Plain Cement Concrete (P C C) Work Procedure -
Plain cement concrete is
the mixture of cement, fine aggregate(sand) and coarse aggregate without steel.
PCC is an important component of a building which is laid on the soil surface
to avoid direct contact of reinforcement of concrete with soil and water.
In this article, we study
the procedure of laying PCC, dos and don’ts and advantages of laying plain
cement concrete.
Material Used in Plain Cement Concrete
The general specifications
of materials used in PCC are –
1. Coarse Aggregate
Coarse aggregate used in
the PCC must be of hard broken stone of granite or similar stone, free from
dust, dirt and other foreign matter. The stone ballast shall be 20 mm in size
and smaller. All the coarse material should be retained in a 5mm square mesh
and should be well graded so that the voids do not exceed 42%.
. Fine Aggregate
Fine aggregate shall be of
coarse sand consisting of hard, sharp and angular grains and shall pass through
a screen of 5 mm square mesh. Sand shall be of standard specifications, clean
and free from dust, dirt and organic matter. Sea sand shall not be used.
3. Cement
Portland Pozzolana cement
(P.P.C) is normally used for plain cement concrete. It should conform to the
specifications and shall have the required tensile and compressive stresses and
fineness.
Water
Water used shall be clean
and reasonably free from injurious quantities of deleterious materials such as
oils, acids, alkalis, salts and vegetable growth. Generally, potable water
shall be used having a pH value not less than 6. The maximum permissible limits
for solids shall be as per IS 456:2000 Clause 5.4, Page No 15.
Proportioning of Plain Cement Concrete
1.
The proportioning is done based on the requirement or given
specification. Generally 1:2:4 or 1:3:6 mix is used.
2.
The measurement of material can be done by weight batching or volume
batching.
3.
In volume batching, coarse aggregate and sand shall be measured by
measuring box of 30cmx30cmx38cm of a suitable size equivalent to one bag cement
of 1/30 m3or 0.035 m3.
4.
Sand shall be measured on the basis of its dry volume.
1.
While measuring the aggregate, sacking, ramming or hammering shall not be
done.
Mixing of Plain Cement Concrete
Mixing of PCC can be done
either manually or by the means of machines :
Hand Mixing
1.
PCC is allowed to be done by hand-mixing only for small-scale works.
2.
The base on which the concrete is mixed must be clean, watertight slab or
a steel platform.
3.
Sand and cement are mixed thoroughly followed with the addition of coarse
aggregate. Lastly, water is added and the mixture is mixed properly to gain an
even colour and consistency.
Machine Mixing
1. A measured quantity of
dry coarse aggregate, fine aggregate and cement shall be placed in their
respective hoppers.
2. The dry materials shall
be mixed in the mixing drum for at least four turns of the drum after which the
correct quantity of water shall be added gradually while the drum is in motion.
3. The total quantity of
water for the mixing shall be introduced before 25% of the mixing time has
elapsed and shall be regulated to achieve the specific water-cement ratio.
4. The mixing shall be
thorough to have a plastic mix of uniform colour.
Note: The quantity of water to be used for
each mix of 50kg cement to give the required consistency shall be as follows:
o Not more than 34 lit –
1:3:6 mix
o Not more than 30 lit –
1:2:4 mix
o Not more than 27 lit –
1:1 ½:3 mix
o Not more than 25 lit –
1:1:2 mix.
Laying of Plain Cement Concrete
1.
The PCC is laid in the layers of not more than 150mm thick and
thoroughly vibrated by the means of mechanical vibrators till a dense concrete
is obtained.
2.
Wherever needed, hand compaction shall be done with the help of wooden
tamping rods so that concrete is thoroughly compacted and completely walked
into the corners of the formwork.
3.
Compaction shall be completed before the initial setting starts that is
within thirty minutes of addition of water to the dry mixture.
Curing of Plain Cement Concrete
1.
Freshly laid concrete shall be protected from rain by suitable covering.
