introdution to concrete and concrete materials (concrete technology)

CHAPTER – I

INTRODUCTION TO CONCRETE AND CONCRETE MATERIALS

“These days, there are two commonly used structural materials: concrete and steel.

They sometimes complement one another, and sometimes compete with one another,

so that many structures of a similar type and function can be built in either of these

materials. And yet, universities, polytechnics and colleges teach much less about

concrete than about steel. This in itself would not matter were it not for the fact that,

in actual practice, the man on the job needs to know more about concrete than about

steel.”

- - Concrete Technology (A. M. Neville CBE, J. J. Brooks)

1.1 Use of Concrete in Structure and Types of Concrete

Concrete, in the broadest sense, is any product or mass made by the use of a

cementing medium. Generally, this medium is the product of reaction between

hydraulic cement and water.

Concrete has been the most common building material for many years. It is expected

to remain so in the coming decades. Concreting is widely used in domestic, rural,

commercial, recreational and educational constructions. Communities around the

world rely on concrete as a safe, strong and simple building material.

Concrete is used in many types of civil engineering structures such as such as

buildings, bridges, dam, plates and shell structures, etc.

Based on ingredients present in concrete, it can be classified into two types:

a. Normal Concrete

b. Special Concrete

Normal Concrete has the ingredients namely cement, sand, aggregates (coarse) and

water mixed in some proportion to achieve concrete of desired strength and property.

Modern concrete, also called special concrete, invariably has additional components

other than these ingredients namely admixtures. Admixtures are added to concrete to

achieve special properties like ultra high strength or resistance to tensile forces.

Shuvanjan Dahal (068-BCE-147)

Based on strength, concrete can be classified as:

S.

N.

Types of Concrete Grade

Designation

Characteristic Strength

(N/mm2)

1. Ordinary Concrete M10 10

M15 15

M20 20

2. Standard Concrete M25 25

M30 30

M35 35

M40 40

M45 45

M50 50

M55 55

3. High Strength Concrete M60 60

M65 65

M70 70

M75 75

M80 80

Designation of Concrete:

Concrete is designated through 28-day Standard Cube Compressive Strength.

Each cube has a side of 150 mm.

The cube is cast, cured and tested in a standard manner.

Concrete designated as M25 has a 28 day characteristic standard cube

strength of 25 MPa.

Advantages of Concrete:

1. Lower life cycle cost (production + maintenance): Although cement is costly,

cement paste occupies only 25 to 40 % of total volume of concrete.

2. Mould-ability: Initially, the concrete is in plastic stage so that it can be cast

into any shape.

3. Robustness: property associated with the massiveness of concrete

4. Can be designed for desired property: by changing proportions of ingredients

Disadvantages of Concrete:


1. Low tensile strength – Concrete is derived from particulate system –

particulate systems are bonded by some type of bonding which is mostly

physical type e.g. Van der Waal’s forces. This can be overcome by conventional

reinforcements like steel (reinforced concrete) or by using small discrete

randomly oriented fibres (fiber reinforced concrete).

2. Low ductility (brittle) – not much capacity to deform before failure unlike steel

These disadvantages can be overcome by adding additional materials.

1.2 Concrete Materials – Role of Different Materials

(Aggregates, Cement, Water and Admixtures)

1.2.1 Aggregates – Properties of Aggregates and their Gradation

Crushed or uncrushed materials derived from natural or artificial sources as in

the form of boulder, gravel or sand for production of concrete are known as

aggregates.

Aggregates occupy 70 to 80 % of the volume of concrete.

Earlier, aggregates were considered as chemically inert materials but now it

has been recognized that some of the aggregates are chemically active and

also that certain aggregates exhibit chemical bond at the interface of

aggregate and paste.

Aggregates should be free from wild acids, alkalies and inorganic materials.

Classification

Aggregates can be classified as: (i) Normal weight aggregates, (ii) Light weight

aggregates and (iii) Heavy weight aggregates.


Almost all natural aggregate materials originate from bed rocks. Since there are

three types of rocks, normal weight aggregates can also be classified into three types

based on the rock these are derived from:

i. Aggregates from Igneous Rocks: Bulk of the concrete aggregates is igneous

in origin. These are the most chemically active concrete aggregates. They

are also highly satisfactory because of hard, tough and dense nature. E.g.

