height of 12 inches.

IS: 2720 part VII recommends essentially the same specification as in Standard Proctor test, some minor

modifications. The mould recommended is of 100mm diameter, 127.3 mm height and 1000ml capacity.

The rammer recommended is of 2.6 kg mass with a free drop of 310mm and a face diameter of 50mm.

The soil is compacted in three layers. The mould is fixed to the detachable base plate. The collar is of

60mm height.

Procedure

About 3kg of air dried soil is taken for the test. It is mixed with 8% water content and filled in the mouldin three layers and giving 25 blows to each layer. The volume of the mould and mass of the compacted

soil is taken. The bulk density is calculated from the observations. A representative sample is placed in

the oven for determination of water content. The dry density id found out from the bulk density and

water content. The same procedure is repeated by increasing the water content.

Presentation of results


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Compaction curve

A compaction curve is plotted between the water content as abscissa and the corresponding dry density

as ordinate. It is observed that the dry density initially increases with an increase in water content till

the maximum density is attained. With further increase in water content the dry density decreases. The

water content corresponding to maximum dry density is known as the optimum water content (O.W.C)

or the optimum moisture content (O.M.C).

At a water content more than the optimum, the additional water reduces the dry density as it occupies

the space that might have been occupied by the solid particles.

For a given water content, theoretical maximum density is obtained corresponding to the condition

when there are no air voids (degree of saturation is 100%). The theoretical maximum density is also

known as saturated dry density. The line indicating theoretical maximum density can be plotted along

with the compaction curve. It is known as the zero air void line.

MODIFIED PROCTOR TEST

The modified Proctor test was developed to represent heavier compaction than that in the standard

Proctor test. The test is used to simulate field conditions where heavy rollers are used. The test was

standardized by American association of State Highway Officials and is, therefore also known as

modified AASHO test.

In this, the mould used is same as that in the Std Proctor test. However, the rammer used is much

heavier and has a greater drop than that in the Std Proctor test. Its mass is 4.89 kg and the free drop is

450mm. The soil is compacted in five equal layers; each layer is given 25 blows. The compactive effort in


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modified Proctor test is 4.56 times greater than in the Std Proctor test. The rest of the procedure is

same

FACTORS AFFECTING COMPACTION

o Water Content

At low water content, the soil is stiff and offers more resistance to compaction. As the water content is

increased, the soil particles get lubricated. The soil mass becomes more workable and the particles have

closer packing. The dry density of the soil increases with an increase in the water content till the O.M.C

is reached.

o Amount of compaction

The increase in compactive effort will increase the dry density at lower water content to a certain

extent.

o Type of soil

The dry density achieved depends upon the type of soil. The O.M.C and dry density for different soils are

different

o Method of compaction

The dry density achieved depends on the method of compaction

EFFECT OF COMPACTION ON PROPERTIES OF SOILS

1. Soil Structure

Soils compacted at a water content less than the optimum generally have a flocculated structure. Soils

compacted at water content more than the optimum usually have a dispersed structure.

2. Permeability

The permeability of a soil depends upon the size of voids. The permeability of a soil decreases with an

increase in water content on the dry side of optimum water content.

3. Swelling

4. Pore water pressure

5. Shrinkage

6. Compressibility

7. Stress-strain relationship

8. Shear strength


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METHODS OF COMPACTION USED IN THE FIELD

Several methods are used in the field for compaction of soils. The choice of method will depend upon

the soil type, the maximum dry density required and economic consideration. The commonly used

methods are

1. Tampers

2. Rollers

3. Vibratory compactors

The compaction depends upon the following factors

o Contact pressure

o Number of passes

o Layer thickness

o Speed of roller

Types of rollers

o Smooth-wheel rollers

o Pneumatic tyred rollers

o Sheep foot rollers

COMPACTION CONTROL

Compaction control is done by measuring the dry density and the water content of compacted soil in the

field

o Dry density

The dry density is measured by core cutter method and sand replacement method

o Water content

For the measurement of water content, oven drying method, sand bath method, calcium carbidemethod etc are used. Proctor needle is also used for this.

