Cement concrete may be defined as building material obtained by mixing cement, aggregates and water in suitable proportions and then curing this plastic mixture to a hard mass. Hardened concrete resembles stone in weight, hardness and strength to a great extent. The properties of concrete depend on the quality and proportions of the ingredients used in the mix and the control exercised during various operations starting from mixing of aggregates to its placement in formwork and curing. Plain cement concrete has considerable strength in compression but it has very little strength in tension. Hence the use of plain cement concrete is normally restricted to situations where high compressive strength and weight are the primary requirements. Plain cement concrete is therefore used in the construction of bed blocks, massive gravity dams, dock walls, gravity walls, columns, and arches etc.
It is seen that in most of the cases, structural members are subjected to
bending, shear and torsion stresses. Inspite of its low strength, it has been
found possible to use cement concrete in construction of members subjected to
high tensile stresses. For this purpose, steel reinforcement is placed in the
tensile zone of the member before it is concreted. The material thus obtained
is termed as reinforced cement concrete or R.C.C. The principal materials used
in making concrete are described below.
CEMENT
Cement in its broadcast term means any substance which act as a binding
agent for materials. Cement as applied to construction and engineering is
produced by calcining at high temperature an intimate mixture of calcareous,
siliceous and aluminous substance and crushing the resulting clinkers to a fine
powder. Cement is the most expensive ingredient in concrete and it is available
in a variety of different form. The properties of cements depend upon the
chemical composition, the process of manufacture and the degree of fineness to
which they are ground. When cement is mixed with water, a chemical reaction
takes place as a result of which the cement paste first sets and then hardness
to a stone like mass. Depending upon their chemical composition setting and
hardening properties, cements can be broadly divided following two categories:
1.Portland cement
2. Special cement
PORTLAND CEMENT
This is the most widely used type of cement and is so named because of
the resemblance of it properties with a well-known natral stone quarried at
Portland. Joseph Aspdin, a Yorkshire brick layer is regarded as the discoverer
of Portland cement.
Composition of Portland cement:
Lime, silica and alumina are considered to be the three principal
constituents of cement. In addition, most of the cements contains iron oxide,
magnesia, sulphur trioxide and alkalies in small proportions. Cement is
manufactured by burning to white heat an intimate mixture of the above
ingredients and then grinding he resulting clinkers with gypsum to an extremely
fine powder.
Lime
60-67%
Silica
17-25%
Alumina
3-8%
Iron oxide
0.5-6%
Magnesia
0.1-4%
Sulphur trioxide 1-2.75%
Alkalies 0.5-1%
Setting and hardening of Portland
cement
If a well burnt Portland cement clinker is ground to a fine powder and
examined, it will be found to be composed of four important mineral compounds
given below:
1.Tri calcium silicate
2. Di calcium silicate
3. tri calcium aluminate
4. Tetra calcium alumina ferrite
If the powdered Portland cement clinker is mixed with 2 or 3 percent of
gypsum and water is added to the mixture to form a paste, it will be observed
that the soft paste will slowly lose its plasticity and will gradually stiffen
and harden into a solid mass. During the process of hardening from paste to a
hard mass, cement undergoes two types of sets. In the first set, also known as
the initial set, the cement paste loses its plasticity, and becomes sufficiently
coherent to withstand a certain arbitrary pressure. After that set, cement
hardens still further becomes much more rigid and in course of time it becomes
strong enough to be capable of withstanding higher intensity of pressure. At
such a stage, the cement is said to have undergone final set.
The setting and hardening of cement paste is mainly due to the hydration
and hydrolysis of the four compounds mentioned above. Out of the four, tri
silicate act as the best cementing material and greater its percentage in the
cement, the better cement it will make. The aluminates are the first to set and
harden and are responsible for the initial setting of cement. Thus the rapidity
of the initial set of cement depends upon the proportion of the aluminates
present. Setting time can however be regulated by the addition of 1 to 3% of
gypsum.
