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ASSIGNMENT

BASIC CIVIL ENGINEERING

TOPIC: CONCRETE & AGGREGATES

Submitted By

Abhijith M V

Amal M D

Anju Mary Jose

Fathimath Shahziya

EC1A, Model Engineering College,

Thrikkakara



Assignment

Basic Civil Engineering

2

CONCRETE & AGGREGATES

CONCRETE

The term concrete is used for an artificial material obtained by mixing together

cementing materials, coarse aggregates, fine aggregates and water. If cement is used as

cementing material in the mix, it is known as plain cement concrete. If instead of cement lime is used then it is known as lime concrete. However cement concrete is used

for civil engineering construction.

PLAIN CEMENT CONCRETE (PCC)

Concrete is used for foundations are of plain cement concrete. Concrete formed by

using cement, coarse and fine aggregates and water is called plain cement concrete. It is

used in structural members subjected to pure compression and for flooring and

foundations etc.

REINFORCED CEMNET CONCRETE (RCC)

PCC is very strong in compression but at the same time it is very weak in tension. Hence

PCC cannot be used at the places where tension forces develop. Steel is equally strong

in compression and tension. But a long steel bar can develop its full tensile stress where

as it cannot carry equal amount of compressive forces due to its buckling which is

caused by the slenderness. A combination of steel and concrete is ideally suited because

the two materials are employed to resist the stresses they are most suitable. The

combination of concrete and steel in construction is known as reinforced cement

concrete.



Assignment


3

PROPERTIES OF REINFORCED CONCRETE

i. The concrete develops very good bond with the surface of steel bars.

Hence the concrete transmits the excessive stresses which it cannot resist

itself to the steel reinforcement.

ii. Coefficient of linear expansion of the concrete and the steel are same.

Hence internal stresses do not develop in reinforced concrete due to

temperature variations.

iii. The cement grout protects the reinforcement from corrosion and at the

same time it does not react with the reinforcement.

ADVANTAGES OF RCC

i. It is durable and fire resisting.

ii. It is not attacked by vermin, termites, fungus, or any other insects.

iii. Well compacted RCC structures are impermeable to water and moisture.

iv. Its maintenance cost is negligible.

v. In the long run it proves to be economicalvi. Reinforced concrete can be molded in any desired shape.

vii. The monolithic character gives more rigidity to the structure

viii. The materials used in RCC are easily available.

ix. Concrete can be molded to any desired shape.

R.B.C Floor

Plan

R.B

Steel rod



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STEEL IS USED IN RCC AS DUE TO REASONS AS FOLLOWS

i. Its tensile strength.

ii. It can develop good bond with concrete.

iii. Its coefficient of expansion is nearly same as for concreteiv. It is freely available

AGGREGATES OF CONCRETE

Hydraulic cement concrete is a cement and water paste in which aggregate particles are

embedded. Aggregate is granular materials such as sand, gravel, crushed stone, blast

furnace slag, and light weight aggregate that usually occupies approximately 60-75 % of

the volume of the concrete. These are inert or chemically inactive materials. These

aggregate bound together by means of cement.

CLASSIFICATION OF AGGREGATES

The aggregates are classified as follows

i. BASED ON SIZE

The material which passes through 4.75 mm sieve (BIS test sieve no. 475) is termed

as fine aggregate. Usually the natural river sand is used as fine aggregate.

The material which is retained on 4.75 mm sieve is termed as coarse aggregate. The

nature of work decides the maximum size of the coarse aggregate. For thin slabs and

walls, the maximum size of coarse aggregate should be limited to 1/3rd

the thickness

of the concrete section.

ii. BASED ON SHAPE

1. Rounded e.g.: River sand

2. Irregular or partly round e.g.: Pit & Gravel

3. Flaky (Thickness is less than other dimensions) e.g.: Cut rocks / laminated

rocks

4. Angular e.g.: Crushed rock

5. Elongated(Length is greater than other dimensions) e.g.: Cut rocks



Assignment


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iii. BASED ON WEIGHT

1. Heavy weight:- concrete having densities 3000 kg/m3

to 5700 kg/m3

can be

obtained from heavy weight aggregates( e.g.: Iron and steel in the form of

shots).

