INTRODUCTION
Concrete is a widely used
construction material. It is produced by mixing properly proportioned
quantities of aggregates, cement, and water by weight. Admixtures may be added
in the mixture to produce concrete of specified quality. Concrete may be molded
into any desired shape or size. Concrete is reinforced with steel to make a
composite material that combines the ceramic properties of concrete with the
tensile strength of steel. The integrity of reinforced cement concrete depends
to a large extent on the reinforcing steel, plain or deformed.
Concrete in its plastic state must
remain workable until it is placed in position and compacted. Hardened concrete
must be durable against the process of deterioration. there are different types
of concrete, such as shotcrete, light-/heavy weight concrete, ready-mixed concrete,
high performance concrete, self-compacting concrete, polymer-modified concrete,
fiber reinforced concrete. The influence of the elastic incompatibility of
steel and concrete is avoidable in pre-stressed concrete.
There are specification and guideline
for production transportation, and placing of concrete in the extreme hot/cold
weather. Underwater concreting is a special operation that may have to be
carried out in remote and difficult areas in unusual environment. Timber or
metal forms are used to mould the concrete as per the equipment. Stripping of
forms from the set concrete demands the same care that goes into the
fabrication and erection of forms. Curing process is a very important factor in
deterring the strength of concrete.
Quality of concrete is to be
assured right from the stage of procurement of the ingredient of concrete
including testing of water available at the site. Quality of concrete is to be
checked by carrying out both destructive and non-destructive tests.
2.
DEFINITION
OF CONCRETE
Concrete is a versatile
construction material. It is defined as a properly proportioned, homogeneous,
and dense mixture of fine and coarse aggregates, cement, and water with or
without admixtures. The aggregates are considered as economic filler materials
generally inert in nature. Cement and water comprised a continuous binder phase
that, after hardening, holds the aggregates together into a compact mass having
load-bearing capacity. There are many binders, such as asphalt, sulphur, epoxy
resin, and so on but the unqualified term ‘concrete’ means that the binder is
principally hardened cement paste, which is the product of the chemical
reaction of the ordinary Portland cement and water. In common parlance, the
term concrete implies ordinary Portland cement concrete despite wide acceptable
of the blended cements.
An admixture is a material other
than the essential ingredients such as water, aggregates, and Portland cement
(ordinary/blended) used as an ingredient of concrete and added to it
immediately before or during its production.
Concrete is the most widely used
material, second only to water, which is used in larger quantities. Concrete is
a material that has been used in some form since the ancient times because:
It possesses excellent resistance
to water unlike wood and steel. It can be easily cast or formed into any
predetermined shape or size. But the area of modern concrete dates from the
middle of the nineteenth century with the advent of the first truly ’Portland’ cement.
Although aggregates constitute about 70% of the produced concrete, it is the
cement paste that is responsible for most of the good and bad qualities of
concrete. For a given aggregate, physical and chemical characteristics of the
cement paste determine the workability of plastic concrete such as strength,
durability and dimensional stability. However, it has certain weakness. It is
brittle and very poor in tension. Ductility and toughness are also poor.
Despite its deceptive uniformity,
cement is not a unified chemical entity. It comprises at least four phases,
which retain their different chemical identities during the hydration process.
Concrete strength and durability are affected significantly by the relative
proportion of these four phases of cement with some minor constituents of
cement influencing concrete durability. It is well known that reinforced
concrete is a composite material, combining the ceramic properties of concrete
with the tensile strength of steel. The quality of both these components is
essential to increase the maintenance free life of reinforced concrete
structures in marine conditions, severe atmosphere pollution, and other extreme
conditions.
3.
HISTORY
The word concrete comes from the
Latin word" concretus " (meaning compact or condensed), the perfect
passive participle of" concrescere ", from " con-"
(together) and" crescere " (to grow) Perhaps the earliest known
occurrence of cement was twelve million years ago, when a natural deposit
formed after an occurrence of oil shale naturally combusted while adjacent to a
bed of limestone. These ancient deposits were investigated in the 1960s and
1970s. On a human time-scale, lime mortars were used in Greece, Crete, and
Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of fully
waterproof concrete. German archaeologist Heinrich Schliemann found concrete
floors, which were made of lime and pebbles, in the royal palace of Tiryns,
Greece, which dates roughly to 1400-1200 BC. Concrete was used for construction
in many ancient structures.
