Cement Concrete

1. Constituents – Cement, Aggregate, Water

2. Quality and permissible limits of deleterious materials
3. Water-Cement Ratio
4. Properties of concrete: Strength, Durability, Workability
5. Concreting Processes – Batching, Mixing, Transporting, Placing, Compaction, Curing,
Finishing
6. Concrete additives and admixtures
7. Principal types of concrete construction - Plain Cement Concrete (PCC), Reinforced Cement
Concrete (RCC), Pre-cast Concrete, Pre-stressed concrete, Special Concrete
8. Defects of concrete and their curing measures

1 Classification of Concrete
1.1 Based on Cementing Material
Concretes are classified as lime concrete, gypsum concrete and cement concrete.

1.2 Based on Grade

Depending upon the strength (N/mm2) of concrete cubes (150 mm side) at 28 days, concrete is classified
as given in table 1 below.
Table 1. Grades of Cement Concrete

Grade M5 M10 M15 M20 M25 M30 M35 M40 M45 M50 M55
Strength 5 10 15 20 25 30 35 40 45 50 55

It is further classified as low strength concrete (< 20 N/mm2), medium strength concrete (20–40
N/mm2) and high strength concrete (>40 N/mm2).

1.3 Based on Bulk Density

On the basis of density, concrete is classified as super heavy (over 2500 kg/m3), dense (1800-2500
kg/m3), light weight (500–1800 kg/m3) and extra light weight concrete (below 500 kg/m3).

1.4 Based on Place of Casting

When concrete is made and placed in position at the site it is known as in-situ concrete and when used
as a material for making prefabricated units in a factory is known as precast concrete.

2 Concrete additives /admixtures

2.1 What are admixtures?
Admixtures are used to modify the properties of concrete or mortar to make them more suitable for the
work at hand or for economy or for such other purposes as saving energy.
As per BIS (IS – 9103: 1999), Concrete Admixture is defined as a material (other than water, aggregates
and hydraulic cement and additives like Pozzolana or slag and fibre reinforcement) used as over
ingredient of concrete or mortar and added to the batch immediately before or during its mixing to
modify one or more of the properties of concrete in the plastic or hardened state.
2.2 Purpose of using admixtures
Some of the important purposes for which admixtures are used are:
1. To modify properties of fresh concrete, mortar and grout to
i. Increase workability without increasing water content or decrease water content at the
same workability.
ii. Retard or accelerate time of initial setting.
iii. Reduce or prevent settlement.
iv. Modify the rate or capacity for bleedings.
v. Reduce segregation.
vi. Improve pumpability.
2. To modify the properties of hardened concrete, mortar and grout to:
i. Retard or reduce heat evolution (heat of hydration) during early hardening.
ii. Accelerate the rate of strength development at early ages.
iii. Increase strength (compressive, tensile or flexural).
iv. Increase durability or resistance to severe condition of exposure.
v. Decrease permeability of concrete.
vi. Control expansion caused by the reaction of alkalis with certain aggregate constituents.
vii. Increase bond of concrete to steel reinforcement.
viii. Increase bond between existing and new concrete.
ix. Improve impact resistance and abrasion resistance.
x. Inhibit corrosion of embedded metal.
xi. Produce coloured concrete or mortar

2.3 Types of Admixtures

Types of admixtures as per American
Concrete Institute Committee report
and IS 9103: 1999 are:
1. Accelerating admixtures
2. Retarding admixtures
3. Water-reducing and set
controlling admixtures
4. Air-entraining admixtures
5. Super plasticizing admixtures
6. Admixtures for flowing
concrete
7. Miscellaneous admixtures Figure 1. Types of Admixtures
Source: Concrete Admixtures: Use and Applications” edited by M. R. Rixom

