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1.0 INTRODUCTION:

A typical flexible Pavement consists of 4 components. They are as follows

1. Soil Subgrade

2. Sub Base course

3. Base course

4. Surface course

Fig (1) Layers of a flexible pavement

The flexible pavement layers transmit the vertical or compressive stresses to the lower

layers by Grain to Grain transfer through the points of contact in the Granular structure. The

load spreading ability of the granular layer depends on the type of the materials and the Mix

Design Factors.

Due to the ability to transfer stresses to a larger area in the shape of a truncated cone, the

Stresses get decreased at the lower layers. Hence the Lower Layers have to take up lesser

magnitudes of stresses. This Stress distribution character of Flexible Pavement leads to the

development of “Layer system concept”.

Hence the Design and construction of a Flexible Pavement is based on the stress

distribution character. Flexible Pavements are commonly designed using empirical design charts

or equations taking into account some of the design factors. There are also semi-emperical and

theoretical design methods.

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2.0 PAVEMENT COMPONENTS:

A typical cross section of a Flexible Pavement consists of Wearing surface at the top,

below which is the Base course followed by the Sub-base course and the lowest layer consists of

the soil Subgrade which has lowest stability among the four typical Flexible pavement

components.

2.1 Soil Subgrade:

The soil Subgrade is a layer of natural soil prepared to receive the loads of various layers

of materials placed over it. The load on the Pavement is ultimately received by the soil Subgrade

for the dispersion to the Earth mass. It is essential that at no time, the soil Subgrade is Over-

stressed.

It is desirable that at least top 50cm layer of the Subgrade soil is well compacted under

controlled conditions of Optimum Moisture Content and Maximum Dry Density. It is necessary

to evaluate the strength properties of soil Subgrade. This helps the designer to adopt the suitable

values of Strength parameter for design purposes. In case the supporting layer doesn’t come up

to the expectation the same is treated a stabilized to suit the requirements.

Many tests are known for measuring the strength properties of Subgrade. The common strength

tests for the evaluation are:

1. California Bearing Ratio test

2. California Resistance Value test

3. Triaxial Compression test

4. Plate Bearing test

2.2 Sub-Base and Base course:

These layers are made of broken stone, bound or unbound aggregate. Sometimes in Sub-

base course a layer of stabilized soil or selected granular soil are also used. In the Sub-base

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course, it is desirable to use smaller size graded aggregate or soil aggregate mixes or soft

aggregates instead of large boulder stone soling courses of brick on edge soling course, as these

have no proper interlocking and therefore have lesser resistance to sinking into the weak

Subgrade soil when wet.

When the Subgrade consists of fine grained soil and when the pavement carries heavy

wheel loads, there is a tendency for these boulder stones to penetrate into wet soil, resulting in

formation of undulations and uneven pavement surface. Sub-Base course primarily has the

similar function as that of Base course and is provided with inferior materials than of Base

course.

The functions of Base course vary according to type of Pavement. Base course and Sub

base courses are used under Flexible Pavement primarily to improve the load supporting

Capacity by distributing the load through a finite thickness. Thus the fundamental purpose of a

Base course and Sub-Base course is to provide a stress transmitting medium to spread the surface

wheel load in such a manner as to prevent shear and consolidation deformations.

2.3 Wearing course:

The purpose of Wearing course is to give a smooth riding surface that is dense. It resists

pressure exerted by tyres and takes up Wear and tear due to the traffic. Wearing course also

offers a water tight layer against the surface water infiltration. In Flexible Pavement, normally a

bituminous surfacing is used as a wearing course. The type of surface depends upon the

availability of materials, plant and equipment and upon the magnitude of surface loads.

There is no test for evaluating the structural stability of Wearing course. Most popular

test in use is Marshall Stability test wherein the optimum content of bitumen binder is worked

out based on the Stability density, VMA and VFB of the given grading of the aggregate

mixtures.