The Romans used concrete extensively from300 BC to 476 AD, a span of more than
seven hundred years. During the Roman Empire, Roman concrete (or opus
caementicium) was made from quicklime, pozzolana and an aggregate of pumice.
Its widespread use in many Roman structures, a key event in the history of
architecture termed the Roman Architectural Revolution, freed Roman
construction from the restrictions of stone and brick material and allowed for
revolutionary new designs in terms of both structural complexity and dimension.
“Concrete as roman knew it, was a
new and revolutionary material. Laid in the shape of arches, vaults and domes,
it quickly hardened into a rigid mass, free from many of the internal thrusts
and strains that troubled the builders of similar structures in stone or brick”
Modern tests show that opus
caementicium had as much compressive strength as modern Portland-cement
concrete (ca. 200 kg/cm 2). However, due to the absence of
reinforcement, its tensile strength was far lower than modern reinforced
concrete, and its mode of application was also different:
“Modern structural concrete differs
from Roman concrete in two important details. First, its mi x consistency is
fluid and homogeneous, allowing it to be poured into forms rather than
requiring hand-layering together with the placement of aggregates, which in
Roman practice, often consisted of rubble. Second, integral reinforcing steel
gives modern concrete assemblies great strength in tension, whereas roman
concrete could depend only upon the strength of the concrete bonding to resist
tension.
The widespread use of concrete in
many Roman structures has ensured that many survive to the present day. The
Baths of Caracalla in Rome are just one example. Many Roman aqueducts and
bridges have masonry cladding on a concrete core, as does the dome the Pantheon
.After the Roman Empire, the use of burning lime and pozzolana was greatly
reduced until the technique was all but forgotten between 500 AD and the 1300s.
Between the 1300s until the mid-1700s, the use of cement gradually returned.
The Canal du Midi was built using concrete in 1670, and there are concrete
structures in Finland that date from the 16th century.
Perhaps the greatest driver behind
the modern usage of concrete was the third Eddy stone Lighthouse in Devon,
England. To create this structure, between 1756 and 1793, British engineer John
Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and
powdered brick as aggregate. A method for producing Portland cement was patented
by Joseph Aspdin on 1824. In 1889 the first concrete reinforced bridge was
built, and the first large concrete dams were built in 1936, Hoover Dam and
Grand Coulee Dam. Reinforced concrete was invented in 1849 by Joseph Monier.
4.
COMPOSITION
OF CONCRETE
Concrete is a composite material
composed of coarse granular material (the aggregate or filler) embedded in a
hard matrix of material (the cement or binder) that fills the space between the
aggregate particles and glues them together. We can also consider concrete as a
composite material that consists essentially of a binding medium within which
are embedded particles or fragments of aggregates. The simplest representation
of concrete is:
Concrete = Filler + Binder.
According to the type of binder used, there
are many different kinds of concrete. For instance, Portland cement concrete,
asphalt concrete, and epoxy concrete. In concrete construction, the Portland
cement concrete is utilized the most.
The composition can be presented as
follows
Cement (+ Admixture) +Water→ Cement
paste + fine aggregate →mortar+ coarse aggregate →concrete
Here we should indicate that
admixtures are almost always used in modern practice and thus become an
essential component of modern concrete. Admixtures are defined as materials
other than aggregate (fine and coarse), water, fibre and cement, which are
added into concrete batch immediately before or during mixing. The widespread
use of admixture is mainly due to the many benefits made possible by their
application. For instance, chemical admixtures can modify the setting and
hardening characteristic of cement paste by influencing the rate of cement
hydration. Water-reducing admixture can plasticize fresh concrete mixtures by
reducing surface tension of water, air-entraining admixtures can improve the
durability of concrete, and mineral admixtures such as pozzolans (materials
containing reactive silica) can reduce thermal cracking. A detailed description
of admixtures will be given in latter sections
There are many types of concrete
available, created by varying the proportions of the main ingredients below. In
this way or by substitution for the cementitious and aggregate phases, the
finished products can be tailored to its application with varying strength,
density, or chemical and thermal resistance properties.
