Batching of concrete

Prepared by:- Prof. Anuj Chandiwala

Leading cement manufacturing companies of India

Associated Cement Company (ACC)

Birla Group of Company

Ambuja Cement Company

Sanghi Cement Company

Shree Digvijay Cement Co. Ltd.

Binani Cement Company

Hathi Cement Company

Grasim Cement Company

Ultratech Cement Company

The raw materials used for the manufacture of Ordinary Portland Cement contains mainly lime, silica,

alumina and iron oxide.

These oxides interact with one another in the kiln at high temperature to form more complex

compounds.

The relative proportions of these oxide compounds are responsible for various physical properties of

cement.

Rate of cooling and fineness of grinding also affect the properties of cement.

When water is added to cement, ingredients of cement react chemically with water and form various

complicated chemical compounds.

The chemical reaction that take place between cement and water is referred as hydration of cement.

Anhydrous cement does not bind fine and coarse aggregates.

It acquires adhesive property only when mixed with water.

The silicates and aluminates of cement react with water and form hydro silicates and hydro aluminates.

These products are thick and sticky. It is called Gel.

Gel possess adhesive property and binds aggregates and sand together. It also fills the voids between sand

and aggregates.

The hydration of cement may happen in two ways. The one is through ‘solution’ mechanism,

in which the cement compounds dissolve in water to produce a supersaturated solution from

which different hydrated products get precipitated. The second theory is that water attack

cement compounds in the solid state converting them into hydrated products.

The reaction of cement with water is exothermic.

The reaction liberates a considerable quantity of heat.

The quality of heat per gram of unhydrated cement, evolved upon complete hydration at a given

temperature is defined as heat of hydration.

The temperature at which hydration occurs significantly affects the rate of heat evolution, which is of

more importance then the total heat of hydration.

In ordinary Portland cement about 50 % of total heat is liberated between 1 and 3 days, about 75 % in 7

days and about 90 % in 6 months.

Setting of cement Hardening of cement

Setting is the term used to describe the stiffening of the cement paste.

Hardening refers to the gain of strength of a set cement paste.

It refers to a change from a fluid to a rigid state.

It refers to formation of solid mass possessing good compressive strength.

The setting of cement starts after 30 minutes from the instant when water is added to cement and completed within 10 hours.

The process of hardening of cement continues for a period more than 1 year.

To know the setting of cement, initial setting time test and final setting time test are conducted.

To know the hardening of cement, compressive strength test is conducted.

1. Ordinary Portland Cement (OPC)

OPC (33 Grade) – IS 269: 1989

OPC (43 Grade) - IS 8112:1989

2. Rapid Hardening Cement (RHC) – IS 8041 : 1990

3. Extra-rapid Hardening Cement

4. Quick Setting Cement

5. Low Heat Cement – IS 12600 : 1989

6. Sulphate Resisting Cement – IS 12330 : 1988

7. Super Sulphated Cement – IS 6909 : 1990

8. Portland Pozzolana Cement

IS 1489 (Part-I) : 1991 (Fly ash based)

IS 1489 (Part- II) : 1991 (Calcined Clay based)

9. Portland Slag Cement- IS 455 : 1989

10. Coloured Cement (white Cement) – IS 8042 : 1989

11. Hydrophobic Cement – IS 8043 : 1991

12. Air Entraining Cement

13. Masonry Cement – IS 3466 : 1988

14. Oil Well Cement – IS 8229 : 1986

15. Expansive Cement

16. High Alumina Cement – IS 6452 : 1989

17. Concrete Sleeper Grade Cement – IRS-T 40 : 1985

18. Waterproof Cement

19. Rediset Cement

20. Very High Strength Cement

Situation Use of cement recommended

All types of ordinary constructions OPC

Pre-fabricated concrete construction Rapid Hardening Cement

Road repair works Rapid Hardening Cement, Rediset Cement

For early removal of form work Rapid Hardening Cement, Rediset Cement

For cold weather concreting Rapid Hardening Cement

Under water construction Quick setting Cement

Grouting operations Quick setting Cement

Situation Use of cement recommended

Mass concrete construction

(Dam, Retaining Wall, raft etc.)