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3.0 DESIGN FACTORS CONSIDERED IN A FLEXIBLE PAVEMENT:

Pavement design consists of two parts:

5. Mix design of materials to be used in each pavement component layer.

6. Thickness of design of the pavement and the component layers.

The following are the different factors that are to be considered for the design of pavements:

1. Design Wheel Load

2. Sub grade soil

3. Climatic factors

4. Pavement component materials

5. Environmental factors

6. Special factors

3.1 Design wheel load:

The design of pavement primarily depends on the design wheel load. While considering

the design wheel load the various wheel load factors to be considered are:

Maximum wheel load

Contact pressure

Dual or multiple wheel loads and Equivalent Single Wheel Load

Repetition of loads

3.1.1 Maximum Wheel Load:

The wheel load configurations are important to know the way in which the loads of a

given vehicle are applied on the pavement surface.

For highways, the maximum legal axle load as specified by Indian Road Congress is

8170kg with a maximum equivalent single wheel load of 4085kg. Total load influences the

thickness requirements of pavements. Tyre pressure influences the quality of surface course.

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3.1.2 Contact Pressure:

The influence of tyre pressure is predominating in the upper layers. At a greater depth the

effect of tyre pressure diminishes and the total load exhibits a considerable influence on the

vertical stress magnitudes. Tyre pressure of high magnitudes therefore demand high quality of

materials in upper layers of pavement. Generally the wheel load is assumed to be distributed over

a circular area.

Three terms in use with reference to tyre pressure are:

1. Tyre pressure

2. Inflation pressure

3. Contact pressure

Theoretically, all these terms should mean the same thing.

Tyre pressure and Inflation pressure mean exactly the same.

The contact pressure is found to be more than tyre pressure when the tyre pressure is less

than 7 kg/cm² and it is vice versa when the tyre pressure exceeds this value.

Contact pressure is given by the relation:

Contact pressure = Load on wheel/ Contact area

The ratio of contact pressure to tyre pressure is defined as Rigidity factor. Thus

value of rigidity factor is 1.0 for an average tyre pressure of 7 kg/cm².

Rigidity factor is higher than unity for lower tyre pressures and less than unity for

tyre pressures higher than 7kg/cm²

3.1.3 Equivalent Single Wheel Load (ESWL):

To maintain the maximum wheel load within the specified limit and to carry greater load,

it is necessary to provide dual wheel assembly to the rear axles of the road vehicles. In doing so

the effect on the pavement through a dual wheel assembly is obviously not equal to two times the

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load on any one wheel. The effect is in between the single load and two times the load on any

one wheel. The following figure shows the stress distribution in pavement.

Fig 2 Distribution of tyre pressure

In the dual wheel load wheel assembly, let ‘d’ be the clear gap between the two wheels,

‘S’ be the spacing between the centres of the wheels and ‘a’ be the radius of circular contact area

of each wheel.

Then,

S= d+2a.

Up to depth of d/2, each wheel load P acts independently and after this point the stresses

induced due to each load begins to overlap. At depth 2S and above, the stresses are induced due

to the effect of both wheels as the area of overlap is considerable. So the total stresses due to the

dual wheels at any depth greater than 2S is considered to be equivalent to a single wheel load of

magnitude 2P.

Equivalent Single Wheel Load (ESWL) may be determined based on either equivalent

deflection or equivalent stress criterion. Multiple wheel loads are converted to ESWL and this

value is used in pavement design.

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3.1.4 Repetition of Loads:

The deformation of pavement or subgrade due to single wheel application of wheel load

may be small. But due to repeated application of the load there would be increased magnitude of

plastic and elastic deformations and the accumulated unrecovered or permanent deformations

may even result in pavement failure.

3.2 Strength of Subgrade Soil:

Subgrade soil is an integral part of the road pavement structure as it provides the support

to the pavement from beneath. The subgrade soil and its properties are important in the design of

pavement structure. The main function of subgrade is to give adequate support to the pavement

and for this the subgrade should possess sufficient stability under adverse climatic and loading

conditions. The strength of soil subgrade plays a major role in pavement design.

The factors on which the strength of soil depends are:

1. Soil type

2. Moisture content

3. Dry density

4. Internal structure of soil

5. Type & mode of stress application

There are different tests that are being used in evaluation of strength characteristics of sub

grade soil. They are grouped in to following three categories. They are:

1. Shear tests

2. Bearing tests

3. Penetration tests

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1. Shear tests are usually carried out on relatively small soil samples in the

laboratory. Some of the commonly known shear tests are Direct shear test,

Triaxial compression test, Vane shear test and unconfined compression test.