2.
After the concrete has begun to harden- that is about one to two hours
after it has been laid- it shall be protected with moist gunny bags, sand or
any other materials against quick drying.
3.
After 24 hours of laying of concrete, the surface shall be cured by
flooding with water of about 25mm depth or by covering with weight absorbent
materials.
4.
Curing shall be done for a minimum period of 14 days.
Dos and Don’ts of PCC Works
Dos
1.
PCC shuttering should be of the exact size and thickness
2.
Water should be mixed with a bucket, in a measured quantity, as per w/c
(water/ cement) ratio.
3.
Use the chute or additional labor to pour the concrete where the depth is
more.
4.
Remove any loose material from the sides of the pit, so that no soil or
other material will collapse in the pit during concreting.
5.
If the water table is high, then de-watering should be carried out at the
same time during concreting.
Don’ts
1.
Do not mix the materials on bare land.
2.
3.
Do not allow the PCC without formwork.
4.
Do not pour concrete without levelling and compacting.
5.
Do not pour concrete in the pit from a height of more than 1.5 m.
6.
Do not allow extra cement mortar on top of PCC. for smooth finishing.
Advantages of Laying Plain Cement Concrete
1.
Required cover to bottom reinforcement is ensured, as cover blocks rest
on a firm PCC.
2.
The effective depth of RCC members is achieved as the formworks can be
easily, uniformly and sturdily fixed, resulting in better dimension accuracy of
foundation RCC member.
3.
Reinforcement steel bars placed on PCC, are never in touch with the
ground soil, which may be chemically active and may lead to steel corrosion in
the immediate future.
4.
Ease in the placement of steel cages and increased productivity.
5.
Concrete does not bond with ground soil (clay); PCC acts as a barrier to
soil and bond well to overlayed structural grade concrete.
Plain
Cement Concrete – How to Produce and Construct
Plain cement concrete is used to provide a rigid impervious bed to RCC in
the foundation where the earth is soft and yielding. It can be used over brick
flat soling or without brick flat soling.
The short name of plain cement concrete is PCC. It is also defined
as Cement Concrete (CC) or Blinding
Concrete.
In this post, I’ll share about plain cement concrete, its properties, and
uses. I’ll also discuss how to produce and construct plain cement concrete.
What is Plain Cement Concrete?
When we say reinforced cement
concrete, we know that there are reinforcements inside that concrete. when
we don’t use any reinforcement inside the concrete, that is the plain
cement concrete. It’s just a mix of concrete
ingredients.
Properties of Plain Cement Concrete
The following are the most important properties of plain cement concrete:
·
Compressive
strength: 200 to 500 kilogram/square centimeter
·
Tensile
strength: 50 to 100 kilogram/square centimeter
·
Density: 2200
to 2500 kilogram/cubic meter
·
Durability: Excellent
Uses of Plain Cement Concrete
PCC is mainly used in footings, grade slabs, and concrete roads. The
purpose of using it in footings is to provide firm, rigid, non-porous,
impervious and leveled base for RCC footings.
We sometimes make
brick flat soling below PCC if the base soil is soft and
yielding.
Materials Used to Produce PCC
Cement: We generally use
Portland cement as bonding material in PCC.
Fine Aggregate: Sand is used as fine
aggregate. The FM of sand should be 1.2 to 1.5.
Coarse Aggregate: We normally use brick
chips for
producing PCC. Stone chips can also be used for this purpose. The
size of the coarse aggregate is 20mm downgrade.
Water: Pure drinkable water should be used in PCC.
How to Produce and Construct PPC?
Use the following techniques to produce and construct plain cement
concrete.
Tools Used for Producing PCC:
We normally use the following tools for producing Plain Cement Concrete-
·
Wooden or Steel rammer
·
Mixture machine (if any)
The Thickness of PCC:
The thickness of PCC is normally 50mm over
Brick Flat Soling (BFS). If you don’t use BFS below PCC then the thickness
should be 75mm.
