Introduction: Cement Concrete is composite product obtained by mixing cement, water and an

inert matrix of sand and gravel or crushed stone. When the aggregate is mixed together with the

dry cement and water, they form a fluid mass that is easily mouldable into any shape. The

cement reacts chemically with the water and other ingredients to form a hard matrix, which binds

all the materials together into a durable stone-like material, which hardens over time.

The ingredients of concrete fall into two groups namely:-

(a) active ingredients: cement and water

(b) inactive ingredients: Fine and coarse aggregate

Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman

Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and

concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of

concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete

dome. Today, large concrete structures (for example, dams and multi-storey car parks) are

usually made with reinforced concrete.

Today, concrete is the most widely used man-made material (measured by tonnage).

History

The word concrete comes from the Latin word "concretus" (meaning compact or condensed), the

perfect passive participle of "concrescere", from "con-" (together) and "crescere" (to grow).

Perhaps the earliest known occurrence of cement was twelve million years ago.

On a human time-scale, small usages of concrete go back for thousands of years. The ancient

Nabatea culture was using materials roughly analogous to concrete at least eight thousand years

ago, some structures of which survive to this day.

German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and

pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400-1200 BC. Lime

mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688

BC) made use of waterproof concrete. Concrete was used for construction in many ancient

structures. Aggregate:  The inert filler material that makes up the bulk of concrete. Usually sand,

gravel, and rocks.  Fibers and reinforcing bars are not considered aggregate.

Terminology

Bleeding:  An undesirable process of mix water separating from the fresh cement paste or

concrete while it is being placed or consolidated.

Cement:  This word is used colloquially to mean several very different things:  the dry unreacted

powder that comes in a sack, the sticky fluid stuff formed just after water is added, and the

rocklike substance that forms later on.  As noted above, people also tend to use it to refer to

concrete.  Obviously this won’t work for people who want to have technical discussions.  For our

purposes, the word cement used by itself refers to the dry unreacted powder.

Cement paste:  Cement (see above) that has been mixed with water.  Usually the term implies

that it has already become hard (see Fresh).

Concrete:  A mixture of sand, gravel, and rocks held together by cement paste.  The world’s most

widely-used man-made material. 

Curing/Hardening:  Essentially interchangeable terms that mean the process of continued

strength gain after the cement paste has set due to chemical reactions between cement and

water.

Fresh:  Refers to cement paste or concrete that has been recently mixed and is still fluid.  This is

what those big trucks with the rotating container on the back are full of.  (These are often called

“cement mixers” but now you know why they should be called “concrete mixers”).

Hardened:  Refers to cement paste or concrete that has gained enough strength to bear some

load.

Heat of hydration:  Like most spontaneous chemical reactions, the hydration reactions between

cement and water are exothermic, meaning that they release heat.  Large volumes of concrete

can warm up considerably during the first few days after mixing when hydration is rapid.  This is

generally a bad thing, for reasons that will be discussed.

Hydration:  The chemical reactions between cement and water.  Hydration is what causes cement

paste to first set and then harden. 

Hydration products:  The new solid phases that are formed by hydration.

Mature: Refers to cement paste or concrete that has reached close to its full strength and is

reacting very slowly, if at all.  An age of 28 days is a very rough rule of thumb for reaching

maturity.

Mortar:  A mixture of cement paste and sand used in thin layers to hold together bricks or stones. 

Technically, mortar is just a specific type of concrete with a small maximum aggregate size.

Placing:  The process of transferring fresh concrete from the mixer to the formwork that defines

its final location and shape

Segregation:  An undesirable process of the aggregate particles becoming unevenly distributed

within the fresh cement paste while the concrete is being placed or consolidated.

Set:  The transition from fresh cement paste to hardened cement paste.  The terms “initial set”

and “final set” refer to specific times when the paste becomes no longer workable and completely

rigid, respectively.  “Setting” is the process by which this occurs.

Application of Cement Concrete:-

Roads Sidewalks houses bricks/blocks Bridges walls Beams Foundations floors sewer pipes water mains containment of nuclear waste

Canals missile silos solidification of hazardous wastes Dams Churches garden ornaments Caskets Monuments subways Tombs indoor furniture patio bricks swimming pools airport runways sculptures Canoes Barges grave vaultsTunnels parking garagesholding tanks Chimneysflower pots & planters bath tubs

Composition of concrete

There are many types of concrete available, created by varying the proportions of the main

ingredients below. In this way or by substitution for the cementations and aggregate phases, the

finished product can be tailored to its application with varying strength, density, or chemical and

thermal resistance properties.

The main ingredients of the cement concrete are listed below:-

1) Cement – The Glue in Concrete

     The responsible for bonding, the word cement by itself only to mean the dry unreacted

powder. Once water is added, it becomes cement paste - the glue that holds concrete together.

This is justified because vastly more Portland cement is used throughout the world than all other

types of cement combined. The name "Portland cement" arose in the 1820s because one of the

early developers of modern calcium silicate cements, and Englishman named Joseph Aspdin,

thought that the hardened paste bore a resemblance to Portland limestone, a commonly used

building stone quarried on the Isle of Portland. Giving a man-made building material a name that

connotes the hardness and durability of stone was of course a shrewd marketing move.

The Cement used shall be any of the following and the type selected should be appropriate for

the intended use(As per IS 456:2000)

a) 33 Grade ordinary Portland cement conforming to IS 269

b) 43 Grade ordinary Portland cement conforming to IS 8112

c) 33 Grade ordinary Portland cement conforming to IS 12269

d) Rapid hardening Portland cement conforming to IS 8041

e) Portland slag cement conforming to IS 455

f) Portland pozzolana cement (fly ash based) conforming to IS 1489 (Part 1)

g) Portland Pozzolana cement (Calcined clay based) conforming to IS 1489 (Part-2)

h) Hydrophobic cement conforming to IS 8043

i) Low heat Portland cement conforming to IS 12600

Components of CementComparison of Chemical and Physical Characteristics

PropertyPortlandCement

Siliceous(ASTM C618 Class F)Fly Ash

Calcareous(ASTM C618 Class C)Fly Ash

SlagCement

SilicaFume

SiO2 content (%) 21 52 35 35 85–97

Al2O3 content (%)

5 23 18 12 —

Fe2O3 content (%)

3 11 6 1 —

CaO content (%) 62 5 21 40 < 1

Specific surfaceb(m2/kg)

370 420 420 40015,000–30,000

Specific gravity 3.15 2.38 2.65 2.94 2.22

General usein concrete

Primarybinder

Cementreplacement

Cementreplacement

Cementreplacement

Propertyenhancer

aValues shown are approximate: those of a specific material may vary.bSpecific surface measurements for silica fume by nitrogen adsorption (BET) method,others by air permeability method (Blaine).