2. Bearing tests are loading tests carried on subgrade soils in-situ with a load

bearing area.

3. Penetration tests may be considered as small scale bearing tests in which the

size of the loaded area is relatively much smaller and ratio of penetration to

size of loaded area is much greater than the ratios in bearing tests.

In this study of Flexible pavement, the strength of sub grade soil is evaluated by means of

conducting a Penetration test. The test used was California Bearing Ratio test, commonly known

as CBR test.

3.3 Climatic factors:

The climatic variations cause the following major effects.

1. Variation in moisture condition

2. Frost action

3. Variation in temperature

The pavement performance is very much affected by the variation in moisture and the frost.

Variation in temperature generally affects the pavement materials like bitumen and concrete.

1. The surface water during rains may enter the subgrade either through the

pavement edges or through the pavement itself, if it is porous, thereby leading

to the increase in moisture content.

2. The stability of most of the subgrade soil is decreased under adverse moisture

conditions.

3. Variation in moisture content may lead to dangerous phenomenon like

swelling and shrinkage.

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4. Frost action refers to the adverse effective due to frost heave, frost melting or

thaw and the alternate cycles of freezing and thawing.

5. The held water in subgrade soil forms ice crystals if freezing temperatures

continue for certain period.

6. These ice crystals grow further in size and results in raising the pavement

structure, which is known as frost heave. Non-uniform heaving causes many

damages.

7. The load carrying capacity of subgrade considerably decreases and under

heavy traffic, the pavement deflects excessively causing progressive failure.

In this way the climatic variations affect the strength of pavement in many ways. One of the

most effective and practical methods to decrease the damaging effects due to climatic variations

is to install proper surface and sub surface drainage systems.

3.4 Special factors:

The special factors include environmental and as well the miscellaneous ones also.

The following are the environmental factors affecting the pavement:

1. Height of embankment

2. Depth of ground water table

3. Foundation details

The choice of bituminous binder and the performance of bitumen should also be taken in

to due consideration.

The formation of shrinkage cracks, fatigue behaviour should also be studied before

arriving at the design.

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4.0 DESIGN OF FLEXIBLE PAVEMENT:

Considering the stress behaviour of the pavement, flexible pavements are designed in

layers. In the design process, it is to be ensured that under the application of load, none of the

layers is over stressed.

Various approaches of flexible pavement design are classified in to three groups. They are:

1. Emperical methods.

2. Semi emperical or semi theoretical methods.

3. Theoretical methods.

The design adopted in this study is the “IRC method of designing the flexible

pavement.”This method is also called as the “CBR method of design.”

4.1 Design procedure of IRC method:

In 1928, California division of Highways developed CBR method for pavement design.

One of the chief advantages of CBR method is the simplicity of the test procedure. Based on the

extensive CBR test data collected on pavement which behaved satisfactorily and those which

failed, an empirical design chart was developed correlating the CBR value and the pavement

thickness. Design curves correlating the CBR value with total pavement thickness cover were

developed by the California state Highway department.

The Indian Road Congress has recommended a CBR design chart for tentative use in

India. The design charts are presented in the following figure.

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The design procedure is as follows:

1. In order to design a pavement by CBR method, first the soaked CBR value of

soil subgrade is evaluated.

2. The appropriate design curve is chosen by taking the design wheel load or by

taking the anticipated traffic in to consideration.

3. Thus the total thickness of flexible pavement needed to cover the sub grade of

the known CBR value is obtained.

4. In case there is material superior than subgrade, such that it may be used as

sub-base course then the thickness of construction over this material could be

obtained from the design chart knowing the CBR value of sub base.

5. Thickness of sub base course is the total thickness minus the thickness over

the sub base.

4.2 IRC recommendations to CBR method:

Some of the important points recommended by IRC for CBR method of design are as

follows:

1. CBR tests should be performed on remoulded soils in the laboratory. In-situ

tests are not recommended for design purposes.

2. For the design of new roads, the sub grade soil sample should be compacted at

OMC to Proctor density. In case of existing roads, the sample should be

compacted to field density of sub grade soil.

3. In new constructions, the CBR test samples may be soaked in water for four

days period before testing. Wherever possible, the most adverse moisture

condition of the sub grade should be determined.

4. The top 50cm of sub grade should be compacted at least up to 95 to 100 % of

Proctor density.