When the PCC is used in car parking area then the thickness should be 75mmover BFS.
Ingredients Proportion of PCC:
The proportion of cement, sand and brick chips in foundation or basement
is 1:3:6. But, when used in the car parking area, the
proportion should be 1:2:4.
The Procedure of Manufacturing PCC:
If you want to use ready-mix concrete then you should skip this step. If you
want to produce PCC by mixture machine then click “How
to mix concrete by mixture machine”. If you don’t have a concrete mixture
machine and want to mix concrete by hand read “how
to mix concrete by hand”.
Placing and Compaction of PCC:
·
Make sure brick soling/sand bed level for PCC is ok.
·
Make formwork for PCC with wooden planks as per specified dimensions.
·
Clean dust and foreign materials from concreting area.
·
Spread polythene over the bed of PCC.
·
Make level pillars of fresh concrete in the area at suitable intervals
but not more than 2m c/c both ways.
·
Place the concrete gently from one side. Use the mixed concrete within 45
minutes after the water is added.
·
Use wooden rammer for compaction and finishing of PCC.
· Make the surface of PCC roughen for joining future work before the
concrete becomes harden.
Curing of PCC:
After 24 hours of laying PCC, moist the concrete surface by flooding with water
or by covering with moist gunny bags at least for seven days.
Checking and Inspection:
·
Check the dimensions of formwork of PCC before mixing concrete.
·
Check polythene sheet is laid over PCC bed.
·
Check the concrete slump (maximum slump should be 75mm)
·
Check the thickness and level of PCC before casting by putting steel pegs
in concreting area or putting a level pillar of fresh concrete at a suitable
distance.
·
Check the finish level of PCC by thread fixing with nails in formwork.
·
Inspect if the concrete is placing gently.
What is Plain Cement Concrete (PCC) in Construction of Foundation?
Concrete has
been used for many wonderful things throughout history, for either architecture
or for infrastructure work and more.
The term PCC stands for
plain cement concrete. The mixture of cement, fine aggregate (sand) and coarse
aggregate are generally called plain cement concrete (PCC). Before starting any
R.C.Cor masonry work directly on the excavated soil, PCC is done to form a
levelled surface and to avoid laying concrete on soil directly so as to avoid
mixing/ with soil and also to prevent soil extracting water from PCC thereby
weakening it.
How to Prepare Reinforcement Steel
before Repairing of the Concrete
Plain cement concrete can
also be called only “cement concrete (CC)” or “binding concrete”.
Some even call it “Mud Mat”.
Cement:
Portland Pozzolona cement
(P.P.C) is normally used for Plain cement concrete. It should conform to the
specifications and tests.
Sand:
Sand to be used for
concrete work should be clean, well graded, hard, strong, durable, and should
meet the requirements specified for its use.
Ways to Check the Quality of Sand at Site -
Aggregate:
Aggregate shall be of an
inert material and should be clean, dense, hard, Sound, Durable, Non-absorbent
and capable of developing a good bond with mortar.
Why Aggregates Have an Important
Role in Concrete?
Coarse Aggregate:
The size of the aggregate
used for PCC varies from 10-12 mm to 40 mm depending on where they are to be
used.
f the size of the aggregate
is more, it results in the reduction of cement consumption.
Coarse aggregate shall be
clean and free from elongated, flaky or laminated pieces.
It should be free from
adhering coat, clay lump, coal residue, clinkers, slag, alkali, mica, organic
matter or other substances
Coarse aggregate shall be
of hard broken stone of granite or similar stone, which is free from dust, dirt
and other foreign matters. The smaller size of the stone is 6.3 mm. All the
course material should be retained in a 6.3 mm square mesh and should be well
graded such that the void does not exceed 42%.
Fine Aggregate:
Fine aggregate shall
be of coarse sand consisting of hard, sharp and angular grains and shall pass through
a screen of 4.75mm square mesh.
