1

What is Concrete?

Concrete is made by mixing

• Cement

• Fine aggregates

• Coarse aggregates

and

• Water

2

Concrete Admixture

• An additional material, known as admixture, is

sometimes added to modify certain properties of

concrete.

3

Advantages of Concrete

• Good compressive strength

• Protection of embedded steel

• Flexibility of application

4

Disadvantages of Concrete

• Low tensile strength

• Rather high density

• Prone to chemical

attack

5

Cement

• Cement is a fine powder which sets after a few

hours when mixed with water, and then hardens

in a few days into a solid, strong material.

• Cement hardens when water is added.

• Basic constituents for cement are:

– limestone

– clay

6

A Two-Step Process

• Basically, cement is produced in two steps:

• Step 1

– Clinker is produced from raw materials

• Step 2

– Cement is produced from clinker

7

Cement Production Process

• Basic Process

8

Hydration of Cement

• When cement comes in contact with water, the

water molecules react with the individual

compounds.

• These products in time produce a firm mass

which give cement its bonding properties,

strength and hardness.

• This process is termed as hydration. The

individual compounds have different rates of

hydration.

9

Heat of Hydration

• Hydration process of cement is accompanied by

heat generation.

• The generated heat in mass concrete may result

in expansion and cracking.

• This can be overcome by using suitable cement

type.

10

Fineness of Cement

• As hydration takes place at the surface of thecement particles, it is the surface area of cementparticles which provide the material available forhydration.

• The rate of hydration is controlled by thefineness of cement.

• For a rapid rate of hydration, a higher fineness isnecessary.

11

Types of Cement

• Ordinary Portland Cement (OPC)

• Rapid-hardening Portland Cement (RHPC)

• Low-heat Portland Cement (LHPC)

• Sulfate-resisting Portland Cement (SRPC)

• Portland Blastfurnace Cement (PBFC)

• Waterproof and Water Repellant Cement

12

Portland Cement

• Portland cement is the most widely used.

• The others being used where concrete with

special properties are required.

13

Cement Storage

• Cement must be kept dry at all times in itspowdery form to ensure that its properties do notchange.

• The shed for storing bagged cement shouldhave water proofed roof and walls and a dry andraised floor, free from ingress of any dampness.

• The storage of bagged cement in the open isnot a recommended practice.

14

Bulk Cement

• Bulk cement is ideal for large projects at which

concrete batching plants are erected.

• However if cement is stored in a silo for a long

period of time (more than 3 months) there is an

increased chance of developing a thin hardened

layer on the top surface, and of cement lump on

the inner surface of the silo.

15

First In, First Out

• Have the cement bags stacked so that the

first batch in can be the first out.

• Make sure that the cement really is used

in the same order that it is delivered.

• The important thing with cement is always

to keep it dry.

16

Aggregates

• Aggregates are the inert particles that boundtogether by the cement paste to form mortar.

• The performance of concrete depends largely onthe physical, thermal and chemical properties ofits aggregate.

• Since at least 60 – 80% of the volume ofconcrete is made-up of aggregates, they can bealso be called filler materials.

17

Classification of Aggregates

• The general classification of aggregates is as

follows:

– Heavyweight aggregate

– Normal aggregate

– Lightweight aggregate

18

Functions of Aggregates in Concrete

1. Reduce costs

2. Modify properties of concrete

3. Reduce dry shrinkage

19

Water in Concrete

• Water is used in concrete to react with cementand thus cause it to set and harden.

• It also facilitates mixing, placing and compactingof the fresh concrete. It is also used for washingthe aggregates and for curing purposes.

• The amount of water content is very important todecide the properties of fresh and hardenedconcrete.

20

Admixtures

• Admixtures are substances that are introducedinto a batch of concrete, during or immediatelybefore its mixing, in order to alter or improve theproperties of the fresh or hardened concrete orboth.

21

Uses of Admixtures

• to improve the workability of the fresh concrete

• to reduce water content for a given workabilitythereby increasing the strength

• to increase the durability of hardened concrete

• to retard setting time or to increase it

• to impart colour to concrete

• to maintain volume stability by reducing oroffsetting shrinkage during concreting

• to increase concrete resistance to freezing andthawing.