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5. An estimate of the traffic to be carried by the road pavements at the end of

expected life should be made keeping in view the existing traffic and probable

growth rate of traffic.

The following formula can be used in estimating the design traffic.

Where

A = no. of heavy vehicles per day for design.

P = no. of heavy vehicles per day at least count.

r = annual rate of increase of heavy vehicles

x = no. of years between the last count and the year of completion of construction.

4.3 Design of pavement as per IRC 37:2001:

The pavement has been designed as per IRC 37:2001 code. The traffic is estimated in

terms of million standard axles (msa).

As per the code,

Where

N = cumulative no. of standard axles to be catered for in the design in terms of msa

A = initial traffic in the year of completion of construction in terms of no. of commercial

vehicles per day.

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D = lane distribution factor.

F = vehicle damage factor

r = annual growth rate of commercial vehicles

n = design life in years.

If reliable value of growth factor ‘r’ is not available, a value of 7.5% may be assumed for roads

in rural areas.

According to the survey,

P = 2415

x = 3 yrs, r = 0.075, n= 15 yrs

As per equation 1,

A = 2415 (1 + 0.075) ³

= 3000

As per table 1 of IRC 37:2001, value of ‘F’ for plain areas is 4.5 for A > 1500.

Since A = 3000, value of F = 4.5

The pavement is considered as two lane single carriage way. Hence as per IRC 37:2001, the

design should be based on 75% of total no. of vehicles in both directions.

So, D = 0.75

Using the equation 2,

= 96523681.56

= 96.5 msa

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Hence on a safe side, we design for 100 msa traffic.

As we estimated the design traffic, now the thickness of pavement layers will be computed.

The CBR value of the sub grade was found to be 10 %. Using the IRC charts for CBR value of

10 % and for traffic of 100 msa,

The pavement thickness is given as 630mm.

1. As per clause 4.2.1.4 of IRC 37:2001, the thickness of sub base shouldn’t be

less than 150mm for design traffic less than 10msa and 200mm for more than

10msa.

2. As per clause 4.2.2.2 of IRC 37:2001 and MORTH specifications, the

recommended minimum thickness of granular base is 225mm for traffic up to

2msa and 250mm for for traffic exceeding 2msa.

Taking the above specifications in to consideration,

Thickness of sub base = 200mm

Thickness of base = 250mm

Bituminous course = 50mm

Dense Bituminous macadam = 130mm

Hence the total pavement thickness for traffic of 100msa on soil of CBR 10% is 630mm.

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5.0 FIELD WORK:

The flexible pavement is laid in four layers namely Sub grade, Sub base, base course and

Wearing course. The details of the materials used and the field work is included in the following

sections.

5.1 Subgrade:

Subgrade soil is an integral part of the road pavement structure as it provides support to

pavement from beneath. Generally, the subgrade material is considered as the naturally existing

soil at the site. The materials which can be used for subgrade are soil, moorum, Gravel or a

mixture of the above materials. Here, in this case, the Subgrade used is Gravel.

As per IRC specifications, the CBR value of sub grade shall be in between 2-10%. The

sub grade soil sample is collected from the site and soaked CBR test has been conducted in the

laboratory. The CBR value of the Gravel was found to be 10%. Using the relevant IRC charts,

the thickness of individual layers have been obtained. Proctor compaction test was also

conducted on the soil sample collected from the site to get the Maximum Dry Density (MDD)

and Optimum Moisture Content (OMC).

The maximum dry density thus obtained was found to be 1.92g/cc and the Optimum

Moisture Content was found to be 15.2%. To check the field density, core cutter method was

used. The field density was found to be 1.88g/cc. the results of the respective tests were included

in the later sections.

The sub grade material is spread in layers of uniform thickness not exceeding 200mm.

The moisture content was checked before carrying out the compaction. The degree of

compaction is given by

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The sub grade is compacted to the required degree of compaction using a \Smooth wheeled

roller.

Fig.3 Compacted Subgrade (Gravel)

The Smooth wheeled rollers are generally used for the densification of granular soils.

They consist of three wheels, two in the rear and one in the front.

The weight of the roller is around 5-15t. The wheels exert high static pressure which

helps in easy and quick compaction. Hence the sub grade is compacted to desired density and is

ready to take up the other layers of pavement.

