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.
1. It can be moulded into any size and shape of durable structural
member.
2. It is possible to control the properties of cement concrete.
3. It is possible to mechanise completely its preparation and placing
processes.
4. It possesses adequate plasticity for mechanical working.
The cement concrete has the following properties
1. It has high compressive strength.
2. It is free from corrosion.
3. It hardens with age and continues for a long time after concrete
has attained sufficient strength.
4. It is proved to be economical than steel
.
5. 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.
6. It forms a hard surface, capable of resisting abrasion stresses.
This is called reinforced cement.
7. 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
following properties.
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 concrete.
4) It should be fit for drinking purpose.
Function of Water
1. It acts as lubricant for fine and coarse aggregates.
2. It acts chemically with cement to form binding paste with coarse
aggregates and reinforcement.
3. It is necessary to flux the cementing material over the surface of
the aggregates.
4. It is employed to damp the concrete in order to prevent them
absorbing water vitally necessary for chemical action.
5. It enables the concrete mix to blow into moulds.
Uses of Concrete:
1) 1:2:2 - For heavy loaded R.C.C columns and R.C.C arches of long
spans.
2) 1:2:2 - For small pre cast members of concrete like fencing poles,
telegraph poles etc. watertight construction.
3) 1:2:3 - For water tanks, bridges, sewers etc.
4) 1:2½:3½ - For foot path, concrete roads.
5)1:2:4 - For general work of RCC such as stairs, beams, columns,
slabs, etc
6)1:4:8 /
1:5:10 For mass concrete for heavy walls, foundation footings etc.
Preparation of concrete mix:
There are two types of concrete mixing
(i) Hand mixing
(ii) Machine mixing
1. 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.
2. Machine Mixing: - The machine used for mixing concrete is termed
as concrete mixer. Two types of concrete mixers are in common are
1. Continuous mixers
2. 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 the discharge.
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.
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 Purposes:
(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.