Basalt, Granite

ii. Aggregates from Sedimentary Rocks: Quality varies depending upon the

cementing material and the pressure under which the rocks were originally

compacted. E.g. Limestone, Sandstone

iii. Aggregates from Metamorphic Rocks: E.g. Quartzite, Gneiss. If the

thickness of the foliation in metamorphic rocks is less, then individual

aggregate may exhibit foliation which is not a desirable characteristic in

aggregate.

Based on Size:

Based on size, aggregates can be classified into coarse (retained on IS 4.75 sieve)

and fine (passing through IS 4.75 sieve). The largest maximum size of aggregate

practicable to handle under a given set of conditions should be used. This results in:

i. Reduction in the cement content

ii. Reduction in water requirement

iii. Reduction of drying shrinkage


Normal Weight Aggregates

Natural E.g. derived from river bed,

mountain quarry

ArtificialE.g. derived from pieces of

burnt bricks, air-cooled slag, sintered fly ash

The maximum size of aggregates that can be used in any given condition may be

limited by the following conditions:

i. Thickness of section

ii. Spacing of reinforcement

iii. Clear cover

iv. Mixing, handling and placing techniques

Maximum size of aggregate < 1/4th of maximum thickness of the member.

Based on Shape:

Classification Description Examples

1. Rounded Fully water worn or completely shaped by

attrition

River or seashore

gravels; desert,

seashore and wind

blown sands

2. Irregular or

Partly Rounded

Naturally irregular or partly shaped by

attrition, having rounded edges

Pit sands and

gravels; land or dug

flints; cuboidal rock

3. Flaky Thickness is small relative to the width and/or

length

Thickness (d) < 0.6 x mean size (average of

lengths)

Laminated rocks

4. Elongated Mean Size = Mean Thickness x 1.8

Based on Density:

1. Light weight aggregates: Density ≤ 1120 kg/m3

2. Normal weight aggregates: Density = 1520 to 1680 kg/m3

3. Heavy weight aggregates: Density ≥ 2080 kg/m3

Based on Mineralogical Composition:

1. Siliceous – containing silica

2. Calcareous – containing clay

Properties of Aggregates


1. Physical Properties

2. Chemical Properties

3. Mechanical Properties

Mechanical Properties

1. Strength

The properties of concrete are based primarily on the quality of the cement

paste.

The strength is dependent also on the bond between the cement paste and

the segregate.

If either the strength of the paste or the bond between the paste and

aggregate is low, a concrete of poor quality will be obtained irrespective of

the strength of the rock or aggregate.

AGGREGATE CRUSHING VALUE

The test specimen is cylindrical in shape of size 25 mm diameter and 25 mm

height.

The aggregate is placed in the cylindrical mould and a load of 40 tonnes is

applied gradually through a plunger.

The material crushed to finer than 2.36 mm is separated and expressed as a

percentage of original weight taken in the mould. This % is referred as

aggregate crushing value.

The crushing value of aggregate is restricted to 30% for concrete used for

roads and pavements and 45% may be permitted for other structures.

The crushing value of aggregate is rather insensitive to the variation in

strength of weaker aggregate. This is so because having been crushed before


the application of the full load of 40 tons, the weaker materials become

compacted, so that the amount of crushing during later stages of the test is

reduced.

2. Hardness

Abrasion strength or hardness is defined as the resistance to wear, which is

expressed in percentage loss in weight on abrasion. It can be determined by Los

Angeles Abrasion Value Test (LAAVT).

LOS ANGELES ABRASION VALUE TEST (LAAVT)

5 to 10 kg of aggregate is placed in a cylindrical drum mounted horizontally

with a shelf inside.

11 spheres each of 11 to 14 kg are charged.

The drum is rotated for about 500 cycles.

The tumbling and dropping of the aggregate and of the balls result abrasion

and attrition of the aggregate.

The proportion of broken material expressed as a percentage is measured.

3. Toughness

Toughness can be defined as the resistance of aggregate to failure by impact.

AGGREGATE IMPACT VALUE (ATV)


Weight of aggregate taken – 5 to 10 kg

A sample of standard aggregate kept in a mould is subjected to 15 blows of a

metal hammer of weight 14 kgs falling from a height of 38 centimeters.

The quantity of finer material (passing through 2.36 mm sieve) resulting from

pounding will indicate the toughness of the sample of aggregate.