Sand for PCC work shall be
clean and free from dust, dirt and organic matter or based on standard
specifications. Never used sea sand in foundation work.
Water:
Water shall be clean
and free from alkaline and substances should be suitable for drinking purposes.
Hand Mixing:
Mixing should be done on a
masonry platform or sheet iron tray.
Machine Mixing:
Stone ballast, sand and
cement shall be poured/ mixed in a concrete mixer to maintain the required
proportions. Machine mixing is always preferred, but under unavoidable
circumstances, hand mixing can be done but all quality check needs to be
assured.
Slump:
For maintaining the
required consistency and control the addition of water, the slump test should
be carried out regularly. A slump of 7.5cm to 10cm may be allowed for
foundation work.
Formwork:
Formwork centring and
shuttering shall be provided as per the standard specification before laying
concrete to confine to support or to keep the concrete in position. To prevent
sticking of concrete, the inner surface of shuttering shall be oiled.
Curing Method:
·
After two hours of laying of concrete, when the concrete has begun to
harden, it shall be kept damp by covering with wet gunny bags for 24 hours.
- After 24 hours of placing PCC, keep the
concrete surface moist by flooding with water or by covering with moist
gunny bags.
- Curing of PCC may be done by either pouring
water with buckets or with pipes.
- Curing should be carried out for a minimum 14
days.
A Strength of PCC/ Nominal Mix:
A strength of PCC is
defined as compressive strength after 28 days, expressed as M15, M20, where M
stands for Mix and 15 stands for 15 N/mm2 compressive strength
at 28 days. The proportions of materials (cement, sand, coarse aggregate) for
nominal mix/design mix concrete that are normally used are 1:3:6 or 1:4:8. As
strength is not a major criterion, the design mix is generally avoided.
Do’s
·
Check and get the soil strata approved from the structural Consultant
before doing of PCCin foundation.
·
PCC shuttering should be of the exact size and thickness
·
Dry materials should be uniformly mixed before adding water to it.
·
Water should be mixed with a bucket, in a measured quantity, as per w/c
(water/ cement) ratio.
·
Use the chute or additional labour to pour the concrete where the depth
is more.
·
Remove any loose material from the sides of the pit, so that no soil or
other material will collapse in the pit during concreting.
·
If the water table is high, then de-watering should be carried out at the
same time during concreting.
·
Compact and level the concrete properly.
·
Maintain the levels of PCC.
·
The next day, mark the centre of the column.
Don’ts
·
Do not mix the materials on bare land.
·
Do not just-pour the water in materials by means of a pipe. i.e. without
any measurement control.
·
Do not allow the PCC without formwork.
do not pour concrete without levelling and compacting.
·
o not pour concrete in the pit from a height of more than 1.5 m.
·
Do not allow extra cement mortar on top of PCC. for smooth finishing.
Plain Cement Concrete
Other
Uses of Plain Cement Concrete are listed below:
Plain cement concrete is
generally used for the foundation. However, its other uses are:
·
As bed concrete below the wall footings, column footings and on walls
below beams.
·
As sill concrete to get a hard and even surface at window and ventilator
sills.
·
As to coping concrete over the parapet and compound walls.
·
For flagging the area around the buildings.
·
To make pavements.
- To make tennis courts, basketball courts etc.
- Plinth Protection
- Storm/ Sewer at drains, small retaining walls.
·
Plain Cement Concrete
(PCC) consists of cement, fine aggregate, coarse aggregate and water. Concrete
admixtures are also added if required.
·
A strength of PCC is
defined as compressive strength after 28 days, expressed as M15, M20, etc.
where M stands for Mix and 15 stands for 15 N/mm2 compressive strength at 28
days. The proportions of materials (cement, sand, coarse aggregate) for nominal
mix/design mix concrete that are normally used are 1:3:6 or 1:4:8. The PCC must
be cured for 14 days.
Its uses are:
·
As bed concrete below the
wall footings, column footings and on walls below beams.