2. Normal weights:- weight of concrete produced ranges between 2300 kg/m3 to

2600 kg/m3(e.g.: sand, gravel, crushed rock)

3. Light weight:- concrete having unit weight of 1450 kg/m3

to 1750 kg/m3

can

be produced from light weight aggregate. The light weight aggregate are clay,

shale and slate.

REQUIREMENT OF GOOD AGGREGATE (Quality of aggregate)

Natural aggregate used for concrete construction is required to comply with norms

laid down in IS: 383-1970 ‘Specification of coarse and fine aggregate from naturalsources of concrete’. Some of the important characteristics of aggregates are

1. Strength

2. Size

3. Particle shape

4. Surface texture

5. Grading

6.

Impermeability

7. Cleanliness

8. Chemical inertness

9. Physical and chemical stability

10. Coefficient of thermal expansion

And

11. Cost

Aggregate should be chemically inert, strong, hard durable, of limited porosity, free

from adherent coating, clay limps, coal and coal residues and should contains no organic

or other admixture that may cause corrosion of the reinforcement or impair the

strength or durability of concrete.

For a concrete of given workability rounded aggregate require least, water cement ratio,

while angular aggregates require highest water cement ratio; but it is less for round

shaped aggregates. Particle shape is thus important.



Assignment


6

GRADING OF AGGREGATE

Grading refers to the distribution of the particle size present in an aggregate. One of the

most important factors for producing workable concrete is good gradation of

aggregates. Good grading implies that a sample of aggregates contains all standardfractions of aggregates in required proportion such that the sample contains minimum

voids. A sample of the well graded aggregate contains minimum voids will require

minimum part to fill up the voids in the aggregate. Minimum paste means less quantity

of water, which further means increased economy, higher strength, lower shrinkage and

greater durability.

FINENESS MODULUS

The fineness modulus of an aggregate is an index number which roughly proportional to

the average size of the particles in the aggregate. When aggregate is coarser fineness

modulus is higher. The fineness modulus is obtained by adding the percentage of the

weight of the materials retained by adding the percentage of the weight of the materials

retained on the following is sieve and dividing it by 100.

(80mm, 40mm, 20mm, 10mm, 4.75mm, 2.36mm, 1.18mm,600 micron,300 micron, and

150 micron. Total 10 sieves)

INGREDIENT OF CEMENT CONCRETE

Cement concrete is prepared by mixing together ordinary Portland cement, sand,

crushed stone, and water. Cement concrete is an important construction material and

hence it should carefully designed, mixed, placed and cured

i. CEMENT

There are many type of cement which is being manufactured artificially and each type of

cement is used under certain condition due to its special properties. Some of them are

listed as

1. Portland cement

2. Rapid hardening cement

3. Blast furnace slag cement

4. High alumina cement

5. Low heat cement



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6. Quick setting cement

7. Colored cement

However for ordinary construction generally Portland cement is used. The composition

of Portland cement is as follows.

Lime 60-67%

Silica 17-25%

Alumina 3-8%

Iron oxide 0.5% - 6%

Magnesia 0.1% - 4%

Soda & Potash 0.2% - 1%

Sulfur dioxide 1% - 2.75%

When water is mixed with cement, chemical reaction takes place as a result of

which the cement paste first loses its plasticity and becomes shift, at the same time

acquires hardness and strength.

ii. FINE AGGREGATE

This is the inert and chemically inactive material, most of which passes 4.75mm

sieve and contains not more than 5% coarse material. They may be classified as follows.

a) Natural sand:

Fine aggregate resulting from the natural disintegration of rock and which

has been deposited by streams or glacial agencies.

b) Crushed stone sand:

Fine aggregate produced by crushing of hard stone

c) Crushed gravel stone:

Fine aggregate produced by crushing of natural gravel

iii. COARSE AGGREGATE

The inert material most of which is retained on 4.75 mm sieve and contains not

more than 0-10% of finer material known as coarse aggregate. They may be put

under the following categories,



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a) Uncrushed gravel or stone which results natural disintegration of rocks.

b) Crushed gravel or stone when it results from crushing of gravel or hard

stone

c) Partially crushed gravel or stone when it is a product of the mixture of the

above two types

iv. WATER

This is the least expensive but most important ingredient of concrete. The water

which used for making concrete should be clean and free from harmful impurities such

as oil, alkali, acid etc. In general the water which is fit for drinking should be used for

making concrete.

