UNIT 1: INTRODUCTION TO CONCRETE MATERIALS

Cement:
Cement is a binder, a substance used for construction that sets, hardens, and adheres to
other materials to bind them together. Cement is seldom used on its own, but rather to bind sand
and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry,
or with sand and gravel, produces concrete.

Cements used in construction are usually inorganic, often lime or calcium silicate based,
and can be characterized as either hydraulic or non-hydraulic, depending on the ability of the
cement to set in the presence of water.

Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical
reaction between the dry ingredients and water. Portland cement gets its strength from chemical
reactions between the cement and water. The process is known as hydration. Non-hydraulic
cement does not set in wet conditions or under water. Rather, it sets as it dries and reacts
with carbon dioxide in the air. It is resistant to attack by chemicals after setting.

The materials, without the gypsum, are proportioned to produce a mixture with the
desired chemical composition and then ground and blended by one of two processes - dry
process or wet process. The materials are then fed through a kiln at 2,600º F to produce grayish-
black pellets known as clinker. The alumina and iron act as fluxing agents which lower the
melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized
and gypsum added to regulate setting time. It is then ground extremely fine to produce cement.

Manufacture of cement:
Portland cement is manufactured by crushing, milling and proportioning the following materials:

 Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
 Silica, SiO2: from sand, old bottles, clay or argillaceous rock
 Alumina, Al2O3: from bauxite, recycled aluminum, clay
 Iron, Fe2O3: from clay, iron ore, scrap iron and fly ash
 Gypsum, CaSO4.2H20: found together with limestone

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Chemical composition of cement:

Table 1:- Approximate oxide composition limits of ordinary Portland cement

The identification of the major compounds of cement is largely based on Bogue’s equations and
hence it is called “Bogue’s Compounds”. The four compounds usually regarded as major
compounds.
Table 2: Major compounds of cement

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 Table 3:- percentage composition of cement clinker

Hydration of cement:
The chemical reaction that takes place between the cement and water is referred as
hydration of the cement. The hydration reaction is an exothermic reaction. The reaction will
result in several by-products that will contribute to the strength of the concrete or mortar formed.
The cement hydration will liberate a considerable amount of heat. This is called as Heat of
liberation or Heat of Hydration. The mixing of cement with water will result in rapid evolution
of heat that will last for few minutes. This evolution of heat is probably due to the reaction of a
solution of aluminates and the sulfates.
Stages of hydration:
There are five stages of cement hydration:

 Stage 1: Initial Hydrolysis

 Stage 2: Induction Period or the Dormant Period
 Stage 3: Acceleration
 Stage 4: Deceleration
 Stage 5: Steady State

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 Fig 1:- Stages of hydration

Hydration of Bogue’s Compounds:

1. Tricalcium silicate (C3S):

 This is also called as Alite.
 This is one that readily reacts with the water and results in more heat of hydration.
 This is also responsible for the early strength of the concrete.
 The cement that has more C3S content is good for cold weathering concrete.
 Alite is responsible for 7-day strength and the hardness.
 The heat of hydration is 500J/Cal. This will produce a higher quantity of Ca(OH)2 or
calcium hydroxide.

2. Dicalcium Silicate (C2S):

 This compound will be undergoing reaction slowly.
 This is hence the factor responsible for the progressive strength of concrete (later
strength).
 This is also called as Belit.
 It will impart strength to the chemical attack.
 The heat of hydration is 260J/Cal.

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 Higher amount C-S-H is produced by this compound than C3S.
 This demands less amount of water for hydration than C3S.

3. Tricalcium aluminate (C3A) :

 Celit is the quickest one to react when the water is added to the cement.
 This is celit which is responsible for the flash setting (which is stiffening without the
development of strength) thus prevents the hydration of Dicalcium silicate (C2S)
and Tricalcium silicate (C3S).
 The hardening of concrete is generally speed up by this compound.
 The increase of this content will help in the manufacture of Quick Setting Cement.
 It provides weak resistance against sulphate attack and its contribution to the
development of strength of cement is perhaps less significant.
 The heat of hydration is 865J/Cal.

4. Tetracalcium Aluminoferrite (C4AF):

 This is called as Felit.
 The heat of hydration is 420J/Cal.
 It has
 The poorest cementing value.

Two silicates, namely C3S and C2S, control the most of the strength giving properties.
Upon hydration, both C3S and C2S give the same product called calcium silicate hydrate
(C-S-H gel) and calcium hydroxide Ca(OH) 2. Calcium silicate hydrate is the most
important product which the good properties of concrete. It makes up 50-60% of the
volume of solids in completely hydrated state.
On the other hand calcium hydroxide constitutes 20-25% of the volume of solid in the
hydrated paste. The lack of durability of concrete is due to presence of calcium
hydroxide. It reacts with sulphates present in water to form calcium sulphate which
further reacts with C3A and cause deterioration of concrete. This is known as sulphate
attack.