Aggregates: Aggregate should be as per with the requirement of IS 383. As far possible

preference shall be given to natural aggregate.

a. Other types of aggregate such as slag and cursed over burnt bricks or tile which may be

found suitable with regard to strength durability of concrete and freedom from harmful effects may

be used for plain concrete members but such aggregate should be used not contain more then

0.5 percent of sulphates as So, and should not absorb more than 10 percent of their own mass

of water.

b. Heavy weight aggregates or light weight aggregates such as bloated aggregates and sintered

fly ash aggregate may also be used provided the engineer in charge is satisfied with the data on

the properties of concrete made with them.

Size of Aggregate :

a.) The nominal maximum size of coarse aggregate should be as large as possible within the

limits specified but in no case greater than one-fourth of the minimum thickness of the member,

provided that the concrete can be placed without difficulty so as to surround all reinforcement

thoroughly and fill the corners of the form.

b)The nominal maximum size of coarse aggregate should be as large as possible within the limits

specified but in no case greater that one fourth of the minimum thickness of the members

provided that the concrete can be placed without difficulty so as to surround all reinforcement

thoroughly and fill the corners of the form. For most work 20mm aggregate is suitable where there

is no restriction to the flow of concrete into section 40mm or larger size may be permitted in

concrete elements with thin sections closely spaced or small cover. Consideration should be

given to the use to 10mm nominal maximum size. Plums above 160mm and up to any reasonable

size may be used in plain concrete work up to a maximum limit of 20 percent by volume of

concrete when specifically permitted by the engineer in charge. The plums shall be distributed

evenly and shall be not closer than 150 mm from the surface

c) For heavily reinforced concrete members as in the case of the ribs of main beams the

nominal maximum size of the aggregate should usually be restricted to 5 mm less that the

minimum clear distance between the main bars or 5mm whichever is smaller.

d) Coarse and fine aggregate shall be batched separately All-in aggregate shall be used

only where specifically permitted by the engineer in charge.

e) Construction of Pavement Quality Concrete (PQM) nominal size of 31.5 Mineral

Admixture

a) Pozzolana materials conforming to relevant indian standard may be used with the

permission of the engineer in charge provided uniform blending with cement is ensured

b) Fly ash conforming to Grade 1 to IS 3812 may be used as part replacement of ordinary

Portland cement provided uniform blending with cement is ensured

c) Silica fume conforming to a standard approved by the deciding authority may be used as

part replacement of cement provided uniform with the cement is ensured.

d) Rice husk ash giving required performance and uniformity characteristics may be used

with the approval of the deciding authority.

e) Metakaoline having fineness between 700 to 900 m2/kg may be used as pozzolanic

material in concrete.

f) Ground granulated blast furnace slag obtained by grinding granulated blast furnace slag

conforming to IS 12089 may be used as part replacement of ordinary Portland cements provided

uniform blending with cement is ensued.

g) Other types of aggregates such as slag and crushed overburnt bricks or tile, which may

be found suitable with regard to strength durability of concrete and freedom from harmful effects

may be used for plain concrete members but such aggregates should not contain more than 0.5

percent of sulphates should as So and should not absorb more than 10 percent of their own

mass of water.

Chemical admixtures

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete

to give it certain characteristics not obtainable with plain concrete mixes. In normal use,

admixture dosages are less than 5% by mass of cement and are added to the concrete at the

time of batching/mixing. (See the section on Concrete Production, below.)The common types of

admixtures] are as follows.

Accelerators speed up the hydration (hardening) of the concrete. Typical materials used

are CaCl

2, Ca(NO3)2 and NaNO3. However, use of chlorides may cause corrosion in steel

reinforcing and is prohibited in some countries, so that nitrates may be favored.

Accelerating admixtures are especially useful for modifying the properties of concrete in

cold weather.

Retarders slow the hydration of concrete and are used in large or difficult pours where

partial setting before the pour is complete is undesirable. Typical polyol retarders are

sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

Air entraining agents add and entrain tiny air bubbles in the concrete, which reduces

damage during freeze-thaw cycles, increasing durability. However, entrained air entails a

trade off with strength, as each 1% of air may decrease compressive strength 5%. If too

much air becomes trapped in the concrete as a result of the mixing process, Defoamers

can be used to encourage the air bubble to agglomerate, rise to the surface of the wet

concrete and then disperse.

Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed

more easily, with less consolidating effort. A typical plasticizer is lignosulfonate.

Plasticizers can be used to reduce the water content of a concrete while maintaining

workability and are sometimes called water-reducers due to this use. Such treatment

improves its strength and durability characteristics.

Superplasticizers (also called high-range water-reducers) are a class of plasticizers that

have fewer deleterious effects and can be used to increase workability more than is

practical with traditional plasticizers. Compounds used as superplasticizers include

sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde

condensate, acetone formaldehyde condensate and polycarboxylate ethers.

Pigments can be used to change the color of concrete, for aesthetics.

Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in

concrete.

Bonding agents are used to create a bond between old and new concrete (typically a

type of polymer) with wide temperature tolerance and corrosion resistance.

Pumping aids improve pumpability, thicken the paste and reduce separation and

bleeding.

Water – The Activator in Concrete

 Water is then mixed with this dry /aggregate blend, which produces a semi-liquid that workers

can shape (typically by pouring it into a form). The concrete solidifies and hardens through a

chemical process called hydration. Hydration involves many different reactions, often occurring at

the same time. As the reactions proceed, the products of the cement hydration process gradually

bond together the individual sand and gravel particles and other components of the concrete to

form a solid mass.

Reaction:

Cement chemist notation C3S + H → C-S-H + CH

Standard notation: Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2

Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2

The water reacts with the cement, which bonds the other components together, creating a robust

stone-like material.

Effects of Water Cement Ratio :-

  Another important issue associated with the mix water is the amount that is added in relation to

the amount of cement. This important parameter is called the water/cement ratio, or "w/c", and it

always refers to the weights of water and cement.. Although there are many aspects of the

concrete mix design and the curing process that affect the final properties of the concrete, the w/c

is probably the most important. If the w/c is too low, the concrete will be stiff and clumpy and will

be difficult to place. However, the lower the w/c, the stronger and more durable the final concrete.