The strengthening effect of the aluminates is maximum during the first 24
hours and it does not extend beyond 28 days. During reaction with water, the
aluminates generate considerable heat which contributes towards many desirable
properties of the material. A cement having low percentage of aluminate
compound exhibits less tendency towards volume changes and formation of cracks,
generate less heat, resists sulphate solution attack better and possesses high
ultimate strength.
Although the process of setting and hardening continues simultaneously
yet there is a difference between the two. Setting is the change of cement
paste from a plastic state to a stiff solid state. Immediately, after setting
the mass is not strong and has very low compressive strength. With the passage
of time, when the hydration proceeds, the product becomes harder and its
compressive strength increases. This is termed as hardening of cement.
Types of Portland cement
Portland cement can be further sub divided into the following different
types. The difference in the properties of the various types of cement are
basically on account of variance in the proportions of its ingredients and
degree of fineness to which the clinkers are ground.
1.Ordinary Portland cement:
This is the basic type of cement which is used on large scale in all
general type of construction work. The details regarding composition and
properties of this type of cement are highlighted in IS:269.
2. Portland Pozzolana cement:
This can be defined as a general purpose cement which can be used in all
situations where ordinary Portland cement is considered suitable. Portland
pozzolana cement is manufactured either by grinding together, Portland cement
clinkers and pozzolana with addition of gypsum or calcium sulphate or by intimately
and uniformly blending Portland cement and fine Pozzolana.
3. Rapid hardening Portland cement:
The main difference between the ordinary Portland cement is that the
latter attains greater strength at an early stage. The property of high early
strength is achieved almost entirely due to higher degree of fineness in
grinding, burning at higher temperature and increased lime content in the
composition of cement. The strength development of this cement at the age of 3
days and 7 days is almost the same as the respective 7 days and 28 days
strength of ordinary Portland cement with the same water cement ratio. The main
advantage of using rapid hardening cement is that the formwork can be removed
earlier and reused in other areas thereby effecting considerable saving in cost
of formwork. It also permits faster completion of the work and thereby the
structure can be put to use early.
4. Low heat cement:
Considerable quantity of heat is evolved during the chemical reaction
that takes place during the setting and hardening of cement. In case of
ordinary structures, it gets lost by dissipation from the surface of the member
and as such produces no harmful effect. However, in case of mass concrete work
such as dams, massive retaining walls, bridge abutments etc. heat of hydration
generated proves dangerous. In low heat Portland cement tri calcium aluminate
and tri calcium silicate which have the maximum contribution towards heat
generation are kept to the minimum and the amount of di calcium silicate is
increased. Low heat Portland cement has not only lesser and slower heat
generation but also greater resistance to cracking, less rapid strength
development with equal strength at advanced ages and greater sulphate
resistance due to decreased tri calcium aluminate. Such a cement is, however,
not suitable for structures of ordinary nature.
5. White or coloured Portland cement:
White cement is a type of ordinary Portland cement which is pure white in
colour, and hence very useful for cast stone, terrazzo flooring or face plaster
to walls and other ornamental works. It is also used for traffic curbs, bridge
rails and aerodrome markings. The strength of white cement is slightly less
than that of ordinary cement and it is 4 to 5 times costlier than the latter.
In this case special care is taken to exclude iron oxides which impart
grey colour to the cement during manufacture. Raw materials used are china clay
and pure limestone. Only oil is used as fuel for burning and the rotary kiln is
lined with special fire clay bricks so that the mixture does not come in
contact with iron or steel during its manufacture.
6. Sulphate resisting Portland
cement:
This type of cement is used in the construction of foundation in soil
where sub soil contains very high proportion of sulphates. The constituents of
sulphate resisting Portland cement are so adjusted that the tri calcium
aluminate and tetra calcium aluminate ferrite are kept very small. Cements with
such composition have excellent resistance to sulphate attack.
SPECIAL CEMENT
Special cements are generally manufactured by adding certain admixtures
to the cements during the process of mixing or grinding.