2.3.1 Accelerating admixtures

Accelerating admixtures are used for quicker setting times of concrete. It provides higher early strength
development in freshly cast concrete.
Usage
i. These admixtures are suitable for concreting in winter conditions
ii. During any emergency repair work
iii. In case of early removal of formwork
Disadvantages
i. It has increased drying shrinkage
ii. It offers reduced resistance to sulphate attack
iii. CaCl2 high risk of corrosion of steel – not permitted in reinforced concrete
iv. It is more expensive and less effective
2.3.2 Retarding admixtures

2.3.3 Water-reducing and set controlling admixtures

Concrete having greater workability be made without the need for more water and so strength losses
are not encountered
By maintaining same workability, but at a lower water content, concrete strengths may be increased
without the need for further cement addition

2.3.4 Air-entraining admixtures

These are generally used to improve workability, ease of placing, increased durability, better resistance
to frost action and reduction in bleeding. The common Air-Entraining agents are natural wood resins,
neutralized vinsol resins, polyethylene oxide polymers and sulfonated compounds.
2.3.5 Super plasticizing admixtures
2.3.6 Admixtures for flowing concrete
2.3.7 Miscellaneous admixtures
3 Concreting Processes
A good quality concrete is essentially a homogeneous mixture of cement, coarse and fine aggregates
and water which consolidates into a hard mass due to chemical action between the cement and water.
Each of the four constituents has a specific function.
a) The coarser aggregate acts as a filler.
b) The fine aggregate fills up the voids between the paste and the coarse aggregate.
c) The cement in conjunction with water acts as a binder.
The mobility of the mixture is aided by the cement paste, fines and nowadays, increasingly by the use
of admixtures.
Most of the properties of the hardened concrete depend on the care exercised at every stage of the
manufacture of concrete. A rational proportioning of the ingredients of concrete is the essence of the
mix design. However, it may not guarantee of having achieved the objective of the quality concrete
work. The aim of quality control is to ensure the production of concrete of uniform strength from batch
to batch.
The stages of concrete production are:

1. Batching or measurement of materials

2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Curing
7. Finishing

3.1 Batching
For good quality concrete a proper and accurate quantity of all the ingredients should be used. The
aggregates, cement and water should be measured with an accuracy of ± 3 per cent of batch quantity.
There are two prevalent methods of batching materials, the volume batching and the weigh batching.
3.1.1 Volume batching
Volume batching is generally recommended for small jobs only. The amount
of each solid ingredient is measured by loose volume using standard box
known as gauge box. (Fig.1).
The volume of a bag of cement (50 kg) is 0.035 m3. The volume of one gauge
box is made equal to 0.035m3. Correction to the effect of bulking of fine
aggregate should be made if volume batching is adopted since the density of
water is in kg/L, water is measured either in kg or litres. Figure 2. Wooden box for
Gauging Aggregates
Bulking of Sand:
The increase in the volume of sand due to increase in moisture content is known as bulking of sand. A
film of water is created around the sand particles which forces the particles to get a side from each other
and thus the volume is increased.
The volume increase in dry sand is known as the bulking if sand. Bulking of sand depends on the
quantity of moisture in the sand and also the size of the particles. Five to eight percent of the increase
in moisture in the sand can increase the volume of sand up to 20-40 %. Again the finer the sand is more
will be the increase in volume and increase in volume will be relatively less for coarser sand.
Bulking of sand depends on the moisture in the sand.
But when the moisture is increased by adding more
water, the particles of sand gets packed near each
other as the film around the sand particles breaks and
the bulking of sand is reduced. Dry sand and the send
completely filled with water will have the exact
volume.
Sand is used in concrete for reduction of segregation
and fill out the pores between cement and coarse
aggregates. For example, we need 1 m3 of sand in
concrete, we need to know the approximate sand
bulkage value. If the given sample has a bulkage of
25% then we need to take 25% more sand or 1.25
times of the sand while volume batching to get 1 m3
of sand for concrete.
3.1.2 Weigh batching
For all important and critical works weigh batching is used. For smaller works manual batching is done.
Automatic batching plants ranging from small to large capacity and manually or electrically operated are
available. In weigh batching water is not added by graduated buckets as the water may spill over during its
addition. A horizontal or vertical tank is fitted to the mixer. The filling is so designed to have a control to
admit any desired quantity of water.
3.2 Mixing
The objective of mixing is to make the concrete mass homogeneous and uniform in colour and
consistency. All the aggregate particles should have a coat of cement paste and all the ingredients of
the concrete should blend into a uniform mass. The mixing is done either by hand or by machine called
mixer.

