631861624-Concrete-Technology VERY-pptx.pptx

CONTENTS
1. INRODUCTION
2. CEMENT
3. PROPERTIES OF CONCRETE
4. CONCRETE MIX DESIGN

5. QUALITY CONTROL OF CONCRETE
6. EXTREMEME WEATHER CONCRETEING
AND CHEMICAL ADMIXTURE IN
CONCRETE
7. PROPERTIES OF SPECIAL CONCRETE

CEMENT
Cement is a commonly used building material in construction. It is the main constituent of concrete. It can be
defined as a material having cohesive and adhesive properties which make it possible to bind with other
materials to form a compact mass. It is obtained by burning together a mixture of calcareous (calcium) and
argillaceous (alumina) material at a very high temperature and then grinding the product (clinker) to a fine
powder.

CEMENT CONCRETE
A mixture of cement and sand when mixed with water forms a paste known as cement mortar whereas the
product obtained by mixing cement, sand, water and gravel or crushed stone is called cement concrete.

COMPOSITION OF CEMENT CONCRETE
CEMENT
WATER
FINE AGGREGATES
COARSE AGGREGATES

ADVANTAGES OF CONCRETE
Concrete is considered superior than other construction materials due to the following reasons:
Concrete possesses a high compressive strength
and the corrosive and weathering effects are
minimal. It is considered as an artificial stone.
Freshly prepared concrete can be moulded in any
desired shape easily.
Concrete can be mixed at places many kilometres
away from the actual site of work and used
without any loss of property of the good concrete.

The strength of concrete can be varied by using
different grades of concrete.
The ingredients required for its manufacture are
easily available.
It can be pumped and placed even in very difficult
locations.
It can be sprayed and can be used to rectify small
cracks and repair works by guniting or
shotcreting.

Its maintenance cost is negligible. Hence it is
economical in long run.
It is fire resistant and can also be used as sound
proof by replacing the coarse aggregate either by
light weight concrete or using foam concrete.
It provides good architectural look to the
structure.

DISADVANTAGES OF CONCRETE
Concrete has the following disadvantages:
Concrete is weak in tension. The tensile strength
of concrete is of the order of 10% of the
compressive strength. Thus it develops cracks.
It is dimensionally not stable. It expands with the
increase in temperature and shrinks with the fall in
temperature.
Concrete is not entirely impervious to moisture
and contains soluble salts which may cause
efflorescence.

Concrete is liable to disintegrate by alkali and
sulphate attack.
The self-weight of the concrete is very high.
Due to more weight of the concrete, more
reinforcement is needed, which increases the cost
of the structure.
Concrete under sustained loading undergoes
creep, resulting in the reduction of prestress in
the prestressed concrete construction.

TYPES OF CONCRETE
Classification of concrete based on density
CLASSIFICATION DENSITY (kN/m3
) MATERIALS
Normal weight concrete 24 Natural sand and crushed stone
(granite)
Light weight concrete 18 Light weight aggregates such as
pumice, or pyro-processed and
bloated aggregates
Heavy weight concrete 32 High density aggregates such as
hematite or scrap steel pieces
Concrete can be classified into various categories depending on its density and strength recommended by IS 456: 2000.

Classification of concrete based on strength
CLASSIFICATION MAXIMUM STRENGTH (MPa) TYPE
Ordinary concrete < 20 Low strength
Standard concrete 25-55 Medium strength
High strength concrete 60-80 High strength concrete

COMPOSITION OF CEMENT
The raw materials used for the manufacture of cement are lime, silica, alumina and iron oxide. The relative
proportions of these oxide compositions are responsible for influencing the various properties of cement.
OXIDE PERCENTAGE CONTENT (%)
Lime (CaO) 60 – 67
Silica (SiO2) 17 – 25
Alumina (Al2O3) 3.0 – 8.0
Iron oxide (Fe2O3) 3.0 – 8.0
Magnesia (MgO) 0.1 – 4.0
Alkalies (Na2O, K2O) 0.4 – 1.3
Sulphur trioxide (SO3) 1.3 – 3.0

The oxides present in the raw materials when subjected to a very high temperature of 1450°C fuses to form more
complex compounds known as “clinkers”. The identification of these major compounds or “clinkers” is largely based
on R.H. Bogue’s work and hence it is called “Bogue’s Compounds”. The major compounds or Bogue’s Compounds
are:
NAME OF COMPOUND FORMULA ABBREVIATION
Tricalcium silicate 3CaO.SiO2 C3S
Dicalcium silicate 2CaO.SiO2 C2S
Tricalcium aluminate 3CaO.Al2O3 C2A
Tetracalcium aluminoferrite 4CaO.Al2O3.Fe2O3 C4AF

ORDINARY PORTLAND CEMENT (OPC)
Ordinary Portland Cement (OPC) is one of the most important and most common type of cement used in India.
There was only one grade of OPC prior to 1987 governed by IS: 269-1976. After 1987, higher grade cements were
introduced in India and was classified into three grades as under:
GRADE Minimum Compressive strength at 28 days as per IS:
4031-1988
33 grade 33 N/mm2
43 grade 43 N/mm2
53 grade 53 N/mm2
In the modern construction activities, higher grade cements have become so popular that 33 grade cement is almost
out of the market. Although, they are little costlier than the low grade cement, they offer many benefits like 10-20 %
savings in cement consumption, faster rate of development of strength and higher strength. This type of cement is
suitable for use in general concrete construction wherever it is not exposed to sulphates in the soil or in ground
water.

HYDRATION OF CEMENT
On adding water to cement, the silicates and aluminates present in the cement start a chemical reaction and form a spongy gel.
The chemical reaction that takes place between cement and water is referred to as hydration of cement. During this process, a
large quantity of heat is evolved. The quantity of heat in calories, liberated on complete hydration of cement is called heat of
hydration. The different cement compounds hydrate at different rates and liberate different quantities of heat. The quantity of
heat liberated depends upon the amount of different constituents in the cement. There are two ways in which the compounds
present in the cement may react with water. In the first case, on addition of water, cement compounds dissolve to produce a
super saturated solution from which different hydrated products are precipitated. In the second type of reaction the water is
hydrolyzed i.e. the water attracts the cement compounds in the solid state converting the compounds into hydrated products.
HYDRATION PRODUCTS
The following are the important products of hydration of cement:
(1) Calcium Silicate Hydrate (C-S-H)
(2) Calcium Aluminate Hydrates
(3) Calcium Hydroxide [Ca(OH)2]

Development of strength of pure compounds Rate of hydration of pure compounds

FINENESS OF CEMENT
The fineness of cement is a measure of the size of particles of cement and is expressed in terms of specific
surface of the cement. Since the hydration starts at the surface of the cement particles, it is the total surface of
cement that represents the material available for hydration. For a given weight of cement, the surface area is
more for finer cement than for a coarser cement. The finer the cement, the higher is the rate of hydration as
more surface area is available for chemical reaction. This results in the early development of strength.

INITIAL SETTING TIME OF CEMENT
Initial setting time is regarded as the time elapsed between the moment that the water is added to the cement,
to the time that the paste starts losing its plasticity.
FINAL SETTING TIME OF CEMENT
The final setting time is the time elapsed between the moment the water is added to the cement, and the time
when the paste has completely lost its plasticity and has attained sufficient firmness to resist certain definite
pressure.

COMPRESSIVE STRENGTH OF CEMENT
Compressive strength is one of the important properties of cement. Strength of cement is indirectly found on cement sand
mortar in specific proportions. The standard sand is used for finding the strength of cement. It shall conform to IS 650-1991.
Cement mortar cubes having an area of 7.06 cm x 7.06 cm are used for the determination of compressive strength of cement.
SOUNDNESS OF CEMENT
It is very important that the cement after setting shall not undergo any appreciable change of volume. Certain cements have been
found to undergo a large expansion after setting causing disruption of the set and hardened mass. This will cause serious
difficulties for the durability of the structures when such cement is used. The unsoundness in cement is due to the presence of
excess of lime than that could be combined with acidic oxide at the kiln. This is also due to inadequate burning or insufficiency in
fineness of grinding or thorough mixing of raw materials. It is also likely that too high a proportion of magnesium content or
calcium sulphate content may cause unsoundness in cement. For this reason, the magnesia content allowed in cement is limited
to 4 percent. To prevent flash set, calcium sulphate (gypsum) is added to the clinker while grinding. The quantity of gypsum
added will vary from 3 to 5 percent depending upon the C3A content. If addition of gypsum is more than that could be combined
with C3A, excess of gypsum will remain in the cement in free state. This excess of gypsum leads to an expansion and consequent
disruption of the set cement paste.

GRADES OF OPC
Ordinary Portland Cement (OPC) is one of the most important and most common type of cement used in India.
There was only one grade of OPC prior to 1987 governed by IS: 269-1976. After 1987, higher grade cements were
introduced in India and was classified into three grades as under:
4031-1988
33 grade 33 N/mm2
43 grade 43 N/mm2
53 grade 53 N/mm2
In the modern construction activities, higher grade cements have become so popular that 33 grade cement is almost
out of the market. Although, they are little costlier than the low grade cement, they offer many benefits like 10-20%
savings in cement consumption, faster rate of development of strength and higher strength. This type of cement is
suitable for use in general concrete construction wherever it is not exposed to sulphates in the soil or in ground
water.

FIELD TESTS OF CEMENT
The following field tests are necessary to perform, to ascertain the quality of cement at site:
1. The colour of the cement should be uniform. It should be typical cement colour i.e. grey colour with a greenish shade.
2. The cement should feel smooth when touched or rubbed in between fingers. If it is felt rough, it indicates adulteration
with sand.
3. If hand is inserted in a bag or heap of cement, it should feel cool and not warm.
4. If a small quantity of cement is thrown in a bucket of water, it should sink and should not float on the surface.
5. A thin paste of cement with water should feel sticky between the fingers.
6. The cement should be free from any hard lumps. Such lumps are formed by the absorption of moisture from the
atmosphere. Any bag of cement containing such lumps should be rejected.
7. Take about 100 gm of cement, add some water and prepare a stiff paste. From the stiff paste, pat a cake with sharp
edges. Put it on the glass plate and slowly take it under water in a bucket. The shape of the cake should not be disturbed,
while taking it down to the bottom of the bucket. After 24 hours the cake should regain its original shape and at the same
time it should also set and gain some strength.
If a sample of cement satisfies the above field tests it may be concluded that the cement is not bad. The above tests do not
really indicate the cement is really good for important works. For using cement in important and major works it is incumbent
on the part of the user to test the cement in the laboratory to confirm the requirements of the Indian Standard specifications
with respect to its physical and chemical properties.

LABORATORY TESTS OF CEMENT
The following tests are usually conducted in the laboratory.
Fineness test
Standard consistency test
Initial and final setting time test
Strength test
Soundness test

STORING OF CEMENT AT SITE
It is often necessary to store cement for a long period, particularly when deliveries are uncertain. Although cement retains its
quality almost indefinitely if moisture is kept away from it, the cement exposed to air absorbs moisture slowly, and gets
deteriorated. An absorption of 1 to 2 percent of water has no appreciable effect, but a further amount of absorption retards
the hardening of cement and reduces its strength. The more finely a cement is ground the more reactive it is, and
consequently the more rapidly does it absorb moisture from damp surroundings. Thus following precautions should be
observed while storing cement.
1. The floor level of the storage house should be at least 1.2 m higher than the general ground level, so that any water
collected nearby may not seep by capillary action.
2. The walls of the storage house should be made of water proof concrete masonry or brick work plastered with cement
mortar on both faces.
3. The plinth level of the storage house should be such that a truck can back conveniently to the door for loading and
unloading the cement.
4. Cement bags should not be piled touching the walls, but a 30 cm space between the wall and cement bags pile should be
left all around.
5. Cement bags should not be directly placed on the floor, but on wooden planks. However, if the floor is made of concrete
and is fully dry, then cement bags may be placed directly on it.
6. Not more than 15 bags should be piled in a stack. The maximum width of a stack should not be more than 3 m.

7. If more than 7 bags are to be put in a stack, then they should be arranged alternately as header and stretcher.
8. The cement store house should have minimum no of doors and windows, so that air circulation should be minimum. A
1.2 m wide passage should be provided so that labour can take out a cement bag putting on his back easily.
9. At the time of taking out cement, ‘first in, first out’ rule should be adopted, that is oldest stored cement should be taken
out first.
10. 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.

VARIOUS TYPES OF CEMENT
1. Ordinary Portland Cement (OPC)
Ordinary Portland Cement (OPC) is one of the most important and most common type of cement used in India. There was only
one grade of OPC prior to 1987 governed by IS: 269-1976. After 1987, higher grade cements were introduced in India and was
classified into three grades as under:
4031-1988
33 grade 33 N/mm2
43 grade 43 N/mm2
53 grade 53 N/mm2
In the modern construction activities, higher grade cements have become so popular that 33 grade cement is almost out of the
market. Although, they are little costlier than the low grade cement, they offer many benefits like 10-20% savings in cement
consumption, faster rate of development of strength and higher strength. This type of cement is suitable for use in general
concrete construction wherever it is not exposed to sulphates in the soil or in ground water.

2. Rapid Hardening Cement (RHC) (IS: 8041-1990)
Rapid Hardening Cement (RHC), as the name indicates it develops strength very rapidly. RHC develops strength at the age of
3 days, the same strength as is expected of OPC at the seven days. The rapid rate of strength development is due to the
higher fineness of grinding, higher C3S and lower C2S. A higher fineness of cement results in greater surface area for the
action of water and also higher proportion of C3S results in quicker hydration. Consequently, rapid hardening cement gives
out much more heat of hydration during the early period. The rate of heat development varies from 53 Cal/g to 93.2 Cal/g at
4°C to 41°C. As the rate of heat development is high, therefore, rapid hardening cement should not be used for mass
concrete constructions. The use of rapid hardening cement is recommended in the following situations:
(a) In pre-fabricated concrete construction.
(b) For road, air-strip and bridge repairs etc.
(c) Where form-work is required to be removed early for re-use elsewhere.
(d) In cold concreting.
(e) Wall sealing etc.

3. Extra Rapid Hardening Cement
Extra rapid hardening cement is obtained by inter-grinding calcium chloride with rapid hardening cement. The quantity of
calcium chloride should not exceed 2 percent. It is necessary that the concrete made by using extra rapid hardening cement
should be transported, placed, compacted and finished within 20 minutes. Extra rapid hardening cement accelerates the
setting and hardening process. A large quantity of heat is evolved in a very short time after placing. As a result, this type of
cement is very suitable for concreting in cold weather. The strength of extra rapid hardening cement is about 25% higher
than that of the rapid hardening cement at one or two days and 10-20% higher at 7 days. The gain of strength will disappear
with age and at 90 days the strength of extra-rapid hardening cement of the ordinary Portland cement may be nearly the
same.
4. Quick Setting Cement
This cement as the name indicates sets very early. The early setting property is brought out by reducing gypsum content at
the time of clinker grinding. The cement is required to be mixed, placed and compacted very early. It is used mostly in water
construction. Quick setting cement may also find its use in some typical grouting operations.

