Concrete

Introduction • The most widely used construction material.

• Second only to water as the most consumed substance on earth.

Q? Why concrete is most popular construction material

among common people?

Fine Aggregates:

25 - 30%

Coarse Aggregates:

30 to 50%

Matrix (paste): • Water: 15 – 20%

• Cementitious materials

(cement, pozzolans & slag):

7 – 15%

• Air: 1 – 8%

• Chemical Admixtures: < 2%

Constituents of Concrete

Production of Concrete:

Material

selection and

Proportions

Process

Mixing

Transportation

Placement

Compaction

Curing

MATERIAL

Cement Cement is the mixture of calcareous, siliceous,

argillaceous and other substances. Cement is used as a binding material in mortar, concrete, etc.

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 is mainly used to bind fine sand and coarse aggregates together in concrete. Cement is a hydraulic binder, i.e. it hardens when water is added.

Chemical Composition of cement

Lime 63%

Silica 22%

Alumina 06%

Iron oxide 03%

Gypsum 02 to 05%

Function of composition of

cement

(i) Lime(CaO):

Lime forms nearly two-third (2/3) of the cement.

Therefore sufficient quantity of the lime must be

in the raw materials for the manufacturing of

cement. Its proportion has an important effect on

the cement. Sufficient quantity of lime forms di-

calcium silicate and tri-calcium silicate in the

manufacturing of cement.

Lime in excess, causes the cement to expand

and disintegrate.

(ii) Silica (SiO2):

The quantity of silica should be enough to form di-calcium silicate and tri-calcium silicate in the manufacturing of cement. Silica gives strength to the cement. Silica in excess causes the cement to set slowly.

(iii) Alumina (Al2O3):

Alumina supports to set quickly to the cement. It also lowers the clinkering temperature. Alumina in excess, reduces the strength of the cement.

(iv) Iron Oxide (Fe2O3): Iron oxide gives colour to the cement.

(v) Calcium Sulphate (or) Gypsum (Ca SO4) :

At the final stage of manufacturing, gypsum is added to increase the setting of cement.

MANUFACTURING OF CEMENT

(1) Mixing and crushing of raw

materials

a.Dry process

b.Wet process

(2) Burning

(3) Grinding

(a) Dry process:

In this process, the raw materials are changed to powdered form in the absence of water.

In this process calcareous material such as lime stone (calcium carbonate) and argillaceous material such as clay are ground separately to fine powder in the absence of water and then are mixed together in the desired proportions.

Water is then added to it for getting thick paste and then its cakes are formed, dried and burnt in kilns.

This process is usually used when raw materials are very strong and hard.

(b) Wet process:

In this process, the raw materials are changed to powdered form in the presence of water.

In this process, raw materials are pulverized by using a Ball mill, which is a rotary steel cylinder with hardened steel balls. When the mill rotates, steel balls pulverize the raw materials which form slurry (liquid mixture).

The slurry is then passed into storage tanks, where correct proportioning is done. Proper composition of raw materials can be ensured by using wet process than dry process. Corrected slurry is then fed into rotary kiln for burning.

This process is generally used when raw materials are soft because complete mixing is not possible unless water is added.

Among wet process and dry

process, which is economical?

(2) Burning: The raw slurry (wet Process) or raw meal (dry process),

obtained by one of wet or dry process is called charge.

Charge is introduced into a rotary Kiln. The rotary kiln consists of a steel cylinder about 150meters long and 4meter diameter and rotates 30 to 60 turns per hour.

At one end of the cylinder a screw conveyer is arranged which slowly allows the charge into the cylinder.

In the other end of the cylinder, a burner is arranged. Coal or burning oil is burnt at this end.

The charge entering the cylinder slowly moves towards the hot end. At the burning end of the kiln, the temperature is around 1600 to 1900 degrees centigrade.

At this end some chemical reactions takes place between oxides of calcium , aluminium and silica.

Mixture of calcium silicates and calcium aluminates is formed.

The resultant product consists of grey hard balls called clinker cement.

The percentage of important compound formed in cement is given below:

(bogue's compound of cement)

(3) Grinding:

• Clinker cement is cooled, ground to fine

powder and mixed with 2 to 5 percent of

gypsum (Calcium sulphate Ca SO4) .

(added for controlling the setting time of

cement)

• Finally, fine ground cement is stored in

storage tanks from where it is drawn for

packing.