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 It is likely that both C3S and C2S phases contribute equally to the eventual strength of
cement but C3S liberates nearly 3 times as much calcium hydroxide chemical reaction as
C2S. However, C2S provides more resistance to chemical attack.

Aggregates:-
Aggregates are the inert granular materials such as sand, gravel, or crushed stone that are
mixed in fixed proportions with a binding material to produce concrete. These act as fillers or
volume increasing components on the one hand and are responsible for strength, hardness and
durability of the concrete on other hand. Aggregates account for 70-80 percent of the total
volume of concrete which influences various characteristics and properties of concrete.

Classification of aggregates:

Aggregate are variously classified on the basis of their grain size, shape their origin and their
volume-weight as follows:

1. Based on Grain Size.

This is the most common classification, where in two types of aggregates are distinguished, fine
aggregate and coarse aggregate.

Fine Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it
called as fine aggregate. Natural sand is generally used as fine aggregate, silt and clay are also
come under this category. The purpose of the fine aggregate is to fill the voids in the coarse
aggregate and to act as a workability agent.

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Coarse Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called
coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size
aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used
for normal strengths and 20mm size is used for high strength concrete.

2. Based on Shapes of aggregate:

Aggregates are classified according to shape into the following types;-

 Rounded aggregates
 Irregular or partly rounded aggregates
 Angular aggregates
 Flaky aggregates
 Elongated aggregates
 Flaky and elongated aggregates

3. Based on origin of aggregate:

 Natural: These include all those types of fine and coarse aggregates, that are available in
almost ready to use form, from natural resources. Examples are sands from river, gravels
from river banks.
 Bye-product: These include materials obtained as wastes from some industrial and
metallurgical engineering Operations, which possess suitable properties for being used as
aggregate. Examples: Cinder obtained from burning of coal in locomotives and kilns.
And Slag is obtained from blast furnaces as Scum is the best example from this category.
 Processed: These form a special class in Aggregate. They are specifically manufactured
for use in making Quality Concretes. Examples they include burnt clay, Shales etc.

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 4. Basis on Density of aggregate:

Three types of aggregates are distinguished on the basis of their weight per unit volume.

 Standard or Normal: These types of aggregate gives strength and weighting to the
concrete of around 2300 to 2500 kg/m3. Gravels, Sand and Crushed stone are all classed
as Standard or Normal Aggregates.

 High-Density Aggregates: These are that type of Aggregates, which is used in standard
proportions yield in heavy weight concretes. Such concretes are especially useful as
shields against X-rays and radiations in atomic power plant. Concretes with such
aggregate usually weight above 4000 kg/m3. Examples: Baryle – a natural mineral with
specific gravity of 4.3 is an example.

 Light weight Aggregate. They consist of natural and artificial materials of very low
density so that the resulting concrete is also quite light in weight, generally with in a
range of 350 to 750 kg/m3. They are specially used in sound proofing and fire proofing
constructions. They are also used extensively in the manufacture of light weight Pre-Cast
concrete blocks.

Soundness of aggregate (IS-2386: Part-5):

 Soundness of aggregate is the bailey to resist change of volume due to change of physical
conditions. These physical conditions include freezing and thawing, temperature change,
alternate change of drying and wetting in normal or in salt water. The aggregate which
are weak, porous and containing undesirable materials undergo large volume change in
change of those physical conditions.

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  The proper weight of the sample for each fraction is weighed and placed in separate
containers for the test. The samples are immersed in the prepared solution of sodium
sulphate or magnesium sulphate for 16 to 18 hours in such a manner that the solution
covers them to a depth of at least 15mm. after the immersion period, the aggregates are
removed from the solution, drained for about 15 minutes, and placed in the drying oven
maintained at a temperature of 1050C to 1100C.

 The samples are dries to a constant weight at this temperature by checking the weights
after 4 hours up to 18 hours. When the successive weights differ by less than 1 gram, it
may be considered that constant weight has been attained and then it may again immersed
in the prepared solution for the next cycle of immersion and drying. The number of
cycles of alternate immersion and drying are minimum 5 for road aggregate.

 After completion of the final cycle, the sample is cooled washed free from sulphate. This
may be determined when there is no more reaction of the wash water with barium
chloride (i.e., when there is no white precipitation when barium chloride is added to wash
water, it can said that there is no sulphate with wash water). Each fraction of sample then
dried to constant temperature of 1050C to 1100C and weighed. It can be taken that the
average loss of weight after 5cycles should not exceed 12 % and 18% when tested with
sodium sulphate and magnesium sulphate respectively.