As a general guide the following concentrations represents the maximum

permissible values.

a) To neutralize 200ml,sample it should not require more than 2ml of 0.1 normal

NaOH

b) To neutralize 200ml sample it should not require more than 10ml of 0.1 normal

HCL

c) Percentage of solid should not exceed the following.

Organic 0.02%

Inorganic 0.30%

Sulfate 0.05%

Alkali chlorides 0.10%

The concrete mixed with water proposed to be used should not have a

compressive strength of concrete mixed with distilled water. Sea water does not

appear to have much effect upon the strength of concrete, but is liable to cause

efflorescence.

Several factors must be taken into consideration in determining the

quantity to be used. E.g.: Workability, strength.



Assignment


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PROPORTIONING OF CONCRETE

The process of selection of relative proportion of cement, sand coarse

aggregate and water, so as to obtain desired quality is known as proportioning of

concrete.The properties of the coarse aggregate, fine aggregate, cement and water

should be such that the resulting concrete has the following properties.

i. When concrete is fresh it should have enough workability so that it can be

placed in the form work economically.

ii. The concrete must possess maximum density or in other words, it should

be strongest and most water tight

iii. The cost and labour required to form the concrete should be minimum.

Different methods of proportioning concrete includes,

Arbitrary method

Fineness modules method

Minimum void method

Maximum density method

Water element ratio method

GRADES OF CONCRETE

The Indian Standard 456 designates concrete as M10, M15, M20, M25etc, where M stands

for mix; 10,15,20,25 etc as the compressive strength in N/mm2

at 28th

day.

Designation of

Concrete

Proportions of

cement: sand: aggregate

Strength of

concrete after 28

days

Used for

M10 1:3:6 10 N/mm2

Mass concreting

M15 1:2:4 15 N/mm2

General

reinforcement

Concrete workM20

1:1.5:3 20 N/mm2

Used for piles

M25 1:1:2 25 N/mm2

Used for front faces

of dams and water

tanks



Assignment


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PROPERTIES OF CONCRETE

i. In fresh state concrete can be used to all members having any shape

ii. In hardened state concrete posses very high compressive strength

iii. It is durableiv. Fire resistant

v. Density is 2410 kg/m3

vi. One of the undesirable properties of concrete is that it will shrink as the water in

it evaporate

WATER CEMENT RATIO

Cement and the water are the only two chemically active elements in the

concrete. By their combination they form glue like binder paste which coasts and

surrounds the particle of inert minerals aggregates, sets and upon hardening binds the

entire product into a composite mass. Next only to cement, water is the most important

element in governing all the properties associated with cement concrete like durability,

strength and water tightness.

The functions of water are

i. To damp the aggregates and prevent them adsorbing the water vitally necessary

for chemical combination between cement and water (Hydration of cement)

ii. Water lubricates the aggregates and it facilitates the passage of cement through

voids of the aggregates.

iii. To make concrete workable so that it can be placed easily and uniformly between

the reinforcing bars and in the corners

Water cement ratio is the ratio of water to the cement used in the

concrete mixture. Water and cement are taken by volume.



Assignment


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WORKABILITY TESTS

i. SLUMP TEST

The test modified is a frustum of a cone, 20cm diameter at bottom, 10cm

diameter at the top and 30cm height. The mould is open at top and bottom. It is placed

on flat, smooth and non-porous plate.

i.