Hydration of cement The chemical reactions that take place between

cement and water is referred as hydration of cement.

On account of hydration certain products are formed. These products are important because they have cementing or adhesive value.

Out of all cement compounds (bogue's compound of cement), the strength of cement is contributed mainly by silicates.

Silicates react with water and produce a gel called Calcium Silicate Hydrate or ‘C-S-H’ gel.

This gel is initially weak and porous, but with the passage of time it becomes stronger and less porous.

Q? Is it desirable to put in as much cement as

possible in a concrete mix provided cost is not a

constraint.

Q? What is the maximum cement content to be used

in concrete? [cl. 8.2.4.2, pg 19, IS456]

In the order of reaction with water, C3A is the first to react with it and imparts setting to the cement paste. Hence C3A is responsible for setting.

Strength contribution by C3A is negligible and therefore can very well be neglected.

Strength of cement is mainly contributed by silicates i.e. C3S and C2S.

In the category of silicates, C3S is quicker in reacting with water as compared to C2S. Therefore the initial strength up to 7 days is mainly given by C3S.

After 7 days when most of C3S has already exhausted, C2S also start reacting with water. The strength between 7 and 28 days is contributed mainly by C2S and a part is contributed by C3S

Which cement to use?

The choice of the cement depends upon the

nature of work, local environment, method of

construction etc.

The different type of cement has been achieved by

different methods like :

Types of cement

(a) Ordinary Portland Cement (OPC):

It is the most commonly produced and used cement. It is available in three different grades.

(b) Rapid Hardening cement (RHC): It is also called ‘Early Strength Cement’ because its 3 days strength is almost equal to 7 days strength of OPC. One type of this cement is manufactured by adding calcium chloride (CaCl2) to the O.P.C in small proportions. Calcium chloride (CaCl2) should not be more than 02%.

In RHC, strength development is very fast. This is because of following reasons:

Higher fineness of cement. The specific surface of this cement is increased to 320 m2/kg as compared to 225 m2/kg for OPC.

Higher quantity of C3S in cement as compared to C2S. C3S is more reactive in comparison to C2S.

The sulphate present in the soil or surrounding environment reacts with free Ca(OH)2 available in the concrete and CaSO4 is formed. There is no dearth of free Ca(OH)2 as it is available in abundance in the set cement. The CaSO4 thus produced reacts with hydrate of calcium aluminate and form an expansive compound called calcium sulpho-aluminate which causes expansion and cracks in the set cement. Sulphate attack is further accelerated if it is accompanied by alternate wetting and drying also, which normally takes place in marine structures of the tidal zone.

(d) Sulphate Resistant Cement (SRC):

It is modified form of O.P.C and is specially manufactured to resist the sulphates. In certain regions/areas where water and soil may have alkaline contents and O.P.C is liable to disintegrate, because of un favorable chemical reaction between cement and water, S.R.C is used. This cement contains a low %age of C3A not more than 05%.

The quantity of C3A can be controlled simply by blending OPC with slag cement.

Limitation:

This cement requires longer period of curing (why?). It develops strength slowly, but ultimately it is as strong as O.P.C.

(e) Portland slag cement:

It is produced by blending OPC clinkers with slag in suitable proportion (20-25%) and grinding together.

The slag can be separately added to OPC while making concrete.

Limitation of slag cement:

It develops strength slowly, but ultimately it is as strong as O.P.C.

(e) Portland Pozzolana cement:

It is produced by blending OPC clinkers with pozzolana

in suitable proportion (20-25%) and grinding together.

It develops strength slowly, but ultimately it is as strong

as O.P.C.

opaline is a man-made

'crystal'

Diatomaceous earth deposit

(f) QUICK SETTING CEMENT:

When concrete is to be laid under water, quick

setting cement is to used. This cement is manufactured

by adding small %age of aluminum sulphate (Al2SO4)

which accelerates the setting action. This cement can

also be produced by not adding gypsum to OPC The

setting action of such cement starts with in 05 minutes

after addition of water and it becomes stone hard in less

than half an hour.

(h) LOW HEAT CEMENT:

In this cement the heat of hydration is reduced by

tri calcium aluminate (C3A ) content. It contains less

%age of lime than ordinary port land cement. It is used

for mass concrete works such as dams etc.