AIV – ratio of the weight of the fines formed to the weight of the total sample

taken

Physical Properties

1. Specific Gravity

Specific gravity is defined as the ratio of mass (or weight) in air of a unit volume of

material to the mass of the same volume of water at the stated temperature. With the

specific gravity of each constituent known, its weight can be converted into solid

volume and hence a theoretical yield of concrete per unit volume can be calculated.

Specific gravity of aggregate is also required in calculating the compacting factor in

connection with the workability measurements.

2. Bulk Density

The bulk density or unit weight of aggregate is measured by filling a container of

known volume in a standard manner and weighing it. Bulk density shows how densely


the aggregate is packed when filled in a standard manner. The bulk density depends

on the particle size distribution and shape of the particles.

3. Absorption and Moisture Content

The porosity, permeability and absorption of aggregate influence the bond between it

and the cement paste, the resistance of concrete to freezing and thawing as well as

chemical stability, resistance to abrasion and specific gravity.

When we deal with aggregates in concrete, the 24 hour absorption during may not be

of much significance; on the other hand, the percentage of water absorption during

the time interval equal of final set of cement may be of more significance. The

aggregate absorbs water in concrete and thus affects the workability and final

volume of concrete.

Bulking of Sand

Bulking occurs when there is a change in volume due to the addition of water to the

sand particles.

When dry sand comes in contact with moisture, thin film is formed around the

particles, which causes them to get apart from each other. This results in increasing

the volume of sand. This phenomenon is known as “bulking of sand”.

When mixes are specified by volume, the sand is assumed to be dry. The volume of a

given weight of sand, however, varies according to its moisture content.

The water molecules grab onto the sand particle surface and also fill the air spaces to

create a bulking effect. Fine sands tend to bulk more than coarse sands due to the

increased ratio of surface area to volume available for water molecules to interact

with the sand particles.


The extent of bulking can be estimated by a simple field test. A sample of moist fine

aggregate is filled into a measuring cylinder in the normal manner. Let the level be

h1. Water is poured into the measuring cylinder and sand is completely inundated.

The cylinder is shaken. Since the volume of the saturated sand is the same as that of

the dry sand, the inundated sand completely offsets the bulking process. Let the level

of sand be h2. The difference of these two levels shows the bulking of the sample of

sand under test.

Percentage of Bulking=h1−h2h2

x100

Fineness Modulus of Aggregates

It is a numerical index to measure the average particle size of aggregates.

F .M .of aggregates=Summationof cumulative%of wt . retained on I . S . sieve100

Higher the F.M., higher will be coarser particles.

Many a time, fine aggregates are designated as coarse sand, medium sand and fine

sand. These classifications do not give any precise meaning. What the supplier terms

as fine sand may be really medium or even coarse sand. To avoid this ambiguity,

fineness modulus could be used as a yardstick to indicate the fineness of sand.

The following limits may be taken as guidance:

F.M. of Fine Sand: 2.2 – 2.6

F.M. of Medium Sand: 2.6 – 2.9


F.M. of Coarse Sand: 2.9 – 3.2

Sand having a fineness modulus more than 3.2 will be unsuitable for making

satisfactory concrete.

F.M. for coarse aggregates (10 kg) F.M. for fine aggregates (1 kg)

I.S.

Sieve

Wt.

retained

(kg)

Cumulati

ve wt.

retained

(kg)

% of

cumulati

ve wt.

retained

I.S.

Sieve

Wt.

retained

(kg)

Cumulati

ve wt.

retained

(kg)

% of

cumulati

ve wt.

retained

80 mm 0.0 0.0 0 80 mm 0.0 0.0 0

40 mm 0.0 0.0 0 40 mm 0.0 0.0 0

20 mm 3.5 3.5 35 20 mm 0.0 0.0 0

10 mm 3.0 6.5 65 10 mm 0.0 0.0 0

4.75

mm

2.8 9.3 93 4.75

mm

0.0 0.0 0

2.36

mm

0.70 10.0 100 2.36

mm

0.1 0.1 10

1.18

mm

0.0 10.0 100 1.18

mm

0.25 0.35 35

600 μ 0.0 10.0 100 600 μ 0.35 0.70 70

300 μ 0.0 10.0 100 300 μ 0.20 0.90 90

150 μ 0.0 10.0 100 150 μ 0.10 1.0 100

Σ=693 Σ=305

F.M. for coarse aggregates = 693/100 = 6.93

F.M. for fine aggregates = 305/100 = 3.05

1.2.2 Cement – Manufacturing of Cement, Compound Composition of

Portland Cement, Structure and Reactivity of Compounds

Cement is a material having cohesive and adhesive properties which provides

bonding action to other concrete ingredients.