Compaction is the application of mechanical energy to a soil to rearrange the particles and reduce the void ratio. By doing compaction, we are i ncreasing the shearstrength of soils. There are different methods of compaction which are discussed here. The purpose, laboratory tests, effect of moisture etc has been discussedhere.

Purpose of Compaction

o The principal reason for compacting soil is to reduce subsequent settlement under working loads.


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o Compaction increases the shear strength of the soil.

o Compaction reduces the voids ratio making it more difficult for water to flow through soil. This is important if the soil is being used to retain

water such as would be required for an earth dam.

o Compaction can prevent the build up of large water pressures that cause soil to liquefy during earthquakes.

Factors affecting Compaction

o Water content of the soil

o The type of soil being compacted

o The amount of compactive energy used

Laboratory Compaction tests

There are several types of test which can be used to study the compactive properties of soils. Because of the importance of compaction in

most earth works standard procedures have been developed. These generally involve compacting soil into a mould at various moisture

contents.

o Standard Compaction Test AS 1289-E1.1

Soil is compacted into a mould in 3-5 equal layers, each layer receiving 25 blows of a hammer of standard weight. The apparatus is shown in

Figure 1 below. The energy (compactive effort) supplied in this test is 595 kJ/m3. The important dimensions are

Volume of mould Hammer mass Drop of hammer

1000 cm3 2.5 kg 300 mm

Because of the benefits from compaction, contractors have built larger and heavier machines to increase the amount of compaction of the

soil. It was found that the Standard Compaction test could not reproduce the densities measured in the field and this led to the development

of the Modified Compaction test.

o Modified Compaction Test AS 1289-E2.1

The procedure and equipment is essentially the same as that used for the Standard test except that 5 layers of soil must be used. To provide

the increased compactive effort (energy supplied = 2072 kJ/m3) a heavier hammer and a greater drop height for the hammer are used. The

key dimensions for the Modified test are

Volume of mould Hammer mass Drop of hammer

1000 cm3 4.9 kg 450 mm


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Presentation of Results

To assess the degree of compaction it is important to use the dry unit weight, dry, because we are interested in the weight of solid soil

particles in a given volume, not the amount of solid, air and water in a given volume (which is the bulk unit weight). From the relationships

derived previously we have

which can be rearranged to give

Because Gsand ware constants it can be seen that increasing dry density means decreasing voids ratio and a more compact soil .

In the test the dry density cannot be measured directly, what are measured are the bulk density and the moisture content. From the

definitions we have


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This allows us to plot the variation of dry unit weight with moisture content, giving the typical reponse shown in Figure 2 below. From this

graph we can determine the optimum moisture content, mopt, for the maximum dry unit weight, (dry)max.


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Figure 2 A typical compaction test result

If the soil were to contain a constant percentage, A, of voids containing air where

writing Vaas V VwVswe obtain

then a theoretical relationship between dryand m for a given value of A can be derived as follows

If the percentage of air voids is zero, that is, the soil is totally saturated, then this equation becomes

From this equation we see that there is a limiting dry unit weight for any moisture content and this occurs when the voids are full of water.

Increasing the water content for a saturated soil results in a reduction in dry unit weight. The relation between the moisture content and dry

unit weight for saturated soil is shown on the graph in Figure 3. This line is known as the zero air voids line.


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Figure 3 Typical compaction curve showing no-air-voids line

Effects of water content during compaction

As water is added to a soil ( at low moisture content) it becomes easier for the particles to move past one another during the application of

the compacting forces. As the soil compacts the voids are reduced and this causes the dry unit weight ( or dry density) to increase. Initially

then, as the moisture content increases so does the dry unit weight. However, the increase cannot occur indefinitely because the soil state

approaches the zero air voids line which gives the maximum dry unit weight for a given moisture content. Thus as the state approaches the

no air voidsline further moisture content increases must result in a reduction in dry unit weight. As the state approaches the no air voids line

a maximum dry unit weight is reached and the moisture content at this maximum is called the optimum moisture content.