Properties of fresh and hardened concrete:

Properties of Fresh Concrete:

 Work-ability - is the ease with which freshly prepared concrete can be transported and
placed for the job and compacted to a dense mass.
 Setting - When concrete changes its state from fresh to hardened then this process is
called setting.
 Segregation - The separation of concrete ingredients from each other is known as
segregation.

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  Plastic Shrinkage - This is the shrinkage that the fresh concrete undergoes till it sets
completely. It may be also called initial shrinkage.
 Thermal Shrinkage - This may be due to falling in temperature of concrete-mix from
the time it laid to the time it sets completely.
 Thermal Expansion - In massive concrete works, when the upper layers are laid
before the lower layers have completely set, there may arise a phenomenon of thermal
expansions – in the lower layers.
 Water Cement Ratio - The comprehensive strength decreases, in general, with
increasing water cement ratio and vice versa.

Properties of hardened concrete:

 Strength
 Creep
 Durability
 Shrinkage
 Modulus of Elasticity
 Water Tightness

Strength:

 When we refer to concrete strength, we generally talk about compressive strength of

concrete. Because, concrete is strong in compression but relatively weak in tension and
bending. Concrete compressive strength is measured in N/mm2. Compressive strength
mostly depends upon amount and type of cement used in concrete mix. It is also affected
by the water-cement ratio, mixing method, placing and curing. Concrete tensile strength
ranges from 7% to 12% of compressive strength. Both tensile strength and bending
strength can be increased by adding reinforcement.

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Creep:

 Deformation of concrete structure under sustained load is defined as concrete creep. Long
term pressure or stress on concrete can make it change shape. This deformation usually
occurs in the direction the force is applied.

Durability:

 Durability might be defined as the ability to maintain satisfactory performance over and
extended service life.

Shrinkage:

 Shrinkage is the volume decrease of concrete caused by drying and chemical changes. In
another word, the reduction of volume for the setting and hardening of concrete is
defined as shrinkage.

Modulus of Elasticity:

 The modulus of Elasticity of concrete depends on the Modulus of Elasticity of the

concrete ingredients and their mix proportions. As per IS-456:2000, the modulus of
elasticity is calculated using formula 5000√fck. Where, fck = 28 day compressive strength
of standard cube.

Water tightness (permeability):

 Another property of concrete is water tightness. Sometime, it’s called impermeability of

concrete. Water tightness of concrete is directly related to the durability of concrete. The
lesser the permeability, the more the durability of concrete.

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ADMIXTURES:-
Concrete Admixture is defined as a material other than water, aggregates and hydraulic
cement and additives like Pozzolana or slag and fiber reinforcement used as an ingredient of
concrete or mortar and added to the batch immediately before or during its mixing to modify one
or more of the properties of concrete in the plastic or hardened state. Some of the important
purposes for which admixtures are used are:

Purpose of using admixture:-

The major reasons for using admixtures are:
 To reduce the cost of concrete construction
 To achieve certain properties in concrete more effectively than by other means
 To maintain the quality of concrete during the stages of mixing, transporting, placing, and
curing in adverse weather conditions
 To overcome certain emergencies during concreting operations

To modify properties of fresh concrete

 Increase workability without increasing water content or decrease water content at the
same workability.
 Retard or accelerate time of initial setting.
 Reduce or prevent settlement.
 Modify the rate or capacity for bleedings.
 Reduce segregation.
 Improve pumpability.
 Reduce the rate of slump loss.

To modify the properties of hardened concrete

 Retard or reduce heat evaluation during early hardening.

 Accelerate the rate of strength development at early ages.
 Increase strength (compressive, tensile or flexural).
 Increase durability or resistance to severe condition of exposure.

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  Decrease permeability of concrete.
 Control expansion caused by the reaction of alkalies with certain aggregate constituents.
 Increase bond of concrete to steel reinforcement.
 Increase bond between existing and new concrete.
 Improve impact resistance and abrasion resistance.
 Inhibit corrosion of embedded metal.
 Produce colored concrete or mortar

Types of Concrete Admixtures:-

 Chemical Admixtures
1. Plasticizers
2. Super Plasticizers
3. Accelerators
4. Set Retarders
 Mineral Admixtures
5. Cementitious
6. Pozzolanic
7. Blast Furnace Slag
8. Flyash
9. Silica Fume
10. Rice Husk

Admixtures are classified according to the Indian Standard (IS 9103: 1999) are as follows:

a) Accelerating admixtures

b) Retarding admixtures

c) Water-reducing admixtures

d) Air-entraining admixtures

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 e) Super plasticizing admixtures

Chemical admixtures:-

1. Water-reducing admixture / Plasticizers:

These admixtures are used for following purposes:

1. To achieve a higher strength by decreasing the water cement ratio at the same workability
as an admixture free mix.
2. To achieve the same workability by decreasing the cement content so as to reduce the
heat of hydration in mass concrete.
3. To increase the workability so as to ease placing in accessible locations
4. Water reduction more than 5% but less than 12%
5. The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.
6. Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived
from wood processing in the paper industry

Actions involved:

1. Dispersion:

Surface active agents alter the physic chemical forces at the interface. They are adsorbed on
the cement particles, giving them a negative charge which leads to repulsion between the
particles. Electrostatic forces are developed causing disintegration and the free water become
available for workability.