It is the most important ingredient of concrete.

Water is required for chemical action (known as hydration).

Manufacture of Portland CementShuvanjan Dahal (068-BCE-147)

The raw materials required for manufacture of Portland cement are calcareous

materials, such as limestone and chalk, and argillaceous materials such as shale or

clay. Cement factories are established where these raw materials are available in

plenty.

The process of manufacture of cement consists of:

Grinding the raw materials.

Mixing them intimately in certain proportions depending upon their purity and

composition.

Burning them in a kiln at a temperature of about 1300 to 1500°C (at this

temperature, the material sinters and partially fuses to form nodular shaped

clinker).

The clinker is cooled and ground to fine powder with addition of about 3 to 5 %

Gypsum.

The product formed by using this procedure is Portland Cement.

Compound Composition of Portland cement


ASTM C 150 defines Portland cement as hydraulic cement produced by pulverizing

clinkers consisting essentially of hydraulic calcium silicates. Clinkers are 5 to 25 mm

diameter nodules of a sintered material which is produced when a raw mixture of

predetermined composition is heated to a high temperature.

The raw materials used for the manufacture of cement consist mainly of lime, silica,

alumina and iron oxide. These oxides interact with one another in the kiln at high

temperature to form more complex compounds.

Ingredients of OPC Percentage

Lime (CaO) 60 – 67

Silica (SiO2) 17 – 25

Alumina (Al2O3) 3 – 8

Iron Oxide (Fe2O3) 0.5 – 6

Magnesia (MgO) 0.1 – 4

Soda and Potash (K2O and Na2O) 0.2 – 3

Sulphur Trioxide (SO3) 1 – 3

Free Lime 0 – 1

Indian Standard specification for 33 grade cement specifies the following chemical

requirements:

(a) Ratio of percentage of lime to percentage of silica, alumina and iron oxide,

known as Lime Saturation Factor, when calculated by the formula

CaO−0.7S O3

2.8SiO 2+1.2 A l2O3+0.65 Fe2O3

should not be greater than 1.02 and not less than

0.66.

(b) Ratio of percentage of alumina to that of iron oxide – not less than 0.66

(c) Weight of insoluble residue - not more than 4 per cent

(d) Weight of magnesia - not more than 6 per cent

(e) Total sulphur content, calculated as sulphuric anhydride (SO3) – not more than

2.5% when C3A is 5% or less. Not more than 3% when C3A is more than 5%.

(f) Total loss on ignition - not more than 5 per cent

Bogue’s Compounds

The oxides present in the raw materials when subjected to high clinkering

temperature combine with each other to form complex compounds. The calculation of


the potential composition of Portland cement is based on the work of R. H. Bogue and

others, and is often referred to as ‘Bogue’s Compounds’.

S.

N.

Name of Compound Chemical

Formula

Abbreviation Commercial

Name

1. Tricalcium Silicate 3CaO.SiO2 C3S Alite

2. Dicalcium Silicate 2CaO.SiO2 C2S Bellite

3. Tricalcium Aluminate 3CaO.Al2O3 C3A Cellite

4. Tetracalcium alumino-

ferrite

4CaO.A

l2O3.Fe2O3

C4AF Fellite

The equations suggested by Bogue for calculating the percentages of major

compounds are given below.

C3S=4.07 (CaO )−7.60 (SiO2 )−6.72 ( A l2O3 )−1.43 (F e2O3 )−2.85 (SO3 )

C2S=2.87 (SiO2 )−0.754 (3CaO.SiO2)

C3 A=2.65 (A l2O3 )−1.69 (F e2O3 )

C4 AF=3.04 (F e2O3 )

The oxide shown within the brackets represent the percentage of the same in the raw

materials.

The silicates, C3S and C2S, are the most important compounds, which are

responsible for the strength of hydrated cement paste.

The presence of C3A in cement is undesirable; it contributes little or nothing to

the strength of cement except at early ages, and when hardened cement paste

is attacked by sulphates, the formation of calcium sulphoaluminate (ettringite)

may cause disruption. However, Tricalcium aluminate is beneficial in the

manufacture of cement in that it facilitates the combination of lime and silica.

C4AF reacts with gypsum to form calcium sulphoferrite and its presence may

accelerate the hydration of the silicates.