Low Heat Cement,

Portland Pozzolana Cement,

Super Sulphated Cement

Portland Slag Cement

To resist sulphate attack Sulphate Resisting Cement

Super Sulphated Cement

Portland Slag Cement

Low Heat Cement

Hot weather Concreting Low Heat Cement

Sewage Treatment Plant, RCC Pipes Sulphate Resisting Cement

Open the bag of cement and take a good look at the cement. There should not any visible

lump.

The colour of the cement should be greenish grey.

When hand is inserted in cement bag it should give cool feeling.

Take a pinch of cement and feel between the fingers. It should give a smooth feeling and

hot a gritty feeling.

Take a handful of cement and throw it on a bucket full of water, the particles should float

on water for some time before sink.

Bagged cement should be stored in waterproof shed with nonporous walls and floors.

The plinth level should be well above ground level.

Number of opening like doors, windows and ventilators should be minimum and kept tightly shut.

Drainage should be provided if necessary to prevent accumulation of water in the vicinity of the

shed.

Cement bags should be kept 30 cm away from walls.

Not more than 14 bags should be kept one above other.

To reduce air circulation no gap is desirable between rows of cement bags.

In moist area cement bags should be placed on wooden planks kept above

floor.

Old bags should be used first. For beams and slab casting use fresh bags.

Once the cement has been properly stored it should not be disturbed until it is

to be used. The practice of moving and restacking the bags, exposes fresh

cement to air.

Fineness

Standard consistency

Initial and final setting time

Compressive strength

soundness

The aggregates occupy about 75 % of the volume of concrete and hence their influence

on various properties of concrete is considerable.

Properties of aggregate greatly affect the properties of concrete such as workability,

strength, durability and economy.

Aggregate are generally cheaper than cement and impart greater volume stability and

durability to concrete.

The aggregate is used primarily for the purpose of providing bulk to the concrete.

Normal weight aggregates.

Light weight aggregates.

Heavy weight aggregates.

• The specific gravity of these aggregates is about 1.5 to 2.7.

• The density of concrete produce using normal weight aggregates is about 2300 Kg/m³

to 2600 Kg/m³ ( 23 KN/m³ to 26 KN/m³).

• The density of light weight aggregate concrete is about 1200 Kg/m³.

• They reduces the weight of concrete.

• They possess good thermal insulation and fire resistance properties.

• The density of light weight aggregate concrete is about 5000 Kg/m³.

• The iron aggregates absorb newtrons.

• This type of concrete is used in the construction of nuclear reactors.

The size of aggregate 4.75 mm and smaller is called fine aggregate.

Sand is generally considered to have a lower size limit of about 0.07 mm.

Material between 0.06 mm and 0.002 mm is classified as silt.

Particles smaller than 0.002 mm are termed as clay.

Loam is a soft deposit consisting of sand, silt and clay in about equal proportions.

IS :383-1970 has divided the fine aggregates in to four grading zones. The grading

zones become progressively finer from grading zone I to grading zone IV.

The size of aggregate bigger than 4.75 mm is called coarse aggregate.

The grade coarse aggregate is described by its nominal size, i.e. 40 mm, 20 mm, 16 mm and 12.5 mm

etc.

For example, a graded aggregate of nominal size 12.5 mm means an aggregate most of which passes

the 12.5 mm IS sieve.

80 mm size is the maximum size that could be conveniently used for making concrete. Using the

largest possible maximum size will result in,

• Reduction of the cement content.

• Reduction in water requirement.

• Reduction of drying shrinkage.

Generally, the maximum size of aggregate should be as large as possible within the

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

the member as specified in IS : 456- 2000.

For heavily reinforced concrete member the nominal maximum size of 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 reinforcement, whichever is

smaller. Generally, for reinforced concrete work, aggregates having a maximum size

of 20 mm are considered satisfactory.