2. Lubrication:

As these agents are organic by nature, thus they lubricate the mix reducing the friction and
increasing the workability.

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 3. Retardation:

A thin layer is formed over the cement particles protecting them from hydration and
increasing the setting time. Most normal plasticizers give some retardation, 30–90 minutes

2. Super Plasticizer admixture:

These are more recent and more effective type of water reducing admixtures also known
as high range water reducer. The main benefits of super plasticizers can be summarized as
follows:

Increased fluidity:

 Flowing
 Self-leveling
 Self-compacting concrete
 Penetration and compaction round dense reinforcement

Reduced W/C ratio:

 Very high early strength, >200% at 24 hours or earlier

 Very high later age strengths, >100 MPa or 15000 psi.
 Reduced shrinkage, especially if combined with reduced cement content.
 Improved durability by removing water to reduce permeability and diffusion.

The commonly used Super Plasticizers are as follows:

 Sulphonated melamine formaldehyde condensates (SMF)

 Give 16–25% water reduction.

 SMF gives little or no retardation, which makes them very effective at low temperatures
or where early strength is most critical.

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  Sulphonated naphthalene formaldehyde condensates (SNF)

 Typically give 16–25% water reduction.

 They tend to increase the entrapment of larger, unstable air bubbles.
 Retardation is more than with SMF but will still not normally exceed 90 minutes. SNF is
a very cost-effective.

 Polycarboxylate ether superplasticizers (PCE)

 Typically give 20–35% water reduction.

 They are relatively expensive per liter but are very powerful so a lower dose (or more
dilute solution) is normally used.
 In general the dosage levels are usually higher than with conventional water reducers.

3. Accelerator admixtures:

An admixture which, when added to concrete, mortar, or grout, increases the rate of
hydration of hydraulic cement, shortens the time of set in concrete, or increases the rate of
hardening or strength development. Accelerating admixtures can be divided into groups based on
their performance and application:

1. Set Accelerating Admixtures,

 Reduce the time for the mix to change from the plastic to the hardened state. Set
accelerators have relatively limited use, mainly to produce an early set.

2. Hardening Accelerators,

 Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least
130% at 48 hours.
 Hardening accelerators find use where early stripping of shuttering or very early access to
pavements is required.
 They are often used in combination with a high range water reducer, especially in cold
conditions.

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  Calcium chloride is the most effective accelerator and gives both set and hardening
characteristics.
 However, is limited due to acceleration of corrosion of steel reinforcement. For this
reason, it should not be used in concrete where any steel will be embedded but may be
used in plain unreinforced concrete.
 They find most use at low temperatures where concrete strength gain may be very slow
so that the relative benefit of the admixture becomes more apparent.
 A hardening accelerator may be appropriate for strength gain up to 24 hours at low
temperature and up to 12 hours at ambient temperatures. Beyond these times, a high
range water reducer alone will usually be more cost-effective.

4. Set Retarder admixtures:

 The function of retarder is to delay or extend the setting time of cement paste in concrete.
 These are helpful for concrete that has to be transported to long distance, and helpful in
placing the concrete at high temperatures.
 Retarding admixtures delay the end of the dormant period and the start of setting and
hardening.
 The mechanism of set retards is based on absorption. The large admixture anions and
molecules are absorbed on the surface of cement particles, which hinders further
reactions between cement and water i.e. retards setting.
 The commonly known retards are Calcium Ligno-sulphonates and Carbohydrates
derivatives used in fraction of percent by weight of cement.

5. Air Entrained Admixtures:

 An addition for hydraulic cement or an admixture for concrete or mortar which causes
air, usually in small quantity, to be incorporated in the form of minute bubbles in the
concrete or mortar during mixing, usually to increase its workability and frost
resistance.
 Air-entraining admixtures are surfactants that change the surface tension of the water.

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  Traditionally, they were based on fatty acid salts or vinsol resin but these have largely
been replaced by synthetic surfactants or blends of surfactants to give improved stability
and void characteristics to the entrained air.
 Air entrainment is used to produce a number of effects in both the plastic and the
hardened concrete. These include:

 Resistance to freeze–thaw action in the hardened concrete.

 Increased cohesion, reducing the tendency to bleed and segregation in the plastic
concrete.
 Compaction of low workability mixes including semi-dry concrete.
 Stability of extruded concrete.
 Cohesion and handling properties in bedding mortars.