·
As sill concrete to get a
hard and even surface at window and ventilator sills.
·
As to coping concrete
over the parapet and compound walls.
·
To make pavements.
·
Plinth Protection
·
Storm/ Sewer at drains,
small retaining walls.
·
Concrete is a composite
material which is a mixture of sand nothing but fine aggregate, coarse
aggregate and for bond formation some amount of cement is added.
·
To get a mixture we want
to add water for making paste and the product obtained finally is called plain
cement concrete.
·
This mixture is used for
different purposes in construction fields. It is also called as PCC in short
form.
·
Sometimes we may confuse
about the difference between PCC and RCC (reinforced cement concrete) it means
providing reinforcement bars to the mixture. We can observe reinforced concrete
in pillars.
Plain Cement Concrete (PCC) is also called as Cement
Concrete (CC) or Blinding Concrete.It is used
for leveling, bedding for footings, grade slabs, concrete roads etc.PCC is used
to provide non-porous, rigid, impervious, firm and leveled bed for laying RCC,
where earth is soft and yielding.PCC can be use over brick flat soling or
without brick flat soling.PCC also used as filler like lump concrete; this is a
mix of PCC and boulder.It consists of cement, sand and coarse
aggregates mixed with water in the specified proportions
Plain Cement Concrete is basically a construction
material that is generally used as binding materials and is composed of cement,
and other cementitious materials like fly ash and slag cement, aggregate,
water, and chemical admixtures.
Plain cement concrete
(PCC) is used to provide rigid impervious bed to RCC in foundation where the
earth is soft and yielding. PCC can be used over brick flat soling or without
brick flat soling.
Plain cement concrete can
also called only “cement concrete (CC)” or “binding concrete”.
Materials used for
producing PCC:
Cement: we generally use Portland cement as bonding material in PCC.
Fine Aggregate: Sand is used as fine aggregate. The FM of sand should be 1.2 to 1.5.
Course Aggregate: We normally use brick chips for producing PCC. Stone chips can also be
used for this purpose. The size of the course aggregate is 20mm downgrade.
Water: Pure drinkable water should be used in PCC.
Plain concrete is *just* concrete. Plain concrete,
such as in a sidewalk, is merely concrete poured into a form and troweled or
otherwise finished on the surface. It is quite durable, but is prone to
fracturing as the subsoil shifts, or if frost or tree roots press upward from
below. Basement floors in a private home are usually made of plain concrete,
because the floor loading is likely to be modest, the subsoil is unlikely to
shift, and there will likely not be a problem with frost or roots.
Concrete is a very
complex recipe. Sure it has 4 main ingredients cement, sand, aggregate and
water. However there are also other liquid additives that deal with micro air
bubbles (entrained & entrapped) within the mixture that have a large impact
on strength.
So “Plain” cement-concrete I would assume to be a given recipe of
sand/cement/aggregate and water without the liquid additives.
Specifications for Plain Cement Concrete
(PCC) | Building Estimation and Costing
Plain Cement Concrete (PCC) is a construction material generally
used as a binding materials and is composed of cement, (commonly Portland
Cement) and other cementitious materials such as fly ash and slag cement,
aggregate (generally a coarse aggregate made of gravels or crushed rocks
such as limestone or granite, plus a fine aggregate such as sand), water,
and chemical admixtures.
Materials Specifications -
Aggregate shall be of invert
materials and should be clean, dense, hard, sound, durable, non-absorbent and
capable of developing good bond with mortar.
Coarse aggregate shall be of hard
broken stone of granite or similar stone, free from dust, dirt and other
foreign matters. The stone ballast shall be of 20mm size and smaller. All the
coarse material should be retained in a 5mm square mesh and should be well
graded such that the voids do not exceed 42%.
Fine aggregate shall be of coarse
sand consisting of hard, sharp and angular grains and shall pass through a
screen of 5mm square mesh. Sand shall be of standard specifications, clean and
free from dust, dirt and organic matter. Sea sand shall not be used.