Microstructure

The chemical composition of compounds present in the industrial Portland cements is

not exactly what is expressed by the commonly used formulas, C3S, C2S, C3A and


C4AF. This is because at the high temperatures during clinker formation, the

elements present in the system, including the impurities such as magnesium, sodium,

potassium and sulphur possess the capability of entering into solid solutions with

each of the major compounds in clinker. Small amount of impurities in solid solution

may not significantly alter the crystallographic nature and the reactivity of a

compound with water, but larger amounts can do so. The crystal structure of cement

compounds is highly complex; however some of their essential features that account

for differences in the reactivity are described here.

CALCIUM SILICATES

Tricalcium Silicate (C3S) and beta-dicalcium silicate (βC2S) are the two hydraulic

silicates commonly found in industrial Portland cement clinkers. Both invariably

contain small amounts of magnesium, aluminium, iron, potassium, sodium and

sulphur ions; the impure forms of C3S and βC2S are known as alite and belite

respectively.

Normally, C3S is found in largest quantity in any cement. It occurs in small

dimensional colourless grains. On cooling below 1250° C, it decomposes slowly, but if

cooling is not very slow, it remains unchanged and relatively is stable at ordinary

temperatures. C2S is found to have three or even four forms depending on

temperature, such as αC2S, which exists at high temperatures; αC2S changes to βC2S

at about 1450° C, βC2S further undergoes change to γC2S at about 670° C. However,

at the cooling rate of commercial cements, βC2S is preserved in the clinker. It forms

rounded grains.

CALCIUM ALUMINATE AND FERROALUMINATE

Several hydraulic calcium aluminates can occur in the CaO-Al2O3 system; however,

C3A is the principal aluminate compound in Portland cement clinker. Calcium ferrites

are not found in normal Portland cement clinker; instead, calcium ferroaluminates

which belong to the C2A-C2F series are formed and the most common compound

corresponds approximately to the equimolecular composition C4AF.

Similar to the calcium silicates, both C3A and C4AF in industrial clinkers contain in

their crystal structures significant amounts of such impurities as magnesium, sodium,

potassium and silica. The crystal structure of pure C3A is cubic; however, C4AF and

C3A containing large amounts of alkalies are orthorhombic. The crystal structures are


very complex but are characterized by large structural holes which account for high

reactivity.

MAGNESIUM OXIDE AND CALCIUM OXIDE

The source of magnesium oxide in cement is usually dolomite, which is present as an

impurity in most limestone. A part of the total MgO in Portland cement clinker (i.e.

up to 2 percent) may enter into solid solution with the various compounds; however,

the rest occurs as crystalline MgO, also called periclase. Hydration of periclase to

magnesium hydroxide is a slow and expansive reaction which under certain

conditions can cause unsoundness (i.e. cracking and pop-outs n hardened concrete).

Uncombined or free calcium oxide is rarely present in significant amounts in

modern Portland cements. Improper proportioning of raw materials, inadequate

grinding and homogenization and insufficient temperature or hold time in the kiln

burning zone are among the principal factors that account for the presence of free or

crystalline calcium oxide in Portland cement clinker. Like MgO, a crystalline CaO

exposed to higher temperature in the cement kiln hydrates slowly and the hydration

reaction is capable of causing unsoundness in hardened concrete.

Both MgO and CaO form cubic structure, with each magnesium or calcium ion

surrounded by six oxygen in a regular octahedron. The size of the Mg2+ ion is such

that in the MgO structure the oxygen ions are in close contact with the Mg2+ ion and

well packed in the interstices. However, in the case of the CaO structure, due to the

much larger size of the Ca2+ ion, the oxygen ions are forced apart so that the calcium

ions are not well packed. Consequently, the crystalline MgO formed from a high

temperature (>1400° C) melt in a Portland cement kiln is much less reactive with

water than the crystalline CaO, which has been exposed to the same temperature

condition. This is the reason why under ordinary curing temperatures, the presence

of significant amount of crystalline free CaO in Portland cement may cause

unsoundness in concrete, whereas a similar amount of crystalline free MgO generally

proves harmless.

ALKALI AND SULPHATE COMPOUNDS

The alkalies, sodium and potassium in Portland cement clinker are derived mainly

from the clay components present in the raw mix and coal. Their total amount,

expressed as Na2O equivalent (Na2O + 0.64 K2O), may range from 0.3 to 1.5 percent.