This is easy to understand when one realizes that any space in the fresh concrete that is

originally occupied by the mix water will end up as porosity in the hardened concrete. Porosity

lowers the intrinsic strength and makes it easier for the concrete to corrode, crack, and spall. For

this reason, the w/c should be a low as possible, meaning just high enough so that the concrete

can be placed properly. This will depend on many factors, such as the amount, size, and shape of

the aggregate , the fineness of the cement, the type of form or mold the concrete is being placed

into, and the type of reinforcement.

A lower water-to-cement ratio yields a stronger, more durable concrete, whereas more water

gives a freer-flowing concrete with a higher slump. Impure water used to make concrete can

cause problems when setting or in causing premature failure of the structure.

Due to increase in water cement ratio, effect on properties of concrete has been illustrated in following table:-

Mix Proportion

Water-Cement Ratio

Age (day)

Weight of Cube (g)

Density of Cube (g/cm3)

Crushing Load (KN)

Compressive Strength (N/mm2)

1:2:4 0.55 7 8100 2.400 245 10.89 1:2:4 0.60 7 7850 2.326 238 10.58 1:2:4 0.65 7 7799 2.311 237 10.53 1:2:4 0.70 7 7499 2.222 218 9.69 1:2:4 0.80 7 7401 2.193 207 9.20 1:2:4 0.55 14 8300 2.459 360 16.00 1:2:4 0.60 14 8000 2.370 323 14.36 1:2:4 0.65 14 7897 2.340 305 13.56 1:2:4 0.70 14 7600 2.252 290 12.89

1:2:4 0.80 14 7450 2.207 281 12.49 1:2:4 0.55 28 8397 2.488 450 20.00 1:2:4 0.60 28 8100 2.400 390 17.33 1:2:4 0.65 28 8000 2.370 385 17.11 1:2:4 0.70 28 7698 2.281 367 16.31 1:2:4 0.80 28 7600 2.252 360 16.00

TYPES OF CONCRETE WITH APPLICATIONS

Types of concrete with applications for different structural components like beams, columns, slabs, foundations are explained here. Special concrete with uses.

Light weight concrete

One of the main advantages of conventional concrete is the self weight of concrete. Density of

normal concrete is of the order of 2200 to 2600. This self weight will make it to some extend an

uneconomical structural material.

1. Self weight of light weight concrete varies from 300 to 1850 kg/m3.

2. It helps reduce the dead load, increase the progress of building and lowers the hauling

and handling cost.

3. The weight of building on foundation is an important factor in the design , particularly in

case of weak soil and tall structures. In framed structure , the beam and column have to

carry load of wall and floor. If these wall and floor are made of light weight concrete it will

result in considerable economy.

4. Light weight concrete have low thermal conductivity.( In extreme climatic condition where

air condition is to installed the use of light weight concrete with low thermal conductivity is

advantageous from the point of thermal comfort and low power consumption.

5. Only method for making concrete light by inclusion of air. This is achieved by a) replacing

original mineral aggregate by light weight aggregate, b) By introducing gas or air bubble

in mortar c) By omitting sand fraction from concrete. This is called no – fine concrete. No

fine concrete is made up of only coarse aggregate , cement and water.These type of

concrete is used for load bearing cast in situ external walls for building. They are also

used for temporary structures because of low initial cost and can be reused as

aggregate.

6. Light weight aggregate include pumice, saw dust rice husk, thermocole beads, formed

slag. Etc

7. Light weight concrete aggregate exhibit high fire resistance.

8. Structural lightweight aggregate’s cellular structure provides internal curing through water

entrainment which is especially beneficial for high-performance concrete

9.  lightweight aggregate has better thermal properties, better fire ratings, reduced

shrinkage, excellent freezing and thawing durability, improved contact between

aggregate and cement matrix, less micro-cracking as a result of better elastic

compatibility, more blast resistant, and has better shock and sound absorption, High-

Performance lightweight aggregate concrete also has less cracking, improved skid

resistance and is readily placed by the concrete pumping method

High density concrete

1. The density of high density concrete varies from 3360 kg/m3 to 3840 kg/m3.They can

however be produced with density upto 5820 kg/m3 using iron as both fine and coarse

aggregate.

2. Heavyweight concrete uses heavy natural aggregates such as barites or magnetite or

manufactured aggregates such as iron or lead shot. The density achieved will depend on

the type of aggregate used. Typically using barites the density will be in the region of

3,500kg/m3, which is 45% greater than that of normal concrete, while with magnetite the

density will be 3,900kg/m3, or 60% greater than normal concrete. Very heavy concretes can

be achieved with iron or lead shot as aggregate, is 5,900kg/m3 and 8,900kg/m3

respectively.

1. They are mainly used in the construction of radiation shields (medical or nuclear).

Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures

2. The ideal property of normal and high density concrete are high modulus of elasticity ,

low thermal expansion , and creep deformation

3. Because of high density of concrete there will be tendency for segregation. To avoid this

pre placed aggregate method of concreting is adopted.

4. High Modulus of Elasticity, Low thermal Expansion ,Low elasticity and creep deformation

are ideal properties.

5. The high density. Concrete is used in construction of radiation shields. They are effective

and economic construction material for permanent shielding purpose.

6. Most of the aggregate specific gravity is more than 3.5

Mass concrete

Mass concrete is defined in ACI as “any volume of concrete with dimensions large enough to

require that measures be taken to cope with generation of heat from hydration of the cement and

attendant volume change to minimize cracking.” The design of mass concrete structures is

generally based on durability, economy, and thermal action, with strength often being a

secondary, rather than a primary, concern. The one characteristic that distinguishes mass

concrete from other concrete work is thermal behavior. Because the cement-water reaction is

exothermic by nature, the temperature rise within a large concrete mass, where the heat is not

quickly dissipated, can be quite high. Significant tensile stresses and strains may result from the

restrained volume change associated with a decline in temperature as heat of hydration is

dissipated. Measures should be taken where cracking due to thermal behavior may cause a loss

of structural integrity and monolithic action, excessive seepage and shortening of the service life

of the structure, or be aesthetically objectionable. Many of the principles in mass concrete

practice can also be applied to general concrete work, whereby economic and other benefits may

be realized. Mass concreting practices were developed largely from concrete dam construction,

where temperature-related cracking was first identified. Temperature-related cracking has also

been experienced in other thick-section concrete structures, including mat foundations, pile caps,

bridge piers, thick walls, and tunnel linings

Ready-mix Concrete

Ready-mix concrete has cement, aggregates, water   and   other   ingredients,   which   are

weigh-batched   at a   centrally located   plant. This is   then  delivered   to the   construction site

in truck mounted transit mixers and can be used straight away without any further treatment. This

results in a precise mixture, allowing specialty concrete mixtures to be developed and

implemented on construction sites. Ready-mix concrete is sometimes preferred over on-site

concrete mixing because of the precision of the mixture and reduced worksite confusion.

