Basic and most important material of
Most widely used and major building
Why concrete is so widely used ?
It can be easily moulded/casted in various
sizes and shapes
2. It can easily be modified and changed as per
the requirements of the structure.
3. It is simple to prepare and place; also readily
TYPES OF CONCRETE
Plain cement concrete (PCC):- It is simple mix
of Cement : Fine aggregate : Coarse aggregate
used for preparing level surfaces, stages, filling of
Reinforced cement concrete (RCC):- It is
similar to PCC only steel (also wood, resin,
plastic, etc. in special cases) is reinforced in
designed manner increasing the tensile strength
Various proportion of Cement:Sand:Aggregate
are used to prepare concrete as per its
importance and uses.
Following are the various methods used for
determining the volumetric proportion of concrete.
Fineness modulus method
Minimum voids method
Maximum density method
Water-cement ratio method.
Concrete mixes and purpose
Type of concrete
Proportion of mix
Nature of work.
1 : 5 : 10
Mass concrete for
Mass concrete and
foundations of less
For general RCC
works (slab, beams,
1 : 11/2 : 3
structures, piles, and
Heavily loaded RCC
structure long span
slab, beams etc.
Batching of Concrete
There are two
methods of batching
1. Volume batching
2. Weight batching
Depending on the
site conditions and
method is decided.
Procedure for preparing
After procuring the materials and deciding the
proportion (mix-design) following are two methods
of mixing: Hand mixing
In hand mixing similar to mortar cement : fine
aggregates : Coarse aggregates. Are taken in
desired quantity on water tight clean platform and
dry mixed thoroughly 3 times.
Then water is added as per the water cement
ratio and thoroughly mixed by spade.
It is suitable for only small quantities
and where quality can be monitored on
It has to be consumed within 30 mins
of it’s preparation.
Advantages and Disadvantages of
hand mixing of concrete.
1. Process is done on
site hence no time
constraint can be
prepared as per the
No power is required
No heavy machinery
Process is less noisy.
Quality can be
monitored on site
to use 10% more
cement then specified.
1. Cannot be adopted for
large quantity of
Since the mixing is done
by hand proper
supervision on mixing is
The work area gets
untidy and cleaning
work is increased.
All the materials are added into the mechanical
mixer and rotated for certain time(1- 4 mins). The
mixture is then taken out of the drum and
It is to be noted that if water is added 1st in the
mixer it ensures evenly distribution of water in the
The speed and time of mixing is crucial(important)
and has to be supervised because it affects the
quality and strength of concrete.
The concrete discharged from mixer should also
be consumed within 30 mins.
Advantages and Disadvantages of
machine mixing of concrete.
The quality of
concrete is good and
2. Since mixing is done
in machine; area is
less untidy and
cleaning is easy.
3. Since machine is
concrete can be
produced at various
location on site
cleaning is difficult,
blades need to be
checked for any
Requires power and
Skilled operator is
required for operation.
Transportation of concrete.
The concrete ready to use should be placed and
finished within 30 min of its preparation; hence
transportation of concrete is also an important
The type of equipment for transportation of
concrete depends on the nature of work, height
above/below ground level and distance between
point of preparation and placing of concrete.
For ordinary work human ladder is formed and
concrete is conveyed(passed on) in pans hand to
hand. For important works various mechanical
devices such as dumpers, trucks, pumps, hoist,
Precautions to be taken while
transportation of concrete.
Concrete should be transferred such that there is
no segregation of aggregates.
Under no circumstances water should be added
to concrete during its passage from mixer to
Spillage and wastage should be minimum.
It should be easily accessible at both the points.
Placing of Concrete
After transporting the concrete another activity is
placing of concrete. Concrete needs to be placed
with great care to avoid segregation and to
achieve proper compaction.
It is advisable to deposit small small quantity of
concrete at various location rather than deposit
huge chunk at one location.
Precautions to be taken while placing
Formwork should be cleaned and well watered
if applicable shuttering oil should be applied
before laying reinforcement to avoid its contact
Concrete to be deposited as near as possible to
its final position.
Concrete to be deposited from a maximum
height of 1.5m.
The mix should be hard enough or else it will
start bleeding form the formwork.
Concrete should be laid in layers ranging from
150 mm to 500 mm thick.
1. Compaction of concrete to be done properly with
various compacting equipments to avoid honey
2. No concrete should be placed after 30 mins of its
3. During placing care should be taken of
reinforcement and cover .
Placing of concrete on slopes.
While placing on slopes some extra care has
to be taken following are the various points.