Mineral Admixtures of Concrete:

Types of Mineral Admixtures

1. Cementitious Admixtures of Concrete

These have cementing properties themselves. For example:

 Ground granulated blast furnace slag (GGBFS)

 natural cement and hydraulic hydrated lime

2. Pozzolanic Admixtures of Concrete:

 A pozzolan is a material which, when combined with calcium hydroxide (lime), exhibits
cementitious properties. Pozzolans are commonly used as an to Portland cement concrete
mixtures to increase the long-term strength and other material properties of Portland
cement concrete and in some cases reduce the material cost of concrete.
 Pozzolans are used to improve the workability and quality of concrete, to effect economy,
and to protect against disruptive expansion caused by the reaction between different
constituents of mass concrete.

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  Natural pozzolanic materials occur in the form of obsidian, pumicite, volcanic ashes,
tuffs, clays, shales, and diatomaceous earth. Most of these pozzolans require grinding.
Examples are:-

 Fly ash
 Silica Fume
 Rice Husk Ash
 Metakaolin

Pozzolanic Action:

The additive act in three ways

1. Filler
2. Nucleating
3. Pozzolanic

1. Filler:

These additives/admixtures are finer than cement, so when added to concrete they occupy the
small pores previously left vacant.

2. Nucleating:

These fine particles accelerate the rate of hydration and precipitation starts.

3. Pozzolanic:

When cementing material reacts with water the following reaction take place:

C2S + H CSH + CH

C3S + H CSH + CH

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CSH is responsible for strength while CH is a soluble material reacts and dissolves in water
leaving behind pores. So when admixture is added

SiO3 or Al2O3+CH CSH

Thus it reduces the amount of CH & increase CSH

Conditions to Declare a Material Pozzolan:

 Having silica + Alumina oxide+ ferrous oxide more than 70%.

 Surface area on normal admixture is more than 300m²/kg.
 Surface area should be more than cement used.

3. Ground Granulated Blast Furnace Slag (GGBFS) admixture:

 Ground granulated blast-furnace slag is the granular material formed when molten iron
blast furnace slag (a by-product of iron and steel making) is rapidly chilled (quenched) by
immersion in water.
 It is a granular product, highly cementitious in nature and, ground to cement fineness,
hydrates like Portland cement. A by-product of steel manufacture which is sometimes
used as a substitute for Portland cement.
 GGBFS is used to make durable concrete structures in combination with ordinary
Portland cement and/or other pozzolanic materials.
 Concrete made with GGBFS cement sets more slowly than concrete made with ordinary
Portland cement but continues to gain strength over a longer period in production
conditions.
 This results in lower heat of hydration and lower temperature rises but may also affect
construction schedules where quick setting is required.
 Use of GGBFS significantly reduces the risk of damages caused by alkali-silica reaction
(ASR), provides higher resistance to chloride ingress, reducing the risk of reinforcement
corrosion, and provides higher resistance to attacks by sulfate and other chemicals.

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Benefits:

1. Durability

 GGBFS cement is routinely specified in concrete to provide protection against both

sulphate attack and chloride attack
 GGBFS is also routinely used to limit the temperature rise in large concrete pours. The
more gradual hydration of GGBFS cement generates both lower peak and less total
overall heat than Portland cement.

2. Appearance

 In contrast to the stony grey of concrete made with Portland cement, the near-white color
of GGBFS cement permits architects to achieve a lighter colour for exposed fair-faced
concrete finishes, at no extra cost.

3. Strength

 Concrete containing GGBFS cement has a higher ultimate strength than concrete made
with Portland cement. It has a higher proportion of the strength-enhancing calcium
silicate hydrates (CSH) than concrete made with Portland cement only, and a reduced
content of free lime, which does not contribute to concrete strength. Concrete made with
GGBFS continues to gain strength over time, and has been shown to double its 28 day
strength over periods of 10 to 12 years.

4. Fly Ash admixture:

The finely divided residue resulting from the combustion of ground or powdered coal.
Fly ash is generally captured from the chimneys of coal-fired power plants; it has pozzolanic
properties, and is sometimes blended with cement for this reason. Fly ash includes substantial
amounts of silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO).
Toxic constituents include arsenic, beryllium, boron, cadmium, chromium, cobalt, lead,
manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium.

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 In addition to economic and ecological benefits, the use of fly ash in concrete improves
its workability, reduces segregation, bleeding, heat evolution and permeability, inhibits alkali-
aggregate reaction, and enhances sulfate resistance. Even though the use of fly ash in concrete
has increased in the last 20 years, less than 20% of the fly ash collected was used in the cement
and concrete industries.