5. Low Heat Cement (IS: 12600-1989)
The reaction of cement with water is exothermic and produces large quantities of heat. This heat of hydration leads to the
formation of cracks in the body of the concrete. As a result, low heat cement was developed in U.S.A. in 1930 for use in mass
concrete construction, such as dams. A low heat cement is achieved by reducing the contents of C3S and C3A which are the
compounds evolving the maximum heat of hydration and increasing C2S. The rate of strength development of this cement is
initially lower, but the ultimate strength is unaffected i.e. same as that of the ordinary Portland cement. The use of low heat
cement is recommended in the following situations:
(a) Mass concrete constructions.
(b) Where it is necessary to produce resistance to sulphate attack.
(c) Hot weather concreting.

6. Sulphate Resisting Cement (IS: 12330-1988)
Ordinary Portland cement is susceptible to the attack of sulphates, in particular to the action of magnesium sulphate. Solid
sulphate do not attack the cement compounds. Sulphates in solution permeate into the hardened concrete and attack
calcium hydroxide and hydrated calcium aluminate to form calcium sulphoaluminate, the volume of which is about 227% of
the volume of the original aluminates. Their expansion within the framework of hardened cement paste results in cracks
and subsequent disruption. This phenomenon is known as sulphate attack. Sulphate attack is greatly accelerated if
accompanied by alternate wetting and drying, as in the case for instance, in a marine structure in the zone between the
tides. To remedy the sulphate attack, the use of cement with low C3A and comparatively low C4AF is found to be effective.
The use of sulphate resisting cement is recommended under the following conditions:
(a) Concrete to be used in marine conditions.
(b) Concrete to be used in the construction of sewage treatment plants.
(c) Concrete used for fabrication of RCC pipes which are likely to be buried in marshy region.
(d) Concrete to be used in foundations and basements where soil is infested with sulphates.
(e) Concrete to be used in the construction of chemical factory.

7. Portland Pozzolana Cement (PPC) (IS: 1489-1991)
Portland Pozzolana Cement (PPC) is manufactured by the intergrinding of OPC clinker with 15-35% of pozzolanic material. A
pozzolanic material is essentially a silicious and aluminous material which possesses no cementitious properties. This
pozzolanic material in the finely divided form and in the presence of water react with calcium hydroxide, liberated in the
hydration process to form compounds possessing cementitious properties.
The hydration of C3S and C2S produce considerable quantity of calcium hydroxide which makes the concrete porous, weak
and undurable. If such useless mass could be converted into a useful cementitious product, it considerably improves quality
of concrete. The pozzolanic materials generally used for the manufacture of PPC are calcined clay or fly ash. The pozzolanic
action is shown below:

8. Air-Entraining Cement
This cement is made by mixing a small quantity of an air-entraining agent with OPC clinker at the time of grinding. The air-
entraining agents could be:
(a) Alkali salts of wood resins.
(b) Synthetic detergents of alkyl-aryl sulphonate type.
(c) Calcium lignosulphate derived from sulphite process in paper making.
(d) Animal and vegetable fats, oils etc.
This cement is a special cement that can be used with good results for a variety of conditions. It has been developed to
produce concrete that is resistant to freeze-thaw action, and to scaling caused by chemicals applied for severe frost and ice
removal. Concrete made with this cement contains tiny, well-distributed and completely separated air bubbles. The bubbles
are so small that there may be millions of them in a cubic foot of concrete. The air bubbles provide space for freezing water
to expand without damaging the concrete. Air-entrained concrete has been used in pavements in the northern states for
about 25 years with excellent results. Air-entrained concrete also reduces both the amount of water loss and the capillary/
water channel structure.

9. Expansive Cement
The cement which does not suffer overall change in volume on drying is known as expansive cement. This type of cement
has been developed by using an expanding agent and stabilizer very carefully. Concrete shrinks while setting due to loss of
free water. This is known as drying shrinkage. Cements used for grouting pre-stressed concrete ducts, for grouting anchor
bolts or grouting machine foundations, if shrinks, the purpose for which the grout is used will be defeated to some extent.
Hence to overcome this short coming of cement the necessity of expanding cement is felt. Expansive cement may be
obtained by mixing about 8 to 20 parts of the sulpho-aluminate clinker with 100 parts of OPC and 15 parts of the stabilizer.
As the expansion takes place in the presence of moisture, curing of concrete should be carefully controlled. Generally two
types of expansive cement are used- shrinkage compensating cement and self-compensating cement. The shrinkage
compensating cement, restrain expansion, induces compressive stresses and offset the tensile stresses. The self-stressing
cement also induces compressive stresses after drying shrinkage.

10.Coloured Cement (White Cement) (IS: 8042-
1989)
The greyish colour of Portland cement is due to the presence of iron oxide. The process of manufacturing of white cement is
same as that of Portland cement but the amount of iron oxide is limited to less than 1 percent. Generally clay is used
together with limestone or chalk free from impurities. Further, to avoid contamination with coal ash in kiln, oil is used as fuel
in place of pulverized coal. The elimination of iron oxide needs higher temperature in kiln to fuse the raw materials as iron
oxide acts as a flux. To obtain higher temperature in kiln, more fuel is needed which is not economical. Hence, to lower
down the temperature, cryolite (sodium alumino fluoride) is added as a flux. To prevent contamination of the cement with
iron during grinding, nickel and molybdenum alloy balls are used in place of ordinary iron grinding balls. Thus the cost of
grinding is higher. Due to higher grinding cost and expensive raw materials, this cement is about 3 times costlier than
ordinary Portland cement.
For the manufacture of coloured cements either grey or white Portland cement is used as a base. With grey cement only red
or brown colour can be given successively and for other colours white cement is used. Usually 5 to 10% pigment is
intimately grounded with the Portland cement clinker. The pigment should be chemically inert with cement and should have
fast colour.

11. Portland-Slag Cement (PSC) (IS: 455-1989)
12. High Alumina Cement (IS: 6452-1989)
This type of cement is made by intergrinding 35 to 65% of ordinary Portland cement clinker and ground granulated blast
furnace slag. Blast furnace slag is a waste product consisting of a mixture of lime, silica and alumina obtained in the
manufacture of pig iron. Its oxide composition is similar to Portland cement so far as oxides of calcium, aluminium and silicon
are concerned, but it contains less calcium oxide. This cement is less reactive than OPC and gains strength a little more slowly
during the first 28 days. It has the advantages in generating heat less quickly than OPC. It is suitable for mass concreting but
unsuitable in cold climate. Because of its fairly high sulphate resistance it is used in sea water construction.
High alumina cement is obtained by fusing a mixture of alumina and calcareous materials in suitable proportions and
grinding the resultant mixture into a fine powder. The raw materials used for the manufacture of high alumina cement are
limestone and bauxite. These raw materials with the required proportion of coke were charged into the furnace. The furnace
is fired with pulverized coal or oil with a hot air blast. The fusion takes place at a temperature of about 1550-1600°C. The
cement is maintained in a liquid state in the furnace. Afterwards, the molten cement is run into the moulds and cooled. After
cooling the cement mass forms a hard rock mass. It is then ground to a fine powder. The high alumina cement is highly
resistant to chemical attack and hence, they are suitable for marine construction.

DIFFERENCES BETWEEN OPC AND PPC
It is economical compared to OPC, as costly clinker is replaced by
cheaper pozzolanic material.
Pozzalanic material converts soluble Ca(OH)2 into insoluble cemetitious
products. Hence, durability and permeability of concrete are improved
as compared to that of OPC.
It generates lower heat of hydration as compared to OPC.
It improves pore size distribution and also reduces the micro cracks in
the cement paste at the transition zone due to pozzolanic action and
being finer than OPC.

WATER
QUALITY OF MIXING WATER
The quality of the water is also important as it affects the quality of the resulting concrete. For example, impurities in the
water may affect the setting time of cement, strength of concrete and may cause corrosion of the reinforcement. A popular
yard-stick to the suitability of water for mixing concrete is that, if water is fit for drinking it is fit for making concrete. This
does not appear to be a true statement for all conditions. Drinking water may be unsuitable as mixing water when the water
has a high concentration of sodium or potassium and there is a danger of alkali aggregate reaction. 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 and mixing water. Instead of depending upon pH
value and other chemical composition, the best course to find out whether a particular source of water is suitable for
concrete mixing or not, is to make concrete cubes with this water and compare its 7 days’ and 28 days’ strength with
companion cubes made with distilled water. If the compressive strength is upto 90 percent, the source of water may be
accepted.
QUALITY OF WATER TO BE USED IN CONCRETE

WATER FOR CURING OF CONCRETE
Water used for curing concrete shall be free of materials that significantly affect the hydration reactions of Portland cement
or that otherwise interfere with the phenomena that are intended to occur during the curing of the concrete. Curing water
intended for use on structures made with reactive aggregate and low-alkali cement but without a pozzolanic admixture
should not contain sufficient amounts of dissolved sodium and potassium salts to endanger exceeding the conditions for
which the low-alkali cement was specified. Generally water suitable for mixing concrete, is also suitable for curing of
concrete. However, following points should be noted regarding the use of water for curing of concrete. Generally curing
water should be free from following impurities:
1. Iron or organic matter:
The presence of these matters may cause staining of concrete particularly if the flow of water over the concrete is slow and
evaporation is rapid.
2. Presence of carbon-dioxide (CO2):
i. Water should be free from CO2 as it reacts with Ca(OH) 2 in concrete to form CaCO3. This process is called carbonation
which is the reversal of calcination in the kiln. It has two effects:
a) Increases mechanical strength.
b) Decreases alkalinity and reduces pH value to 10.
As a result of decrease in pH value, corrosion of reinforcements takes place.
ii. Water formed by melting ice or by condensation should not be used for curing as it contains little CO2. This CO2 dissolves
in water to form H2CO3 (carbonic acid) which reduces the alkalinity. As a result the corrosion of the reinforcements takes
place.

CHAPTER 3
PROPERTIES OF
CONCRETE

DIFFERENT GRADES OF CONCRETE
(IS 456-2000)
CLASSIFICATION MAXIMUM STRENGTH (MPa) TYPE
Ordinary concrete < 20 Low strength
Standard concrete 25-55 Medium strength
High strength concrete 60-80 High strength concrete

MINIMUM GRADE OF CONCRETE FOR DIFFERENT EXPOSURE
CONDITIONS FOR PLAIN CONCRETE AND R.C.C.
Minimum Cement Content, Maximum Water/Cement ratio and Minimum grade of Concrete for Different exposures with Normal
weight aggregates of 20 mm Nominal Maximum Size (IS: 456-2000)

DURABILITY OF CONCRETE
The durability of cement concrete can be defined as its ability to resist weathering action, chemical attack,
abrasion or any other process of deterioration. A durable concrete will maintain its original form, quality and
serviceability when exposed to adverse environment.

WATER/CEMENT RATIO
The water/cement ratio is defined as the weight of the mixing water divided by the weight of the cement. High
quality concrete is produced by lowering the water/cement ratio as much as possible without sacrificing the
workability of fresh concrete. Actually water/cement ratio is an index of the strength of concrete. The strength
of concrete mainly depends upon the strength of the cement paste and the cement paste strength depends
upon the dilution of cement paste. In other words the strength of cement paste increases with cement content
and decreases with water and air content. For a fully compacted concrete, its strength is taken to be inversely
proportional to the water/cement ratio.

MAXIMUM W/C RATIO FOR DIFFERENT GRADES OF CONCRETE
FOR DIFFERENT EXPOSURE CONDITIONS
Minimum Cement Content, Maximum Water/Cement ratio and Minimum grade of Concrete for Different exposures with Normal
weight aggregates of 20 mm Nominal Maximum Size (IS: 456-2000)

DEFINITION OF WORKABILITY
The diverse requirements of mixability, stability, transportability, placeability, mobility, compactibility and
finishability of fresh concrete are collectively known as workability. The workability of fresh concrete is thus a
composite property. IS 6461 (Part VII): 1973 defines workability as that property of freshly mixed concrete or
mortar which determines the ease and homogeneity with which it can be mixed, placed, compacted and
finished.
PROPERTIES OF FRESH CONCRETE

FACTORS AFFECTING WORKABILITY OF CONCRETE
• Water content in a given volume of concrete will have significant influences on the
workability. An increase in water content results in monotonous increase in workability
but eventually a state is reached where segregation and bleeding takes place.
Water Content
• Aggregate/cement ratio is an important factor influencing workability. The higher the
aggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity of
cement paste is available for providing lubrication per unit surface area of aggregate and
hence the mobility of aggregate is restrained. On the other hand, in case of rich concrete,
with lower aggregate/cement ratio, more paste is available to make the concrete mix
cohesive and fatty to give better workability.
Mix
Proportions
• The bigger the size of aggregate, the less is the surface area and less is the internal
friction between the aggregates. Hence, less amount of water is required for lubricating
the surface of the aggregates to reduce internal friction. For a given quantity of water
and cement, bigger size of aggregates will give higher workability.
Size of
Aggregates

• The shape of aggregates influences the workability in good measure. Angular, elongated or flaky
aggregate makes the concrete very harsh when compared to rounded aggregates. Contribution to
better workability of rounded aggregates will come from the fact that for the given volume or weight
it will have less surface area and less voids than angular or flaky aggregate. Moreover, the internal
friction between the rounded particles is very less compared to angular or flaky aggregates and
hence less water is required for increasing the lubricating effect of the particles. Hence, rounded
aggregates provide better workability than angular or flaky aggregates.
Shape of
Aggregates
• The influence of surface texture on workability is again due to the fact that the total surface area of
rough textured aggregate is more than the surface area of smooth rounded aggregate of same
volume. The rough textured aggregate show poor workability and smooth or glassy textured
aggregate will give better workability. A reduction of inter-particle frictional resistance offered by
smooth aggregates also contributes to higher workability.
Surface
Texture
• This is one of the factors which will have maximum influence on the workability. A well graded
(continuous grading) aggregate is the one which has least amount of voids in a given volume. Other
factors being constant, when the total voids are less, excess paste is available to give better
lubricating effect. With excess amount of paste, the mixture becomes cohesive and fatty which
prevents segregation of particles. Aggregate particles will slide past each other with fewer amounts
of compacting efforts. The better the grading, the less is the void content and higher is the
workability.
Grading of
Aggregates

• The presence and nature of admixtures and mineral additives affect the workability
considerably. The use of plasticizers and super-plasticizers improve the workability. Use
of air-entraining agent being surface active, reduces the internal friction between the
particles. They also act as artificial fine aggregates of very smooth surface. It can be
viewed that air bubbles act as a sort of ball bearing between the particles to slide past
each other and give easy mobility to the particles.
Use of
Admixtures
• The workability of a concrete mix is also affected by the temperature of concrete and,
therefore, by the ambient temperature. On a hot day, it becomes necessary to increase
the water content of the concrete mix in order to maintain the desired workability.
Effect of
Environmental
Conditions
• The fresh concrete loses workability with time mainly because of the loss of moisture
due to evaporation. The loss of workability varies with the type of cement, the
concrete mix proportions, the initial workability and the temperature of the concrete.
Effect of time

DETERMINATION OF WORKABILITY OF
CONCRETE BY SLUMP CONE TEST
This test is used extensively at the site of work all over the world. The slump test shows the behaviour of a compacted cone
under the action of gravitational force. It is an indirect measurement of concrete consistency or stiffness. The consistency or
stiffness indicates how much water has been used in the mix.
The apparatus for conducting the slump test essentially consists of a metallic mould in the form of a frustum of a cone
having the internal dimensions as under:
Bottom diameter of the mould = 20 cm.
Top diameter of the mould = 10 cm.
Height of the mould = 30 cm.