WHITE CEMENT:

This cement is called snowcrete. As iron

oxide gives the grey colour to cement, it is

therefore necessary for white cement to

keep the content of iron oxide as low as

possible. Lime stone and china clay free from

iron oxide are suitable for its manufacturing.

This cement is costlier than O.P.C. It is mainly

used for architectural finishing in the

buildings.

Tests on Cement Field Test

Laboratory test

Field Test (a) Date of Manufacture

(b) One feels cool by thrusting one’s hand in the cement bag.

(c) It is smooth when rubbed in between fingers.

(d) A handful of cement thrown in a bucket of water should float.

Laboratory test

(Self Study)

(1) Fineness Test. (why?)

(2) Consistency test. (why?)

(3) Setting Time Test. (why?)

(4) Soundness test. (why?)

(4) Compressive strength test. (why?)

Q? How would you differentiate

between Coarse Aggregate and fine

aggregate.

Aggregate

Aggregate

What size of aggregate should be used?

The nominal maximum size of coarse aggregate in no case

greater than one-fourth of the minimum thickness of the

member, provided that the concrete can be placed

without difficulty so as to surround all reinforcement

thoroughly and fill the comers of the form.

For heavily reinforced concrete members as in the case of

ribs of main beams, the nominal maximum size of the

aggregate should usually be restricted to 5 mm less than

the minimum clear distance between the main bars or 5

mm less than the minimum cover to the reinforcement whichever is smaller.

• Generally represent 60 to 75% of the total volume of concrete strong influence on the properties, proportioning, cost and the performance of the concrete mixtures.

• Generally divided in two groups:

• Fine aggregates: natural or manufactured sand. Generally, sand particles almost entirely pass the 4.75mm sieve and are predominantly retained on the 75µm sieve.

• Coarse aggregates: natural gravel or manufactured material. The particles are predominantly retained on the 4.75mm sieve.

Aggregates

Properties of aggregate

Inherited Properties

Chemical and mineral composition

Specific gravity

Hardness

Strength

Color etc.

Acquired properties

Shape

Size

Surface texture

Water absorption

IMPORTANCE OF ANGULARITY NUMBER

The normal aggregate which are suitable

for making concrete may have angularity

number anything from 0 to 11.

Angularity number 0 represents the most

practicable rounded aggregate

Angularity number 11 indicates the most

angular aggregate that could be used for

making concrete.

Flaky and elongated particles may have adverse effects on

concrete. For instance, flaky and elongated particles tend to lower

the workability of concrete mix which may impair the long-term

durability.

Flakiness Index is It is the percentage by weight of flaky particles in a

sample.

Elongation Index is the percentage by weight of elongated particles

in a sample.

Effort should be made to use minimum volume of paste in the concrete. It should just be sufficient to fill the voids left out by

the aggregates. This can be achieved by using well graded aggregates so that the voids are minimum.

Grading of aggregate

Type of Sand Fineness Modulus

Range

Fine Sand 2.2 – 2.6

Medium Sand 2.6 – 2.9

Coarse Sand 2.9 – 3.2

WHY TO DETERMINE FINENESS

MODULUS? •Fineness modulus is generally used to get an idea of how coarse

or fine the aggregate is. More fineness modulus value indicates that

the aggregate is coarser and small value of fineness modulus

indicates that the aggregate is finer.

•Fineness modulus of different type of sand is as per given below.

•Generally sand having fineness modulus more than 3.2 is not used

for making good concrete.

Fineness Modulus = 246/100 = 2.46

Bulking of sand The volume increase of fine aggregate due to

presence of moisture content is known as

bulking.

Bulking increases with increase in moisture

content up to a certain limit and beyond that

the further increase in moisture content results in

decrease in volume.

WHAT CAUSES BULKING OF AGGREGATE?

The moisture present in aggregate forms a film around each particle.

These films of moisture exert a force, known as surface tension, on

each particle. Due to this surface tension each particles gets away

from each other. Because of this no direct contact is possible among

individual particles and this causes bulking of the volume.

Bulking of aggregate is dependent upon two factors,

Percentage of moisture content

Particle size of fine aggregate

WHY TO DETERMINE PERCENTAGE OF BULKING?

Due to bulking, fine aggregate shows completely unrealistic volume.

Therefore, it is absolutely necessary that consideration must be given

to the effect of bulking in proportioning the concrete by volume.