Rubbles 160 mm size or up to any reasonable size may be used in plain concrete. Such

a concrete is called plum concrete. The rubbles used are called ‘ Plums’.

The quantity of rubble up to a maximum limit of 20 % by volume of concrete is used

when specially permitted.

The rubbles are placed on about 60 cm thick plastic concrete at certain distance apart

and then the plastic concrete is vibrated by powerful internal vibrators.

The rubbles sink into the concrete.

Sometimes combined aggregates comprising different fractions of fine aggregates and

coarse aggregates are available in nature.

They are called all-in-aggregate.

In such cases adjustments often become necessary to supplement the grading by

addition of respective size fraction which may be deficient in the aggregate.

Generally, all-in-one aggregates are not used for making high quality concrete.

IS Sieve

Designation

Percentage by weights passing

40 mm Nominal size 20 mm Nominal size

80 mm 100 -

40 mm 95-100 100

20 mm 45-75 95-100

4.75 mm 25-75 30-50

600 micron 8-30 10-35

150 micron 0-6 0-6

Grading Limits of All –in- Aggregates

When aggregates comprising majority of particles are of nearly same size are called

single size aggregates.

For example, a 20 mm single size aggregate means an aggregate most of which passes

through a 20 mm IS sieve size and major portion of which is retained on a 10 mm IS

sieve.

Rounded aggregate

Irregular aggregate

Angular aggregate

Flaky aggregate

Elongated aggregate

Fully water worn or completely shaped by attrition.

Contains minimum voids ranging from 32 to 33%.

It gives minimum ratio of surface area to volume, thus requiring minimum cement paste

to make good concrete.

It gives better workability.

The interlocking between the particles is less and hence the development of bond is poor

making it unsuitable for high strength concrete.

Example:- river or seashore gravel.

Naturally irregular or partly shaped by attrition, having rounded

edges.

Contains higher percentage of voids ranging from 35 to 38.

It requires more cement paste for a given workability.

The interlocking between the particles, though better than that

obtained with the rounded aggregate, is inadequate for high

strength concrete.

Example:- pit sands and gravels, land or dug flints, cuboid rock.

Possessing well defined edges formed at the intersection of roughly planner faces.

Contains a maximum percentage of voids ranging from 38 to 40.

The interlocking between the particles is good, thereby providing a good bond.

They requires more cement paste to make workable concrete of high strength than that

required by rounded aggregates.

The angular aggregates are suitable for high strength concrete and pavements subjected to

tension.

Example:- crushed rocks of all types, talus , screes.

The aggregate whose least dimension is less than 3/5 (0.6) of its mean dimension is termed

as flaky aggregates.

The mean dimension of the aggregate is the average of the sieve through which the

particle pass and retained, respectively.

They reduces workability of concrete.

The flaky particles also adversely affect the durability of concrete as they tend to be

oriented in one plane with water and air voids forming underneath. They also reduces

strength of concrete.

The presence of these particles should not be more than 15 % in the mass of aggregates.

Examples:- Laminated, rocks.

The aggregate whose greatest dimension (length) is greater

than 9/5th (1.8) of its mean dimension is called elongated

aggregate.

They reduces workability of concrete.

The elongated particles also adversely affect the durability

of concrete and strength of concrete.

The presence of these particles should not be more than 15

% in the mass of aggregates.

Examples:- Laminated rocks.

Angularity number is the method of expressing the angularity of aggregates qualitatively.

It was suggested by Shergold.

It is based on the percentage of voids in sample compacted in a standard manner.

The method of determination of angularity number is described in IS : 2386 (Part-I)- 1963.

The angularity number is based on the concept that most rounded aggregate will have voids 33 % and

the angularity number measures the percentage of voids in excess of that in the rounded gravel.

If the voids are 40 % the angularity number of such aggregate is considered as 7.

If the voids are 33 % the angularity number of such aggregate is considered 0.