Slump Apparatus
The thickness of the mould should not be thinner than 1.6 mm. Sometimes, the mould is provided with suitable guides for
lifting vertically up. For tamping the concrete, a steel tamping rod 16 mm diameter, 600 mm long with bullet end is used.

Procedure:
1. The internal surface of the mould is thoroughly cleaned and the mould is placed on a smooth, horizontal, rigid and non-
absorbent surface.
2. The mould is then filled in three layers, each approximately 1/3 of the height of the mould.
3. Each layer is tamped 25 times by the tamping rod.
4. When the mould is completely filled with concrete, the top surface is struck off level with a trowel and tamping rod.
5. The mould is firmly held against its base during the entire operation so that it could not move due to the poring of
concrete and this can be done by means of handles or foot- rest brazed to the mould.
6. The mould is then immediately removed from the concrete by raising it slowly and carefully in a vertical direction.
7. This allows the concrete to subside. This subsidence is referred to as Slump of Concrete.
8. The difference in level between the height of the mould and that of the highest point of the subsided concrete is
measured. The difference in height in mm is taken as Slump of Concrete.

Kinds of slump:
The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as:
1. Collapse Slump
2. Shear Slump
3. True Slump
Kinds of slump

Collapse Slump
In a collapse slump the concrete collapses completely. A collapse slump will generally mean that the mix is too wet or that it
is a high workability mix, for which slump test is not appropriate.
Shear Slump
If one-half of the cone slides down an inclined plane, the slump is said to be a shear slump.
1) If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated.
2) If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix and
indicates segregation characteristics.
True slump
If the concrete slumps evenly, then it is called true slump. This is the only slump which is used in various tests.

CONCRETE BY COMPACTION FACTOR TEST
The compacting factor test is designed primarily for use in the laboratory where maximum size of aggregate does not
exceed 40 mm; however, it may also be used in the field. It is more precise and sensitive than slump test and is particularly
useful for concrete mixes of very low workability as are normally used when concrete is to be compacted by vibration. Such
dry concrete are insensitive to slump test.

The essential dimensions of the hoppers and mould and the distances between them are shown in table below:
DETAILS DIMENSIONS (cm)
Upper Hopper A
Top internal diameter 25.4
Bottom internal diameter 12.7
Internal height 27.9
Lower hopper B
Top internal diameter 22.9
Bottom internal diameter 12.7
Cylinder C
Internal diameter 15.2
Distance between bottom of upper hopper and top of lower
hopper
20.3
Distance between bottom of lower hopper and top of
cylinder
20.3

Procedure:
1. The sample of concrete to be tested is placed in the upper hopper upto the brim.
2. The trap door is opened so that the concrete falls into the lower hopper.
3. Then the trap door of the lower hopper is opened and the concrete is allowed to fall into the cylinder.
4. The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades.
5. The outside of the cylinder is wiped clean and it is weighted. This weight is known as “Weight of partially compacted
concrete”.
6. The cylinder is emptied and then refilled with the concrete from the same sample in layers of approximately 5 cm deep.
7. The layers are heavily rammed so as to obtain full compaction.
8. The top surface of the fully compacted concrete is then carefully struck off level with the top of the cylinder and
weighted. This weight is known as “Weight of fully compacted concrete”.
9. The ratio of the two densities or weights of partially compacted concrete and fully compacted concrete will give its
compacting factor.
Compacting factor = Weight of partially compacted concrete/Weight of fully compacted concrete

CONCRETE BY VEE BEE CONSISTOMETER TEST
This is a good laboratory test to measure indirectly the workability of concrete. This test consists of a vibrating table, a metal pot,
a sheet metal cone, a standard iron rod.
Vee-Bee Consistometer

Procedure:
1. A conventional Slump test as performed, placing the slump cone inside the sheet metal cylindrical pot of the
consistometer.
2. The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot.
3. The electrical vibrator is then switched on and a stop watch is started simultaneously.
4. The vibration is continued till such a time as the conical shape of the concrete disappears and the concrete assumes a
conical shape.
5. When the concrete fully assumes a conical shape, the stop watch is switched off immediately. The time is noted.
6. The time required for the shape of concrete to change from slump cone shape to cylindrical shape in seconds in known
as Vee Bee Degree.
This method is very suitable for very dry concrete whose slump cannot be measured by slump test, but the vibration is too
vigorous for concrete with a slump greater than about 50 mm.

CONCRETE BY FLOW TABLE TEST
This is a laboratory test which gives an indication of the quality of concrete with respect to consistency, cohesiveness and
proneness to segregation.
The apparatus consists of a flow table about 76 cm in diameter over which concentric circles are marked. A mould made of
smooth metal casting in the form of a frustum of a cone is used with the following internal dimensions. The base is 25 cm in
diameter, upper surface 17 cm in diameter and height of the cone is 12 cm.
Flow Table Test Apparatus

Procedure:
1. The mould is kept on the centre of the table, firmly held and filled in two layers.
2. Each layer is rodded 25 times with a tamping rod 1.6 cm diameter and 61 cm long rounded at the lower tamping end.
3. After the top layer is rodded evenly, the excess of concrete which has overflowed the mould is removed.
4. The mould is lifted vertically upward and the concrete stands on its own without support.
5. The table is then raised and dropped 12.5 mm 15 times in about 15 seconds.
6. The diameter of the spread concrete is measured in about 6 directions to the nearest 5 mm and the average spread is
noted.
7. The flow of concrete is the percentage increase in the average diameter of the spread concrete over the base diameter
of the mould.
Flow % age = [(Spread diameter in cm−25)/25] x 100

Workability, Slump and Compacting Factor of Concretes with 20 mm or 40 mm Maximum Size of Aggregate
RANGE VALUES OF WORKABILITY REQUIREMENT FOR DIFFERENT
TYPES OF CONCRETE WORKS

SEGREGATION
Segregation can be defined as the separation of the ingredients of concrete mix, so that the mix is no longer in a
homogenous and stable condition. If a sample of concrete exhibits a tendency for segregation of say, coarse aggregate from
the rest of the ingredients, then, that sample is said to be showing the tendency for segregation. Such concrete is not only
weak but also undurable.
Segregation of concrete

TYPES OF SEGREGATION
Segregation may be of three types:
Firstly, the coarse aggregate separating out or settling down from the rest of the matrix,
Secondly, the paste or matrix separating away from the coarse aggregate, and
Thirdly, water separating out from the rest of the material. This is known as bleeding.
CAUSES OF SEGREGATION
The various causes of segregation are listed as under:
1. Badly proportioned mix where sufficient matrix (paste) is not available to bind and contain the aggregates.
2. Insufficiently mixed concrete with higher water content.
3. Dropping concrete from a height more than one metre.
4. Excessive vibration used for spreading concrete.
5. Concrete discharged from a badly designed mixer or from a mixer of worn out blades.

REMEDIAL MEASURES
The risk of segregation can be minimized by taking the following precautions:
1. Using correctly proportioned mix.
2. Reducing the height of drop of concrete.
3. Reducing the continued vibration over a long time.
4. The use of air-entraining agents appreciably reduces segregation.
5. Use of certain workability agents, pozzolanic materials makes the mix cohesive and greatly helps in reducing segregation.
6. At any stage, if segregation is observed, remixing for a short time would make the concrete again homogenous.

HARSHNESS
Harshness is defined as the resistance offered by the freshly laid concrete to surface finishing.
Harshness of concrete may be due to
(i) Insufficient quantity of water
(ii) Lesser quantity of fines
(iii) Use of poorly graded aggregates
(iv) Lesser quantity of cement
Harsh quantity will result in porous concrete and poor surface finish.

BLEEDING
Bleeding is sometimes referred to as water gain. It is a particular form of segregation in which some of the water from the
concrete comes out to the surface of the concrete. It causes formation of a porous, weak and non-durable concrete layer at
the top of the concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently
mixed concrete.
Bleeding is due to rise of water in the mix to the surface because of the inability of the solid particles in the mix to hold all
the mixing water during settling of the particles under the effect of compaction. In case of lean mixes, water while rising up,
create capillary channels. If water/cement ratio is more than 0.7, these capillary channels remain continuous and un-
segmented by the development of gel, increasing the permeability of concrete.
Bleeding water while coming from bottom to top brings certain quantity of cement to the surface. This formation of cement
paste at the surface is known as ‘Laitance’ or ‘scum’. The excess mortar at the top causes plastic shrinkage cracks. The
laitance formed on roads produces dust in summer and mud in rainy season.
When concrete is placed in different layers and each layer is compacted and a certain time is allowed to lapse before the
next layer is laid, bleeding can cause creation of a weak horizontal layer of non-durable concrete in between the two layers,
thus causing a plane of weakness. This can be avoided by delaying the placing of the next layer until all the bleed water has
evaporated. Over compacting the surface should be avoided. Any laitance formed should be removed by brushing and
washing before a new layer is added.

Excessive bleeding may also cause trapping of rising water below the aggregate reducing the bond between the aggregate
and the paste. This aspect is more pronounced in the case of flaky aggregate. Similarly, rising water that accumulates below
the reinforcing bars reduces the bond between the reinforcement and the concrete.
REMEDIAL MEASURES
Bleeding is an inherent phenomenon in concrete. It can be reduced by:
1. Proper proportioning and uniform and complete mixing.
2. Use of finely divided pozzolanic materials create a longer path for the water to traverse and reduces bleeding.
3. Use of air entraining admixtures.
4. Using finer cement or cement with low alkali content.

COHESIVENESS
Cohesiveness of concrete describes the tendency to resist segregation and bleeding.

PROPERTIES OF HARDENED CONCRETE
The strength may be defined as the ability to resist force. With regard to concrete for structural purposes it can be defined
as the unit force required causing rupture. Strength is a good index of most of the other properties of practical importance.
In general stronger concretes are stiffer, more water tight and more resistant to weathering etc. Strength of concrete may be
measured as strength in tension, strength in compression, in flexural or in shear. Strength may be classified as follows:
1. Compressive Strength: It is the resistance of the concrete to crushing. A compressive strength of 150 kg/cm2
is normally
specified for concrete in building construction. For roads and bridges, concrete of strength 200 to 400 kg/cm2
is
specified.
2. Flexural Strength: It is the resistance of the concrete to tension under flexural loading. Concrete is weak in tension and
its flexural tensile strength is quantitatively about one-eighth to one-tenth of the compressive strength.
3. Bond Strength: When concrete surrounds the steel as in the case of reinforced concrete, it firmly grips the steel. This
property of adhesion between steel and concrete is called bond strength. In general, higher the compressive strength
better is the bond strength.

DEFINITION OF COMPRESSIVE STRENGTH
Compressive Strength is the resistance of the concrete to crushing. A compressive strength of
150 kg/cm2
is normally specified for concrete in building construction. For roads and bridges,
concrete of strength 200 to 400 kg/cm2
is specified.

IMPERMEABILITY ELASTIC PROPERTIES
OF CONCRETE
In the theory of reinforced concrete, it is assumed that concrete is elastic, isotropic, homogenous and that it conforms to
Hooke’s law. Actually none of these assumptions are strictly true and concrete is not a perfectly elastic material. Concrete
deforms when load is applied but this deformation does not follow any simple set rule. The deformation depends upon the
magnitude of the load, the rate at which the load is applied and the elapsed time after which the observation is made. In
other words, the rheological behaviour of concrete i.e., the response of concrete to applied load is quite complex.
The knowledge of rheological properties of concrete is necessary to calculate deflection of structures, and design of
concrete members with respect to their section, quantity of steel and stress analysis. When reinforced concrete is designed
by elastic theory it is assumed that a perfect bond exists between concrete and steel. The stress in steel is “m” times the
stress in concrete where “m” is the ratio between modulus of elasticity of steel and concrete, known as modular ratio. The
accuracy of design will naturally be dependent upon the value of the modulus of elasticity of concrete, because the
modulus of elasticity of steel is more or less a definite quantity.

MODULUS OF ELASTICITY OF CONCRETE
In view of the peculiar and complex behaviour of stress-strain relationship, the modulus of elasticity of concrete is defined in
somewhat arbitrary manner. The modulus of elasticity of concrete is designated in various ways and they have been
illustrated on the stress-strain curve. The term Young’s modulus of elasticity can strictly be applied only to the straight part of
stress-strain curve. In the case of concrete, since no part of the graph is straight, the modulus of elasticity is found out with
reference to the tangent drawn to the curve at the origin. The modulus found from this tangent is referred as initial tangent
modulus. This gives satisfactory results only at low stress value. For higher stress value it gives a misleading picture.

CREEP
Concrete creep is defined as the deformation of concrete structure under sustained load.
Basically, long term pressure or stress on concrete can make it change shape. This
deformation usually occurs in the direction the force is being applied. Like a concrete column
getting more compressed or a beam bending.

FACTORS AFECTING CREEP
• The creep in aggregate is very less. It is the paste which is responsible
for the creep. However, the aggregate influences the creep of concrete
through a restraining effect on the magnitude of creep. The paste
which is creeping under load is restrained by aggregate which do not
creep. The stronger the aggregate, the more is the restraining effect
and hence the less is the magnitude of creep.
Aggregate
• The amount of paste content and its quality is one of the most
important factors influencing creep. A paste structure which is poorer
undergoes higher creep. Therefore, it can be said that creep increases
with increase in water/cement ratio. In other words, it can also be said
that creep is inversely proportional to the strength of concrete.
Mix
Proportions

• It has been observed that for a given type of concrete, the
creep decreases as the age at the time of application of load
increases as the strength increases with age.
Age
• Fineness of cement affects the strength development at the
early ages and thus influences the creep. In the early ages the
strength development is very less in finest cement hence the
creep of concrete is the greatest, but after 1000 days it
becomes least due to the high gain of strength.
Fineness of
cement
• The greater the degree of hydration of the cement at the time
of load application, the greater will be the development of
strength and as creep varies inversely with strength so lower
will be the rate and total amount of creep.
Degree of
Hydration

SHRINKAGE
The term shrinkage is loosely used to describe the various aspects of volume changes in concrete due to loss of moisture at
different stages due to different reasons. To understand this aspect more closely, shrinkage can be classified in the following
way:
(a ) Plastic Shrinkage
(b) Drying Shrinkage
(c) Autogenous Shrinkage
(d) Carbonation Shrinkage.