5.2 Subbase Course:

The next layer that is to be laid over the subgrade is the subbase. The materials generally

used for sub-base are gravel, crushed stone, moorum, soil etc. Here, the material which is used as

sub-base is crushed stone, which is generally termed as Quarry Dust. Since the CBR value of the

subgrade is enough to take up the loads, the CBR of sub-base need not be considered. However,

as per IRC: 37, the CBR of sub base should be between 20-30%.

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Fig. 4 Curing of Subbase Course

Quarry dust has been used as sub-base since it has given a higher maximum dry density

value. The quarry dust consists of fine powder of aggregates passing through 2.36mm and

retained on 75mm. The sub-base is laid on the prepared subgrade and is watered to maintain the

desired moisture content. The sub-base is then compacted to attain a degree of compaction of

98%.

5.3 Base Course:

The base course can be constructed using two types of materials:

1. Water Bound Macadam (WBM)

2. Wet Mix Macadam (WMM)

Here the Base course is made up of Wet Mix Macadam (WMM). WMM is a pavement

layer wherein crushed graded aggregates and granular material like graded coarse sand are mixed

with water in mixing plant and rolled to a dense mass on a prepared surface. It has many

advantages over WBM construction. They are as follows:

1. Superior gradation of aggregates

2. Faster rate of Construction

3. Higher standard of densification.

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The sub-base layer on which WMM has to be laid is made free from dust and specified

camber is maintained. All the ruts and soft yielding places are corrected by rollers until firm

surface is obtained.

Fig.5 Preparation of Water Mix Macadam

The coarse aggregates of grade II i.e., size ranging from 63mm to 45mm have been

mixed with controlled amount of water in the mix plant thoroughly. The abrasion and impact

tests were carried out on the aggregates before using them in WMM. The test results were

included in the later sections. The mix is then transported to the site and is spread using a Paver

finisher.

Fig.6 Laying the Wet Mix Base course

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The mix should be properly laid over the sub base and is compacted to the desired density

using a smooth wheeled roller. The layer is then cured for a period of 7 days using water and

then dried for 24hrs before laying bitumen layer.

Fig 7 Compaction with Smooth Wheeled Roller

5.4 Wearing Course:

The wearing course is also called as surface course. The surface course is

generally laid using bitumen. Since the pavement is being designed for traffic of 100msa, as per

IRC specifications, a Dense Bitumen Macadam (DBM) is provided as Wearing course. As per

annexure 5 of IRC 37:2001, for DBM in traffic of 100msa, a layer of 40mm thick Bituminous

Concrete (BC) is also provided over DBM.

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Fig 8 Prepared Bituminous surfacing

The grade of bitumen used is 80/100. The bitumen dressing is done in two coats

over the WMM layer. The heated bitumen is applied on the WMM at the rate of 4.9 to

9.8kg/10m2 area. The required aggregates and bituminous binder are mixed thoroughly and are

heated to the specified temperature and are then placed in the mixer. The mixing is done till a

homogenous mixture is obtained. The bituminous mixture is then placed immediately on the

desired location. The rolling is then carried out from the edges towards the centre longitudinally.

Thus the Flexible pavement has been designed and constructed as per IRC

specifications.

6.0 LABORATORY TESTS AND RESULTS:

There are different tests to be carried out before using the materials in pavement

construction. The test used to find out the strength of sub grade is CBR test. The next test to be

carried out is on the aggregates used in WMM. The different tests performed on aggregates are

1. Los Angeles abrasion test

2. Impact test

3. Crushing test

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Each of these tests is discussed in detail in the following pages. The corresponding lab results are

also included.

6.1 California Bearing Ratio test:

This is a penetration test developed by the California division of Highways, as a

method for evaluating the stability of soil sub grade and other flexible pavement materials. The

test may be conducted in the laboratory or in the field.

The apparatus used for conducting the experiment is as shown in the above figure.

Normally, there are two kinds of samples considered in this test. They are:

1. Soaked sample

2. Un-soaked sample

For pavements, the test conducted is soaked CBR test.

The laboratory CBR apparatus consists of a mould 150mm diameter with a base plate and

a collar, a loading frame with the cylindrical plunger of 50mm diameter and dial gauges for

measuring the expansion on soaking and the penetration values.