• The grading, the shape and the texture of aggregates can significantly influence concrete workability.

• The amount of water required for a target workability is related to aggregate properties:

Nominal maximum size of the coarse aggregate.

Shape and texture of particles of fine and coarse aggregates.

Grading of coarse aggregate.

Some Properties of Aggregates Affecting Fresh Concrete Properties

Properties of Aggregates Affecting Fresh Concrete Properties

• Angular sand (manufactured sand) can significantly increase the water demand and the cement content for a required workability.

• Very coarse sands and coarse aggregates can produce harsh, unworkable mixes.

• Changes in grading (or the shape / texture) of the aggregates can cause changes in the water demand of concrete, segregation and affect uniformity of concrete from batch to batch.

• The shape and grading of the fine aggregate can have significant effect on the bleeding and the finishing properties of concrete the finer the sand, the lower the bleeding.

• The temperature of aggregates can strongly impact on the setting time of the concrete (T° ↑ setting time ↓).

Properties of Aggregates Affecting Fresh Concrete Properties

• For concrete with compressive strengths < 20 MPa, the strength ↑ with ↑ maximum size of the coarse aggregate.

• For concrete of higher compressive strengths, there is an optimum maximum size fraction for each strength level. For high-performance concrete, the maximum coarse aggregate size is often limited to 10 – 14 mm.

Properties of Aggregates Affecting Hardened Concrete Properties

• Generally,

• The modulus of elasticity of concrete ↑ with ↑ modulus of elasticity of the aggregates;

• With ↑ modulus of elasticity of the aggregate, the creep of concrete ↓.

• For similar compressive strength levels, better flexural strengths are obtained when using aggregates with higher angularity and good surface roughness.

Properties of Aggregates Affecting Hardened Concrete Properties

Some Aggregate Characteristics Affecting Concrete

Properties

Specific gravity and bulk density

Mix proportioning calculations and concrete density

Size and grading, particle shape, surface texture

Workability of fresh concrete, economy (mixture proportioning), strength, bleeding, finish-ability, pumping

Absorption and surface moisture

Affect the net water content in concrete workability, strength will vary

High absorption could reduce durability (freezing and thawing)

Cleanness Dirty aggregates poor fresh and hardened (aggregate – paste bond) concrete properties

Aggregate Characteristics Affecting Concrete Properties

Hardness, toughness and wear resistance

Affects the mechanical properties

Abrasion resistance of concrete function of aggregate type. Hard aggregates with good micro- and macrotextures are better (related to mineralogy).

Soundness Durability, resistance to weathering

Particle strength and elasticity

Resistance to abrasion, creep & shrinkage; the effect is generally relatively limited for conventional (normal) strength concrete

Volume stability

Drying shrinkage aggregates with high absorption properties may have high shrinkage properties on drying (e.g. sandstone, shale, slate, greywacke)

Requirements of water used in concrete

Water used for mixing and curing shall be clean and free

from injurious amounts of Oils, Acids, Alkalis, Salts, Sugar,

Organic materials

Potable water is generally considered satisfactory for mixing

concrete

Mixing and curing with sea water shall not be permitted.

The pH value shall not be less than 6

The permissible limits for solids in water

Solids Permissible Limits (Max)

Organic 200 mg/lit

Inorganic 3000 mg/lit

Sulphates (SO4) 500 mg/lit

Chlorides (Cl) 500 mg/lit

Suspended matter 2000 mg/lit

Requirements of water used in concrete

What if water does not meet the above

requirements????

Effect of Sea Water

Salinity of sea water is approximately 3.5%. If sea

water is used, the main concern will be the

corrosion of steel and reduction in strength.

In addition, it also accelerates the setting time of

cement, causes efflorescence and persistent

dampness. Therefore use of sea water should be

avoided for concrete works.

Hydration

Concrete achieves its strength through a

chemical process called Hydration.

Hydration is a complex process but in

simple terms, is the reaction between water

and the cement in the mix.

Water/Cement Ratio and Strength The most important indicator of strength

Lower the w/c ratio, the higher the final concrete strength

Concept was developed by Duff Abrams

A reduction in the water-cement ratio generally results in an increased quality of concrete, in terms of density, strength, impermeability, reduced shrinkage and creep, etc.