FACTORS AFECTING SHRINKAGE
• One of the most important factors that affects shrinkage is the drying condition or in
other words, the relative humidity of the atmosphere at which the concrete specimen is
kept. If the concrete is placed in 100 per cent relative humidity for any length of time,
there will not be any shrinkage, instead there will be a slight swelling. The magnitude of
shrinkage increases with time and also with the reduction of relative humidity.
RELATIVE HUMIDITY
OF THE
ATMOSPHERE
• The richness of the concrete also has a significant influence on shrinkage.
WATER/CEMENT
RATIO
• The quantum of an aggregate, its size, and its modulus of elasticity influence the magnitude of drying
shrinkage. The grading of aggregate by itself may not directly make any significant influence. But since
it affects the quantum of paste and water/cement ratio, it definitely influences the drying shrinkage
indirectly. The aggregate particles restrain the shrinkage of the paste. The harder aggregate does not
shrink in unison with the shrinking of the paste whereby it results in higher shrinkage stresses, but low
magnitude of total shrinkage. But a softer aggregate yields to the shrinkage stresses of the paste and
thereby experiences lower magnitude of shrinkage stresses within the body, but greater magnitude of
total shrinkage.
AGGREGATE

OBJECTIVES OF MIX DESIGN
Mix design can be defined as the process of selecting suitable ingredients of concrete and determining their relative
proportions with the object of producing concrete of certain minimum strength and durability as economically as possible.
The purpose of designing as can be seen from the above definitions is two-fold. The first object is to achieve the stipulated
minimum strength and durability. The second object is to make the concrete in the most economical manner. Cost wise all
concretes depend primarily on two factors; namely cost of material and cost of labour. Labour cost, by way of formworks,
batching, mixing, transporting, and curing is nearly same for good concrete and bad concrete. Therefore attention is mainly
directed to the cost of materials. Since the cost of cement is many times more than the cost of other ingredients, attention
is mainly directed to the use of as little cement as possible consistent with strength and durability.

LIST OF DIFFERENT METHODS OF MIX DESIGN
American Concrete Institute Method of Mix Design
Road Note No. 4 Method
DOE Method of Concrete Mix Design
Mix Design for Pumpable Concrete
Indian Standard Recommended Method of Concrete Mix Design
(IS 10262 – 2009)
Rapid Method

STUDY OF MIX DESIGN PROCEDURE BY I.S. METHOD
AS PER I.S. 10262-2009
REQUIREMENTS OF CONCRETE MIX DESIGN
The requirements which form the basis of selection and proportioning of mix ingredients are:
a) The minimum compressive strength required from structural consideration.
b) The adequate workability necessary for full compaction with the compacting equipment available.
c) Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site
conditions.
d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete.

DATA REQUIRED FOR MIX PROPORTIONING
The following data are required for mix proportioning of a particular grade of concrete:
a) Grade designation;
b) Type of cement;
c) Maximum nominal size of aggregate;
d) Minimum cement content;
e) Maximum water-cement ratio;
f) Workability;
g) Exposure condition as per Table 4 and Table 5 of IS 456: 2000;
h) Maximum temperature of concrete at the time of placing;
i) Method of transporting and placing;
j) Early age strength requirements, if required;
k) Type of aggregate;
l) Maximum cement content; and
m) Whether an admixture shall or shall not be used and the type of admixture and the condition of use.

PROCEDURE FOR MIX PROPORTIONING
1. Target Strength for Mix Proportioning
The target mean compressive strength f’ck at 28 days is given by the relation,
f’ck = fck + 1.65s
where f’ck = target mean compressive strength at 28 days in N/mm2
,
fck = characteristic compressive strength at 28 days in N/mm2
, and
s = standard deviation N/mm2
.
The value of standard deviation can be calculated from table 8 of IS 456: 2000
Assumed Standard Deviation (IS 456: 2000)

2. SELECTION OF MIX PROPORTIONS
2.1. Selection of Water/cement ratio
The water-cement ratio given in Table 5 of IS 456: 2000 for respective environmental conditions may be used as a starting
point.
Minimum cement content, Maximum water-cement ratio and Minimum grade of concrete for different Exposures with Normal
Weight Aggregates of 20 mm Nominal Maximum Size (IS 456: 2000)

Adjustments to minimum cement contents for aggregates other than 20 mm nominal maximum size (IS 456: 2000)
The free water-cement ratio selected according to 2.1 should be checked against the limiting water-cement ratio for the
requirements of durability and the lower of the two values adopted.

2.2. Selection of Water Content
The water content of concrete is influenced by a number of factors, such as aggregate size, aggregate shape, aggregate
texture, workability, water-cement ratio, cement and other supplementary cementitious material type and content,
chemical admixture and environmental conditions. An increase in aggregate size, a reduction in water-cement ratio and
slump, and use of rounded aggregate and water reducing admixtures will reduce the water demand. On the other hand
increased temperature, cement content, slump, water-cement ratio, aggregate angularity and a decrease in the proportion
of the coarse aggregate to fine aggregate will increase water demand.
The quantity of maximum mixing water per unit volume of concrete may be determined from Table 2 of IS 10262: 2009.
Maximum Water Content per cubic metre of Concrete for Nominal Maximum size of Aggregate. (IS 10262: 2009)

The water content in Table 2 is for angular coarse aggregate and for 25 to 50 mm slump range. The water estimate in Table 2
can be reduced by approximately 10 kg for sub-angular aggregates, 20 kg for gravel with some crushed particles and 25 kg
for rounded gravel to produce same workability. For the desired workability (other than 25 to 50 mm slump range), the
required water content may be established by trial or an increase of about 3 percent for every additional 25 mm slump.
Water reducing admixtures or super-plasticizing admixtures usually decrease water content by 5 to 10 percent and 20
percent and above respectively at proper dosages.
2.3. Calculation of Cementitious Material Content
The cement and supplementary cementitious material content per unit volume of concrete may be calculated from the free
water-cement ratio and the quantity of water per unit volume of concrete as
The cementitious material content so calculated shall be checked against the minimum content for the requirements of
durability and greater of the two values adopted. The maximum cement content shall be in accordance with IS 456: 2000.
According to Clause 8.2.4.2 of IS 456: 2000 cement content not including fly ash and ground granulated blast furnace slag in
excess of 450 kg/m3 should not be used unless special consideration has been given in design to the increased risk of
cracking due to drying shrinkage in thin sections, or to early thermal cracking and to the increased risk of damage due to
alkali silica reactions.

2.4. Estimation of Coarse Aggregate Proportion
Aggregates of essentially the same nominal maximum size, type and grading will produce concrete of satisfactory workability
when a given volume of coarse aggregate per unit volume of total aggregate is used. Approximate values for this aggregate
volume are given in Table 3 of IS 10262: 2009 for a water-cement ratio of 0.5, which may be suitably adjusted for other
water-cement ratios. For water-cement ratios other than 0.5 at the rate of -/+ 0.01 for every +/- 0.05 change in water-
cement ratio.
Volume of Coarse Aggregate per unit Volume of Total Aggregate for different zones of Fine Aggregate. (IS 10262: 2009)

2.5 Estimation of Fine Aggregate Proportion
With the completion of procedure given in 2.4, all the ingredients have been estimated except the coarse and fine aggregate
content. These quantities are determined by finding out the absolute volume of cementitious material, water and the
chemical admixture; by dividing their mass by their respective specific gravity, multiplying by 1/1000 and subtracting the
result of their summation from unit volume. The values so obtained are divided into Coarse and Fine Aggregate fractions by
volume in accordance with coarse aggregate proportion already determined in 2.4. The coarse and fine aggregate contents
are then determined by multiplying with their respective specific gravities and multiplying by 1000 (1 gm/cc = 1000 kg/m3).

TESTING OF CONCRETE
Testing of hardened concrete plays an important role in controlling and confirming the quality of cement
concrete works. Systematic testing of raw materials, fresh concrete and hardened concrete are inseparable part
of any quality control programme for concrete, which helps to achieve higher efficiency of the material used
and greater assurance of the performance of the concrete with regard to both strength and durability. The test
methods should be simple, direct and convenient to apply.

SIGNIFICANCE OF TESTING
One of the purposes of testing hardened concrete is to confirm that the concrete used at site has developed the required
strength. As the hardening of the concrete takes time, one will not come to know, the actual strength of concrete for some
time. This is an inherent disadvantage in conventional test. But, if strength of concrete is to be known at an early period,
accelerated strength test can be carried out to predict 28 days strength. But mostly when correct materials are used and
careful steps are taken at every stage of the work, concretes normally give the required strength. The tests also have a
deterring effect on those responsible for construction work. The results of the test on hardened concrete, even if they are
known late, helps to reveal the quality of concrete and enable adjustments to be made in the production of further
concretes. Tests are made by casting cubes or cylinder from the representative concrete or cores cut from the actual
concrete. It is to be remembered that standard compression test specimens give a measure of the potential strength of the
concrete, and not of the strength of the concrete in structure. Knowledge of the strength of concrete in structure can not
be directly obtained from tests on separately made specimens.

COMPRESSION TEST
Compression test is the most common test conducted on hardened concrete, partly because it is an easy test to perform,
and partly because most of the desirable characteristic properties of concrete are qualitatively related to its compressive
strength. The compression test is carried out on specimens cubical or cylindrical in shape. Prism is also sometimes used, but
it is not common in our country. Sometimes, the compression strength of concrete is determined using parts of a beam
tested in flexure. The cube specimen is of the size 15 x 15 x 15 cm. If the largest nominal size of the aggregate does not
exceed 20 mm, 10 cm size cubes may also be used as an alternative.
Moulds
Metal moulds, preferably steel or cast iron, thick enough to prevent distortion are required. They are made in such a
manner as to facilitate the removal of the moulded specimen without damage and are so machined that, when it is
assembled ready for use, the dimensions and internal faces are required to be accurate within the following limits.

The height of the mould and the distance between the opposite faces are of the specified size ± 0.2 mm. The angle
between adjacent internal faces and between internal faces and top and bottom planes of the mould is required to be 90°
± 0.5°. The interior faces of the mould, are plane surfaces with a permissible variation of 0.03 mm. Each mould is provided
with a metal base plate having a plane surface. The base plate is of such dimensions as to support the mould during the
filling without leakage and it is preferably attached to the mould by springs or screws. The parts of the mould, when
assembled, are positively and rigidly held together, and suitable methods of ensuring this, both during the filling and on
subsequent handling of the filled mould, are required to be provided.
In assembling the mould for use, the joints between the sections of the mould are thinly coated with mould oil and a
similar coating of mould oil is applied between the contact surface of the bottom of the mould and the base plate in order
to ensure that no water escapes during the filling. The interior surfaces of the assembled mould is also required to be thinly
coated with mould oil to prevent adhesion of concrete.
Compacting
The test cube specimens are made as soon as practicable after mixing and in such a way as to produce full compaction of
the concrete with neither segregation nor excessive laitance. The concrete is filled into the mould in layers approximately 5
cm deep. In placing each scoopful of concrete, the scoop is required to be moved around the top edge of the mould as the
concrete slides from it, in order to ensure a symmetrical distribution of the concrete within the mould. Each layer is
compacted either by hand or by vibration. After the top layer has been compacted the surface of the concrete is brought to
the finished level with the top of the mould, using a trowel. The top is covered with a glass or metal plate to prevent
evaporation.

Curing
The test specimens are stored in place free from vibration, in moist air of at least 90% relative humidity and at a
temperature of 27° ± 2°C for 24 hours ± 1/2 hour from the time of addition of water to the dry ingredients. After this period,
the specimens are marked and removed from the moulds and unless required for test within 24 hours, immediately
submerged in clean fresh water or saturated lime solution and kept there until taken out just prior to test. The water or
solution in which the specimens are submerged, are renewed every seven days and are maintained at a temperature of 27°
± 2°C. The specimens are not to be allowed to become dry at any time until they have been tested.
Making and Curing Compression Test Specimen in the Field
The test specimens are stored on the site at a place free from vibration, under damp matting, sacks or other similar material
for 24 hours ± 1/2 hour from the time of addition of water to the other ingredients. The temperature of the place of storage
should be within the range of 22° to 32°C. After the period of 24 hours, they should be marked for later identification
removed from the moulds and unless required for testing within 24 hours, stored in clean water at a temperature of 24° to
30°C until they are transported to the testing laboratory. They should be sent to the testing laboratory well packed in damp
sand, damp sacks, or other suitable material so as to arrive there in a damp condition not less than 24 hours before the time
of test. On arrival at the testing laboratory, the specimens are stored in water at a temperature of 27° ± 2°C until the time of
test. Records of the daily maximum and minimum temperature should be kept both during the period the specimens remain
on the site and in the laboratory particularly in cold weather regions.

NON-DESTRUCTIVE TESTING (NDT) OF
CONCRETE
Non-destructive testing (NDT) relates to the examination of materials for flaws without harming the object being tested.
As an industrial test method, NDT provides a cost effective means of testing while protecting the objects usability for its
design purpose. In the non-destructive testing methods of testing, the specimens are not loaded to failure and as such the
strength inferred or estimated cannot be expected to yield absolute values of strength. These methods, therefore, attempt
to measure some other properties of concrete from which an estimate of its strength, durability and elastic parameters are
obtained. Some such properties of concrete are hardness, resistance to penetration of projectiles, rebound number,
resonant frequency and ability to allow ultrasonic pulse velocity to propagate through it. The electrical properties of
concrete, its ability to absorb, scatter and transmit X-rays and gamma rays, its response to nuclear activation and its
acoustic emission allow us to estimate its moisture content, density, thickness and its cement content.

METHODS OF NDT
Surface hardness test
Rebound test
Penetration and pull-out techniques
Dynamic or vibration tests

Combined methods
Radioactive and nuclear methods
Magnetic and electrical methods
Acoustic emission techniques

REBOUND HAMMER TEST
The Schmidt Rebound Hammer is practically a surface hardness tester. It works on the principle that the rebound of an
elastic mass depends on the hardness of the surface against which the mass impinges.
Schmidt Rebound Hammer

The hammer weights about 1.8 kg and is suitable for use both in the laboratory and in the field. The main components
include the outer body (tubular housing), the plunger, the hammer mass and the main spring. Other features include a
latching mechanism that unlocks the hammer mass to the plunger rod and a sliding rider to measure the rebound of the
hammer mass. The rebound distance is measured on an arbitrary scale marked from 10 to 100. The rebound distance is
recorded as a “rebound number” corresponding to the position of the rider on the scale.
Schematic cross section of rebound hammer showing operating
principle

The plunger is first held perpendicular to the concrete surface (a).
The body is then pushed towards the concrete (b).
This movement extends the spring holding the mass to the body. When the maximum extension of the spring is reached,
the latch releases and the mass is pulled towards the surface by the spring (c).
The mass hits the shoulder of the plunger rod and rebounds because the rod is pushed hard against the concrete (d).
During rebound the slide indicator travels with the hammer mass and stops at the maximum distance the mass reaches
after rebounding. A button on the side of the body is pushed to lock the plunger into the retracted position and the
rebound number is read from a scale on the body.

DETERMINATION OF REBOUND INDEX OF CONCRETE BY
REBOUND HAMMER TEST AS PER I.S. 13311
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 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.