Cement shall be fresh
Portland cement of standard ISI specifications and shall have the
required tensile and compressive stresses and fineness.
Water shall be clean and
free from alkaline and acid matters and suitable for drinking purposes.
Proportion Specifications
1:2:4 (cement : sand : stone ballast) by volume when specified. Minimum
compressive strength of concrete of 1:2:4 proportion shall be 140 kg/cm2 in 7
days.
Hand mixing -
Mixing shall be done on masonry platform or sheet iron tray.
Machine mixing -
Stone ballast, sand and cement shall be put into cement concrete mixer to have the required proportions.
Slump
Regular slump test should be carried out to control the addition of water
and to maintain the required consistency. A slump of 7.5cm to 10cm may be
allowed for building work.
Formwork -
Formwork centering and shuttering shall be provided as required as per
the standard specification before laying concrete to confine to support or to
keep the concrete in position. The inner surface of shuttering shall be oiled
to prevent concrete sticking to it.
Laying Technique -
Concrete shall be laid gently (not thrown) in layers not exceeding 15cm
and compacted by pinning with rods and tamping with wooden tampers or with
mechanical vibrating machine until a dense concrete is obtained.
Curing Method
After about two hours of laying of concrete, when the concrete has begun
to harden, it shall be kept damp by covering with wet gunny bags or wet sand
for 24 hours.
Why Is Plain Cement Concrete (PCC) Laid Before Any
Reinforced Cement Concrete?
Plain Cement Concrete of
M15 grade (thickness of 100mm) are mandatorily placed on a ground/soil
strata, to give a firm,clean base for placing reinforcement
cage,as we start building foundation of a RCC structure.
It is also help in
following ways:
1.Required cover to bottom
reinforcement is ensured ,as cover blocks rest on a firm PCC .
2.The effective depth of
RCC members is achieved as the formworks can be easily,uniformly and sturdily
fixed ,resulting in better dimension accuracy of foundation RCC member.
3.Reinforcement steel
placed on a PCC, are never in touch with ground soil ,which may be chemically
active may lead steel corrossion in immediate future (when in contact with
ground soil )
4.Ease placement of steel
cage and increase productivity.
5. Concrete don’t bond with
ground soil (clay),PCC acts a barrier to soil and bond well to overlayed
structural grade concrete.
Note : Ensure good curing
of PCC too.
The mixture of cement, FINE aggregate (sand) and coarse aggregate is
called plain cement concrete (PCC) PCC is mainly used for following
purposes
1. It is
used as a protective layer for the RCC above so that water from the RCC is not
absorbed by the earth below.
2. Provides
a base for the concrete and also helps workers to set out the structure above
in a easier way
3.act as a
cover to reinforced cement concrete i.e. resist corrosion of steel bars in
footings
4.moisture
available in soil should not absorbed by R.C.C footings which causes corrosion
of reinforcemen
Concrete is good in compression but least in tension, so we need to form
platform as PCC below footing which take the load of the entire structure which
minimize the effect of tension in RCC works like footings, beams etc. Also
steel is used in bottom of concrete bed to resist vertical tensile stresses. By
these two principal structure is safe.
First is the compaction sub base may contain deleterious substances,
alkaline, harmful salts etc. which will causes corroded the reinforcement of
RCC.
No.2 -is it
may porous & the cavity would soak water & cement slurry that may
decrease the strength of RCC,
No.3-PCC
support RCC as a hard & level surface against the soft & unduled sub
base
No.4-load
of RCC will uniformly distributed sub-grade & below sub base.
DETAIL AND GENERAL SPECIFICATION
FOR THE PLAIN CEMENT CONCRETE (PCC) WORK IN ISOLATED COLUMN FOOTING -
GENERAL SPECIFICATION
·
In plain cement concrete coarse aggregate should be hard durable and free
from impurities.