Natural sand used in making concrete is normally obtained from river. In the years to

come natural sand will not be available in large quantity for mega infrastructural

projects.

When rock is crushed in the normal way it is likely to yield flaky fine aggregate.

Improved version of crushers are used to produce cubical shaped well graded fine

aggregate.

This method of production of artificial sand is being practiced for high rise building

projects at Mumbai and for construction of Mumbai-Pune Express Highway.

The compressive strength of concrete cannot exceed that of the bulk of the aggregate

contained there in.

The compressive strength of concrete depends upon the quality of cement paste and the

bond between the cement paste and the aggregate.

Strength of aggregate plays important role in quality and strength of concrete.

Aggregate crushing value test

Aggregate impact value test

Aggregate abrasion value test

Ten percent fines value test

Bulking is the phenomenon of increase in the volume of fine aggregates caused by the

presence of free moisture.

Free moisture forms a film of water around each particle. This film of moisture exerts

what is known as surface tension which keeps the neighbouring particles away from it.

Therefore, no point contact is possible between the particles. This cause increase in

volume of the mass of fine aggregates.

The extent of bulking depends upon the percentage of moisture present in the sand and

the fineness of sand particles.

It is interesting to note that the bulking increases with the increase in moisture content

up to a certain limit and beyond that the further increase in the moisture content results

in the decrease in the volume.

For ordinary sand the bulking usually between 15-30 %.

Fine sand bulks more and the maximum bulking is obtained at a higher water content

than the coarse sand.

Extremely fine sand and particularly the manufactured fine aggregate bulks as much as

about 40 %.

Sand F.M.Fine sand 2.2 – 2.6Medium sand 2.6 – 2.9Coarse sand 2.9 – 3.2

Carbonates and bicarbonates of sodium and potassium affect the setting time. While sodium

carbonate may cause quick setting, the bicarbonates may either accelerate or retard the setting.

Alkali carbonates and bicarbonates should not exceed 1000 ppm.

Brakish water contains chlorides and sulphates. When chloride does not exceed 500 ppm and

SO3 does not exceed 400 ppm the water is harmless.

Salts of manganese, zinc, copper, tin and lead cause a marked reduction in strength of concrete.

The salts present in sea water reduces strength of concrete by 10 to 20 % chlorides caused

corrosion of reinforcing steel and efflorescence of concrete.

Suspended particles of clay and silt are undesirable as they interfere with setting,

hardening and bond characteristics.

It is defined as a material other than the basic ingredients of concrete cement,

aggregates and water, added to the concrete mix immediately before or during mixing

to modify some properties of concrete in the fresh or hardened state.

They should not adversely affect any property of concrete.

Admixture are no substitute for good concrete practice.

The use of admixtures like accelerators, retarders, air- entraining agents, pozzolanic

materials, water proofing admixtures etc. is being practiced by Indian construction

industry since long back.

The properties commonly modified using admixtures are setting time, workability, air-entrainment, dispersion etc.

The admixture is generally added in a relatively small quantity ranging from 0.005 to 2 % by weight of cement.

Overuse of admixtures have detrimental effects on the properties of concrete.

To increase the strength of concrete.

To accelerate the initial setting time of concrete.

To retard the initial setting of concrete.

To improve workability of concrete.

To increase durability of concrete.

To reduce heat of hydration.

To make light weight concrete.

To reduce permeability of concrete.

To increase the resistance to sulphate attack.

To increase the bond between old and new concrete.

To reduce segregation and bleeding of concrete.

To produce coloured concrete or mortar.

To control the corrosion of concrete.

One of the common plasticizer generally used is lignosulphonic acid in the form of calcium

or sodium salt. At higher dosages, it may cause retardation in setting time.

Higher dosage of super-plasticizer affect the shrinkage and creep properties of concrete.

Higher dosage of plasticizer may cause segregation and premature stiffening under certain

conditions.

Higher dosages of super-plasticizer may increase rate of loss of workability.

Excess use of accelerators cause more heat evolution and there are chances of cracks in the

concrete.