DETERMINATION OF COMPRESSIVE STRENGTH OF CONCRETE BY
REBOUND HAMMER TEST AS PER I.S. 13311
Investigations have shown that there is a general correlation between compressive strength of concrete and rebound
number; however, there is a wide degree of disagreement among various research workers regarding the accuracy of
estimation of strength from rebound readings. The variation of strength of a properly calibrated hammer may lie between
±15% and ±20%. The figure below shows the relationship between compressive strength of concrete cylinders and rebound
numbers.

DETERMINATION OF QUALITY OF CONCRETE
BY ULTRASONIC PULSE VELOCITY TEST
Ultrasonic pulse velocity method consists of measuring the time of travel of an ultrasonic pulse, passing through the
concrete to be tested. The pulse generator circuit consists of electronic circuit for generating pulses and a transducer for
transforming these electronic pulses into mechanical energy having vibration frequencies in the range of 15 to 50 kHz. The
time of travel between initial onset and the reception of the pulse is measured electronically. The path length between
transducer divided by the time of travel gives the average velocity of wave propagation.
Velocity criterion for concrete quality grading (IS: 13311-Part I)

CHAPTER 5
QUALITY CONTROL OF
CONCRETE

BATCHING
The measurement of materials for making concrete is known as batching.
There are two methods of batching:
Volume batching
Weigh batching

VOLUME BATCHING
Volume batching is not a good method for proportioning the material because of the difficulty it offers to measure granular
material in terms of volume. Volume of moist sand in a loose condition weighs much less than the same volume of dry
compacted sand. The amount of solid granular material in a cubic metre is an indefinite quantity. Because of this, for quality
concrete material have to be measured by weight only. However, for unimportant concrete or for any small job, concrete may be
batched by volume.
Cement is always measured by weight. It is never measured in volume. Generally, for each batch mix, one bag of cement is used.
The volume of one bag of cement is taken as thirty five (35) litres. Gauge boxes are used for measuring the fine and coarse
aggregates. The volume of the box is made equal to the volume of one bag of cement i.e., 35 litres or multiple thereof. The gauge
boxes are made comparatively deeper with narrow surface rather than shallow with wider surface to facilitate easy estimation of
top level. Sometimes bottomless gauge-boxes are used. This should be avoided. Correction to the effect of bulking should be
made to cater for bulking of fine aggregate, when the fine aggregate is moist and volume batching is adopted.
Gauge boxes are generally called farmas. They can be made of timber or steel plates. Often in India volume batching is adopted
even for large concreting operations. In a major site it is recommended to have the following gauge boxes at site to cater for
change in Mix Design or bulking of sand. The volume of each gauge box is clearly marked with paint on the external surface.
Water is measured either in kg. or litres as may be convenient. In this case, the two units are same, as the density of water is one
kg. per litre. The quantity of water required is a product of water/cement ratio and the weight of cement; for a example, if the
water/cement ratio of 0.5 is specified, the quantity of mixing water required per bag of cement is 0.5 x 50.00 = 25 kg. or 25 litres.
The quantity is, of course, inclusive of any surface moisture present in the aggregate.

WEIGH BATCHING
Strictly speaking, weigh batching is the correct method of measuring the materials. For important concrete, invariably, weigh
batching system should be adopted. Use of weight system in batching, facilitates accuracy, flexibility and simplicity. Different
types of weigh batchers are available, The particular type to be used, depends upon the nature of the job. Large weigh batching
plants have automatic weighing equipment. The use of this automatic equipment for batching is one of sophistication and
requires qualified and experienced engineers. In this, further complication will come to adjust water content to cater for the
moisture content in the aggregate. In smaller works, the weighing arrangement consists of two weighing buckets, each connected
through a system of levers to spring-loaded dials which indicate the load. The weighing buckets are mounted on a central spindle
about which they rotate. Thus one can be loaded while the other is being discharged into the mixer skip. A simple spring balance
or the common platform weighing machines also can be used for small jobs.
Aggregate weighing machines require regular attention if they are to maintain their accuracy. Check calibrations should always be
made by adding weights in the hopper equal to the full weight of the aggregate in the batch. The error found is adjusted from
time to time.

VOLUME BATCHING FOR NOMINAL MIXES
Batch volume of materials for various mixes

DIFFERENT TYPES OF MIXERS
MIXERS
BATCH
MIXERS
PAN MIXERS
DRUM
MIXERS
TILTING
MIXERS
NON-TILTING
MIXERS
REVERSING
MIXERS
CONTINUOUS
MIXERS

TILTING AND NON-TILTING MIXERS
The shape of the drum, the angle and size of blades, the angle at which the drum is held, affect the efficiency of mixer. It is
seen that tilting drum to some extent is more efficient than non-tilting drum. In non-tilting drum for discharging concrete,
a chute is introduced into the drum by operating a lever. The concrete which is being mixed in the drum, falls into the
inclined chute and gets discharged out. It is seen that a little more of segregation takes place, when a non-tilting mixer is
used. It is observed in practice that, generally, in any type of mixer, even after thorough mixing in the drum, while it is
discharged, more of coarse aggregate comes out first and at the end matrix gets discharged. It is necessary that a little bit
of re-mixing is essential, after discharged from mixer, on the platform to off-set the effect of segregation caused while
concrete is discharged from the mixer.
As per I.S. 1791–1985, concrete mixers are designated by a number representing its nominal mixed batch capacity in litres.
The following are the standardized sizes of three types:
a) Tilting: 85 T, 100 T, 140 T, 200 T
b) Non-Tilting: 200 NT, 280 NT, 375 NT, 500 NT, 1000 NT
c) Reversing: 200 R, 280 R, 375 R, 500 R and 1000 R
The letters T, NT, R denote tilting, non-tilting and reversing respectively.

Diagrammatic illustration of Non-tilting and Tilting type Concrete Mixers

DIFFERENT TYPES OF VIBRATORS
Needle vibrator (Internal vibrator)
Formwork vibrator (External vibrator)
Table vibrator
Platform vibrator
Surface vibrator (Screed vibrator)
Vibratory Roller

NEEDLE VIBRATOR
Of all the vibrators, the internal vibrator is most commonly used. This is also called, “Needle Vibrator”, “Immersion
Vibrator”, or “Poker Vibrator”. This essentially consists of a power unit, a flexible shaft and a needle. The power unit may
be electrically driven or operated by petrol engine or air compressor. The vibrations are caused by eccentric weights
attached to the shaft or the motor or to the rotor of a vibrating element. Electromagnet, pulsating equipment is also
available. The frequency of vibration varies upto 12,000 cycles of vibration per minute. The needle diameter varies from
20 mm to 75 mm and its length varies from 25 cm to 90 cm. The bigger needle is used in the construction of mass
concrete dam. Sometimes, arrangements are available such that the needle can be replaced by a blade of approximately
the same length. This blade facilitates vibration of members, where, due to the congested reinforcement, the needle
would not go in, but this blade can effectively vibrate. They are portable and can be shifted from place to place very easily
during concreting operation. They can also be used in difficult positions and situations.

SURFACE VIBRATOR
Surface vibrators are sometimes knows as, “Screed Board Vibrators”. A small vibrator placed on the screed board gives an
effective method of compacting and levelling of thin concrete members, such as floor slabs, roof slabs and road surface.
Mostly, floor slabs and roof slabs are so thin that internal vibrator or any other type of vibrator cannot be easily employed.
In such cases, the surface vibrator can be effectively used. In general, surface vibrators are not effective beyond about 15
cm. In the modern construction practices like vacuum dewatering technique, or slip-form paving technique, the use of
screed board vibrator are common feature. In the above situations double beam screed board vibrators are often used.

TABLE VIBRATOR
This is the special case of formwork vibrator, where the vibrator is clamped to the table. or table is mounted on springs
which are vibrated transferring the vibration to the table. They are commonly used for vibrating concrete cubes. Any article
kept on the table gets vibrated. This is adopted mostly in the laboratories and in making small but precise prefabricated
R.C.C. members.

FORM WORK
The form work or shuttering is a temporary ancillary construction used as a mould for the structure, in which concrete is
placed and in which it hardens and matures.
FORM WORK FOR COLUMNS
Form work for a column is probably the simplest. It consists of the following main components:
(i) Sheeting all around the column periphery,
(ii) Side yokes and end yokes,
(iii) Wedges, and
(iv) Bolts with washers
The side yokes and end yokes consist of two numbers each, and are suitably spaced along the height of the column. The two
side yokes are comparatively of heavier section, and are connected together by two long bolts of 16mm diameter. Four
wedges, one at each corner, are inserted between the bolts and the end yokes. The sheathing is nailed to the yokes.

Form work for square or rectangular column Form work for octagonal or round column

FORM WORK FOR BEAM AND SLAB FLOOR
The figure shows the form work for beam and slab floor.

The slab is continuous over a number of beams. The slab is supported on 2.5cm thick sheathing laid parallel to the main
beams. The sheathing is supported on wooden battens which are laid between the beams, at some suitable spacing. In
order to reduce the deflection, the battens may be propped at middle of the span through joists. The side forms of the
beam consist of 3cm thick sheathing. The bottom sheathing of the beam form may be 5 to 7cm thick. The ends of the
battens are supported on the ledger which is fixed to the cleats throughout the length. Cleats 10cm x 2cm to 3cm are fixed
to the side forms at the same spacing as that of battens, so that battens may be fixed to them. The beam form is supported
on a head tree. The shore or post is connected to head tree through cleats. At the bottom of share, two wedges of hard
wood are provided over a sole piece.

FORM WORK FOR WALLS
The figure below shows fixed form for walls. The boarding may be 4 to 5cm thick for walls upto 3 to 4m high. The boards
are fixed to 5cm x 10cm posts, known as studs or soldiers, spaced at about 0.8m apart. Horizontal walings of size 7.5cm x
10cm are fixed to the posts at suitable interval. The whole assembly is then strutted as shown, using 7.5cm x 10cm struts.
The two shutters are kept apart equal to the thickness of the wall, by providing a 5cm high concrete kicker at the bottom
and by 2.5cm x 5cm spacers nailed to the posts.

The figure below shows moving form for wall. In these the forms are made up in panel size of 0.6m x 1.8m so that handling
and stripping is easier. A 15mm plywood is commonly used instead of boarding. The panels are erected in such a way that
the lower panels can be removed when concrete is hard and used higher up the wall. Framing of size 5cm x 10cm is used to
ply shutter. The panels are fixed to a central and two end studs. Each stud consists of two pieces of timber, 5cm x 15cm,
blocked apart. The end strut of each panel secures adjacent panel.

MATERIALS USED FOR FORM WORK
Timber
Plywood
Steel
Reinforced Concrete
Plain Concrete

REQUIRMENTS OF GOOD FORM WORK
The material of the form work should be cheap and it should be
suitable for reuse several times.
It should be practically water proof so that it does not absorb water
from concrete. Also, its shrinkage and swelling should be minimum.
It should be strong enough to withstand all loads coming on it, such as
dead load of concrete and live load during its pouring, compaction and
curing.
It should be stiff enough so that deflection is minimum.

It should be as light as possible.
The surface of the form work should be smooth, and it
should afford easy stripping.
All joints of the form work should be stiff so that lateral
deformation under loads is minimum. Also, these joints
should be leak proof.
The form work should rest on non-yielding supports.

STRIPPING TIME FOR THE REMOVAL OF FORMWORK
AS PER I.S. 456- 2000 PROVISIONS
FOR DIFFERENT STRUCTURAL MEMBERS
Formwork should not be removed until the concrete has developed a strength of at least twice the stress to which concrete
may be subjected at the time of removal of formwork. In special circumstances the strength development of concrete can be
assessed by placing companion cubes near the structure and curing the same in the manner simulating curing conditions of
structures. In normal circumstances, where ambient temperature does not fall below 15°C and where ordinary Portland
cement is used and adequate curing is done, following striking period can be considered sufficient as per IS 456 of 2000.
Stripping Time of Formwork
For other cements and lower temperature, the stripping time recommended above may be suitably modified.

TRANSPORTATION OF CONCRETE
Concrete can be transported by a variety of methods and equipments. The precaution to be taken while
transporting concrete is that the homogeneity obtained at the time of mixing should be maintained while being
transported to the final place of deposition.
The methods adopted for transportation of concrete are:
Mortar Pan
Wheel Barrow, Hand Cart
Crane, Bucket and Rope way
Truck Mixer and Dumpers
Belt Conveyors

Chute
Skip and Hoist
Transit Mixer
Pump and Pipe Line
Helicoptor

2. WHEEL BARROW
Use of mortar pan for transportation of concrete is one of the common methods adopted in this country. It is labour
intensive. In this case, concrete is carried in small quantities. While this method nullifies the segregation to some extent,
particularly in thick members, it suffers from the disadvantage that this method exposes greater surface area of concrete
for drying conditions. This results in greater loss of water, particularly, in hot weather concreting and under conditions of
low humidity. It is to be noted that the mortar pans must be wetted to start with and it must be kept clean during the entire
operation of concreting. Mortar pan method of conveyance of concrete can be adopted for concreting at the ground level,
below or above the ground level without much difficulties.
1. MORTAR PAN
Wheel barrows are normally used for transporting concrete to be placed at ground level. This method is employed for
hauling concrete for comparatively longer distance as in the case of concrete road construction. If concrete is conveyed by
wheel barrow over a long distance, on rough ground, it is likely that the concrete gets segregated due to vibration. The
coarse aggregates settle down to the bottom and matrix moves to the top surface. To avoid this situation, sometimes,
wheel barrows are provided with pneumatic wheel to reduce vibration. A wooden plank road is also provided to reduce
vibration and hence segregation.

3. CRANE, BUCKET AND ROPE WAY
A crane and bucket is one of the right equipment for transporting concrete above ground level. Crane can handle concrete in
high rise construction projects and are becoming a familiar sites in big cities. Cranes are fast and versatile to move concrete
horizontally as well as vertically along the boom and allows the placement of concrete at the exact point. Cranes carry skips
or buckets containing concrete. Skips have discharge door at the bottom, whereas buckets are tilted for emptying. For a
medium scale job the bucket capacity may be 0.5 m3
.
4. TRUCK MIXER AND DUMPERS
For large concrete works particularly for concrete to be placed at ground level, trucks and dumpers or ordinary open steel-
body tipping lorries can be used. As they can travel to any part of the work, they have much advantage over the jubilee
wagons, which require rail tracks. Dumpers are of usually 2 to 3 cubic metre capacity, whereas the capacity of truck may be 4
cubic metre or more. Before loading with the concrete, the inside of the body should be just wetted with water. Tarpaulins
or other covers may be provided to cover the wet concrete during transit to prevent evaporation. When the haul is long, it is
advisable to use agitators which prevent segregation and stiffening. The agitators help the mixing process at a slow speed.

5. BELT CONVEYORS
Belt conveyors have very limited applications in concrete construction. The principal objection is the tendency of the
concrete to segregate on steep inclines, at transfer points or change of direction, and at the points where the belt passes
over the rollers. Another disadvantage is that the concrete is exposed over long stretches which causes drying and stiffening
particularly, in hot, dry and windy weather. Segregation also takes place due to the vibration of rubber belt. It is necessary
that the concrete should be remixed at the end of delivery before placing on the final position.
6. CHUTE
Chutes are generally provided for transporting concrete from ground level to a lower level. The sections of chute should be
made of or lined with metal and all runs shall have approximately the same slope, not flatter than 1 vertical to 2.5
horizontal. The lay-out is made in such a way that the concrete will slide evenly in a compact mass without any separation
or segregation. The required consistency of the concrete should not be changed in order to facilitate chuting. If it becomes
necessary to change the consistency the concrete mix will be completely redesigned.