The mould is filled up in four equal layers and each layer is compacted twenty

five times by 60cm long and 16mm diameters bullet pointed steel rod.ii. Top surface is smoothened by trowel and the mould is vertically lifted

immediately without disturbing the concrete

iii. The concrete is allowed to settle slowly

iv. When settlement ceases completely, the vertical settlement (height of cone) is

measured in centimeters. This is known as slump.

v. Value of slump to be used for various works as follows

Plain concrete (Floor, slab etc.) 25 mm to 40 mm

Reinforced Cement Concrete (Beam, Slab etc.) 50 mm to 100 mm Mass concrete 25 mm to 50 mm

Concrete for road work 20 mm to 30 mm

Columns and retaining walls and vertical section 75 mm to 150 mm

Vibrated concrete

Hand compacted concrete

Fully Compacted

Inadequately compacted

w c ratio

Compressive

Strength



Assignment


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ADVANTAGES: - Apparatus cheap, portable, and suitable for site use.

DISADVANTAGES: - Not suitable for concrete in which maximum size of aggregate is

more than 40 mm. Very wet mix produces collapsible slump.

20 cm

30

cm

10 cm

Slump

Slump Test

Slump

Scale

Slump

Cone

Stand

Slumped shape of

concrete

Top view of slumpcone

SLUMP CONE TEST APPARATUS



Assignment


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i. THE COMPACTION FACTOR TEST

The concrete is placed in the top hopper without any compaction. The door at the

bottom of the hopper is opened allowing the concrete to fall from the top to middle

hopper. The door at the bottom of the middle hopper is now opened and the concrete is

allowed to fall in to the bottom cylinder. The concrete is then struck off level and the

contents are weighed (W1), this weight being called the partially compacted weight. The

cylinder is now emptied and refilled with concrete (taken from same sample) with

compaction. The contents in the cylinder are now weighed (W2), this weight being

called the fully compacted weight. Then compaction factor is defined as the ratio of

W1/W2.

Top

Hopper

Bottom

Hopper

Cylindrical

mold

THE COMPACTING FACTOR TEST APPARATUS

COMPACTION FACTOR= W1/W2



Assignment


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For high workability compaction factor is 0.95.

For low workability compaction factor is 0.85.

The compaction factor test has the following merits.

i. Gives satisfactory performance both wet and dry mixes.

ii. Gives more sensitive results than slump test.

The only limitation is that equipment is not portable and hence cannot be used at site.

BATCHING

Batching is the process of weighing or volumetrically measuring and introducing in to

mixer ingredients for a batch of concrete. To produce a uniform quality concrete mix,

measure the ingredients accurately for each batch. Most concrete specifications require

that the batching be performed by weight, rather than by volume, because of

inaccuracies in measuring aggregate, especially damp aggregate. Water and liquid-air

entraining admixtures can be measured accurately by either weight or volume. Batching

by using weight provides greater accuracy and avoids problems. Problems created by

bulking of damp sand. Volumetric batching is used for concrete mixed in a continuous

mixer and the mobile concrete mixer where weighing facilities are not at hand.

Specifications generally require that materials be measured in individual batches within

the following percentages of accuracy:

Cement (1%), Aggregate (2%), Water (1%), Air entraining admixtures (3%).

MIXING

The process of rolling, folding, and spreading at particles is known as the mixing of

concrete. The ingredients of concrete are mixed thoroughly such that the ingredients

are uniformly distributed in the concrete mass. Cement water paste should completely

cover the surface of aggregates. There are two types of concrete mixing.

HAND MIXING

For hand mixing, the materials are stacked on a water tight

platform. The plat form may be of wood, brick, or steel. The

ingredients are thoroughly mixed at least three times, in dry

condition before water poured. The prepared mix should be



Assignment


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consumed within 30 minutes from the time the water is added to it. The mixing by hand

is allowed in case of small works or unimportant works where small quantity of concrete

is required, it in advisable to use 10 % more cement than specified.

MACHINE MIXING

Machine used for mixing concrete are known as concrete mixtures. For machine mixing

all the materials of concrete including water are put in revolving drum and then, the

drum is rotated for a certain period. The resulting mix is then taken out of the drum.