Water/Cement Ratio and Strength

(w/c) Ratio 0.40 0.50 0.60 0.70 0.80

Probable Strength(%) 100 87 70 55 44

Factors Low w/c ratio High w/c ratio

Strength High Low

Permeability Low High

Shrinkage Low High

Water/Cement Ratio and Strength

Adding extra water to concrete!!!

Adding more water creates a diluted paste that is weaker and

more susceptible to cracking and shrinkage

Shrinkage leads to micro-cracks (zones of weakness)

Once the fresh concrete is placed, excess water is squeezed

out of paste by weight of aggregate and cement

The excess water bleeds out onto the surface.

The micro channels and passages that were created inside the

concrete to allow that water to flow become weak zones

Adding extra water to concrete!!!

This affects the compressive, tensile and flexural strengths, the porosity and the shrinkage

Loss of Inherent good qualities like Cohesiveness and Homogeneity

Harmful to Strength and Durability

Sowing the seed of Cancer in concrete

It is an Abuse, It is a Criminal act, Un-engineering ------------------(M.S.Shetty, Eminent Author)

* Increased strength.

* Lower permeability.

* Increased resistance to weathering.

* Better bond between concrete and

reinforcement.

* Reduced drying shrinkage and cracking.

* Less volume change from wetting and drying.

Advantages of low water/cement ratio

Workability

The ease with which freshly mixed concrete can be

transported, placed and finished without

segregation

Influencing factors

Size, Shape, Texture and grading of aggregate

Water Content

ADMIXTURE

It is an optional ingredient of concrete which is

added to modify the properties of fresh as well as

hardened concrete and grout material as per

some specific requirements.

Addition of admixture may alter workability,

pumping qualities, strength development,

appearance etc. in fresh concrete and

permeability, strength, durability etc. in hardened

concrete.

Use of chemical admixture is a must for producing

high grade concrete.

Admixture types Admixtures to enhance workability

Mineral (Fly ash, Silica fume, GGBFS)

Chemical

Air entraining

Chemical and Air-entraining admixtures are Covered by IS:9301-1999

a) Accelerating admixtures

b) Retarding Admixtures

c) Water-reducing admixtures (plasticizers)

d) Air-entraining admixtures and

e) Super-plasticizing admixtures

Water-reducing admixtures An admixture which either increases workability of

freshly mixed mortar or concrete without increasing water content or maintains workability with a reduced amount of water

Role of water reducers is to deflocculate the cement particles agglomerated together and release the water tied up in these agglomerations

Can be categorized according to their active ingredients salts and modifications of hydroxylized carboxylic acids

(HC type)

salts and modifications of lignosulfonic acids and Polymeric materials (PS type)

Reduces water demand 7-10%

Example: PolyHeed 997 -BASF, FLOCRETE N-Don chemicals

Air-entraining admixtures

Which causes air to be incorporated in the

form of minute bubbles in the concrete or

mortar during mixing, usually to increase

workability and resistance to freezing and

thawing and disruptive action of de-icing salts

Reduces bleeding and segregation of fresh

concrete

Can be categorized into four groups: salts of wood resins

synthetic detergents

salts of petroleum acids,

fatty and resinous acids and their salts

MB-AE 90-BASF, Airalon® 3000-Grace

Super-plasticizing admixtures

Which imparts very high workability or allows a

large decrease in water content for a given

workability

Reduce water content by 12 to 30 percent

The effect of superplasticizers lasts only 30 to

60 minutes and is followed by a rapid loss in

workability

Superplasticizers are usually added to

concrete at the jobsite

Example : Glenium-BASF, Supaflo-Don

Chemicals

Production of Concrete:

Material

Activity related to material are there selection and

Proportions

Process

Mixing

Transportation

Placement

Compaction

Curing

Proportioning of concrete

Nominal Mix

The Nominal mixes of grades M10, M15, M20

and M25 correspond approximately to the mix

proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2)

respectively.

Design Mix

Q? Why we are proportioning

Concrete

The goal is to provide the desired strength and

workability at minimum expense.

Curing methods

1. Water curing 2. Steam curing 3. Curing compounds

Water curing

Sea water shall not be used for curing

Seawater shall not come into contact with concrete members unless it has attained adequate strength

Exposed surface of concrete shall be kept continuously in a damp or wet condition by ponding or by covering with a layer of sacks, canvas, Hessian or similar materials and shall be kept constantly wet for a period of not less than 14 days from the date of placing of concrete.

Curing