Class F Fly Ash:

 The burning of harder, older anthracite and bituminous coal typically produces Class F
fly ash. This fly ash is pozzolanic in nature, and contains less than 10% lime (CaO). The
glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland
cement, quicklime, or hydrated lime, with the presence of water in order to react and
produce cementitious compounds.

Class C Fly Ash:

 Fly ash produced from the burning of younger lignite or subbituminous coal, in addition
to having pozzolanic properties, also has some self-cementing properties. In the presence
of water, Class C fly ash will harden and gain strength over time. Class C fly ash
generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C
fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally
higher in Class C fly ashes.

5. Silica Fume admixture:

 By-product of semiconductor industry

 The terms condensed silica fume, microsilica, silica fume and volatilized silica are often
used to describe the by-products extracted from the exhaust gases of silicon, ferrosilicon
and other metal alloy furnaces.
 However, the terms microsilica and silica fume are used to describe those condensed
silica fumes that are of high quality, for use in the cement and concrete industry.
 Silica Fume consists of very fine particles with a surface area ranging from 13000 to
30000 m²/kg, which is approximately 100 times smaller than the average cement particle.

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  Because of its extreme fineness and high silica content, Silica Fume is a highly effective
pozzolanic material. It has been found that Silica Fume improves compressive strength,
bond strength, and abrasion resistance; reduces permeability of concrete to chloride ions;
and therefore helps in protecting reinforcing steel from corrosion, especially in chloride-
rich environments such as coastal regions.

6. Rice Husk Ash admixture:

 This is a bio waste from the husk left from the grains of rice. It is used as a pozzolanic
material in cement to increase durability and strength. The silica is absorbed from the
ground and gathered in the husk where it makes a structure and is filled with cellulose.
When cellulose is burned, only silica is left which is grinded to fine powder which is used
as pozzolana.

Non-Destructive Evaluation / Testing of Concrete

NDT is defined as the use of non- invasive techniques to determine the integrity of a
material, component or a structure. Non-destructive test is a method of testing existing concrete
structures to assess the strength and durability of concrete structure. In the non-destructive
method of testing, without loading the specimen to failure (i.e. without destructing the concrete)
we can measure strength of concrete. Now days this method has become a part of quality control
process. This method of testing also helps us to investigate crack depth, micro cracks and
deterioration of concrete. Non-destructive testing of concrete is a very simple method of testing
but it requires skilled and experienced persons having some special knowledge to interpret and
analyze test results. Non-destructive testing (NDT) is a mechanism used by engineers to detect
defects in materials and structures, either during manufacturing or while in service. Typically,
the methods used are ultrasonic, radiography, magnetic particle, eddy current, dye penetrant and
visual methods.

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Purposes (importance) of Non-destructive Tests are:

 Estimating the in-situ compressive strength

 Estimating the uniformity and homogeneity
 Estimating the quality in relation to standard requirement
 Identifying areas of lower integrity in comparison to other parts
 Detection of presence of cracks, voids and other imperfections
 Monitoring changes in the structure of the concrete which may occur with time
 Identification of reinforcement profile and measurement of cover, bar diameter, etc.
 Condition of prestressing/reinforcement steel with respect to corrosion
 Chloride, sulphate, alkali contents or degree of carbonation
 Measurement of Elastic Modulus
 Condition of grouting in prestressing cable ducts

Methods of NDT:

There are different methods of NDT as on today in the modern practice they are:-

 Visual methods
 Microwave testing
 Thermography
 Tap testing
 Magnetic particle diffraction
 Acoustic microscopy
 X-ray diffraction
 Acoustic emission
 Ultrasonic methods flux
 Leakage techniques
 Magnetic measurements
 Liquid penetrant methods
 Ray application methods
 Eddy current techniques

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  Laser interferometry
 Vibration Analysis

Some Uses of NDT

 Flaw detection and evaluation

 Leak detection
 Location determination
 Dimensional measurements
 Structure and microstructure characterization
 Estimation of mechanical and physical properties
 Stress strain measurements and dynamic response measurements

Six most common NDT methods, which are commonly deployed

1) Visual method
2) Liquid penetrant
3) Magnetic method
4) Ultrasonic
5) Eddy current
6) X-ray diffraction

1. Visual method:-

This method is carried out before any intended non-destructive test. Visual testing is the
most widely used method of non-destructive testing (NDT). Even the more sophisticated
methods require a visual test to be performed. Visual inspections provide information on type of
concrete damage, their possible causes and type of NDT test suitable for further investigation.
Visual inspection is carried out by experienced civil engineers who can interpret information
from the damages in the concrete structure.

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Advantages of visual inspection:-

 It can be a very simple but effective test to perform and often does not need expensive
equipment. Experienced operators and advanced equipment make it possible for visual
inspection to be very sensitive.
 It allows discontinuities to be seen and not be just a blip on the screen.
 Many different surface-breaking discontinuities can be found.
 Training and experience times can be short.
 Virtually any component can be examined anywhere on the surface.