However, using a pre-determined concrete mixture reduces flexibility, both in the supply chain

and in the actual components of the concrete. Ready Mixed Concrete, or RMC as it is popularly

called, refers to concrete that is specifically manufactured for delivery to the customer’s

construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of

Portland cement, water and aggregates comprising sand and gravel or crushed stone. In

traditional work sites, each of these materials is procured separately and mixed in specified

proportions at site to make concrete. Ready Mixed Concrete is bought and sold by volume –

usually expressed in cubic meters. Ready Mixed Concrete is manufactured under computer-

controlled operations and transported and placed at site using sophisticated equipment and

methods. RMC assures its customers numerous benefits.

Advantages of Ready mix Concrete over Site mix Concrete

A centralised concrete batching plant can serve a wide area.

The plants are located in areas zoned for industrial use, and yet the delivery trucks can

service residential districts or inner cities.

Better quality concrete is produced.

Elimination of storage space for basic materials at site.

Elimination of procurement / hiring of plant and machinery

Wastage of basic materials is avoided.

Labor associated with production of concrete is eliminated.

Time required is greatly reduced.

Noise and dust pollution at site is reduced.

Disadvantages of Ready-Mix Concrete

The materials are batched at a central plant, and the mixing begins at that plant, so the

traveling time from the plant to the site is critical over longer distances. Some sites are

just too far away, though this is usually a commercial rather than technical issue.

Access roads and site access have to be able to carry the weight of the truck and load.

Concrete is approx. 2.5tonne per m². This problem can be overcome by utilizing so-called

‘minimix’ companies, using smaller 4m³ capacity mixers able to access more restricted

sites.

Concrete’s limited time span between mixing and going-off means that ready-mix should

be placed within 2 hours of batching at the plant. Concrete is still usable after this point

but may not conform to relevant specifications.

Polymer concrete

Concrete is porous. The porosity is due to air voids , water voids or due to inherent property of gel

structures. On account of porosity strength of concrete is reduced , reduction of porosity result in

increase in strength of concrete. The impregnation of monomer and subsequent polymerization is

the latest technique adopted to reduce inherent porosity of concrete and increase strength and

other properties of concrete

There are mainly 4 types of polymer concrete

1. Polymer impregnated concrete

2. Polymer cement concrete

3. Polymer concrete

4. Partially impregnated and surface coated polymer concrete.

Polymer impregnated concrete

It is a precast conventional concrete cured and dried in oven or by dielectric heating from which

the air in the open cell is removed by vacuum. Then a low viscosity monomer is diffused through

the open cell and polymerized by using radiation, application of heat or by chemical initiation.

Mainly the following type of monomers are used

Methyl methacrlylate(MMA)

1. Acrylonitrile

2. t- butyl styrene

3. Other thermoplastic monomer

4. The amount of monomer that can be loaded into a concrete specimen is limited by the amount

of water and air that has occupied the total void space.

5. PIC require cast in situ structures

Polymer cement concrete

Polymer cement concrete is made by mixing cement, aggregate, water and monomer. Such

plastic mixture is cast in moulds , cured dried and polymerized. The monomer that are used in

PCC are

1. Polyster- styrene

2. Epoxy-styrene

3. Furans

4. Vinyldene chloride

PCC produced in this way have been disappointing. In many cases material poorer than ordinary

concrete is obtained.This is because organic material are incompatable with aqueous systems

and some times interfere with the alkaline cement hydration process. Russians developed a

superior polymer by incorporation of furfuryl alcohol and aniline hydrochloride in the wet mix. This

material is dense and non shrinking and to have high corrosion resistance, low permeability and

high resistance to vibration and axial extension .PCC can be cast in situ for field application.

Polymer concrete

Polymer concrete is an aggregate bound with a polymer binder instead of Portland cement as in

conventional concrete. The main technique in producing PC is to minimize void volume in the

aggregate mass so as to reduce the quantity of polymer needed for binding the aggregate. This is

achieved by properly grading and mixing the aggregate to attain maximum density and minimum

voids

Shotcrete

It is defined as a mortar conveyed through a hose and pneumatically projected at high velocity on

to a surface. There are mainly two different methods namely wet mix and dry mix process. In wet

mix process the material is conveyed after mixing with water. Shotcrete is a process where

concrete is projected or "shot" under pressure using a feeder or "gun" onto a surface to form

structural shapes including walls, floors, and roofs. The surface can be wood, steel, polystyrene,

or any other surface that concrete can be projected onto. The surface can be trowelled smooth

while the concrete is still wet. The properties of both wet and dry process shotcrete can be further

enhanced through the addition of many different additives or admixtures .

a) Wet method – All ingredients, including water, are thoroughly mixed and introduced into the

delivery equipment. Wet material is pumped to the nozzle where compressed air is added to

provide high velocity for placement and consolidation of the material onto the receiving surface.

b) Dry method – Pre-blended dry or damp materials are placed into the delivery equipment.

Compressed air conveys material through a hose at high velocity to the nozzle, where water is

added. Material is consolidated on the receiving surface by the high-impact velocity.

c) Advantages

Shotcrete has high strength, durability, low permeability, excellent bond and limitless shape

possibilities. These properties allow shotcrete to be used in most cases as a structural material.