1. Workability of concrete should be less i.e.
concrete should be stiff.
2. Concrete should be laid in such a manner
that it does not flows downward under the
action of gravity
3. Certain barriers should be made to resist its
4. Finishing should be done as soon as it gets
settled (max after 1 hour).
5. Placing of concrete should always start from
Compaction of concrete.
Compaction of concrete has to be done to
eliminate air bubbles (voids) in concrete and
obtain maximum density
Also to achieve proper contact with
And to get proper shape of the structure by
ensuring that concrete has reached every corners
of the formwork shuttering.
There are two basic methods of compacting
1. Hand Compaction.
2. Machine Compaction.
Hand Compaction of concrete.
It is done for unimportant works by
ramming, tamping, spading, and slicing
with suitable tools.
Concrete should be little low in slump
and highly workable
Hand compaction is preferred because
the use of vibrator may lead to the
segregation of the aggregates.
Mechanical vibrators are machines used to
achieve well compacted concrete
It is easy to compact hard concrete with these
Quality of concrete can be improved
The time taken for compaction is less
It is easy with vibrators to deposit well compacted
concrete in complex designs of structure.
Following are the four types of vibrators
1. Immersion vibrators 2.
3. Shutter vibrators
These are the steel platforms mounted on flexible
springs and they are operated by electromagnetic
action or electric motors.
The frequency of vibrations varies from 3000 to
7200 vibrations per minute.
Frequency α 1 / Time
It means frequency is inversely proportional to the
time of vibration i.e. if frequency is more the
compacted concrete is achieved in less time.
Curing of Concrete.
After attaining the final setting time fresh concrete
is kept moist for certain period (min 7 days and
max 23 days) so as to promote the hardening of
Cement contains C3S, C3A, C2S, C4AF these
elements of cement react with water and hardens
the concrete the content of these elements in
different ratio determines the various property of
Generally the 7 days curing is of utmost
importance because initial 60-80 % strength of
concrete is achieved in this period the other 4020 % strength of design concrete is achieved
within 23 days.
Purpose of Curing
It protects the finished surface from
direct sun and wind.
It helps in chemical reaction of
It reduces the shrinkage, increases
Methods of curing
There are various methods but the main concern
of every method is to reduce heat of hydration,
the temperature should be kept minimum and the
water loss should be prevented.
1. Ponding with water
2. Covering concrete with wet jute bags.
3. Covering concrete with wet sand, saw-dust, etc.
4. Covering concrete with polythene sheets.
5. Intermittent spraying / continuous sprinkling of
6. Applying curing compounds.
Application of curing compound.
Various chemicals used are acrylates,
chlorinated rubbers, bitumens, sodium
silicates, linseed oils, wax, etc.
Precautions for application of curing
1. Concrete should be thoroughly wet,
curing compound containers should be
shaken well before use, it should be
sprayed as soon as possible after casting
of concrete, it should not be applied when
water is on the surface.
Advantages of curing compound
It relives from the continuous monitoring of
It is applied in a single operation and also aids
in the quality control of concrete.
They are used where there is shortage of
They are useful in the areas which are nonaccessible for eg slip form tall chimney’s etc.
They are economical for industries
manufacturing pre-cast products.
Workability of concrete
The property of wet concrete to easily form and
take the shape of formwork/shuttering; its ability
to get casted in the desired shape is known as
Generally to make a workable concrete water is
added in access but it compromises with the
strength of concrete.
Water if added in required optimum quantity
imparts maximum designed strength to concrete
this is due to hydration of cement.
If water is less reaction is incomplete cement gets
wasted; if water is more reaction gets disturbed
and again strength is disturbed.
Workability of concrete is achieved by adding
If the mix is too wet/flowable then by slightly
changing the proportion of fine and coarse
aggregate mix could be made dry.
2. If the mix is too stiff then by adding cement
water paste of 1:1 the mix is corrected
Various admixtures are also added to achieve
workability in modern mix design.
Hydration of cement
The products of the reaction between cement and
water are termed "hydration products." In
concrete (or mortar or other cementitious
materials) there are typically four main types:
Calcium silicate hydrate:
Calcium hydroxide - Ca(OH)2:
AFm and AFt phases:
Finishing of concrete surface
If proper compaction is done on concrete and
proper curing then there is very less chances of
To increase the finish of surface concrete is
prepared by adding more fines to it for e.g. slag,
fly ash, porcelain etc..
It also increases the workability of concrete.
Proper tamping on shuttering while laying of
concrete and compaction also helps in achieving
well finished surface.