Fig 9 CBR apparatus

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The procedure of conducting the experiment is as follows:

1. The specimen to be tested is placed in the cylindrical mould and is subjected

to four days soaking.

2. The swelling and water absorption values of the sample are noted.

3. The surcharge weight is placed on the top of the specimen in the mould and

the assembly is placed under the plunger of the loading frame.

4. The plunger, which is 50mm in diameter, is allowed to penetrate in to the

specimen at 1.25mm/min.

5. The load values corresponding to the penetration values of 0.0, 0.5, 1.0, 1.5,

2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 7.0, 8.0.9.0, 10.0, 11.0 and 12.0 mm

are noted.

6. The values recorded at depth of 2.5mm and 5.0mm are noted. Normally CBR

value for 2.5mm is more than that at 5.0mm and is reported as CBR value of

the material.

7. The results are plotted in the form of graph with penetration on X-axis and

Load on Y-axis.

The formula used for computing the CBR value at 2.5mm penetration is:

The formula used for computing the CBR value at 5.0mm penetration is:

The following Observations are noted from the expirement

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Dial gauge

reading

Penetration depth

(mm)

Proving ring reading

P

Load = P x 6.14

(kg)

50 0.5 2 12.28

100 1.0 8 49.12

150 1.5 12 73.68

200 2.0 18 110.52

250 2.5 22 135.08

300 3.0 24 147.36

350 3.5 25 153.50

400 4.0 27 165.78

450 4.5 28 171.92

500 5.0 29 178.06

550 5.5 33 202.62

600 6.0 37 227.18

700 7.0 40 245.60

800 8.0 43 264.02

900 9.0 44 270.16

1000 10.0 47 288.58

1100 11.0 53 325.42

1200 12.0 55 337.70

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The dimensions of the mould,

Diameter of mould = 150mm

Height of the mould = 125mm

Volume of mould = 22089c.c

Proving ring constant =6.14

Calculations:

Using the observations recorded, a graph is plotted with penetration depth on X-axis and

load on y-axis.

Hence by using the California Bearing Ratio test (CBR), the CBR value of sub grade soil

(gravel) is found to be 10%.

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6.2 Abrasion Test on Aggregates:

Due to the movements of traffic, the road stones used in the surface course are subjected

to wearing action at the top. Hence road stones should be hard enough to resist the abrasion due

to traffic. Abrasion tests are carried out to test the hardness property of stones.

The Abrasion Test on aggregates can be carried out by using any of the following ways:

1. Los Angeles abrasion test

2. Deval Abrasion test

3. Dorry abrasion test

Here the abrasion value of aggregates is determined by Los Angeles Abrasion test method.

The principle of Los Angeles abrasion test is to find the percentage wear due to the

relative rubbing action between the aggregate and steel balls used as abrasive charge. The Los

Angeles machine consists of a hollow cylinder closed at both ends, having inside diameter 70cm

and length 50cm and mounted so as to rotate about its horizontal axis. The abrasive charge

consists of cast iron spheres of approx. diameter of 4.8cm and each of weight 390 to 445gm.

Fig 10 Los Angeles Abrasion Test Apparatus

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The test procedure for finding abrasion value is as follows:

1. Aggregates passing through 50mm and retained on 40mm, weighing 2.5kg are

taken.

2. Aggregates passing through 35mm and retained on 31mm, weighing 2.5kg are

taken along with the above aggregate sample.

3. The sample of 5kg is placed in the machine along with the abrasive charge

(steel balls).

4. The machine is rotated at a speed of 30 to 33 rpm for specified number of

revolutions (500 to 1000).

5. The abraded aggregate is then sieved on 1.7mm IS sieve and the weight of

powdered aggregate passing this sieve is found.

6. The result of the abrasion test is expressed as the percentage wear or

percentage passing 1.7mm sieve in terms of original weight of the sample.

Observations:

Weight of aggregates passing through

63mm and retained on 50mm = 2500gm

Weight of aggregates passing through

50mm and retained on 40mm = 2500gm

Weight of sample passing 1.7mm = 2200gm

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Hence the abrasion value of aggregates was found to be 35%. As per IRC specifications,

the abrasion value of aggregates to be used in Water Mix Macadam is maximum of 40%. Hence

the aggregates can be used in preparing WMM.