1.Quick setting cement:
When concrete is to be laid under still water or in running water, quick
setting cement is used advantageously. The setting action of such a cement
starts within five minutes as it becomes stone hard in less than an hour. Quick
setting cement is mixed with a small percentage of aluminium sulphate and is
ground much finer than ordinary Portland cement. It also contains small
percentage of gypsum.
2. Calcium Chloride cement:
Calcium chloride accelerates the rate of strength development when added
up to 2% in the ordinary cement. It also imparts setting and hardening
properties to the cement in cold weather. A higher percentage, however causes
excessive shrinkage of concrete and also adversely affects the steel
reinforcements in case of R.C.C works.
3. High alumina cement:
This cement is manufactured by fusing together 40% each of bauxite and
lime; 15% of iron oxide and 5% of silica, magnesia etc. and then grinding the
resulting clinkers to a very fine powder. The fineness of grinding is higher
than that of Portland cement.
4. Slag cement:
Slag cements are produced by grinding blast furnace granulated slag with
a hydraulic binding agent like Portland cement clinkers lime or gypsum. When
Portland cement is used with slag it is called Portland sag cement. When lime
is used is called lime slag cement.
5. Expansive cement:
This is a form of hydraulic cement which expands during hardening and
setting. Concrete made with ordinary Portland cement shrinks while hardening
and has tendency to develop shrinkage cracks. This can be overcome by adding
expansive cement to the concrete mix made from ordinary cement. The concrete
thus made will have no tendency of shrinkage of expansion. Expansive cement
cannot be used for the construction of roof slabs, canal lining, tunnel lining,
concrete parchment etc.
STORAGE OF CEMENT
Storage of cement requires extra care to preserve its quality and fitness
for use. To prevent its deterioration, it is necessary to protect it from rain,
sun, winds and moisture.
Storage of cement is also a noteworthy item, because proper arrangements
for storing the cement have to be made in the factories before sale or on large
construction projects before use.
Moisture is the first and the greatest danger to be guarded against.
Cement has great affinity for moisture and hence it should be stored well
shielded from moisture laden current of air. The exposed cement is attacked by
air setting which gradually spreads. It gives rise to the formation of lumps.
If the lumps formed are so hard that they cannot be powered by passing between
the fingers, it should be concluded that the cement has been rendered useless
for any sound construction.
To prevent possibilities of lumping under pressure, the maximum height of
stack should not exceed 15 bags. The width of stacks should not be more than
four bags or 3 m. In stacks more than 8 bags high, the cement bags should be
arranged alternately lengthwise and cross wise so as to ensure stability of the
stacks and to prevent danger of the bags in stack toppling over.
In monsoon or in situations when it is necessary to store cement for
unusually long periods, the cement stacks should be completely enclosed by 700
gauge polyethene sheet or other water proofing membrane materials.
PROPERTIES OF CONCRETE
Strength, durability and workability may be considered as the main
properties of concrete. In addition, good concrete should be able to resist
wear and corrosion and it should be water tight, compact and economical.
1.Strength
The concrete must be strong enough to withstand without injury all the
imposed stresses with the required factor of safety. When the concrete mix has
been designed on the basis of maximum permissible water cement ratio, keeping
in view the requirements of durability, it will develop the required strength
if properly placed in position and cured. After placing concrete should not be
allowed to dry rapidly because moisture is very much essential for the
development for its high strength. Curing temperature also has a great effect
on the strength development of concrete. To develop a given strength longer
time of moist curing is required at lower temperature that is necessary while
curing is done at higher temperature.
2. Durability
It is the property of concrete by virtue of which it is capable of
resisting its disintegration and decay which may be caused due to:
a. Use of unsound cement.
b. Use of less durable cement.
c. Entry of harmful gases and salts through the pores and void present in
the concrete.
d. Freezing and thawing of water sucked through the cracks or crevices by
capillary action.
e. Expansion and resulting from temperature changes and alternate drying
and wetting.