22

Types of Admixtures

1. Air-entraining agents

2. Accelerating agents

3. Retarders

4. Water reducers or plasticizers

5. Super plasticizers

6. Bonding admixtures

7. Water repelling agents

23

Reinforced Concrete (RC)

• Basically made of two materials:

1. plain concrete

2. steel bars embedded in concrete.

• Reinforced concrete is made to combine the

best features of concrete and steel bars.

24

Properties of Reinforced Concrete

Properties Concrete Steel

Strength in tension poor good

Strength in compression good good, but slender bars will

buckle

Strength in shear fair good

Durability good corrodes if unprotected

Fire Resistance good poor – suffers rapid loss of

strength at high temperature

25

Composite Action of RC

• The tensile strength of concrete is only about

10% of the compressive strength.

• Therefore nearly all reinforced concrete

structures are designed on the assumption that

the concrete does not resist any tensile forces.

• Reinforcement is designed to carry these tensile

forces.

26

Combination of Steel & Concrete

The combination of steel and concrete works

readily for several reasons:

• bond prevents slip of the bars

• proper concrete mixes provide adequate

impermeability of the concrete against water

intrusion and bar corrosion

27

Stress-Strain Relation

• As the load is applied, at first strain increases

linearly with stress and the concrete behaves as

an elastic material.

• After that the curve is no longer linear and the

concrete behaves more like plastic material.

• In the plastic range, if the load is removed, the

deformation would not recover.

28

Stress-Strain Curve for Concrete

Stress-strain curve for concrete in compression

29

Concrete Strength

• Concrete generally increases its strength withage.

• A typical variation in strength of an adequatelycured Portland Cement Concrete as suggestedby BS8110 is:

7 days 1 month 2 months 3 months 6months 1 year

20 30 33 35 36 37 N/mm2

• BS 8110 does not permit the use of strengthsgreater than the 28-day value in calculations.

30

Durability of Concrete

The durability of concrete is influenced by:

• the exposure conditions

• the concrete quality

• the cover to the reinforcement

• the width of any cracks

31

Specification of Concrete

• Concrete of a given strength is identified by its‘grade’.

• For example Grade 25 concrete has acharacteristic cube crushing strength of 25N/mm2 after 28 days of curing.

• It is recommended that for normal denseaggregate reinforced concrete, grades of 30, 35and 40 are used.

32

Green Concrete Specifications

33

Batching of Concrete

• Batching refers to the process of measuring the

correct amounts of the constituent materials, in

order that the resultant mix will be of the

desired quality.

34

Advantages of Using Bulk Cement

• Bulk cement is cheaper than bagged cement.

• A gang of men are not needed to off-load thecement.

• The trouble of ensuring adequate protection ofthe cement during storage is avoided.

• The mixer can always be used to capacitywithout the risk of errors which may result fromsplitting bags.

• Changes in mix proportions are made moreeasily.

35

Mixing of Concrete

• The objective of mixing is to coat the surfaces of

all aggregates with cement paste and to blend

all the ingredients of concrete to give a uniform

consistency.

• Machine mixing is generally more economical

and convenient for large quantities and the

concrete produced is also more homogeneous.

36

Transporting of Concrete

• The transport of concrete from the mixing plantto the point at which it is to be placed mustcomply with four main requirements, namely:

– Transport must be rapid so that the concretedoes not dry out or lose its workability duringthe time it elapses between mixing andplacing.

– Segregation must be reduced to a minimumin order to avoid non-uniform concrete. Forthe same reason any loss of fine material orcement or water must be prevented.

37

Transporting of Concrete (Contd)

– The transport should be organised so that

during the placing of any particular lift or

section, delays will not result in the

formation of pour planes, cold joints or

construction joints.

– No additional water should be added to the

concrete.

38

Time Limit

• In case of ready-mixed concrete, the usual

specified requirement is that the concrete shall

be discharged from a truck mixer or agitator

truck within two hours after the time of loading.

• If non-agitating transporting equipment is used,

the time is reduced to one hour after the time of

loading.

39

Method of Transportation

• Various methods are available for transportingconcrete:

– Truck mixers

– Wheel barrows

– Dumpers

– Steel skips and buckets

– Chutes

– Cranes

– Concrete pumps with pipe lines

40

Truck Mixers

• Truck mixers are the usual delivery vehicles for

normal concrete. They provide the facility for

keeping the concrete agitated during long-haul

distances.