The sulphates in a cement kiln generally originate from fuel. Depending on the


amount of sulphate available, soluble double sulphates of alkalies such as langbeinite

and aphthitalite are known to be present in Portland cement clinker. Their presence

is known to have a significant influence on the early hydration reactions of the

cement.

When sufficient sulphate is not present in the kiln system, the alkalies are

preferentially taken up by C3A and C2S, which may then be modified to compositions

of the type NC8A3 and KC23S12, respectively. Sometimes large amounts of sulphate in

the form of gypsum are purposefully added to the raw mix either for lowering the

burning temperature or for modification of the C3A phase to C4A3S, which is an

important constituent of certain types of expansive as well as rapid-hardening

cements.

Chemical Reactions when mixed with Water

On adding water to the cement, the silicates and aluminates present in the cement

start a chemical reaction and form a mass known as gel. During this process, a large

quantity of heat is generated. The quantity of heat generated depends upon the

amount of different constituents in the cement.

The main hydrates of cement can be classified as calcium silicate hydrates and

Tricalcium aluminate hydrate. The two calcium silicate hydrates are the main

cementitious compounds in cement.

HYDRATION OF SILICATES

When water is added in a limited quantity as in the case of cement paste or cement

concrete, C3S and βC2S undergo hydrolysis first, producing calcium silicate of lower

basicity and ultimately C3S2H3 and releases lime as Ca (OH) 2. The reaction for fully

hydrated C3S and C2S pastes may be expressed as:


From the above equations, it is clear that on weight basis, both silicates require

approximately the same amount of water for their hydration, but C3S produces more

than two times the calcium hydroxide as is formed by hydration of C2S. From the

above reaction, it is also seen that if the surface area and consequently, the adhesive

property of hydrated cement paste are mainly due to the formation of calcium silicate

hydrate; it is expected that the ultimate strength of a high – C3S Portland cement

would be lower than a high – C2S cement.

Second, if the durability of a hardened cement paste to acidic and sulphate water is

reduced due to the presence of calcium hydroxide, it may be expected that the

cement containing a higher proportion of C2S will be more durable in acidic and

sulphate environment than the cement containing a higher percentage of C3S.

HYDRATION OF ALUMINATES


a) For C3A

C3 A (100 )+6H (40 )⇒C3 A H 6

i .e .3CaO . A l2O3+6H 2O⇒ 3CaO. A l2O3 .6H 2O+867 Jgram

b) For C4AF

C4 AF+2CH+10H (37)⇒C3 A H 6+C3F H 6

i .e .4CaO. A l2O3 .F e2O3+2Ca (OH )2+10H 2O⇒ 3CaO . A l2O3 .6H 2O+3CaO.F e2O3 .6H 2O+419 Jg

If sufficient water and gypsum are added, then the reaction of C3A is as follows:

3CaO. A l2O3+3CaSO4+32H 2O⇒3CaO. A l2O3 .3CaSO 4 .32H 2O

The product is known as ettringite. The typical characteristic of this reaction is that

due to 32 molecules of water larger volume change takes place for the product on the

right side in comparison to the volume of the compounds on the left side of the

reaction. In plastic (green) stage of concrete, the volume change is not so dangerous.

Volume change in large magnitude is dangerous when it happens in hardened stage.

Such phenomenon happens when C4AF hydrates contacts with sulphate environment.

In such case the reaction is as follows:


3CaO. A l2O3 .6H 2O+3CaSO4+26H 2O⇒ 3CaO . A l2O3 .3CaSO4 .32H 2O

In this reaction, the product is 4.6 times increased in volume. Such reaction in the

hardened cement paste induces stresses and destroys the internal structure

(sulphate corrosion or deterioration).

1.2.3 Introduction to Special Types of Cement

1. Rapid Hardening Cement 7 days OPC Strength = 3 days RPC Strength

Higher C3S content and a higher fineness than OPC.

Used when formwork is to be removed early for re-use or where sufficient

strength for further construction is required quickly.

RPC should not be used in large structural sections because of its higher rate

of heat development.

Provides a satisfactory safeguard against early frost damage.

Used in road repair works.

2. Quick Setting Cement (QSC)Initial Setting Time (IST) – 5 minutes

Final Setting Time (FST) – 30 minutes

This name, as the name suggests, sets very early. The early setting property is

brought out by reducing the gypsum content at the time of clinker grinding. This

cement is required to be mixed, placed and compacted very early. It is used mostly in

under water construction where pumping is involved. Use of QSC in such conditions

reduces the pumping time and makes it economical.