Tilting

Handle

Frame

Wheels

Drum

MIXING MACHINE



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i. Mixing with machine is more efficient and it produces concrete of better

quality in a short time.

ii. The mixer of various types and capacities are available, they are provided

with power operated loading hoppers.

iii. The water should enter the mixture at the same time or before the other

materials are placed. This ensures even distribution of water.

iv. The concrete mixer should be thoroughly washed and cleaned after use.

v. The inside portion of the mixer should be inspected carefully at regular

intervals. The damaged or broken blades should be replaced.

vi. The duration of mixing the materials in the mixer and the speed of the

mixer are very important factors in deciding the strength of concrete.

vii. The mixing time should be at least one minute, but preferably two minutes

viii. Concrete discharged by mixer should be consumed within 30 minutes.

TRASPORTING CONCRETE

The method of transporting concrete from the mixture to the form work depends on the

size of the job, height above ground level at which concrete is to be placed. The quick

transportation of concrete is essential such that the concrete is placed and compactedbefore the initial set of cement starts. Barrows, trucks, dumper, belt conveyers, chutes,

buckets, are used most commonly for the purpose.

For ordinary works, human ladder is formed and concrete is conveyed in pans from

hand to hand. Precautions should be taken during the placing of concrete.

i. The concrete should be transported in such a way that there is no

segregation of aggregate.

ii. Under no circumstances the water should be added to concrete during its

transportations.



Assignment


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PLACING OF CONCRETE

The concrete should be placed and compacted before its setting starts. Following

precautions should be taken while placing the concrete.

i. The frame work or the surface which is to receive the fresh concrete

should be properly cleaned, prepared and well watered.

ii. It is desirable to deposit concrete as near as practicable to its final

position.

iii. The large quantities of concrete should not be deposited at a time

otherwise the concrete will start to flow along the form work and

consequently the resulting concrete will not have uniform composition.

iv. The concrete should be dropped vertically from a reasonable height. For

vertical lying of concrete, care should be taken to use stiff mix. Otherwisethe bleeding of the concrete through crack in forms will take place. The

term bleeding is used to mean the diffusion or running of concrete

through form works.

v. The concrete should be deposited in horizontal layers of about 150 mm

height. For mass concrete the layers may be of 400 mm to 500 mm height.

The accumulation of excess water in the upper layers is known as laitance

and it should be prevented by using shallow layer with stiff mix or putting

dry batches of concrete to absorb excess water.

vi. The concrete should be thoroughly worked around the reinforcement and

topped in such a way that no honey combed surface appears on removal

of form work. (The term honey comb is used to mean comb or mass of

waxy cells formed by bees in which they store their honey.)

vii. The concrete should be placed as soon as possible. But in no case it should

be placed after 30 of its preparation.

viii. During placing it should be seen that all edges and corners surface remains

unbroken, sharp, and straight in line.

ix. The placing of concrete should be carried out uninterrupted between

predetermined construction joints.



Assignment


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COMPATION OF CONCRETE (CONSOLIDATION)

The term consolidation is used to mean the compaction between the aggregate and

aggregate; between aggregate and reinforcement; and between aggregate and forms.

The main aim of consolidation is to eliminate air bubbles and thus to give maximum

density to concrete. An intimate contact between concrete and reinforcement is

ensured by proper consolidation.

The importance of consolidation of concrete can be seen from the fact that presence of

5% of voids reduces 30% strength of concrete. The difference between voids and pores

may be noted. The voids are gaps between two individual particles. The pores represent

the opening within the individual particle. The process of consolidation of concrete is

carried out by either hand or with the help of vibrations.

HAND CONSOLIDATION

For unimportant works the consolidation of concrete is carried out by hand methods

which include ramming, tamping, etc. with suitable tools. The hand method requires use

of a fairly wet concrete.

VIBRATORS

There are mechanical devices which are used to compact concrete in the form work.