“Aggregate " consists of large chunks of material in a concrete
mix, generally coarse gravel or crushed rocks such as limestone, or granite,
along with finer materials such as sand.
“Cement ", commonly Portland cement, and other cementitious
materials such as fly ash and slag cement, serves as a binder for the aggregate.
Water is then mixed with this dry composite, which produces a semi-liquid that
workers can shape (typically by pouring it into a form). The concrete
solidifies and hardens to rock-hard strength through a chemical process called hydration. The water reacts with the
cement, which bonds the other components together, creating a robust stone-like
material.
“Chemical admixtures" are added to achieve varied properties.
These ingredients may speed or slow down the rate at which the concrete
hardens, and impart many other useful properties.
"Reinforcements” are often added to concrete. Concrete can be
formulated with high compressive strength, but always has lower tensile
strength. For this reason, it is usually reinforced with materials that are
strong in tension (often steel).
“Mineral admixtures" are becoming more popular in recent
decades. The use of recycled materials as concrete ingredients has been gaining
popularity because of increasingly stringent environmental legislation, and the
discovery that such materials often have complimentary and valuable properties.
The most conspicuous of these are fly ash, a by-product of coal-fired power
plants, and silica fume, a byproduct of industrial electric furnaces. The use
of these materials in concrete reduces the amount of resources required as the
ash and fume acts as a cementer placement. This displaces some cement
production, an energetically expensive and environmentally problematic process,
while reducing the amount of industrial waste that must be disposed of.
5. THE MIXING DESIGN AND PROCESS
The mix design depends on the type
of structure being built, how the concrete is mixed and delivered, and how it
is placed to form the structure. Thorough mixing is essential for the
production of uniform, high quality concrete. For this reason, equipment and
methods should be capable of effectively mixing concrete materials containing
the largest specified aggregate to produce uniform mixtures of the lowest slump
practical for the work. Separate paste mixing has shown that the mixing of
cement and water into a paste before combining these materials with aggregates
can increase the compressive strength of the resulting concrete. The paste is
generally mixed in a high-speed, shear-type mixer at a w/cm (water to cement
ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures
such as accelerators or retarders, superplasticizers, pigments, or silica fume.
The premixed paste is then blended with aggregates and any remaining batch
water and final mixing is completed in conventional concrete mixing equipment.
5.1. MATERIALS USED ON SITE
At the site there
was concrete plant where all processes of mixing were done; here are some
materials used at the site and some materials described below were not used for
our site but because of their need in some cases of concrete mixing some was
planned and available on site.
Cement
Portland cement is the most common
type of cement in general usage. It is a basic ingredient of concrete, mortar
and plaster. English masonry worker Joseph Aspdin patented Portland cement in
1824. It was named because of the similarity of its color to Portland
limestone, quarried from the English Isle of Portland and used extensively in
London architecture. It consists of a mixture of oxides of calcium, silicon and
aluminum. Portland cement and similar materials are made by heating limestone
(a source of calcium) with clay and grinding this product (called clinker) with
a source of sulfate (most commonly gypsum). In modern cement kilns many
advanced features are used to lower the fuel consumption per ton of clinker
produced.
Water
Combining water with a cementitious
material forms a cement paste by the process of hydration. The cement paste
glues the aggregate together, fills voids within it, and makes it flow more
freely. A lower water to cement ratio yields a stronger, more durable concrete,
while more water gives a freer-flowing concrete with a higher slump. Impure
water used to make concrete can cause problems when setting or in causing
premature failure of the structure. Hydration involves many different
reactions, often occurring at the same time.
As the reactions proceed, the
products of the cement hydration process gradually bond together the individual
sand and gravel particles and other components of the concrete, to form a solid
mass.