Although the hardened properties of shotcrete are similar to conventional cast-in-place concrete,

the nature of the placement process provides additional benefits, such as excellent bond with

most substrates and instant or rapid capabilities, particularly on complex forms or shapes. In

addition to building homes, shotcrete can also be used to build pools

Pre packed concrete

In constructions where the reinforcement is very complicated or where certain arrangements like

pipe, opening or other arrangements are incorporated this type of concreting is adopted. One of

the methods is concrete process in which mortar is made in a high speed double drum and

grouting is done by pouring on prepacked aggregate. This is mainly adopted for pavement slabs

Vacuum concrete

Concrete poured into a framework that is fitted with a vacuum mat to remove water not required

for setting of the cement; in this framework, concrete attains its 28-day strength in 10 days and

has a 25% higher crushing strength. The elastic and shrinkage deformations are considerably

greater than for normal-weight concrete. Specialty Concretes

Pervious concrete

Pervious concrete is a mix of specially graded coarse aggregate, cement, water and little-to-no

fine aggregates. This concrete is also known as "no-fines" or porous concrete. Mixing the

ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate

particles. The hardened concrete contains interconnected air voids totalling approximately 15 to

25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment

admixtures are often used in freeze–thaw climates to minimize the possibility of frost damage.

Nano concrete is created by High-energy mixing (HEM) of cement, sand and water using a

specific consumed power of 30 - 600 watt/kg for a net specific energy consumption of at least 5

kJ/kg of the mix. A plasticizer or a superplasticizer is then added to the activated mixture which

can later be mixed with aggregates in a conventional concrete mixer. In the HEM process sand

provides dissipation of energy and increases shear stresses on the surface of cement particles.

The quasi-laminar flow of the mixture characterized with Reynolds number less than 800 is

necessary to provide more effective energy absorption. This results in the increased volume of

water interacting with cement and acceleration of Calcium Silicate Hydrate (C-S-H) colloid

creation. The initial natural process of cement hydration with formation of colloidal globules about

5 nm in diameter[50] after 3-5 min of HEM spreads out over the entire volume of cement – water

matrix. HEM is the "bottom-up" approach in Nanotechnology of concrete. The liquid activated

mixture is used by itself for casting small architectural details and decorative items, or foamed

(expanded) for lightweight concrete. HEM Nano concrete hardens in low and subzero

temperature conditions and possesses an increased volume of gel, which drastically reduces

capillarity in solid and porous materials.

Microbial concrete

Bacteria such as Bacillus pasteurii, Bacillus pseudofirmus, Bacillus cohnii, Sporosarcina pasteuri,

and Arthrobacter crystallopoietes increase the compression strength of concrete through their

biomass. Not all bacteria increase the strength of concrete significantly with their biomass.

Bacillus sp. CT-5. can reduce corrosion of reinforcement in reinforced concrete by up to four

times. Sporosarcina pasteurii reduces water and chloride permeability. B. pasteurii increases

resistance to acid. Bacillus pasteurii and B. sphaericuscan induce calcium carbonate precipitation

in the surface of cracks, adding compression strength.

Pumped concrete

Pumped concrete must be designed to that it can be easily conveyed by pressure through a rigid

pipe of flexible hose for discharge directly into the desired area.  Pozzocrete use can greatly

improve concrete flow characteristics making it much easier to pump, while enhancing the quality

of the concrete and controlling costs.

REQUIREMENT OF CONCRETE MIX

The designer must be aware of the need to improve the grade and maintain uniformity of the

various materials used in the pumped mix in order to achieve greater homogeneity of the total

mix.  Three mix proportioning methods frequently used to produce pump able concrete are :

Maximum Density of Combined Materials

Maximum Density – Least Voids

Minimum Voids – Minimum Area

Mixes must be designed with several factors in mind:

1. Pumped concrete must be more fluid with enough fine material and water to fill internal voids.

2. Since the surface area and void content of fine material below 300 microns control the liquid

under pressure, there must be a high quantity of fine material in a normal mix.  Generally

speaking, the finer the material, the greater the control.

3. Coarse aggregate grading should be continuous, and often the sand content must be

increased by up to five percent at the expense of the coarser aggregate so as to balance the 500

micron fraction against the finer solids.

Pozzocrete Effective

Unfortunately, adding extra water and fine aggregate leads to a weaker concrete. The usual

remedies for this are either to increase the cement content, which is costly, or to use chemical

admixtures, which can also be costly and may lead to segregation in marginal mixes. There is

another and far more effective alternative:

POZZOCRETE

There are many advantages to including POZZOCRETE in concrete mixes to be pumped. Among

them are :

1. Particle Size. Pozzocrete meets IS 3812 Specification with 66% passing the 325 (45-micron)

sieve and these fine particles are ideal for void filling.  Just a small deficiency in the mix fines can

often prevent successful pumping.

2. Particle Shape. Microscopic examination shows most Pozzocrete particles are spherical and

act like miniature ball bearings aiding the movement of the concrete by reducing frictional losses

in the pump and pining.  Studies have shown that Pozzocrete can be twice as effective as cement

in improving workability and, therefore, improve pumping characteristics.

Pozzolanic Activity:

his chemical reaction combines the Pozzocrete particles with the calcium hydroxide liberated

through the hydration of cement to form additional cementitious compounds which increase

concrete strength.

Water Requirement:

Excess water in pumped mixes resulting in over six inch slumps will often cause material

segregation and result in line blockage.  As in conventionally placed mixes, pumped concrete

mixes with excessive water also contribute to lower strength, increased bleeding and shrinkage.

The use of Pozzocrete in pumped or conventionally placed mixes can reduce the water

requirement by 2% to 10% for any given slump.

Sand/Coarse Aggregate Ratio:

In pumped mixes, the inclusion of liberal quantities of coarse aggregate can be very beneficial

because it reduces the total aggregate surface area, thereby increasing the effectiveness of the

available cementitious paste.  This approach is in keeping with the “minimum voids, minimum

area” proportioning method.  As aggregate size increases, so does the optimum quantity of

coarse aggregate.  Unfortunately, this process is frequently reversed in pump mixes, and sand

would be substituted for coarse aggregate to make pumping easier.  When that happens, there is

a need to increase costly cementitious material to compensate for strength loss.  However, if

Pozzocrete is utilized, its unique workability and pump ability properties permit a better balance of

sand to coarse aggregate resulting in a more economical, pump able concrete.

Types of Mixes

1. Nominal Mixes

In the past the specifications for concrete prescribed the proportions of cement, fine and coarse

aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are

termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of

strength above that specified. However, due to the variability of mix ingredients the nominal

concrete for a given workability varies widely in strength.

2. Standard mixes

The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may

result in under- or over-rich mixes. For this reason, the minimum compressive strength has been

included in many specifications. These mixes are termed standard mixes.

IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20,

M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the

specified 28 day cube strength of mix in N/mm2. The mixes of grades M10, M15, M20 and M25

correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2)

respectively.

3. Designed Mixes

In these mixes the performance of the concrete is specified by the designer but the mix

proportions are determined by the producer of concrete, except that the minimum cement content

can be laid down. This is most rational approach to the selection of mix proportions with specific

materials in mind possessing more or less unique characteristics. The approach results in the

production of concrete with the appropriate properties most economically. However, the designed

mix does not serve as a guide since this does not guarantee the correct mix proportions for the

prescribed performance.

For the concrete with undemanding performance nominal or standard mixes (prescribed in the

codes by quantities of dry ingredients per cubic meter and by slump) may be used only for very

small jobs, when the 28-day strength of concrete does not exceed 30 N/mm2. No control testing is

necessary reliance being placed on the masses of the ingredients.

Factors affecting the choice of mix proportions

The various factors affecting the mix design are:

1. Compressive strength

It is one of the most important properties of concrete and influences many other describable

properties of the hardened concrete. The mean compressive strength required at a specific age,

usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting

the strength of concrete at a given age and cured at a prescribed temperature is the degree of

compaction. According to Abraham’s law the strength of fully compacted concrete is inversely

proportional to the water-cement ratio.

2. Workability

The degree of workability required depends on three factors. These are the size of the section to

be concreted, the amount of reinforcement, and the method of compaction to be used. For the

narrow and complicated section with numerous corners or inaccessible parts, the concrete must

have a high workability so that full compaction can be achieved with a reasonable amount of

effort. This also applies to the embedded steel sections. The desired workability depends on the

compacting equipment available at the site.

3. Durability

The durability of concrete is its resistance to the aggressive environmental conditions. High

strength concrete is generally more durable than low strength concrete. In the situations when the

high strength is not necessary but the conditions of exposure are such that high durability is vital,

the durability requirement will determine the water-cement ratio to be used.

4. Maximum nominal size of aggregate

In general, larger the maximum size of aggregate, smaller is the cement requirement for a

particular water-cement ratio, because the workability of concrete increases with increase in

maximum size of the aggregate. However, the compressive strength tends to increase with the

decrease in size of aggregate.

IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should be as

large as possible.

5. Grading and type of aggregate

The grading of aggregate influences the mix proportions for a specified workability and water-

cement ratio. Coarser the grading leaner will be mix which can be used. Very lean mix is not

desirable since it does not contain enough finer material to make the concrete cohesive.

The type of aggregate influences strongly the aggregate-cement ratio for the desired workability

and stipulated water cement ratio. An important feature of a satisfactory aggregate is the

uniformity of the grading which can be achieved by mixing different size fractions.

Mix Proportion designations

The common method of expressing the proportions of ingredients of a concrete mix is in the

terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of

proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix

contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The

proportions are either by volume or by mass. The water-cement ratio is usually expressed in

mass

Procedure for Mix Design

1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-

day fck and the level of quality control , This equation is for building work .For Bridge work take

the target strength as per table 1700-5 MORT&H Specifications. For road work the design mix is

based on flexure strength as per IRC 44.

ft = fck + 1.65 S

where S is the standard deviation obtained from the Table of approximate contents given after the

design mix.

2. Obtain the water cement ratio for the desired mean target using the empirical relationship between

compressive strength and water cement ratio so chosen is checked against the limiting water

cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio

for the requirements of durability given in table and adopts the lower of the two values.

3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.

4. Select the water content, for the required workability and maximum size of aggregates (for

aggregates in saturated surface dry condition) from table.

5. Determine the percentage of fine aggregate in total aggregate by absolute volume from table for

the concrete using crushed coarse aggregate.

6. Adjust the values of water content and percentage of sand as provided in the table for any

difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate

the values are given in table.

7. Calculate the cement content form the water-cement ratio and the final water content as arrived

after adjustment. Check the cement against the minimum cement content from the requirements

of the durability, and greater of the two values is adopted.

8. From the quantities of water and cement per unit volume of concrete and the percentage of sand

already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates

per unit volume of concrete from the following relations:

where V = absolute volume of concrete

= gross volume (1m3) minus the volume of entrapped air

Sc = specific gravity of cement

W = Mass of water per cubic metre of concrete, kg

C = mass of cement per cubic metre of concrete, kg

p = ratio of fine aggregate to total aggregate by absolute volume

fa, Ca = total masses of fine and coarse aggregates, per cubic metre of concrete,

respectively, kg, and

Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates,

respectively

9. Determine the concrete mix proportions for the first trial mix.

10. Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and

test them wet after 28-days moist curing and check for the strength.

11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.

Concrete production

Concrete plant facility showing a Concrete mixer being filled from the ingredient silos.

Concrete production is the process of mixing together the various ingredients—water, aggregate,

cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the

ingredients are mixed, workers must put the concrete in place before it hardens. In modern

usage, most concrete production takes place in a large type of industrial facility called a concrete

plant, or often a batch plant.

In general usage, concrete plants come in two main types, ready mix plants and central mix

plants. A ready mix plant mixes supply the required grade of concrete as per customer

requirement and supplied by transit mixer loaders by giving necessary dose of retarders , while a

central mix plant mixes all the ingredients including water. A central mix plant offers more

accurate control of the concrete quality through better measurements of the amount of water

added, but must be placed closer to the work site where the concrete will be used, since

hydration begins at the plant.

A concrete plant consists of large torage hoppers for various reactive ingredients like cement,

storage for bulk ingredients like aggregate and water, mechanisms for the addition of various

additives and amendments, machinery to accurately weigh, move, and mix some or all of those

ingredients, and facilities to dispense the mixed concrete, often to a concrete mixer truck.

Modern concrete is usually prepared as a viscous fluid, so that it may be poured into forms, which

are containers erected in the field to give the concrete its desired shape. There are many different

ways in which concrete formwork can be prepared, such as Slip forming and Steel plate

construction. Alternatively, concrete can be mixed into dryer, non-fluid forms and used in factory

settings to manufacture Precast concrete products.

There is a wide variety of equipment for processing concrete, from hand tools to heavy industrial

machinery. Whichever equipment builders use, however, the objective is to produce the desired

building material; ingredients must be properly mixed, placed, shaped, and retained within time

constraints. Once the mix is where it should be, the curing process must be controlled to ensure

that the concrete attains the desired attributes. During concrete preparation, various technical

details may affect the quality and nature of the product.