The advantages of vibrators over hand method as follows.

i. It is possible by means of vibrators to make a stiff concrete mix workable.

ii. The quality of concrete can be improved by use of vibrators as less as water

required.

iii. The use of vibrators results in the reduction of consolidation time.

iv. With the help of vibrators it is possible to deposit concrete in small opening or

places where it will be difficult to deposit concrete by hand method

v. Following are four types of vibrators



Assignment


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I. Internal or immersion vibrators

These vibrators consist of a steel tube (Pocker) which is inserted in fresh

concrete. Pocker is connected to an electric motor or petrol engine

through a flexible tube. The frequency of vibration is about 3000 to 6000

rpm. Skilled and experienced men should handle internal vibrators. These

vibrators are more efficient than other type of vibrators and hence they

are most commonly used.

II. Surface vibrators

These vibrators are mounted on platform or screeds. They are used to

finish concrete surfaces such as bridge, floors, road slabs, station platform,

etc. these vibrators are found to be more effective for compacting very dry

concrete mixture because the vibration acts in the same direction of

gravity.III. Form or shutter vibrators

These vibrators are attached to the form work and external centering of

walls, columns etc. The vibrating action is conveyed to the concrete

through the form work during the transmission of vibrators. Hence they

are not used generally. But they are very much helpful for concrete

sections which are too thin for the use of internal vibrators.

IV. Vibrating tables

There are in the form of a rigidly built platform mounded on flexible

springs and they are operated by electromagnetic action or electric

motors. They are found to be very effective in compacting stiff and harsh

concrete mixer and hence they are invariably used in preparation of

precast structural products in factories and test specimens in laboratories.

CURING OF CONCRETE

The concrete surfaces are kept wet for certain period after placing of concrete so as to

promote the hardening of cement. It consists of control of temperature and moisture

movement from and into the concrete. The term curing of concrete is used to indicate

all such procedures and processes.



Assignment


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PURPOSES OF CURING

1. Curing protects concrete from sun and wind.

2. To retain water in concrete until the concrete has fully hardened.

3. The strength concrete gradually increases with age, if curing is efficient. Thisincrease in strength is sudden and rapid in early stages and it continues slowly for

an indefinite period.

4. By proper curing, the durability and impermeability of concrete are increased and

shrinkage is reduced.

5. The resistance of concrete is considerably increased by proper curing.

PERIOD OF CURING

This depends upon the type of cement and nature of work. For ordinary Portland

cement, the curing period can be considerably reduced.

EFFECT OF IMPROPER CURING

1. Compressive and flexure strength are lowered.

2. Cracks are formed due to plastic shrinkage and thermal effects.

3. The durability decreases due to high permeability.

4. The frost and weathering resistance are decreased.

5. The rate of carbonation increases.

VARIOUS METHOD OF CURING

1. Shading: - The object of shading the concrete is to prevent evaporation of water

from surface.

2. Sprinkling of water: - Water is sprinkled on the surface.

3. Ponding of water: - Water is made to stand on horizontal surface.

4. Membrane Curing: - Surface concrete is kept in contact with concrete. This

membrane prevents evaporation.

5. Steam Curing: - This method is uses for precast membrane. By this methodhydration to concrete is brought about within a very short timing.

6. Covering the surface with well gunny bags and sprinkling water on them

frequently.



Assignment


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PRESTRESSED CONCRETE (PSE)

It is the concrete in which compressive strength is artificially induced in a structure

before it is loaded. As the structure is loaded any tensile stresses which might be caused

by the loads are automatically cancelled and the cracks are eliminated. Pre-stressedconcrete save about 5% of concrete. The quality of steel is also reduced as high tensile

steel. Pre-stressed concrete make economical design compared to ordinary reinforced

concrete.

FERRO CEMENT

Ferro cement is also called armored cement concrete. Concreting structures using iron is

termed as Ferro concrete. Reinforced concrete and reinforced brick concrete are good

example of Ferro concrete. In reinforcement brick concrete steel rod embedded in

cement concrete will take tension, and the comprehension will be taken by both bricksand cement concrete. This is more economical than reinforced concrete. Strengthening

of brick wall is done by using either iron rods or welded steel wire mesh embedded in

cement mortar.

concrete & aggregates as

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