Effects of increasing compactive effort

Increased compactive effort enables greater dry unit weights to be achieved which because of the shape of the no air voids line must occur at

lower optimum moisture contents. The effect of increasing compactive energy can be seen in Figure 4. It should be noted that for moisture

contents greater than the optimum the use of heavier compaction machinery will have only a small effect on increasing dry unit weights. For

this reason it is important to have good control over moisture content during compaction of soil layers in the field.


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Figure 4 Effects of compactive effort on compaction curves

It can be seen from this figure that the compaction curve is not a unique soil characteristic. It depends on the compaction energy. For this

reason it is important when giving values of (dry)maxand moptto also specify the compaction procedure (for example, standard or modified).

Effects of soil type

The table below contains typical values for the different soil types obtained from the Standard Compaction Test.

Note that these are typical values. Because of the variability of soils it is not appropriate to use typical values in design, tests are always

required.

Field specifications


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To control the soil properties of earth constructions (e.g. dams, roads) it is usual to specify that the soil must be compacted to some pre-

determined dry unit weight. This specification is usually that a certain percentage of the maximum dry density, as found from a laboratory

test (Standard or Modified) must be achieved.

For example we could specify that field densities must be greater than 98% of the maximum dry unit weight as determined from the

Standard Compaction Test. It is then up to the Contractor to select machinery, the thickness of each lift (layer of soil added) and to control

moisture contents in order to achieve the specified amount of compaction.

Accept

Reject


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Accept

Reject

(a) (b)

There is a wide range of compaction equipment. For pavements some kind of wheeled roller or vibrating plate is usually used. These only

affect a small depth of soil, and to achieve larger depths vibrating piles and drop weights can be used. The applicability of the equipment

depends on the soil type as indicated in the table below

Equipment Most suitable soils Typical application Least suitable soils

Smooth wheeled rollers, static or

vibrating

Well graded sand-gravel, crushed

rock, asphalt

Running surface, base courses,

subgrades Uniform sands

Rubber tired rollers

Coarse grained soils with some

fines Pavement subgrade Coarse uniform soils and rocks

Grid rollers

Weathered rock, well graded

coarse soils Subgrade, subbase

Clays, silty clays, uniform

materials

Sheepsfoot rollers, static

Fine grained soils with > 20%

fines Dams, embankments, subgrades

Coarse soils, soils with cobbles,

stones

Sheepsfoot rollers, vibratory

as above, but also sand-gravel

mixes subgrade layers

Vibrating plates Coarse soils, 4 to 8% fines Small patches clays and silts

Tampers, rammers All types Difficult access areas

Impact rollers Most saturated and moist soils

Dry, sands and gravels

Sands and gravels

For soils without any fines (sometimes referred to as cohesionless) the standard compaction test is difficult to perform. For these soil types it

is normal to specify a relative density, I d, that must be achieved. The relative density is defined by


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where e is the current voids ratio,

emax, eminare the maximum and minimum voids ratios measured in the laboratory from Standard Tests (AS 1289-5.1)

Note that if e = emin, Id= 1 and the soil is in its densest state

e = emax, Id= 0 and the soil is in its loosest state

The expression for relative density can also be written in terms of the dry unit weights associated with the various voids ratios. From the

definitions we have

and hence

The description of the soil will include a description of the relative density. Generally the terms loose, medium and dense are used where

Note that you cannot determine the unit weight from knowing I d. This is because the values of the maximum and minimum dry unit weights

(void ratios) can vary significantly. They depend on soil type (mineralogy), the particle grading, and the angularity.

Definition of Embankment

An embankment is a levee, dike or other artificial bank or barrier used to hold back or redirect water in order to prevent floodingfrom a river, lake, sea or other water source. Embankments can also be constructed to support transportation services, such asroadways, railways and canals. In the case of roadways and railways, embankments often raise the level of the transportation to keepthem removed from flooding or other natural dangers.

Transportation Embankments

Transportation embankments are built to support transportation. Normally built to allow for a straight, flat and uninterrupted pathof transportation, these embankments are in many ways the opposite of a transportation cutting, where a section of a mountain orhill is cut to make room for a road or railway. In fact, material gathered from transportation cuttings is often used in theconstruction of transportation embankments.


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Bituminous road constructions steps.docx