Mixing concrete

Thorough mixing is essential for the production of uniform, high-quality concrete. For this reason

equipment and methods should be capable of effectively mixing concrete materials containing the

largest specified aggregate to produce uniform mixtures of the lowest slump practical for the

work.

Separate paste mixing has shown that the mixing of cement and water into a paste before

combining these materials with aggregates can increase the compressive strength of the resulting

concrete. The paste is generally mixed in a high-speed, shear-type mixer at a w/c (water to

cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as

accelerators or retarders, superplasticizers, pigments, or silica fume. The premixed paste is then

blended with aggregates and any remaining batch water and final mixing is completed in

conventional concrete mixing equipment.

Placing Of Concrete

Placing is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired

work (vibration) and without reducing the concrete's quality. Placing depends on workability of

concrete (water content, aggregate ,shape and size distribution, cementitious content and

age ,level of hydration) and can be modified by adding chemical admixtures, like superplasticizer.

Raising the water content or adding chemical admixtures increases concrete workability.

Excessive water leads to increased bleeding (surface water) and/or segregation of aggregates

(when the cement and aggregates start to separate), with the resulting concrete having reduced

quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix

design with a very low slump, which cannot readily be made more workable by addition of

reasonable amounts of water.

Workability can be measured by the concrete slump test, a simplistic measure of the plasticity of

a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is

normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The

cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in

three layers of equal volume, with each layer being tamped with a steel rod to consolidate the

layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing

to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches

(25 or 50 mm) out of one foot (305 mm). A relatively wet concrete sample may slump as much as

eight inches. Workability can also be measured by the flow table test.

Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer

without changing the water-cement ratio.Some other admixtures, especially air-entraining

admixture, can increase the slump of a mix.

High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods.

One of these methods includes placing the cone on the narrow end and observing how the mix

flows through the cone while it is gradually lifted.

After mixing, concrete is a fluid and can be pumped to the location where needed.

Curing

A concrete slab ponded while curing.

In all but the least critical applications, care must be taken to properly cure concrete, to achieve

best strength and hardness. This happens after the concrete has been placed. Cement requires a

moist, controlled environment to gain strength and harden fully. The cement paste hardens over

time, initially setting and becoming rigid though very weak and gaining in strength in the weeks

following. In around 4 weeks, typically over 90% of the final strength is reached, though

strengthening may continue for decades. The conversion of calcium hydroxide in the concrete

into calcium carbonate from absorption of CO2 over several decades further strengthens the

concrete and makes it more resistant to damage. However, this reaction, called carbonation,

lowers the pH of the cement pore solution and can cause the reinforcement bars to corrode.

Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying

and shrinkage due to factors such as evaporation from wind during placement may lead to

increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in

greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp

during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-

strength concrete is designed to hydrate faster, often by increased use of cement that increases

shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It

depends on cross-section dimension of elements and conditions of structure exploitation.

During this period concrete must be kept under controlled temperature and humid atmosphere. In

practice, this is achieved by spraying or ponding the concrete surface with water, thereby

protecting the concrete mass from ill effects of ambient conditions. The picture to the right shows

one of many ways to achieve this, ponding – submerging setting concrete in water and wrapping

in plastic to contain the water in the mix. Additional common curing methods include wet burlap

and/or plastic sheeting covering the fresh concrete, or by spraying on a water-impermeable

temporary curing membrane.

Properly curing concrete leads to increased strength and lower permeability and avoids cracking

where the surface dries out prematurely. Care must also be taken to avoid freezing or

overheating due to the exothermic setting of cement. Improper curing can cause scaling,

reduced strength, poor abrasion resistance and cracking.

TESTS FOR CONCRETE QUALITY CHECKING

Tests for checking quality of concrete should be done for the following possible purposes:

1. To detect the variation of quality of concrete being supplied for a given specification.

2. To establish whether the concrete has attained a sufficient strength or concrete has set sufficiently for stripping, stressing, de-propping, opening to traffic etc.

3. To establish whether the concrete has gained sufficient strength for the intended purpose.

There are so many tests available for testing different qualities of concrete. Different tests give

results for their respective quality of concrete. Thus it is not possible to conduct all the tests as it

involves cost and time. Thus, it is very important to be sure about purpose of quality tests for

concrete. The most important test for quality check of concrete is to detect the variation of

concrete quality with the given specification and mix design during concrete mixing and

placement. It will ensure that right quality of concrete is being placed at site and with checks for

concrete placement in place, the quality of constructed concrete members will be as desired.

Following are the lists of various tests conducted for Concrete Quality:

Tests on hardened concrete:

Compressive strength (cylinder, cube, core)

Tensile strength: Direct tension

Modulus of rupture

Indirect (splitting) Test

Density

Shrinkage

Creep

Modulus of elasticity

Absorption

Permeability Tests on Concrete

Freeze/thaw resistance

Resistance to aggressive chemicals

Resistance to abrasion

Bond to reinforcement

Analysis for cement content and proportions

In situ tests: Schmidt Hammer, Concrete pull-out, break-off, cones etc.

Ultrasonic, nuclear.

Tests on fresh concrete:

Workability Tests (slump test and others)

Bleeding

Air content

Setting time

Segregation resistance

Unit weight

Wet analysis

Temperature

Heat generation

Of these many tests for concrete quality, in practice well over 90% of all routine tests on concrete

are concentrated on compression tests and slump tests. It is also desirable to conduct fresh

concrete temperature and hardened concrete density determination tests.

The reasons for the selection of compressive strength test and slump test in practice for

quality control testing of concrete are:

1. All or most other properties of concrete are related to its compressive strength.

2. Compressive strength test is the easiest, most economical or most accurately determinable

test.

3. Compressive strength testing is the best means available to determine the variability of

concrete.

4. Slump tests also checks for variation of construction materials in mix, mainly water-cement

ratio.

5. Slump test is easy and fast to determine quality of concrete before placement based on

recommended slump values for the type of construction.

6. Slump test is most economical because it is done at site and does not require any laboratory or

expensive testing machine.

7. Slump tests is done before the placement of concrete, so the quality of control is high as

rejected mix can be discarded before pouring into the structural member. So, dismantling or

repair of defective concrete members can be avoided. VEE-BEE TEST

To determine the workability of fresh concrete by using a Vee-Bee consistometer as per IS: 1199

– 1959. The apparatus used is Vee-Bee consistometer.

