Cement concrete is a mixture of cement, sand, pebbles or crushed rock
and water. When placed in the skeleton of forms and allowed to cure,
becomes hard like a stone. Cement concrete is important building
material because of the following reasons.
- It can be moulded into any size and shape of durable structural
- It is possible to control the properties of cement concrete.
- It is possible to mechanise completely its preparation and placing
- It possesses adequate plasticity for mechanical working.
The cement concrete has the following properties
- It has high compressive strength
- It is free from corrosion
- It hardens with age and continues for a long time after concrete
has attained sufficient strength
- It is proved to be economical than steel
- It binds rapidly with steel and it is weak in tension, steel
reinforcement is placed in cement concrete at suitable places to
take up tensile concrete or simply R.C.C.
- It forms a hard surface, capable of resisting abrasion stresses.
This is called reinforced cement.
- It has tendency to be porous to avoid this proper grading & consolidation of the aggregates, minimum water-cement ratio should be adopted.
Constituents – Requirements.
The main constituents of concrete are
a) Cement / Lime: Before introduction of ordinary Portland
cement, lime was used as cementing material. At present most
of the cement concrete works in the building construction is
done with ordinary Portland cement. But other special varieties
of cement such as rapid hardening cement, high alumina
cement are used under certain circumstances. The cement
should comply with all standard specifications
b) Fine Aggregates: The material, which is passed through
4.7625mm B.S.test sieve, is termed as fine aggregates. Usually
natural river sand is used as fine aggregates. But places where
natural sand is not available economically, finely crushed stone
may be used as fine aggregates.
c) Coarse Aggregates: The material retained on 4.7625mm size
B.S.test sieve is termed as coarse aggregates. Broken stone is
generally used as coarse aggregates. For thin slabs, and walls,
the maximum size of coarse aggregates should be limited to
one third the thickness of the concrete section
d) Water: Water to be used in the concrete work should have the
1) It should be free from oils
2) It should be free from acids or alkalies
3) It should be free from Iron, Vegetables matter or other
substance, which is likely to have adverse effect on
4) It should be fit for drinking purpose
Function of Water
- It acts as lubricant for fine and coarse aggregates.
- It acts chemically with cement to form binding paste with coarse
aggregates and reinforcement.
- It is necessary to flux the cementing material over the surface of
- It is employed to damp the concrete in order to prevent them
absorbing water vitally necessary for chemical action
- It enables the concrete mix to blow into moulds.
Uses of Concrete:
1:2:2 – For heavy loaded R.C.C columns and R.C.C arches of long
1:2:2 – For small pre cast members of concrete like fencing poles,
telegraph poles etc. watertight construction.
1:2:3 – For water tanks, bridges, sewers etc.
1:2½:3½ – For foot path, concrete roads
1:2:4 – For general work of RCC such as stairs, beams, columns,
Preparation of concrete mix:
There are two types of concrete mixing
(i) Hand mixing
(ii) Machine mixing
- Hand Mixing: This method of mixing concrete is resorted to when
the quantity if concrete to be used in a work is insufficient to warrant the necessity of machine. This is used with advantage in places where
machinery cannot be used on account of their non-availability or in
works near a hospital where the noise of machine is not desirable. Hand mixing is done on a clean, hard and impermeable surface. Cement and sand are first mixed dry with the help of shovels until the mixture attains a uniform colour. Aggregative are then added to this mixture and the whole mixture is then turned by shovels until the stone pieces uniformly spread throughout. After this, desired are quantity of water is poured into the heap from a can fitted with a rose. The mass is then turned until a workable mixture is obtained. It is advised to add 10% extra cement to guard against the possibility of inadequate mixing by this method.
- Machine Mixing: – The machine used for mixing concrete is termed
as concrete mixer. Two types of concrete mixers are in common are
- Continuous mixers
- Batch mixers
Continuous mixers are employed in massive construction
where large and continuous flow of concrete is desired. The
process of feeding the mixing is more or less automatic. The
machine requires careful supervision so as to obtain the concrete
mix of desired consistency.
In batch type of concrete mixer. The desired proportion of
materials are fed into the hopper of a drum in which the materials
get mixed by the series of blades or baffles inside the mixer.
Batch mixers are further two types 1. Tilting drum type 2.
closed drump type. In the first type, components are fed in the
revolving drum in a tilted position and after sometime the
concrete mix is discharged by tilting the drums in the opposite
direction. In the latter type the drum remains rotating in one
direction and emptied by means of hopper which tilts to receive
While using the mixer, coarse aggregates should be fed
first, sand and cement should be put afterwards. In this revolving
state, the components get mixed while water is poured with the
help of can. The concrete should be for atleast 2 minutes, the time
being measured after all the ingredients including water have
been fed into the drum.
Compaction – Methods:
Concrete should be placed and compacted immediately
after mixing. The concrete should be placed within 30 to 40
minutes to prevent the danger of concrete getting its initial set,
before laying the concrete, the shuttering should be cleaned of all
of dust or debris. Crude oil or grease etc is usually applied to the
shuttering before concreting to prevent the shuttering absorbing
the water from the concrete or getting struck to it. In placing the
concrete, care should be taken to see that it should not be thrown
from heights. Concrete should be laid in layers 15 to 30 cm (6” to
12”) in thickness and each layer should be properly compacted
before laying the next one.
Compaction of concrete should be proceed immediately
after placing. The function of compaction of concrete is to expel
the air bubbles in the mass and make it impermeable in addition
to its securing the desired strength. The concrete mass should be
consolidated or compacted till the cream of the cement starts
appearing on the surface. Over compaction may lead to
segregation of concrete while-under-compaction may leave air
voids in concrete and results in honey combing. Compaction may
be done by hand or mechanical device.
(i) Hand compaction: The hand compaction may be
done by rodding, tamping or hammering. Tamping is
usually adopted for compacting concrete for slabs or
other such surfaces. Rodding is done for thin vertical
members. Hammering is done for massive plain
concrete works and for compacting an almost dry
concrete the surface is beaten with heavy flat bottom
rammers till the thin film of mortar start appearing on
(ii) Mechanical compaction: Mechanical compaction is
done by the use of vibrators. Vibrators are of three
types 1. Internal 2. External 3. Surface. Internal
vibrators are commonly used in large works for flat
surface compaction. In this the vibrator is immersed
in the full depth of concrete layer. The vibrator should
be kept in one position for about 3 minutes and then
removed and placed another position. External vibrators are placed against the form work and are only adopted for thin section of members or in places where internal vibrators cannot be used with ease.
Surface vibrators are generally employed in concrete road construction. Compaction of concrete by use of vibrators permits the use of stiff concrete mix of high strength and ensure better compaction than that obtained by the method of hand compaction
Curing of concrete:
Curing of concrete is one of the essential requirement of process
of concreting. Curing is process of keep the set concrete damp for
some days in order to enable the concrete gain more strength
(i) Curing protects concrete surfaces from sun and wind
(ii) Presence of water is essential to cause the chemical action
which a companies the setting of concrete