Disadvantages of visual inspection:-

 Many variables can lead to discontinuities being missed.

 At its worst, it relies totally on the human factor.
 Many organizations pay little attention to the proper training of operators.
 Sub-surface discontinuities will not be seen.

2. Acoustic Emission Testing:

Acoustic Emission Testing (AET) is a Non-Destructive Testing (NDT) method that is

used to analyze emitted sound waves caused by defects or discontinuities. These acoustic waves
are induced by small deformations, corrosion or cracking, which occur prior to structure failure.
It is therefore possible, with AET, to locate structural defects and to monitor the propagation and
development of discontinuities. AET is a method which evaluates the elasticity of waves caused
by discontinuities formed within the specimen. In large-sized structures, several sensors are
placed on the material surface, leaving a space of some meters in between. The information
collected by each of the sensors is monitored through a computer. If defects exist in some areas,
the signal character from the sensor attached nearest to the discontinuity appears in a different
way. By analyzing the discontinuity’s indications, it is possible to ascertain the defect position
and suspect area of the structure. Acoustic emission testing can be performed in the field with
portable instruments or in a stationary laboratory setting. Typically, systems contain a sensor,
preamplifier, filter, and amplifier, along with measurement, display, and storage equipment (e.g.
oscilloscopes, voltmeters, and personal computers). Because of the versatility of Acoustic

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Emission Testing (AET), it has many industrial applications (e.g. assessing structural integrity,
detecting flaws, testing for leaks, or monitoring weld quality) and is used extensively as a
research tool.

Advantages:-

 In-service continuous monitoring with alarms is possible

 Entire structures can be monitored from various locations
 Global testing
 Integrated service of AET and pressure control
 Increased sensitivity compared to conventional testing methods
 Shorter testing time than other NDT techniques
 Real time monitoring

3. Ultra pulse velocity method:-

 The ultrasonic pulse is generated by an electro acoustical transducer. When the pulse is
induced into the concrete from a transducer, it undergoes multiple reflections at the
boundaries of the different material phases within the concrete. A complex system of
stress waves is developed which includes longitudinal (compressional), shear (transverse)
and surface (Rayleigh) waves. The receiving transducer detects the onset of the
longitudinal waves, which is the fastest.
 Because the velocity of the pulses is almost independent of the geometry of the material
through which they pass and depends only on its elastic properties, pulse velocity method
is a convenient technique for investigating structural concrete.
 The underlying principle of assessing the quality of concrete is that comparatively higher
velocities are obtained when the quality of concrete in terms of density, homogeneity and
uniformity is good. In case of poorer quality, lower velocities are obtained. If there is a
crack, void or flaw inside the concrete which comes in the way of transmission of the
pulses, the pulse strength is attenuated and it passes around the discontinuity, thereby
making the path length longer.

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  Quality of concrete in terms of strength, homogeneity, trapped air, internal flaws, cracks,
segregation, honeycombing, compaction, workmanship, and durability can be concluded
from this test. The test can also be used to evaluate the effectiveness of crack
repair. Ultrasonic testing is an indicative and other tests such as destructive testing must
be conducted to find the structural and mechanical properties of the material

𝐰𝐢𝐝𝐭𝐡 𝐨𝐟 𝐭𝐡𝐞 𝐬𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐞

Pulse velocity =
𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧 𝐛𝐲 𝐭𝐡𝐞 𝐩𝐮𝐥𝐬𝐞 𝐭𝐨 𝐠𝐨 𝐭𝐡𝐫𝐨𝐮𝐠𝐡

APPARATUS

The apparatus for ultrasonic pulse velocity measurement shall consist of the following:

a) Electrical pulse generator

b) Transducer - one pair

c) Amplifier

d) Electronic timing device.

Objective:

The ultrasonic pulse velocity method could be used to establish:

 The homogeneity of the concrete

 The presence of cracks, voids and other imperfections
 Changes in the structure of the concrete which may occur with time
 The quality of the concrete in relation to standard requirements
 The quality of one element of concrete in relation to another
 The values of dynamic elastic modulus of the concrete

4. Rebound hammer test:

 It consists of a spring controlled mass that slides on a plunger within a tubular housing.
When the plunger of rebound hammer is pressed against the surface of the concrete, the
spring controlled mass rebounds and the extent of such rebound depends upon the surface

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 hardness of concrete. The surface hardness and therefore the rebound is taken to be
related to the compressive strength of the concrete.
 The rebound is read off along a graduated scale and is designated as the rebound number
or rebound index. The points of impact on the specimen must not be nearer an edge than
20 mm and should be not less than 20 mm from each other. The same points must not be
impacted more than once.
 For taking a measurement, the rebound hammer should be held at right angles to the
surface of the concrete member. The test can thus be conducted horizontally on vertical
surfaces or vertically upwards or downwards on horizontal surfaces.
 The rebound hammer method provides a convenient and rapid indication of the
compressive strength of concrete by means of establishing a suitable correlation between
the rebound index and the compressive strength of concrete.