·
Fine aggregate should contain sharp, angular grain.
·
Cement should be fresh Portland cement.
·
Mixing should be done by hand mixing or by machine mixing.
·
Laying and compaction should be done before setting of concrete i.e.
within 30 min.
·
Curing should be done for minimum 14 days.
DETAIL SPECIFICATION -
MATERIALS
Coarse aggregate:
Aggregates
shall comply with the requirements of IS 383.
It
shall be crushed or broken from hard stone from the approved quarry.
· It shall be hard, strong, dense and durable, clean and free from soft
friable, thin, flat, elongated or laminated, flaky pieces and shall be roughly
cubical in shape.
·
It shall be clean and free from dirt and any other foreign matter.
·
Coarse aggregate may be graveled, may either be river bed shingle or pit
graveled. It shall be sound, hard, clean suitably graded in size.
·
This shall be free from flat particles of shale, powdered clay, slate,
loam and other impurities.
·
Gravel shall have to be washed if it contains soil materials adhering to
it.
·
Unless specially mentioned the size of the coarse aggregate shall be 20
mm graded down and shall be retained in 5mm square mesh so that the voids do
not exceed 42%.
Fine aggregate:
·
Aggregate most of which passes 4.75mm IS sieve is known as fine
aggregate. Sand as fine aggregate shall be coarse, consisting of sharp, angular
grains and be of standard specification.
·
Crushed dust stone may also be used as fine aggregate.
Cement:
·
Cements shall be phrased as Portland cement and confirmed to the ISI
specification.
·
It should have required compressive and tensile strength and fineness.
Water:
·
Water used shall be clean and reasonably free from injurious quantities
of deleterious materials such as oils, acids, alkalis, salts and vegetable
growth.
·
Generally potable water shall be used.
·
The pH value of the water should not be less than 6.
· The maximum permissible limits for solids shall be as per IS 456:2000 Clause 5.4, Page No 15.
PROPORTION
·
Proportioning of cement, sand and coarse aggregate shall be 1:2:4 or as
specified.
·
Coarse aggregate and sand shall be measured by measuring box of
30cmx30cmx38cm of suitable size equivalent to one bag cement of 1/30 m3or
0.035 m3.
·
Cement shall be measured by bag weighing 50kg.
·
Sand shall be measured on the basis of its dry volume.
·
While measuring the aggregate sacking, ramming or hammering shall not be
done.
·
If damp sand is used compensation shall be made by adding additional sand
to the extent required for the bulking of damp sand.
MIXING
Hand mixing:
Hand mixing by batches shall be permitted on small works.
The mixing shall be done on a clean water tight masonry or concrete slab or steel plate platform.
·
Measured quantity of sand shall be sprayed evenly. The cement shall be
dumped on the sand and distributed evenly. The sand and cement shall be mixed
thoroughly with spade turning the mixture over and over again until its even
colour throughout and free from streaks. The measured quantity of coarse
aggregate shall be sprayed out and the sand cement mixture shall be sprayed on
its top.
·
This shall be mixed at least three times by shoveling and turning over by
twist from centre to side then back to the centre and again to the sides.
·
A hollow shall be made in the middle of the mixed pile.
·
Three quarters of the total quantity of water required shall be added
while the material is turned in toward the centre with spades.
·
The remaining water shall be added by water can fitted with rose head
slowly turning the whole mixtures over and over untill a uniform colour and
consistency is obtained throughout the pile.
Machine Mixing -
·
Measured quantity of dry coarse aggregate, fine aggregate and cement
shall be placed in the hopper respectively.
·
The dry materials shall be mixed in the mixing drum for at least four
turns of the drum after which correct quantity of water shall be added
gradually while the drum is in motion.
·
The total quantity of water for the mixing shall be introduced before 25%
of the mixing time has elapsed and shall be regulated to achieve the specific
water cement ratio.
·
The mixing shall be thorough to have a plastic mix of uniform colour.