Reaction:
Cement chemist notation: C 3 S + H
→ C-S-H + CH
Standard notation: Ca3SiO
5 + H2O → (CaO) · (SiO 2) · (H 2O)
(gel) + Ca (OH) 2
Balanced: 2Ca 3SiO5
+ 7H 2O → 3(CaO) ·2(SiO 2) ·4(H 2O)
(gel) +3Ca (OH) 2
Aggregates
Crushed stone aggregate
Fine and coarse aggregates make up
the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used
mainly for this purpose. Recycled aggregates (from construction, demolition and
excavation waste) are increasingly used as partial replacements of natural
aggregates, while a number of manufactured aggregates, including air-cooled
blast furnace slag and bottom ash are also permitted. The presence of aggregate
greatly increases the durability of concrete above that of cement, which is a
brittle material in its pure state. Thus, concrete is a true composite
material. Redistribution of aggregates after compaction often creates
inhomogeneity due to the influence of vibration. This can lead to strength
gradients. Decorative stones such as quartzite, small river stones or crushed
glass are sometimes added to the surface of concrete for a decorative
"exposed aggregate" finish, popular among landscape designers. In
addition to being decorative, exposed aggregate adds robustness to a concrete
driveway.
Reinforcement
Installing rebar in a floor slab during a concrete pour. Concrete is strong in
compression, as the aggregate efficiently carries the compression load.
However, it is weak in tension as the cement holding the aggregate in place can
crack, allowing the structure to fail. Reinforced concrete adds steel
reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile
loads.
Chemical admixtures
In some cases of concrete at the
site the use of admixture was needed, for example when the concrete was needed
at the site of morocco the retarders was added in the concrete. Chemical
admixtures are materials in the form of powder or fluids that are added to the
concrete to give it certain characteristics not obtainable with plain concrete
mixes. In normal use, admixture dosages are less than5% by mass of cement and
are added to the concrete at the time of batching/mixing. The common types of admixtures are as
follows.
1. Accelerators speed up the hydration (hardening) of the concrete.
Typical materials used are CaCl 2, Ca (NO 3) 2 and NaNO3. However,
use of chlorides may cause corrosion in steel reinforcing and is prohibited in
some countries, so that nitrates may be favored.
2. Retarders slow the hydration of concrete and are used in large
or difficult pours where
partial setting before the pour is complete is undesirable. Typical polyol
retarders are sugar,
sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.
3. Air entrainments add and entrain tiny air bubbles in the
concrete, which reduces damage during freeze-thaw cycles, increasing
durability. However, entrained air entails a trade off with strength, as each
1% of air may decrease compressive strength 5%. Plasticizers increase the
workability of plastic or "fresh" concrete, allowing it be placed
more easily, with less consolidating effort. A typical plasticizer is
lignosulfonate. Plasticizers can be used to reduce the water content of a
concrete while maintaining workability and are sometimes called water-reducers
due to this use. Such treatment improves its strength and durability
characteristics. Superplasticizers (also called high-range water-reducers) are
a class of plasticizers that have fewer deleterious effects and can be used to
increase workability more than is practical with traditional plasticizers.
Compounds used as superplasticizers include sulfonated
naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate,
acetone formaldehyde condensate and polycarboxylate ethers Pigments can be used
to change the color of concrete, for aesthetics. Corrosion inhibitors are used
to minimize the corrosion of steel and steel bars in concrete. Bonding agents
are used to create a bond between old and new concrete (typically a type of
polymer). Pumping aids improve pumpability, thicken the paste and reduce
separation and bleeding.
6. EQUIPMENT AND MACHINERY USED
Ø Mixer
machine
Ø Cement
pump
Ø Water
pump
Ø balances
Ø Admixes
pump
Ø Computer
Ø Generators
Ø Bulldozers
Ø Concrete
mixer trucks
Ø Concrete
pump
7. Skills and man power used
·
Engineers
·
Concrete specialists
·
Foremen
·
Students
·
Skilled and unskilled labourers
8. SAFETY PRECAUTIONS
Because cement is dangerous for
human body; during concrete work all workers at site were obliged to wear over
coats, boots, and helmet and grooves to prevent them against any kind of harm.