7. SKIP AND HOIST
This is one of the widely adopted methods for transporting concrete vertically up for multistoreyed building construction.
Employing mortar pan with the staging and human ladder for transporting concrete is not normally possible for more than 3 or 4
storeyed building constructions. For laying concrete in taller structures, chain hoist or platform hoist or skip hoist is adopted. At
the ground level, mixer directly feeds the skip and the skip travels up over rails upto the level where concrete is required. At that
point, the skip discharges the concrete automatically or on manual operation. The quality of concrete i.e. the freedom from
segregation will depend upon the extent of travel and rolling over the rails. If the concrete has travelled a considerable height, it
is necessary that concrete on discharge is required to be turned over before being placed finally.
8. TRANSIT MIXER
Transit mixer is one of the most popular equipments for transporting concrete over a long distance particularly in Ready Mixed
Concrete plant (RMC). In India, today (2000 AD) there are about 35 RMC plants and a number of central batching plants are
working. It is a fair estimate that there are over 600 transit mixers in operation in India. They are truck mounted having a capacity
of 4 to 7 m3. There are two variations. In one, mixed concrete is transported to the site by keeping it agitated all along at a speed
varying between 2 to 6 revolutions per minute. In the other category, the concrete is batched at the central batching plant and
mixing is done in the truck mixer either in transit or immediately prior to discharging the concrete at site. Transit-mixing permits
longer haul and is less vulnerable in case of delay. The truck mixer the speed of rotating of drum is between 4–16 revolution per
minute. A limit of 300 revolutions for both agitating and mixing is laid down by ASTM C 94 or alternatively, the concretes must be
placed within 1.5 of mixing. In case of transit mixing, water need not be added till such time the mixing is commenced. BS 5328 –
1991, restrict the time of 2 hours during which, cement and moist sand are allowed to remain in contact. But the above
restrictions are to be on the safe side. Exceeding these limit is not going to be harmful if the mix remains sufficiently workable for
full compaction.

9. PUMP AND PUMPLINE
Pumping of concrete is universally accepted as one of the main methods of concrete transportation and placing. Adoption
of pumping is increasing throughout the world as pumps become more reliable and also the concrete mixes that enable the
concrete to be pumped are also better understood. The modern concrete pump is a sophisticated, reliable and robust
machine. In the past a simple two-stroke mechanical pump consisted of a receiving hopper, an inlet and an outlet valve, a
piston and a cylinder. The pump was powered by a diesel engine. The pumping action starts with the suction stroke drawing
concrete into the cylinder as the piston moves backwards. During this operation the outlet value is closed. On the forward
stroke, the inlet valve closes and the outlet valve opens to allow concrete to be pushed into the delivery pipe. The modern
concrete pump still operates on the same principles but with lot of improvements and refinements in the whole operations.

PLACING OF CONCRETE
It is not enough that a concrete mix correctly designed, batched, mixed and transported, it is of utmost importance that
the concrete must be placed in systematic manner to yield optimum results. The precautions to be taken and methods
adopted while placing concrete in the under-mentioned situations, will be discussed.
a) Placing concrete within earth mould. (example: Foundation concrete for a wall or column).
b) Placing concrete within large earth mould or timber plank formwork. (example: Road slab and Airfield slab).
c) Placing concrete in layers within timber or steel shutters. (example: Mass concrete in dam construction or construction
of concrete abutment or pier).
d) Placing concrete within usual from work. (example: Columns, beams and floors).
e) Placing concrete under water.
Concrete is invariably laid as foundation bed below the walls or columns. Before placing the concrete in the foundation, all
the loose earth must be removed from the bed. Any root of trees passing through the foundation must be cut, charred or
tarred effectively to prevent its further growth and piercing the concrete at a later date. The surface of the earth, if dry,
must be just made damp so that the earth does not absorb water from concrete. On the other hand if the foundation bed
is too wet and rain-soaked, the water and slush must be removed completely to expose firm bed before placing concrete.
If there is any seepage of water taking place into the foundation trench, effective method for diverting the flow of water
must be adopted before concrete is placed in the trench or pit.

For the construction of road slabs, airfield slabs and ground floor slabs in buildings, concrete is placed in bays. The ground
surface on which the concrete is placed must be free from loose earth, pool of water and other organic matters like grass,
roots, leaves etc. The earth must be properly compacted and made sufficiently damp to prevent the absorption of water
from concrete. If this is not done, the bottom portion of concrete is likely to become weak. Sometimes, to prevent
absorption of moisture from concrete, by the large surface of earth, in case of thin road slabs, use of polyethylene film is
used in between concrete and ground. Concrete is laid in alternative bays giving enough scope for the concrete to
undergo sufficient shrinkage. Provisions for contraction joints and dummy joints are given. It must be remembered that
the concrete must be dumped and not poured. It is also to be ensured that concrete must be placed in just required
thickness. The practice of placing concrete in a heap at one place and then dragging it should be avoided.
When concrete is laid in great thickness, as in the case of concrete raft for a high rise building or in the construction of
concrete pier or abutment or in the construction of mass concrete dam, concrete is placed in layers. The thickness of
layers depends upon the mode of compaction. In reinforced concrete, it is a good practice to place concrete in layers of
about 15 to 30 cm thick and in mass concrete, the thickness of layer may vary anything between 35 to 45 cm. Several such
layers may be placed in succession to form one lift, provided they follow one another quickly enough to avoid cold joints.
The thickness of layer is limited by the method of compaction and size and frequency of vibrator used.

Before placing the concrete, the surface of the previous lift is cleaned thoroughly with water jet and scrubbing by wire
brush. In case of dam, even sand blasting is also adopted. The old surface is sometimes hacked and made rough by
removing all the laitance and loose material. The surface is wetted. Sometimes, a neat cement slurry or a very thin layer of
rich mortar with fine sand is dashed against the old surface, and then the fresh concrete is placed. The whole operation
must be progressed and arranged in such a way that, cold joints are avoided as far as possible. When concrete is laid in
layers, it is better to leave the top of the layer rough, so that the succeeding layer can have a good bond with the previous
layer. Where the concrete is subjected to horizontal thrust, bond bars, bond rails or bond stones are provided to obtain a
good bond between the successive layers. Of course, such arrangements are required for placing mass concrete in layers,
but not for reinforced concrete.
Certain good rules should be observed while placing concrete within the formwork, as in the case of beams and columns.
Firstly, it must be checked that the reinforcement is correctly tied, placed and is having appropriate cover. The joints
between planks, plywoods or sheets must be properly and effectively plugged so that matrix will not escape when the
concrete is vibrated. The inside of the formwork should be applied with mould releasing agents for easy stripping. Such
purpose made mould releasing agents are separately available for steel or timber shuttering. The reinforcement should be
clean and free from oil. Where reinforcement is placed in a congested manner, the concrete must be placed very carefully,
in small quantity at a time so that it does not block the entry of subsequent concrete. The above situation often takes place
in heavily reinforced concrete columns with close lateral ties, at the junction of column and beam and in deep beams.
Generally, difficulties are experienced for placing concrete in the column. Often concrete is required to be poured from a
greater height. When the concrete is poured from a height, against reinforcement and lateral ties, it is likely to segregate or
block the space to prevent further entry of concrete. To avoid this, concrete is directed by tremie, drop chute or by any
other means to direct the concrete within the reinforcement and ties. Sometimes, when the formwork is too narrow, or
reinforcement is too congested to allow the use of tremie or drop chute, a small opening in one of the sides is made and
the concrete is introduced from this opening instead of pouring from the top. It is advisable that care must be taken at the

stage of detailing of reinforcement for the difficulty in pouring concrete. In long span bridges the depth of prestressed concrete
girders may be of the order of even 4 – 5 meters involving congested reinforcement. In such situations planning for placing
concrete in one operation requires serious considerations on the part of designer.
Form work: Form work shall be designed and constructed so as to remain sufficiently rigid during placing and compaction of
concrete. The joints are plugged to prevent the loss of slurry from concrete.
Stripping Time: Formwork should not be removed until the concrete has developed a strength of at least twice the stress to
which concrete may be subjected at the time of removal of formwork. In special circumstances the strength development of
concrete can be assessed by placing companion cubes near the structure and curing the same in the manner simulating curing
conditions of structures. In normal circumstances, where ambient temperature does not fall below 15°C and where ordinary
Portland cement is used and adequate curing is done, following striking period can be considered sufficient as per IS 456 of 2000.
Stripping Time of Formwork
For other cements and lower temperature, the stripping time recommended above may be suitably modified.

Underwater Concreting
Concrete is often required to be placed underwater or in a trench filled with the bentonite slurry. In such cases, use of
bottom dump bucket or tremie pipe is made use of. In the bottom dump bucket concrete is taken through the water in a
water-tight box or bucket and on reaching the final place of deposition the bottom is made to open by some mechanism
and the whole concrete is dumped slowly. This method will not give a satisfactory result as certain amount of washing away
of cement is bound to occur. In some situations, dry or semi-dry mixture of cement, fine and coarse aggregate are filled in
cement bags and such bagged concrete is deposited on the bed below the water. This method also does not give
satisfactory concrete, as the concrete mass will be full of voids interspersed with the putricible gunny bags. The satisfactory
method of placing concrete under water is by the use of tremie pipe.
The word “tremie” is derived from the french word hopper.
A tremie pipe is a pipe having a diameter of about 20 cm capable of easy coupling for increase or decrease of length. A
funnel is fitted to the top end to facilitate pouring of concrete. The bottom end is closed with a plug or thick polyethylene
sheet or such other material and taken below the water and made to rest at the point where the concrete is going to be
placed. Since the end is blocked, no water will have entered the pipe. The concrete having a very high slump of about 15 to
20 cm is poured into the funnel. When the whole length of pipe is filled up with the concrete, the tremie pipe is lifted up
and a slight jerk is given by a winch and pully arrangement. When the pipe is raised and given a jerk, due to the weight of
concrete, the bottom plug falls and the concrete gets discharged. Particular care must be taken at this stage to see that the
end of the tremie pipe remains inside the concrete, so that no water enters into the pipe from the bottom. In other words,
the tremie pipe remains plugged at the lower end by concrete. Again concrete is poured over the funnel and when the

whole length of the tremie pipe is filled with concrete, the pipe is again slightly lifted and given slight jerk. Care is taken all
the time to keep the lower end of the tremie pipe well embedded in the wet concrete. The concrete in the tremie pipe gets
discharged. In this way, concrete work is progressed without topping till the concrete level comes above the water level.
Under Water Concreting by Tremie Method

This method if executed properly, has the advantage that the concrete does not get affected by water except the top layer.
The top layer is scrubbed or cut off to remove the affected concrete at the end of the whole operation.
During the course of concreting, no pumping of water should be permitted. If simultaneous pumping is done, it may suck
the cement particles. Under water concreting need not be compacted, as concrete gets automatically compacted by the
hydrostatic pressure of water. Secondly, the concrete is of such consistency that it does not normally require compaction.
One of the disadvantages of under water concreting in this method is that a high water/cement ratio is required for high
consistency which reduces the strength of concrete. But at present, with the use of superplasticizer, it is not a constraint. A
concrete with as low
a w/c ratio as 0.3 or even less can be placed by tremie method.

COMPACTION OF CONCRETE
Compaction of concrete is the process adopted for expelling the entrapped air from the concrete. In the process of mixing,
transporting and placing of concrete air is likely to get entrapped in the concrete. The lower the workability, higher is the
amount of air entrapped. In other words, stiff concrete mix has high percentage of entrapped air and, therefore , would
need higher compacting efforts than high workable mixes. If this air is not removed fully, the concrete loses strength
considerably. Insufficient compaction increases the permeability of concrete resulting in easy entry for aggressive
chemicals in solution, which attack concrete and reinforcement to reduce the durability of concrete. Therefore, 100 per
cent compaction of concrete is of paramount importance. In order to achieve full compaction and maximum density, with
reasonable compacting efforts available at site, it is necessary to use a mix with adequate workability. It is also of common
knowledge that the mix should not be too wet for easy compaction which also reduces the strength of concrete. For
maximum strength, driest possible concrete should be compacted 100 per cent. The overall economy demands 100 per
cent compaction with a reasonable compacting efforts available in the field.

METHOS OF
COMPACTIO
N OF
CONCRETE
Hand
Compaction
Rodding Ramming Tamping
Compaction
by Vibration
Needle
Vibrator
(Internal
Vibrator)
Formwork
Vibrator
(External
Vibrator)
Table
Vibrator
Platform
Vibrator
Surface
Vibrator
(Screed
Vibrator)
Vibratory
Roller
Compaction
by Pressure
and Jolting
Compaction
by Spinning

FINISHING OF CONCRETE
Finishing operation is the last operation in making concrete. Finishing in real sense does not apply to all concrete operations.
For a beam concreting, finishing may not be applicable, whereas for the concrete road pavement, airfield pavement or for
the flooring of a domestic building, careful finishing is of great importance. Concrete is often dubbed as a drab material,
incapable of offering pleasant architectural appearance and finish. This shortcoming of concrete is being rectified and
concretes these days are made to exhibit pleasant surface finishes. Particularly, many types of prefabricated concrete panels
used as floor slab or wall unit are made in such a way as to give very attractive architectural affect. Even concrete claddings
are made to give attractive look.
TYPES OF FINISHING
Formwork Finishes
Surface Treatment
Applied Finishes

REQUIREMENT OF A GOOD FINISH
A good concrete floor should have a surface which is durable, non-absorptive, suitable texture, free from
cracks, crazing and other defects. In other words, the floor should satisfactorily withstand wear from traffic. It
should be sufficiently impervious to passage of water, oils or other liquids. It should possess a texture in
keeping with the required appearance, should be easy to clean and be safe against slipping. It should
structurally be sound and must act in unison with sub-floor.

CURING OF CONCRETE
Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. As
hydration is an exothermic process, so a large amount of heat is evolved during cement hydration. Due to this heat
evolution, the water in the concrete mix evaporates and stops the hydration process. This leads to shrinkage and cracking
of concrete. The curing period may depend on the properties required of the concrete, the process for which it is to be
used and the ambient conditions i.e. the temperature and the relative humidity of the surrounding atmosphere. It is
observed that about 90 % hydration is complete within 28 days and the rest 10 % hydration takes years to complete.
Hence, a curing period of 28 days is basically used to obtain a high strength concrete.

NECESSITY OF CURING
The main objectives of curing are:
To prevent the loss of water by evaporation and
to maintain the process of hydration.
To reduce the shrinkage of concrete.
To preserve the properties of concrete.