6.3 Impact test on aggregates:

Impact test is designed to evaluate the toughness of stone or the resistance of the

aggregates to fracture under repeated impacts. The aggregate impact value indicates a relative

measure of aggregate to impact, which has a different effect than the resistance to gradually

increasing compressive stress.

The impact testing machine consists of a metal base and cylindrical steel cup of internal

diameter 10.2cm and depth 5cm in which the aggregate specimen is placed. A metal hammer of

weight 13.5-14kg having a free fall from height 38cm is arranged to drop through vertical

guides.

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Test Procedure:

1. The empty weight of the cylindrical steel cup is noted first (W1).

2. Aggregates passing through 12.5mm sieve and retained on 10mm sieve are

taken in to the mould and filled in three layers.

3. Each layer is given 25 blows using a tamping rod.

4. The mould along with aggregates is weighed (W2).

5. The cylindrical mould is then placed in the impact apparatus and the specimen

is subjected to 15 blows by the hammer.

6. The crushed aggregate are taken out and sieved on 2.36mm sieve. The weight

passing through the sieve is noted. (W3)

7. The aggregate impact value is expressed as the percentage of the fine formed

in terms of total weight of sample.

Observations:

Weight of empty mould = 2085 g

Weight of mould with Aggregates = 2785 g

Weight of aggregates = 700 g

Weight of aggregates passing

Through 2.36mm sieve = 170 g

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Hence the impact value of aggregates was found to be 24.2%. As per IRC specifications,

the abrasion value of aggregates to be used in Water Mix Macadam is maximum of 30%. Hence

the aggregates can be used in preparing WMM.

6.4 Crushing Test on Aggregates:

The strength of coarse aggregate may be assessed by aggregate crushing test. The

aggregate crushing value provides a relative measure of resistance to crushing under gradually

applied compressive load. To achieve a high quality of pavement, aggregate possessing high

resistance to crushing or low aggregate crushing value is required.

The apparatus for the standard test consists of a steel cylinder 15.2cm diameter with a

base plate and a plunger, compression testing machines, cylindrical measure of diameter 11.5cm

and height 18cm, tamping rod and sieves.

Fig 11 Crushing apparatus

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Test procedure:

1. The weight of the empty mould is noted. (W1)

2. The mould is filled with dry aggregates passing through 12.5mm sieve and

retained on 10mm sieve, in three layers.

3. Each layer is given 25 blows with the help of a tamping rod.

4. The weight of the mould with the aggregates is noted. (W2)

5. The plunger is placed on the top of the specimen and a load of 40 tones is

applied at a rate of 4 tons per minute by the compression machine.

6. The crushed aggregate is removed and sieved through 2.36mm sieve.

7. The weight of the material passing through the sieve is noted. (W3)

8. The aggregate crushing value is the percentage of crushed material passing

2.36mm sieve in terms of original weight of the specimen.

Observations:

Weight of empty mould = 11.7 kg

Weight of mould with

Aggregates = 15 kg

Weight of aggregates = 3.3 kg

Weight of aggregates passing

Through 2.36mm sieve = 0.735 kg

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7.0 CONCLUSION:

The following conclusions can be drawn from the study:

The flexible pavement have been designed as per IRC : 37-2001 to meet the

safety and serviceability requirements.

The pavement consists of 4 layers namely Subgrade, Subbas course, Base course

and Wearing course.

To determine the strength of the subgrade soil, CBR test was conducted in the

laboratory. The CBR value of existing subgrade was found to be 10%.

Wet Mix Macadam (WMM) was laid down as a Base course. It has capacity to

carry traffic more efficiently when compared with Water Bound Macadam

(WBM)

Relevant tests namely Los Angeles Abrasion test, Aggregate impact test and

Aggregate crushing test were carried out on the aggregates that are used in

WMM.

All the required laboratory results have been included in the study.

As per design charts of IRC : 37-2001, for the obtained CBR value, the thickness

of pavement was found to be 630mm.

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8.0 REFERENCES:

1. “Highway Engineering” by S. K. KHANNA and C. E. G. JUSTO, Nem

Chand & bros publications

2. IRC 37:2001 code

3. IRC 109:1997 code

4. www.en.wikipedia.org