Figure 3. Different types of Concrete Mixers

3.2.1 Hand Mixing
It is used for small jobs. Hand mixing is done over an impervious floor. Measured quantities of coarse
aggregate and fine aggregate are spread over the floor in alternate layers. Then cement is poured over
it and the ingredients are mixed dry with shovel until uniformity in colour is achieved. This mix is
spread out in thickness of 200 mm and water
is sprinkled. The mix is kept on turning over
till a uniform colour is achieved. As the
hand mixing cannot be thorough, it is
desirable to add some more cement (10%)
to cater for the possible inferior concrete
produced by this method.
3.2.2 Machine Mixing
For quality works mixing is carried out by
mixer. Mixers can be broadly classified as
batch mixers and continuous mixers. The
batch mixers produce concrete batch by
batch with time interval, whereas
continuous mixers produce concrete
continuously till plant is working. Batch
mixers are used for small and medium size
works. Continuous mixers are used for large
size works, e.g., dams.
Figure 4. Tilting Batch Mixer
Batch mixer may be of pan type and drum
type. The drum type may be further classified as
i. Tilting (T) (Fig: ),
ii. Non-tilting (NT),
iii. Reversing (R) type
Transit Mixer:
Truck mounted mixers also known as transit mixers (Fig.) are very popular and have replaced the dumpers
and agitator cars used earlier to transport fresh concrete from the batching plant to the site. Transit mixers
of capacity 4 to 12 cum mounted on truck chassis are available.

Figure 5. Transit Mixer

 Figure 6. Interior of Twin Fin Transit Mixer Drum with water spray arrangement

3.3 Transporting
Concrete should be transported to the place of deposition at the earliest without the loss of homogeneity
obtained at the time of mixing. A maximum of 2 hours from the time of mixing is permitted if trucks
with agitator and 1 hour if trucks without agitators are used for transporting concrete. Also it should be
ensured that segregation does not take place during transportation and placement. Some of the methods of
transporting concrete are as below:
3.3.1 Mortar Pan
This is the most common method of transporting concrete. This is labour intensive method wherein the
pans are passed from hand to hand and is slow and expensive method. Since pan conveys small quantity
of concrete, more and more concrete area is exposed to atmosphere during transportation. This may
lead to evaporation of water from concrete particularly in hot weather and under conditions of low
humidity.
3.3.2 Wheel Barrow:
Wheel barrows (Fig. 10.8) are used for transporting
concrete to be placed at ground level. These are used
for concreting rigid payments.
3.3.3 Dumper:
Dumpers, lorries or, trucks are used economically for
hauls up to 5 km. Dumpers are usually of capacity 2 to
3 cu m whereas trucks are of 4 cu m capacity. For long
hauls agitators are used to prevent segregation. The
concrete should be covered with tarpaulins to prevent evaporation of water from concrete (Transit Mixers).
3.3.4 Skip and Hoist:
This is the most useful and advantageous method of transporting concrete for multi-storey buildings.
The mixer feeds the skip which moves up over rails up to the level of deposition. If height of travel is
too much, the concrete may require a turning over before deposition in to the place.
3.3.5 Pumping
Pumping of concrete is done for multi-storey buildings, tunnels, and bridges. The concrete is fed from
the hopper into the pump cylinder largely by gravity, assisted by the vacuum created on the suction
stroke of the piston and forced into the pipe line on the pressure stroke. The pipes are made of steel, or
plastic of sizes ranging from 80 to 200 mm diameter. The aluminium pipes have a drawback of
formation of hydrogen gas and are not recommended. The pumps of capacity 15 to 150 m3/hr are
available. These can pump concrete 400 m horizontally and up to 80 m vertically. The pumpable
concrete should be cohesive and fatty. It should have a slump of 50 to 100 mm or even more. Pumping
compacts the concrete partially and reduces the slump by about 25 per cent at the delivery end.
Sometimes, admixtures are added to offer additional lubrication, reduced bleeding, and segregation.
Generally, air entraining admixtures are used giving an air content of 3 to 5 per cent.