METHODS OF CURING
Shading of concrete works
Covering concrete surfaces by wet gunny bags
Sprinkling method
Ponding method

Membrane curing
Steam curing
Curing by infra-red radiation
Electrical curing
Use of Calcium chloride for curing

1. SHADING OF CONCRETE WORKS
The object of this method is to protect the newly laid fresh concrete from direct sun rays and dry hot wind to avoid rapid
drying and non-uniform temperature development in the mass of concrete. This objective can be achieved by using canvas
by stretching it over frames.
Shading of concreting

2. COVERING CONCRETE SURFACES BY WET
GUNNY BAGS
This method is widely used for structural concrete. In this method the concrete surface is covered with empty cement bags
and water sprinkled at short intervals to keep them moist.
Curing by wet gunny bags

3. SPRINKLING METHOD
This is an excellent method of curing, providing sprinkling of water could be done continuously. This method needs much
more water for curing than any other method. This method is more suitable for floors.
Curing by sprinkling water

4. PONDING METHOD
This method is useful for horizontal members only such as pavements, floors, slabs etc. In this case small dikes are formed of
earth and water is filled in between the dikes. Water may be filled 2 or 3 times a day depending on the climatic condition of
the region. This method has been found more effective in neutralizing the heat of hydration than wet covering. Ponding
method has been found to give more strength than wet covering method of all ages.
Curing by ponding method

5. MEMBRANE CURING
This method is employed at places where water is scarce. Chemical compounds or polythene sheets or water proof papers
etc. are used for curing of concrete. The polythene sheet is spread over the surface as soon as possible without damage to
the concrete surface and the lap joints between adjacent sheets lightly sealed.
Curing by wet polythene sheets

6. STEAM CURING
The development of strength of concrete is a function of not only time but also that of temperature. When concrete is
subjected to higher temperature it accelerates the hydration process resulting in faster development of strength. Concrete
cannot be subjected to dry heat to accelerate the hydration process as the presence of moisture is also an essential
requisite. Therefore, subjecting the concrete to higher temperature and maintaining the required wetness can be achieved
by subjecting the concrete to steam curing.
Steam curing at ordinary pressure:
This method of curing is often adopted for prefabricated concrete elements. Application of steam curing to in situ
construction will be a little difficult task. However, at some places it has been tried for in situ construction by forming a
steam jacket with the help of tarpaulin or thick polyethylene sheets. But this method of application of steam for in situ work
is found to be wasteful and the intended rate of development of strength and benefit is not really achieved.
Steam curing at ordinary pressure is applied mostly on prefabricated elements stored in a chamber. The chamber should be
big enough to hold a day’s production. The door is closed and steam is applied. The steam may be applied either
continuously or intermittently. An accelerated hydration takes place at this higher temperature and the concrete products
attain the 28 days strength of normal concrete in about 3 days. In large prefabricated factories they have tunnel curing
arrangements. The tunnel of sufficient length and size is maintained at different temperature starting from a low
temperature in the beginning of the tunnel to a maximum temperature of about 90°C at the end of the tunnel. The
concrete products mounted on trollies move in a very slow speed subjecting the concrete products progressively to higher
and higher temperature. Alternatively, the trollies are kept stationarily at different zones for some period and finally come
out of tunnel.

High Pressure Steam Curing:
In the steam curing at atmospheric pressure, the temperature of the steam is naturally below 100°C. The steam will get
converted into water, thus it can be called in a way, as hot water curing. This is done in an open atmosphere.
The high pressure steam curing is something different from ordinary steam curing, in that the curing is carried out in a
closed chamber. The superheated steam at high pressure and high temperature is applied on the concrete. This process is
also called “Autoclaving”. The autoclaving process is practised in curing precast concrete products in the factory, particularly,
for the lightweight concrete products. In India, this high pressure steam curing is practised in the manufacture of cellular
concrete products, such as Siporex, Celcrete etc.
7. CURING BY INFRA-RED RADIATION
Curing of concrete by Infra-red Radiation has been practised in very cold climatic regions in Russia. It is claimed that much
more rapid gain of strength can be obtained than with steam curing and that rapid initial temperature does not cause a
decrease in the ultimate strength as in the case of steam curing at ordinary pressure. The system is very often adopted for
the curing of hollow concrete products. The normal operative temperature is kept at about 90°C.

8. ELECTRICAL CURING
Another method of curing concrete, which is applicable mostly to very cold climatic regions is the use of electricity. This
method is not likely to find much application in ordinary climate owing to economic reasons. Concrete can be cured
electrically by passing an alternating current (Electrolysis trouble will be encountered if direct current is used) through the
concrete itself between two electrodes either buried in or applied to the surface of the concrete. Care must be taken to
prevent the moisture from going out leaving the concrete completely dry.
9. USE OF CALCIUM CHLORIDE FOR CURING
Calcium chloride is used either as a surface coating or as an admixture. It has been used satisfactorily as a curing medium.
Both these methods are based on the fact that calcium chloride being a salt, shows affinity for moisture. The salt, not only
absorbs moisture from atmosphere but also retains it at the surface. This moisture held at the surface prevents the mixing
water from evaporation and thereby keeps the concrete wet for a long time to promote hydration.

CHAPTER 6
EXTREME WEATHER CONCRETING
AND
CHEMICAL ADMIXTURE IN
CONCRETE

COLD WEATHER CONCRETING
The concreting operations which are under taken at a temperature of below 5°C are called cold weather
concreting. The production of concrete in cold weather introduces many problems like delay in setting and
hardening process, damage to concrete in plastic stage when exposed to below freezing point due to
formation of ice crystals. Hence, it is essential to maintain the temperature of concrete above 5°C.

EFFECTS OF COLD WEATHER ON CONCRETE
The effects of cold weather on concrete are as follows:
1. Delay in setting and hardening
The rate of hydration of cement depends upon the ambient temperature. So if the temperature is low, the hydration
process will go slow and concrete takes a lower time to set and harden. Thus the rate of development of strength is very
slow. The delay in setting time makes the concrete prone to frost attack and the delay in hardening period causes delay in
the removal of formwork, resulting in the slow rate of progress of the work.
2. Freezing of concrete at early age
When the temperature of concrete falls below freezing point, the free water in the plastic concrete freezes. Freezing of
water not only slow down or prevent the hydration of cement but also leads to expansion in the concrete. This expansion in
the concrete causes disruption, resulting in the loss of strength and quality of concrete.

3. Freezing and thawing
In cold weather regions, when the concrete is subjected to alternate cycles of freezing and thawing, its durability is greatly
impaired. It has been found that even 1 cycle of freezing and thawing during the pre-hardening period may reduce the
compressive strength to 50 percent of what would be expected for normal temperature concrete.
4. Stresses due to temperature differential
In case of mass concreting in cold weather there will be a large temperature differential due to higher temperature inside
the mass, which may promote micro cracking and has a harmful effect on durability of concrete.

The fresh concrete should not be subjected to freezing condition till such time it attains a certain amount of strength. This
time interval is known as ‘pre-hardening period’. IS: 7861-1981 (Part II) recommends the pre-hardening periods as given in
the following table.
Pre-hardening periods for different grades of concrete

MEASURES TO BE TAKEN
TO AVOID FREEZING OF CONCRETE
Following measures suggested by IS: 7861-1981 (Part II) should be adopted:
1. Selection of suitable type of cement
Cements containing higher percentage of fineness, higher percentage of C3S, C3A and comparatively lower percentage of
C2S, hydrates fast and gives out more heat of hydration and early strength. Rapid hardening cements, extra rapid
hardening cements or high alumina cements are such cement that can be used.

2. Temperature control of ingredients
The temperature at the time of setting of concrete can be raised by heating the ingredients of the concrete mix. Hot water is
used for mixing the ingredients of concrete. It is to be kept in mind that the temperature of water used for mixing should
not exceed 65°C as flash setting of concrete takes place. The aggregates are heated by passing steam through pipes
embedded in aggregate storage bins.
3. Use of insulating formwork
The hydration of cement generates considerable amount of heat during the first three days of hydration. Such heat can be
gainfully conserved by having insulating formwork covers capable of maintaining concrete temperature above the desirable
limit.

4. Admixtures of anti-freezing materials
In cold weather concreting the use of accelerating admixtures has been widely used, which identically work as anti-freezer.
The most commonly used accelerator is calcium chloride. Many specifications restrict the use of CaCl2 upto 3% by weight of
cement, for fear of flash set and loss of strength. But, in Russia the use of CaCl2 are extensively employed for concreting at
sub-zero temperature. They have tried about 20% of CaCl2 by weight of mixing water in the construction of Gorky Hydro
Power projects. But the Russian practice is to use the combination of CaCl2 and NaCl to neutralize the effect of temperature
and also to give maximum benefits to the concrete at the plastic stage as well as hardened stage. For open air temperature
(between 0°C to 5°C) the recommended proportions of CaCl2 and NaCl are 3% and 3.5% respectively.
5. Use of air entraining agents
The air-entrained concrete has higher durability than that of ordinary concrete under freezing conditions. It has been found
that even a weak concrete with air-entrainment is more durable under freezing conditions than that of strong concrete
without air-entrainment.

6. Delayed removal of formwork
Because of the slower rate of gain of strength during the cold weather, the formwork is required to be kept in place for a
longer time than in usual concreting practice.
7. Use of light weight aggregate
In cold weather concreting light weight aggregate is advantageous as the light weight aggregate concrete has a lower
thermal conductivity than normal concrete and thus act as a self-insulator. Light weight aggregate concrete has a low
specific heat. Thus the heat of hydration of cement keeps this concrete more effectively against freezing than the normal
weight aggregate concrete.

8. Placing and curing of concrete
Before placing the concrete, all ice, snow, and frost should be completely removed from the surfaces of formwork. During
freezing conditions water curing is not applicable. Low pressure wet steam curing may be useful.
9. Electrical heating of concrete mass
Concrete mass can be heated using a.c. current. Electricity is conducted through reinforcing bars or mats. Sometimes special
electrodes are carefully positioned for uniform heat generation. But, electrical heating reduces the strength of concrete by
about 20% because of loss of water and temperature stresses.
10. Covering the concrete surfaces
During cold weather, all concrete surfaces shall be covered as soon as the concrete has been placed in order to prevent the
loss of heat and to help prevent freezing. The concrete surfaces may be covered by plastic sheets, tarpaulins, clean straw
blankets etc.

HOT WEATHER CONCRETING
Any operation of concreting done at atmospheric temperature above 40°C or where the temperature of the
concrete at the time of placement is expected to be beyond 40°C may be categorized as hot weather
concreting. IS: 7861-1975 (Part I) recommends that concrete is not suitable to be placed at a temperature
above 40°C without proper precautions.

EFFECTS OF HOT WEATHER ON CONCRETE
The effects of hot weather on concrete are as follows:
1. Rapid rate of hydration
The rate of hydration depends upon the temperature. Higher the temperature, higher the rate of hydration. At a higher
temperature, the setting time will be reduced resulting in early stiffening of the concrete and reduction in workability. It has
been reported that with 11°C rise in the temperature of concrete there is an approximately 25 mm decrease in the value of
slump. It has also been found that the gel structure so formed at higher temperature in the early period of hydration is of
poor quality. Concrete placed in hot weather no doubt will develop high early strength but the ultimate strength of the
concrete will decrease.
2. Rapid evaporation of mixing water
The hot weather condition is normally associated with lower relative humidity. Due to lower relative humidity, the water
mixed in the concrete for providing the required workability will be lost due to evaporation and the concrete paste
becomes unworkable. Such concrete cannot be compacted properly and will result in reduction in strength.

3. Greater plastic shrinkage
In hot weather conditions, the rate of evaporation of water from the surface of the concrete will be much faster than the
rate of movement of water from the interior to the surface. This creates a moisture gradient, resulting in the formation of
cracks which are known as plastic shrinkage cracks.
4. Less finishing time
In hot weather conditions, the rate of evaporation of water from the surface of the concrete is much higher. So the finishing
of concrete must be done as early as possible, otherwise due to quicker evaporation and faster stiffening the finishing will
be of poor quality.

5. Absorption of water by subgrade
In hot weather regions, usually the subgrade is dry and absorptive. Thus the subgrade or the surface of formwork has to be
wetted before placing the concrete otherwise the water will be absorbed by the subgrade and formwork leaving very little
water for the hydration process which eventually makes the contact zone of concrete poorer in quality.
6. Difficulty in curing
In hot weather concreting early curing becomes necessary if 53 grade cement is used. Hot weather needs a continuous
curing. If there is any discontinuity in the curing, the surface of the concrete will dry up fast, resulting interruption in the
hydration process. Once the interruption in concrete takes place, the subsequent wetting will not contribute to the
development of full strength.

PRECAUTIONS OF CONCRETING
IN HOT WEATHER
To improve the quality of concrete the temperature of the freshly mixed concrete should be as low as possible. To obtain
such a condition, attempts should be made to lower the temperature of the ingredients of concrete as low as possible. The
following are the precautions that could be taken:
1. Cooling of aggregates
The temperature of the concrete can be kept down by lowering the temperature of the aggregates. Aggregates should be
stockpiled in shade. Water can also be sprinkled over the surface of aggregates before using them in concrete. If possible
aggregates can also be cooled by spraying cold air over the surface just before using them in concrete.

2. Mixing water
The temperature of the mixing water has the greatest effect on the temperature of concrete as the specific heat of water
(1.0) is nearly five times that of normal aggregate (0.22). Moreover, the temperature of water is easier to control than that
of other ingredients. Cooled water may be added to reduce the temperature of concrete. If the temperature is very high, ice
pieces are incorporated directly into the mixer.
3. Production and delivery
a) The temperature of the ingredients of concrete should be maintained at the lowest practical levels so that the
temperature of concrete is below 40°C at the time of placement.
b) The required mixing time of concrete in hot weather should be minimum.
c) When ice is used it must be mixed to such an extent that all ice gets melted.
d) The period between mixing and delivery should be kept to an absolute minimum.
e) Reinforcement, formwork and subgrade should be sprinkled with cold water just prior to placing the concrete.
f) Immediately after finishing, the top of concrete must be covered by plastic sheets, tarpaulins, gunny bags etc. to prevent
the loss of water by evaporation.
g) Black surfaces should be white washed to reduce their heat absorbing capacity.

ADMIXTURES
Admixtures may be defined as the materials other than the basic ingredients of concrete i.e. cement,
aggregates and water added to the concrete mix immediately before or during the mixing process to modify
one or more of the properties of concrete in fresh and hardened state. They are classified as chemical and
mineral admixtures. Chemical admixtures are used in construction industry for building strong, durable and
waterproof structures. Mineral admixtures are silicious materials which are added to the concrete either as a
filler or to improve favourably certain desired properties such as durability. Mineral admixtures are classified
as either pozzolanic or cementitious. They are either natural materials or by-products such as fly ash of
industries.
An additive is a material which is added at the time of grinding cement clinker in the cement factory.