Procedure to determine workability of fresh concrete by Vee-Bee consistometer.

i) A conventional slump test is performed, placing the slump cone inside the cylindrical part of the

consistometer.

ii) The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the

pot.

iii) The electrical vibrator is switched on and a stop-watch is started, simultaneously.

iv) Vibration is continued till the conical shape of the concrete disappears and the concrete

assumes a cylindrical shape.

v) When the concrete fully assumes a cylindrical shape, the stop-watch is switched off

immediately. The time is noted.The consistency of the concrete should be expressed in VB-

degrees, which is equal to the time in seconds recorded above.

COMPACTING FACTOR

Compacting factor of fresh concrete is done to determine the workability of fresh concrete by

compacting factor test as per IS: 1199 – 1959. The apparatus used is Compacting factor

apparatus.

Procedure to determine workability of fresh concrete by compacting factor test.

i) The sample of concrete is placed in the upper hopper up to the brim.

ii) The trap-door is opened so that the concrete falls into the lower hopper.

iii) The trap-door of the lower hopper is opened and the concrete is allowed to fall into the

cylinder.

iv) The excess concrete remaining above the top level of the cylinder is then cut off with the help

of plane blades.

v) The concrete in the cylinder is weighed. This is known as weight of partially compacted

concrete.

vi) The cylinder is filled with a fresh sample of concrete and vibrated to obtain full compaction.

The concrete in the cylinder is weighed again. This weight is known as the weight of fully

compacted concrete.

Compacting factor = (Weight of partially compacted concrete)/(Weight of fully compacted

concrete)

AGGREGATE IMPACT VALUE

This test is done to determine the aggregate impact value of coarse aggregates as per IS: 2386

(Part IV) – 1963. The apparatus used for determining aggregate impact value of coarse

aggregates is Impact testing machine conforming to IS: 2386 (Part IV)- 1963,IS Sieves of sizes –

12.5mm, 10mm and 2.36mm, A cylindrical metal measure of 75mm dia. and 50mm depth, A

tamping rod of 10mm circular cross section and 230mm length, rounded at one end and Oven.

Preparation of Sample

i) The test sample should conform to the following grading:

– Passing through 12.5mm IS Sieve – 100%

– Retention on 10mm IS Sieve – 100%

ii) The sample should be oven-dried for 4hrs. at a temperature of 100 to 110oC and cooled.

iii) The measure should be about one-third full with the prepared aggregates and tamped with 25

strokes of the tamping rod.

A further similar quantity of aggregates should be added and a further tamping of 25 strokes

given. The measure should finally be filled to overflow, tamped 25 times and the surplus

aggregates struck off, using a tamping rod as a straight edge. The net weight of the aggregates in

the measure should be determined to the nearest gram (Weight ‘A’).

Procedure to determine Aggregate Impact Value

i) The cup of the impact testing machine should be fixed firmly in position on the base of the

machine and the whole of the test sample placed in it and compacted by 25 strokes of the

tamping rod.

ii) The hammer should be raised to 380mm above the upper surface of the aggregates in the cup

and allowed to fall freely onto the aggregates. The test sample should be subjected to a total of

15 such blows, each being delivered at an interval of not less than one second.

Reporting of Results

i) The sample should be removed and sieved through a 2.36mm IS Sieve. The fraction passing

through should be weighed (Weight ‘B’). The fraction retained on the sieve should also be

weighed (Weight ‘C’) and if the total weight (B+C) is less than the initial weight (A) by more than

one gram, the result should be discarded and a fresh test done.

ii) The ratio of the weight of the fines formed to the total sample weight should be expressed as a

percentage.

iii) Two such tests should be carried out and the mean of the results should be reported

Aggregate impact value = (B/A) x 100%.

Compression test

The Compression Test is a laboratory test to determine the characteristic strength of the concrete

but the making of test cubes is sometimes carried out by the supervisor on site. This cube test

result is very important to the acceptance of insitu concrete work since it demonstrates the

strength of the design mix.

The procedure of making the test cubes is as follows: –

1. 150 mm standard cube mold is to be used for concrete mix and 100 mm standard cube mold is

to be used for grout mix.

2. Arrange adequate numbers of required cube molds to site in respect with the sampling

sequence for the proposed pour.

3. Make sure the apparatus and associated equipment ( see Fig 7 – 6 ) are clean before test and

free from hardened concrete and superfluous water .

4. Assemble the cube mold correctly and ensure all nuts are tightened.

5. Apply a light coat of proprietary mold oil on the internal faces of the mold.

6. Place the mold on level firm ground and fill with sampled concrete to a layer of about 50 mm

thick.

7. Compact the layer of concrete thoroughly by tamping the whole surface area with the Standard

Tamping Bar. (Note that no less than 35 tamps / layer for 150 mm mold and no less than 25

tamps / layer for 100 mm mold).

8. Repeat Steps 5 & 6 until the mold is all filled. (Note that 3 layers to be proceeded for 150 mm

mold and 2 layers for 100 mm mold).

9. Remove the surplus concrete after the mold is fully filled and trowel the top surface flush with

the mold.

10. Mark the cube surface with an identification number (say simply 1, 2, 3, etc) with a nail or

match stick and record these numbers in respect with the concrete truck and location of pour

where the sampled concrete is obtained.

11. Cover the cube surface with a piece of damp cloth or polythene sheeting and keep the cube

in a place free from vibration for about 24 hours to allow initial set .

12. Strip off the mold pieces in about 24 hours after the respective pour is cast. Press the

concrete surface with the thumb to see any denting to ensure the concrete is sufficiently

hardened, or otherwise de-molding has to be delayed for one more day and this occurrence

should be stated clearly in the Test Report.

13. Mark the test cube a reference number with waterproof felt pen on the molded side, in respect

with the previous identification number.

14. Place the cube and submerge in a clean water bath or preferably a thermostatically controlled

curing tank until it is delivered to the accredited laboratory for testing.

Checking Quality of Fine Aggregates

For checking the quality of fine aggregates, a field test was conducted in which the sand was

placed in a flask containing water. The sand was allowed to settle for some time and then after

few hours the reading of the silt or other impurity layer is takenIf that reading is less than 5% of

the total sand that is put in the flask, then we accept the sand but if it is more than 5% the sand is

rejected.