The resistance of concrete to chemical attack, weathering, abrasion and
fire etc. depends mostly upon conditions of exposure, grade of concrete used,
quality of its materials and the extent of voids and pores present in the
concrete mass. The amount of cover provided over reinforcement and the degree
of imperviousness of concrete mix also influences the durability of the
concrete.
3. Workability
Despite all its importance workability is the most exclusive property of
concrete and is quite difficult to define and measure. It is simplest form of a
concrete is said to be workable if it can be easily mixed, handled,
transported, placed in position and compacted.
Lubricating effect of the cement paste which in turn is solely governed
by the degree of dilution, affects the workability of a concrete mix. A
workable concrete mix must be fluid enough so that it can be compacted with
minimum labour. A workable does not result in bleeding or segregation. Bleeding
of concrete takes place when the excess of water in the mix comes up at the
surface is caused when coarse aggregates have a tendency to separate from the
fine aggregates.
Workability of a concrete mix is appreciably affected by the following:
a. By reducing the proportion of coarse aggregates. It should be well
understood that the finer is the grading the greater will be the workability.
b. Shape of the aggregates has a great effect on the workability of
concrete.
c. By increasing the quantity of cement.
d. By the process of mixing.
REINFORCED CEMENT CONCRETE
Though strong in compression, concrete is extremely weak in tension. Its
resistance is so low that plain concrete can only be used where the member is
in pure compression. Steel on the other hand, is equally strong in compression
and tension, but it is apparent that while a long steel bar can develop its
full strength in tension, it cannot resist equal amount compressive force,
owing to its buckling due to the slenderness. Thus, the combination of concrete
of concrete and steel has proved to be ideal, as the two material are used to
take up the stresses they are most suitable for.
PROPERTIES OF REINFORCED CONCRETE
The combination of steel and concrete becomes practicable or workable or
account the following three reasons:
· The concrete while setting grips very
fast the surface of the steel bars. Therefore, the concrete is able to transmit
to the steel bars those stresses which it cannot resist itself.
· The co efficient of liner expansion
of concrete and steel are almost the same. Therefore, no internal stresses are
set up within reinforced concrete due to variations in temperature.
· The coating of cement grout on the
surface of steel bars protects them from corrosion and does not produce any
adverse chemical effect.
ADVANTAGES OF REINFORCED CONCRETE
· It is economical in ultimate cost.
· Its monolithic character gives much
rigidity to the structure.
· It is almost impermeable to moisture.
· The cost of maintenance of a
reinforced concrete structure is almost nil.
· It is durable and fire resisting. It
does not rot or decay and is not attacked by termites.
BASIC ASSUMPTIONS
· A transverse section plain before
bending remains plain after bending.
· The moduli of electrify of steel and
concrete are constant.
· All tensile stresses are taken up by
steel reinforcement only and the resistance of concrete to tension is nil.
CAUSES OF FAILURE OF REINFORCED
CONCRETE STRUCTURES
A reinforced concrete member can fail mostly in the following cases:
· When the member is subjected to excessive
tension, so as to exceed the permissible stress in steel.
· When the loading is such that the
compressive stress in concrete exceeds its safe permissible value.
· When the concrete is subjected to
excessive shear.
· On account of the slipping of the steel
bars from concrete.
· Due to bad quality of materials used,
shrinkage, creep or thermal effects.
GENERAL DESIGN REQUIREMENTS
An efficient design of a reinforced concrete structure requires that the
materials be economically selected, proportioned and arranged to carry the
required loads without developing stresses which are in excess of the allowable
working stress. The design requirements may be summarised into the following
steps:
· To decide suitable arrangement of
beams, columns, foundations etc. for the structure on the layout plan.
· To determine all types of loads and
forces to which the structure is likely to be subjected to.
· To analyse the structure and to
calculate, stresses, moments and shears etc. in the members.
· To work out safe section and area of
steel for different members.
· To provide sufficient anchorage for
all reinforcements, so that the anchorage and bond requirements are fully met.
0 Comments