3.4 Placing
To achieve quality concrete it should be placed with utmost care securing the homogeneity achieved
during mixing and the avoidance of segregation in transporting. Research has shown that a delayed
placing of concrete results in a gain in ultimate compressive strength provided the concrete can be adequately
compacted. For dry mixes in hot weather delay of half to one hour is allowed whereas for wet mixes in cold
weather it may be several hours.

Before placing the concrete in the foundation all the loose earth, roots of trees etc., are removed. If the
surface is found dry it is made wet so that earth does not absorb water from concrete. On the other hand
if the foundation bed is wet the water and mud is removed and cement is sprinkled before placing
concrete.
The insides of the formworks of beams, columns and slabs should be cleaned and oiled before use to
avoid any sticking of concrete with the forms and making their stripping off difficult. Concrete should
not be dropped but placed in position to prevent segregation. It should be dropped vertically from as
small height as possible. It should be placed at one point in the formwork and allowed to flow sideways.
When the concrete is to be laid in mass as for raft foundation, dam, bridge, pier etc., concrete is placed
in layers of 350–450 mm thickness. Several such layers placed in quick succession form a lift. Before
placing the concrete in the next lift, the surface of the previous lift is cleaned thoroughly with water jets
and scrubbing by wire brush. In case of dams, sand blasting is done. The laitance and loose materials
are removed and cement slurry is applied.

3.5 Compaction
After concrete is placed at the desired location, the next step in the process of concrete production is
its compaction. Compaction consolidates fresh concrete within the moulds or frameworks and around
embedded parts and reinforcement steel. Considerable quantity of air is entrapped in concrete during
its production and there is possible partial segregation also. Both of these adversely affect the quality
of concrete. Compaction of the concrete is the process to get rid of the entrapped air and voids,
elimination of segregation occurred and to form a homogeneous dense mass. It has been found that 5
per cent voids in hardened concrete reduce the strength by over 30 per cent and 10 per cent voids
reduce the strength by over 50 per cent. Therefore, the density and consequently the strength and
durability of concrete largely depend upon the degree of compaction. For maximum strength driest
possible concrete should be compacted 100 per cent. The compaction of concrete can be achieved by the
following methods.
3.5.1 Hand Compaction
This method of compaction is used for small and unimportant jobs. However, this method is extremely
useful for thin elements such as slabs, and for members with congested reinforcements. Hand
compaction is achieved by rodding ramming, or tamping.
Rodding is done with the help of 16 mm diameter, 2 m long steel rod to pack the concrete between the
reinforcement, sharp corners and edges. Rodding is done continuously during concreting. Ramming is
permitted only for unreinforced concrete constructions. The roof and floor slabs are usually tamped for
achieving compaction. The tampers are 100 × 100 mm in section and about 1 m long. Tamping bars not
only compact the concrete but also level the top surface. The limitation of this method is that a large
water-cement ratio is required for full compaction.
3.5.2 Compaction by Vibration
This is the most common and widely used method of compacting concrete for any structural element.
Plastic mixes need less time of vibration than harsh or dry mixes. The vibrations imparted to the fresh
concrete reduce the internal friction between the particles of concrete by setting the particles in motion
and thus produce a dense and compact mass. Vibration helps entrapped air to escape first from between
the coarse aggregate particles and later from the mortar. When vibration continues some more entrapped
air from the mortar is driven out.
The various types of vibrators in use are needle, formwork, table or platform, and surface vibrators.