3. Portland Pozzolana Cement (PPC) Pozzolana is a siliceous or siliceous and aluminous material which in itself

possesses little or no cementitious value but will, in finely divided form and in

the presence of moisture, chemically react with lime at ordinary

temperatures to form compounds possessing cementitious properties.

This cement gains strength slowly but the long-term strength is high.

Requires curing over a comparatively long period.

Used when high strengths at early stages are not required.


4. Coloured CementThese cements fall into two groups; most are derived from pigment addition to white

cement, but some are produced from clinkers having the corresponding colours.

The addition of pigment should not interfere with the strength of the cement.

5. White Cement White cement is made from china clay, which contains little iron oxide and

manganese oxide, together with chalk or limestone free from specified

impurities.

Special precautions are required during the grinding of the clinker so as to

avoid contamination.

The cost of this type of cement is high.

6. Sulphate Resisting Cement Low C3A content so as to avoid sulphate attack from outside the concrete.

Heat developed by this type of cement is not much higher but the cost is high

due to the special composition of the raw materials.

The use of sulphate resisting cement is recommended under the following conditions:

i. Concrete to be used in marine condition;

ii. Concrete to be used in foundation and basement, where soils is infested with

sulphates;

iii. Concrete used for fabrication of pipes which are likely to be buried in marshy

region or sulphate bearing soils;

iv. Concrete to be used in the construction of sewage treatment works.

7. Portland Blast Furnace Slag Cement Made by intergrinding cement clinker with granulated blast furnace slag.

Amount of slag = 25 to 65 % of the mass of the mixture.

Early strengths are generally lower compared to OPC.

Typical uses are in mass concrete because of a lower heat of hydration and in

sea water construction because of a better sulphate resistance.

8. Low Heat Cement In this cement, a low heat evolution is achieved by reducing the contents of

C3A and C3S which are the compounds evolving the maximum heat of hydration

and increasing C2S.

A reduction in temperature will retard the chemical action of hardening and so

further restricts the rate of evolution of heat.


Slow rate of gain of strength but ultimate strength is similar to that achieved

by OPC.

9. Alumina Cement Obtained by grinding high alumina clinker.

High early strength, high heat of hydration and very high durability against

chemical attack.

Used in hot weather is very limited due to high heat of hydration and increased

porosity.

1.2.4 Use of Water in Concrete

Cheapest ingredient of concrete.

Water is required for:

Mixing of ingredients of concrete.

Washing of ingredients of concrete.

Curing of concrete (7 to 10 days).

Requirements of Mixing Water

1. The silt content should be less than 2000 ppm. It affects setting and hardening

of concrete.

2. Drinking water may be unsuitable as mixing water when the water has a high

concentration of sodium or potassium.

3. As a rule, any water with a pH of 6.0 to 8.0 which does not taste saline or

brackish is suitable for use.

4. Water should be free from wild acids, alkalies and organic matter.

5. Alkali carbonates and bicarbonates in water should not exceed 1000 ppm.

6. As a rule, water suitable for drinking is suitable for concrete.

Test of Water

The suitability of water for mixing can be determined by comparing the setting time

of cement and the strength of mortar cubes using the water in question with the

corresponding results obtained using known ‘good’ water or distilled water. BS

3148:1980 suggests a tolerance of 10 per cent to allow for change in variations in

strength.


1.2.5 Admixtures

It is the fifth ingredient of concrete other than cement, sand, aggregate and water

and found invariably in modern day concrete. Admixtures are used to enhance the

properties of concrete.

Local Admixtures used in Nepal: sugar (causes slow setting), edible soda

(accelerating admixture)

Admixtures modify properties either in wet state or after mix have been

hardened.

Added less than 5% of cement (generally <2%) i.e. in very small amount.

Chemical Admixtures/Chemical Plasticizers

Mechanism of Action – Chemical:

Chemical interaction in hydration (accelerators and retarders)

Adsorption on cement surface causing better particle dispersion (plasticizers

and super plasticizers)

Affecting surface tension of water (increased air entrainment)

Affecting rheology of water (viscosity modifier)

1. Plasticizers (Water-reducing Admixtures)These admixtures are used for following purposes:


Admixtures

Mineral Admixture

s

Fly Ash Rice Husk Ash

Chemical Admixture

s

Accelerating

Admixture

Retarding Admixture Plasticizer Super

Plasticizer

i. To achieve a higher strength by decreasing the water cement ratio at the same

workability as an admixture free mix.

ii. To achieve the same workability by decreasing the cement content so as to

reduce the heat of hydration in mass concrete.

iii. To increase the workability so as to ease placing in accessible locations.

iv. Water reduction more than 5 % but less than 12 %.