Objectives:-

The rebound hammer method could be used for:-

a) Assessing the likely compressive strength of concrete with the help of suitable
correlations between rebound index and compressive strength
b) Assessing the uniformity of concrete
c) Assessing the quality of the concrete in relation to standard requirements
d) Assessing the quality of one element of concrete in relation to another.

5. Impact echo method:

 Impact-echo is a method for nondestructive evaluation of concrete and masonry
structures, based on the use of impact-generated stress (sound) waves that propagate
through a structure and are reflected by internal flaws and external surfaces.
 A short duration impact, produced by tapping a small steel sphere against a concrete or
masonry surface, is used to generate low-frequency stress waves (1 to 60 kHz) that
propagate into the structure and are reflected by flaws and external surfaces.
 These stress waves propagate through concrete as though it were a homogeneous elastic
medium. Surface displacements caused by the arrival of reflected waves at the impact

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 surface are recorded by a transducer, located adjacent to the impact site, producing an
analog voltage signal proportional to displacement.
 This voltage signal called waveform is digitalized and transformed to computer where it
is converted into a spectrum of amplitude vs frequency. The dominant frequencies, which
appear as peaks in the spectrum, are associated with multiple reflections of stress waves
within the structure, and they provide information about the thickness of the structure, its
integrity, and the location of flaws.

Objectives:-

 It provides independent, nondestructive measurements of the thickness of concrete slabs

with an accuracy of 3% or better.
 It can determine the location and extent of flaws such as cracks, delamination, voids,
honeycombing, and debonding in plain, reinforced and post-tensioned concrete
structures.
 It can locate voids in the subgrade beneath slabs and pavements.
 For masonry it can determine thickness and locate cracks, voids and other defects where
the brick or block units are bonded together with mortar.
 It is not adversely affected by the presence of steel reinforcing bars.

6. Half-Cell Electrical Potential Method (corrosion measurement):-

 The method of half-cell potential measurements normally involves measuring the
potential of an embedded reinforcing bar relative to a reference half-cell placed on the
concrete surface.
 The half-cell is usually a copper/copper sulphate or silver/silver chloride cell but other
combinations are used. The concrete functions as an electrolyte and the risk of corrosion
of the reinforcement in the immediate region of the test location may be related
empirically to the measured potential difference. In some circumstances, useful
measurements can be obtained between two half-cells on the concrete surface

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APPLICATIONS

 This technique is most likely to be used for assessment of the durability of reinforced
concrete members where reinforcement corrosion is suspected.
 Used in location of areas of high reinforcement corrosion risk in marine structures, bridge
decks and abutments.
 Used in conjunction with other tests, it has been found helpful when investigating
concrete contaminated by salts.

7. Pullout test:

This method of test is intended to provide a standardized procedure for comparison of bond
characteristics between concrete and different types of steel reinforcing bars. In pull-out tests on
plain bars, the maximum load generally represents the bond strength that can be developed
between the concrete and steel. With plain bars the maximum load is not very different from the
load at the first visible slip, but in the case of the deformed bar, the maximum load may
correspond to a large slip which may not in fact be obtained in practice before other types of
failure occur. It is preferable, therefore, when comparing plain and deformed bars to determine
not only the maximum load but also the load at arbitrary amounts of slip and also plot the
complete load-slip curves for the plain and deformed bars under comparison. One such basis of
comparison is the load at a relative movement (slip) between steel and concrete of 0.025 mm at
the free end of the bar in a pull-out test.

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The test specimens shall consist of concrete cubes of size given below, with a single reinforcing
bar embedded vertically along a central axis in each specimen. The bar shall project down for a
distance of about 10 rom from the bottom face of the cube as cast, and shall project upward from
the top face whatever distance is necessary to provide sufficient length of bar to extend through
the bearing blocks and the support of the testing machine and to provide an adequate length to be
gripped for application of load:

Diameter of the Bars (mm) size of the cube (mm)

Up to and including 12 100

Over 12 up to and including 25 mm 150

Over 25 mm 225

The loading shall be continued and readings of movements recorded at appropriate intervals
until:

a) The yield point of the reinforcing bars has been reached

b) The enclosing concrete has failed (the type of failure shall be noted)

c) A minimum slippage of 2-5 mm has occurred at the loaded end

The slip at the loaded end of the bar shall be calculated as the average of the readings of the two
dial gauges. The average bond stress shall be the value obtained for each specimen, by dividing
the applied load at the slip specified, by the surface area of the embedded length of the bar.

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