3.6 Curing
Cement gains strength and hardness because of
the chemical action between cement and water.
This chemical reaction requires moisture,
favourable temperature and time referred to as
the curing period. The variation of compressive
strength with curing period is shown in Fig. 7.
Curing of freshly placed concrete is important for
optimum strength and durability.
1. The major part of the strength in the
initial period is contributed by the
clinker compound C3S and partly by
C2S, and is completed in about three
weeks. The later strength contributed
Figure 7. Development of strength with age
by C2S is gradual and takes long time.
2. Sufficient water should be made
available to concrete to allow it to gain full strength.
3. The process of keeping concrete damp for this purpose is known as curing. The objective is to
prevent the loss of moisture from concrete due to evaporation or any other reason, supply
additional moisture to accelerate the gain of strength.
4. Curing must be done for at least three weeks and in no case for less than ten days.
5. Approximately 14 litres of water is required to hydrate each bag of cement. Soon after the
concrete is placed, the increase in strength is very rapid (3 to 7 days) and continues slowly
thereafter for an indefinite period.
6. Concrete moist cured for 7 days is about 50 per cent stronger than that which is exposed to
dry air for the entire period.
7. If the concrete is kept damp for one month, the strength is about double than that of concrete
exposed only to dry air.
3.6.1 Methods of Curing:
3.6.1.1 Water Curing
Water Curing is done by covering the vertical or inclined concrete surface with gunny bags and then
sprinkling water over them regularly or with water proof paper.
The horizontal surfaces may also be kept wet by storing water over them (ponding) or by damp
gunny bags, straw, etc. Ponding, may, affect the strength if the concrete is flooded too soon.
3.6.1.2 Steam Curing
Curing can be also accomplished by artificial heat while the concrete is maintained in moist
condition. Both of these conditions can be fulfilled by the use of steam curing.
This method of curing is also known as accelerated curing since an increased rate of strength
development can be achieved.
The accelerated process of curing has many advantages in the manufacture of precast concrete products.

3.6.1.3 Curing by IR Radiation

A much more rapid gain of strength can be obtained with the help of infra-red radiation than even
with steam curing. The rapid initial rise of temperature does not affect the ultimate strength. It is
particularly suitable for the manufacture of hollow concrete products in which case the heaters are
placed in the hollow spaces of the product. The normal operative temperature is 90°C.
3.6.1.4 Electrical Curing
Concrete products can be cured by passing alternating current of low voltage and high amperage
through electrodes in the form of plates covering the entire area of two opposite faces of concrete.
Potential difference between 30 and 60 V is generally adopted.
Evaporation is prevented by using an impermeable rubber membrane on the top surface of the
concrete.
By electrical curing, concrete can attain the normal 28-day strength in a period of 3 days.
The technique is expensive.
3.6.1.5 Chemical Curing
Chemical membranes can be sprayed on to cure concrete. Liquid membrane forming curing
compounds such as sodium silicate (water glass) solution retard or prevent evaporation of moisture
from concrete. They form a film, fill the pores, seal the surface voids and prevent evaporation.
The application should be made immediately after the concreting has been finished.
If there is any delay, the concrete should be kept moist until the membrane is applied.
Membrane curing compound should not be applied when there is free water on the surface, because
this water will be absorbed by the concrete and the membranes broken.

3.7 Finishing
Concrete is basically used because of its high compressive strength. However, the finish of the ultimate
product is not that pleasant. In past couple of decades efforts have been made to develop surface finishes
to give a better appearance to concrete surfaces and are as follows.
3.7.1 Formwork Finishes
Concrete takes up the shape of the form
3.7.2 Surface Treatment
The type of surface treatment depends upon the purpose for which the concrete surface is to be used.
For example a pavement surface should be plane but with sufficient roughness to exhibit skid
resistance. The concrete after levelling is broomed or scratched to make the surface rough.
3.7.3 Applied Finishes
The exterior surfaces of concrete elements can be modified to give a pleasant look.
The concrete surface is roughened, cleaned and wetted. Over this a cement mortar of ratio 1:3 is
applied.
This mortar rendering can be given a number of surface finishes such as sand facing, rough cast finish,
pebble dash etc.