TYPES OF ADMIXTURES
TYPES OF
ADMIXTURES
CHEMICAL
ADMIXTURES
MINERAL
ADMIXTURES

CHEMICAL ADMIXTURES
1. Accelerating Admixture or Accelerator
An admixture used to speed up the initial setting of concrete and to acquire strength more rapidly is called an accelerator.
An increase in the early strength development may help in the following purposes:
• Permitting earlier removal of form work.
• Shortening the curing period.
• Opening the structure for use at the earliest opportunity.
• Partially compensate for the retarding effect of low temperature during cold weather concreting.
• In the emergency repair work.
Usually chlorides of calcium, aluminium and sodium, the sulphates of sodium and potassium, caustic soda, certain silicates,
carbonates etc. are used as accelerators. Out of the above substances calcium chloride is the most commonly used
accelerating admixture. When calcium chloride is used 2% by weight of cement, it reduces the initial setting time from
approximately 3 to 1 hour, the final setting time from 6 to 2 hours and at 21°C it approximately doubles the 1 day
compressive strength.
There are some drawbacks with calcium chloride. Calcium chloride in no case should exceed 3% by mass of cementing
material. If over 3% of calcium chloride is added there is an instantaneous or flash setting of cement. It imparts volume
stability. Calcium chloride has been found to increase the corrosion of reinforcement and increases the drying shrinkage
upto 50% of normal concrete.

Sodium chloride may also be used as an accelerating agent. But calcium chloride provides a greater accelerating or the early
strength than equal amount of sodium chloride. The heat of hydration is increased by calcium chloride upto 3 days after
mixing but sodium chloride increases it upto 1 day. Calcium chloride gives high early strength but has little effect on the 28
day strength. But sodium chloride increases the early strength but decreases the 28 day strength.
2. Retarding Admixture or Retarder
A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable
for a longer time than concrete without the retarder. Retarders delay setting time of cement either by forming a thin coating
on the cement particles and thus slowing down their dissolution and reaction with water or by increasing the intra-
molecular distance of reacting silicates and aluminates from water molecules by forming certain transient compounds in the
system. With the formation of silicates and aluminate hydrates, the influence of retarders diminishes and the hydration
process becomes normal. Thus, a retarding admixture holds back the hydration process, leaving more water for workability
and allowing the concrete to be finished and protected before drying out.
Calcium sulphate in the form of gypsum is generally added during the manufacture of cement to retard the setting time. But
the amount of gypsum if added beyond a limited quantity produces unsoundness and other undesirable effects. Another
most effective retarding agent is common sugar. It can be used for delaying the setting time of concrete without any harmful
effect on the ultimate strength of concrete. At normal temperatures, 0.2% addition of sugar can extend the final setting time
to about 72 hours or more.
Ammonium chloride, ferrous and ferric chlorides, calcium borates and oxychlorides, calcium tartarate, sodium bicarbonate,
various forms of starch etc. are some other principal materials which are effectively used to retard the rate of hydration.

3. Air Entraining Admixture
Air entraining admixtures help to incorporate a controlled amount of air, in the form of millions of non-coalescing air
bubbles, which will act as flexible ball bearings and will modify the properties of plastic concrete regarding workability,
segregation, bleeding and finishing quality of concrete. It also modifies the properties of hardened concrete regarding its
resistance to frost action and permeability.
Air entrainment has direct effect on the following properties:
• Resistance to freezing and thawing.
• Improvement in workability
• Reduction in strength.
The rapid fluctuations in temperature cause more deterioration in concrete than extreme hot or extreme cold conditions.
But the drawback with the entrained air is the reduction in the strength of concrete. Hence entrainment of air proves
beneficial where a good surface finish and resistance to frost are more important than the strength of concrete.

Due to entrainment of air concrete becomes more workable and more cohesive even at same slump. The increased
cohesiveness of the concrete makes it less liable to bleeding and segregation. Hence the final finishing can be started
immediately after compaction. The entrainment of air by 5% increases the compacting factor by 0.07 and the corresponding
increase of the slump may be from 1 cm to 5 cm. The increase in workability is more with wet mixes than with dry and with
lean mixes than with rich. The following types of air entraining agents are used for making air entrained concrete:
a) Natural wood resins.
b) Animal and vegetable fats and oils, such as tallow, olive oil and their fatty acids such as stearic and oleic acids.
c) Various wetting agents such as alkali salts or sulphated and sulphonated organic compounds.
d) Water soluble soaps of resin acids, and animal and vegetable fatty acids.
4. Plasticizers
A plasticizer, also known as water reducer is defined as an admixture added to wet concrete mix to impart adequate
workability properties. Plasticizers are used in concrete for the following purposes:
a) To achieve a higher strength by decreasing the water/cement ratio at the same workability as that of an admixture.
b) To achieve the same workability by decreasing the cement and to reduce the heat of hydration in the mass concrete.
c) To increase the workability to provide ease in placing concrete in inaccessible locations.

The following materials are generally used as plasticizers:
a) Ligno sulphates and their derivatives and modifications, salts of sulphonates hydrocarbons.
b) Polyglycol esters, acid of hydroxylated carboxylic acids and their derivatives.
c) Carbohydrates.
Among these materials sodium, calcium and ammonium lingo sulphates are most popoular. The amount of plasticizers
used varies from 0.1% to 0.4% by weight of cement. At constant workability, the use of 0.1% to 0.4% of plasticizer reduces
the mixing water by 5% to 15%, which naturally increases the strength.
A good plasticizer produces fluidity in concrete or mortar in a different way than that of an air entraining agent. However
some plasticizers also entrain some air along with improving the workability. A good plasticizer should not entrain air more
than 1 to 2% as air entrainment reduces the strength of concrete.
The main action of plasticizers is to produce fluidity in the mix and improve the workability of concrete. The mechanisms
involved are:
1) Dispersion
2) Retarding effect

5. Super-plasticizer
A new class of water reducer, chemically different from the normal and mid-range water reducer and capable of reducing
water content by about 20 to 40 percent has been developed. The admixtures belonging to this class are known as high
range water reducers or super-plasticizers. The use of super-plasticizer is practiced for production of flowing, self-levelling,
self-compacting and for the production of high strength and high performance concrete. The super-plasticizers are more
powerful as dispersing agents and they are high range water reducers. It is the use of super-plasticizer which has made it
possible to use water/cement ratio as low as 0.25 or even lower and yet to make flowing concrete to obtain strength of the
order 120 MPa or more. The use of super-plasticizers also made it possible to use fly ash, slag and silica fume to produce
high quality concrete.
The super-plasticizers are used for the following purposes:
1) To produce more workable concrete than the concrete without the use of super-plasticizer at the same water/cement
ratio.
2) For the same workability, the use of super-plasticizer permits the use of lower water/cement ratio.
3) The use of super-plasticizer also permits the reduction in cement content due to the increase in strength.
4) The use of super-plasticizers also produces a homogenous and cohesive concrete without any tendency of segregation
and bleeding.

6. Damp-proofing and Water proofing admixtures
One of the most important requirements of concrete is that it must be impervious to water under the following conditions:
Firstly, when concrete surface is subjected to water pressure on one side.
Secondly, the concrete surface should be impervious to the absorption of surface water by capillary action.
To achieve the above objectives, the use of well-chosen admixtures has been accepted. Water proofing agents may be
obtained in liquid, paste or powdered form and may consist of water repellent or pore filling materials. The various water
repelling materials are resins, vegetable oils, fats, waxes, soda, potash soaps, calcium soaps and coal tar residues. The chief
pore filling materials are silicates of soda, aluminium and zinc sulphates and aluminium and calcium chlorides. These
materials are chemically active pore fillers. They also act as accelerators and accelerate the setting time of concrete and
thus render the concrete more impervious at early stage. The chemically inactive pore filling materials are chalk, earth
fullers and talc and these are used very finely ground. Their chief action is to improve the workability and to facilitate the
reduction of water for given workability and to make dense concrete which is basically impervious.
In some kind of water proofing admixtures, inorganic salts of fatty acids, usually calcium or ammonium stearate or oleate is
added along with lime and calcium chloride. Calcium or ammonium stearate or oleate act mainly as water repelling
materials, lime as pore filling material while calcium chloride acts as an accelerator for the development of early strength
and also helps in efficient curing of concrete. All these factors contribute towards making concrete impervious.

7. Corrosion Inhibiting admixtures
Corrosion inhibiting admixtures slows down the corrosion of steel reinforcements in concrete. They are used as a defensive
strategy for concrete structures constructed in marine facilities, highway bridges, and in industrial environment where
reinforced concrete is exposed to high concentrations of chloride. Dougill of U.K. got a patent for the use of sodium
benzoate as a corrosion inhibiting admixture. A 2% sodium benzoate solution in mixing water may be used to prevent
corrosion of reinforcement. Sometimes 10% benzoate cement slurry is also used to paint the reinforcement for inhibiting
corrosion. A 2-3% of sodium nitrate by weight of cement has been found to be effective in preventing corrosion of
reinforcement.

MINERAL ADMIXTURES
Fly Ash
Rice Husk Ash (RHA)
Calcined clay (Surkhi)
Ground Granulated Blast Furnace Slag (GGBS)

CHAPTER 7
PROPERTIES OF SPECIAL
CONCRETE

READY MIXED CONCRETE
As the name indicates, Ready Mixed Concrete (RMC) is the concrete which is delivered in the ready-to-use manner. RMC is
defined by the American Concrete Institute’s Committee 116R-90 as:
“Concrete that is manufactured for delivery to a purchaser in a plastic and unhardened state.”
The Indian Standard Specification IS: 4926-2003 defines RMC as:
“Concrete mixed in a stationary mixer in a central batching and mixing plant or in a truck mixer and supplied in fresh
condition to the purchaser either at the site or into the purchaser’s vehicle.”
In India, concrete has traditionally been produced on site with the primitive equipments and use of large labour force.
Ready Mixed Concrete is an advanced technology, involving a high degree of mechanization and automation. A typical
RMC plant consists of silos and bins for the storage of cement and aggregates respectively, weigh batchers for
proportioning different ingredients of concrete, high efficiency mixture for thorough mixing of ingredients and a
computerized system controlling the entire production process. The quality of the resulting concrete is much superior to
site-mixed concrete.

ADVANTAGES OF READY MIXED CONCRETE (RMC)
The following are the advantages of ready mixed concrete over site mixed concrete:
1) Quality of concrete: Since RMC is factory produced, the raw material and production process quality is better than
conventional site mixed concrete.
2) Speed of construction: In site mixed concrete, the contractor needs to mobilize labour for mixing as well as placing. In RMC,
fresh concrete is supplied in a placeable condition and can directly be placed by pumping. Hence a faster construction speed
can be achieved.
3) Elimination of storage needs at the construction site: In case of site mixed concrete, all raw materials such as aggregates, sand
and cement have to be stored at the site. In urban situations and when work is progressing close to the highways, there is a
problem of storage of raw materials affecting smooth flow of traffic. In case of RMC, this problem is completely avoided as the
storage of material takes place at the central plant.
4) Easier admixture addition: In RMC, admixtures can be added in a controlled manner because of the use of sophisticated
computer-controlled methods of releasing exact quantities needed. This is not possible in normal concreting.
5) Reduction in wastage: In site mixed concrete job, wastage occurs in handling of all materials including cement. The wastage is
generally of the order of about 2-3 kg per 50 kg bag of cement. All such wastages are considerably minimized at RMC facility.
6) RMC is eco-friendly: The production of RMC is done in an environmentally assessed and licensed central plant. Hence dust
and noise pollution which is inevitable in concrete is avoided.

PRESTRESSED CONCRETE
Prestressed concrete is a form of concrete used in construction which is ‘pre-stressed’ by being placed under compression
prior to supporting any loads beyond its own dead weight. This compression is produced by the tensioning of high-strength
‘tendons’ located within or adjacent to the concrete volume, and is done to improve the performance of the concrete in
service. The essence of prestressed concrete is that once the initial compression has been applied, the resulting material
has the characteristics of high-strength concrete when subject to any subsequent compression forces, and of ductile
strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability
compared to conventionally reinforced concrete in many situations.
APPLICATIONS OF PRESTRESSED CONCRETE
Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow
longer spans, reduced structural thicknesses, and material savings compared to simple reinforced concrete. Typical
applications include high-rise buildings, residential slabs, foundation systems, bridge and dam structures, silos and tanks,
industrial pavements and nuclear containment structures.

FIBRE REINFORCED CONCRETE
Fibre reinforced concrete may be defined as concrete made with hydraulic cement, containing fine or fine and coarse
aggregate and discontinuous discrete fibres. The fibres can be made from natural material like asbestos, cellulose, sisal or
are a manufactured product such as glass, carbon, steel and polymer. The purpose of reinforcing the cement based matrix
with fibres are to increase the tensile strength by delaying the growth of cracks and to increase the toughness by
transmitting stress across a cracked section so that much larger deformation is possible beyond the peak stress than
without fibre reinforcement. Fibre reinforcement improves the impact strength and fatigue strength and also reduces
shrinkage.
Fibre Reinforced Concrete

USES OF FIBRE REINFORCED CONCRETE
The following are the important uses of fibre reinforced concrete:
1) Fibre reinforced concrete can be used for all types of works as road pavements, industrial flooring, bridge decks, canal
lining, explosive resistant structures, refractory linings etc.
2) Fibre reinforced concrete can also be used for the fabrication of precast products like pipes, boats, beams, stair case
steps, wall panels, roof panels, manhole covers etc.
3) Fibre reinforced concrete is also being tried for the manufacture of prefabricated formwork moulds of “U” shape for
casting lintels and small beams.

PRECAST CONCRETE
Precast concrete is a construction product produced by casting concrete in a reusable mould or ‘form’ which is then cured
in a controlled environment, transported to the construction site and lifted into place.
ADVANTAGES OF PRECAST CONCRETE
Using a precast concrete system offers many potential advantages over onsite casting. Precast concrete production is
performed on ground level, which helps with safety throughout a project. There is a greater control over material quality
and workmanship in a precast plant compared to a construction site. The forms used in a precast plant can be reused
hundreds to thousands of times before they have to be replaced, often making it cheaper than onsite casting when looking
at the cost per unit of form work.

HIGH PERFORMANCE CONCRETE
High performance concrete is a concrete mixture, which possesses high durability and high strength when compared to
conventional concrete. This concrete contains one or more of cementitious materials such as Fly ash, Silica fume or Ground
Granulated Blast Furnace Slag and usually a super plasticizer. The term ‘high performance’ is somewhat pretentious
because the essential feature of this concrete is that its ingredients and proportions are specifically chosen so as to have
particularly appropriate properties for the expected use of the structure such as high strength and low permeability. Hence
High Performance Concrete is not a special type of concrete. The use of some mineral and chemical admixtures like Silica
fume and super plasticizer enhance the strength, durability and workability qualities to a very high extent
ADVANTAGES OF HIGH PERFORMANCE CONCRETE
High performance concrete works out to be economical, even though its initial cost is higher than that of conventional
concrete because the use of High Performance concrete in construction enhances the service life of the structure and the
structure suffers less damage which would reduce overall costs.
High performance concrete can be designed to give optimized performance characteristics for a given set of load, usage
and exposure conditions consistent with the requirement of cost, service life and durability. The high performance concrete
does not require special ingredients or special equipments except careful design and production. High performance
concrete has several advantages like improved durability characteristics and much lesser micro cracking than normal
strength concrete.

REVISION,
CLASS TEST
AND
SEMINAR

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