The commonly used admixtures are Ligno-sulphonates and hydro carbolic acid salts.

Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived

from wood processing in the paper industry.

MECHANISM OF ACTION:

1. Dispersion: Water reducers are surface acting agents adsorbed on the cement

and fine particles giving them –ve charges leading to repulsion.

2. Lubrication: The –ve charges also cause development of sheath of oriented

water molecules around molecules.

3. Retardation: A thin layer formed over the cement particles protect them from

hydration and increases the setting time.

2. Super PlasticizersThese 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 around dense reinforcement

Reduced W/C ratio:

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

Very high later age strengths, > 100 MPa

Reduced shrinkage, especially if combined with reduced cement content.

Improved durability by removing water to reduce permeability and diffusion.

Water reduction: 12 to 30 %


Dosage: Low (0.6 to 2 % on cement)

Examples of commonly used super plasticizers are Sulphonated Melamine

Formaldehyde condensates (SMF), Sulphonated Naphthalene Formaldehyde

condensates (SNF) and Prolycarboxylate Ether super plasticizer (PCE).

3. AcceleratorsIt is 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. They can be divided into two groups

based on their performance and application.

a. Set Accelerating Admixtures: They reduce the time for the mix to change

from the plastic state to the hardened state. These admixtures have relatively

limited use, mainly to produce an early set.

b. Hardening Accelerators: They increase the strength at 24 hours by at least

120% at 20° C and at 5° C by at least 130% at 48 hours. These admixtures 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.

E.g. calcium chloride, chloride-free accelerators based on salts of nitrate, nitrite,

formate and thiocyanate.

4. Set RetardersThe 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.

When water is first added to cement there is a rapid initial hydration reaction, after

which there is little formation of further hydrates for typically 2–3 hours. The exact

time depends mainly on the cement type and the temperature. This is called

the dormant period when the concrete is plastic and can be placed. At the end of

the dormant period, the hydration rate increases and a lot of calcium silicate hydrate

and calcium hydroxide is formed relatively quickly. This corresponds to the setting

time of the concrete.

Retarding admixtures delay the end of the dormant period and the start of setting

and hardening. This is useful when used with plasticizers to give workability

retention. Used on their own, retarders allow later vibration of the concrete to


prevent the formation of cold joints between layers of concrete placed with a

significant delay between them.

E.g. Calcium Ligno-sulphonates, Carbohydrate derivatives

Mineral Admixtures

These are generally of two types:

1. Cementitious These have cementing properties themselves. E.g. Ground Granulated Blast Furnace

Slag (GGBFS)

2. PozzolanicA pozzolan is a material which, when combined with calcium hydroxide (lime),

exhibits cementitious properties. Pozzolans are commonly used as an addition (the

technical term is "cement extender") 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. Examples are

Fly ash

Silica Fume

Rice Husk Ash

Metakaolin

Fly AshCoal from mines is generally contaminated with clay. When this coal is grinded to

fine size and then burnt in thermal power plant, it turns into carbon dioxide. The clay

contaminants form ash mainly of silica and alumina. The larger ash settles on the

bottom whereas the finer ones called fly ash fly above.

Thus, fly ash is the finely divided residue resulting form the combustion of ground or

powdered coal. Fly ash is generally captured from the chimneys of coal-fired power

plants.

One of the most important fields of application for fly ash is PCC pavement, where a

large quantity of concrete is used and economy is an important factor in concrete

pavement construction. Fly ash increases cementitious property. These are spherical

particles; thus, ball bearing action helps to reduce water demand.

Silica Fume 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.

Because of its extreme fineness and high silica content, Silica Fume is a highly

effective pozzolanic material. Silica Fume is used in concrete to improve its

properties. 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.

Rice Husk AshThis 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.


introdution to concrete and concrete materials (concrete technology)

Documents

types of concrete concrete

use of concrete

concrete technology

advantages of concrete

modern concrete

concrete of desired

steel reinforced concrete

concrete materialsthese