MIXING TIME:
The materials shall be mixed in a drum for a period of not less than 2
min and until a uniform colour and consistency obtained.
·
The time shall be counted from the moment all the materials have been put
into the drum.
CONSISTENCY
·
The quantity of water to be used for each mix of 50kg cement to give the
required consistency shall be as follows
LAYING -
·
The entire concrete used in the work shall be laid gently in layers not
exceeding 15cm and shall be thoroughly vibrated by means of mechanical
vibrators till a dense concrete is obtained.
·
Hand compaction shall be done with the help of punning rods and tamping
rods and tamping with the wooden tampers so that concrete is thoroughly
compacted and completely walked into the corners of the form work.
·
The layers of concrete shall be so placed that the bottom layer does not
finally sit before the top layer is placed.
· Compaction shall be completed before the initial setting starts that is within thirty minutes of addition of water to the dry mixture.
PROTECTION AND CURING -
· Freshly laid concrete shall be protected from rain by suitable covering
· After the concrete as begun to harden, that is about one to two hours
after its laying it shall be protected with moist gunny bags, sand or any other
materials against quick drying.
· After 24hrs of lying of concrete the surface shall be cured by flooding
with water of about 25mm depth or by covering with weight absorbent materials
.Curing shall be done for a minimum period of 14 days.
FORMWORK -
· If centering and shuttering are required to be done for this work.
·
This shall be done in accordance with the specifications for form work
under R.C.C
·
Concrete is a composite material composed of cement, aggregates (fine and coarse aggregate), and water. Cement and water are
binders for all ingredients in the mix. The aggregate generally is graded
in size from
sand to gravel, with
the maximum gravel
size in structural concrete commonly being
·
¾”.
·
·
Concrete is a mixture
of cement paste
and aggregate, each of which
has an essentially linear
and brittle stress-strain relationship in compression. Plain Concrete is considered brittle because it has no ability to show little
yielding before failure as shown
in Figure1.
·
Stress (σ) is the applied load
(P) divided by the area it is acting on (typically
the cross-sectional area
of the member),
given in equation
1 below:
· σ = P/A
·
Strain (ε) is the elongation or contraction of a material per unit length
of the material. According to Hooke's Law (σ = Eε) stress
is dependent on strain in the
material. The modulus
of elasticity (E) or Young's
modulus of a material is a constant associated with Hooke's Law as shown
in Eq.2.
·
Compressive Strength Tests -
·
Compressive strength
is the amount
of compressive stress
that a material can resist
before failing standard acceptance tests. Measuring the strength of concrete involves
short time compression tests on cylinders 6 inches diameter by 12 inch high as shown in Figure 2.
·
Test cylinders for an acceptance test
must be allowed
to harden in their
molds for 24 hours at the job
site at 60-80
⁰F and then cured in a moist
room at 73 ⁰F or immersed in water saturated with lime.
· The standard test is carried out when the concrete is 28 days old. The standard strength “test” is the average of the strength of two cylinders from the sample test.
·
The tensile strength of concrete varies
between 8-15% of the compressive strength.
·
Two types of tests
available, the modulus
of rupture or flexural test,
in which a plain
concrete beam generally 6” x 6” x 30” Long is loaded in flexure at third
point of a 24 inch
span until it fails a shown in Figure 3 below.
·
Flexural tensile or modulus
of rupture, fr is calculated using equation 3 below.
0.37
· Where, is the concrete compressive strength in ksi.
·
The second test
is the split cylinder test
in which a standard 6inch
x 12 inch compression
· test is placed on its side and loaded in compression along a diameter as shown in Figure 4.
·
Concrete Slump Test -
·
This test is used to measure
the workability of concrete. A slump cone
is placed and filled
to 1/3 of the cone.
A rod is used to tamp the
concrete 25 times. Fill another
1/3 and tamp
25 times. Fill
the rest of the way
and make sure its smooth
on top. Slowly
remove the cone
and place it by side
and measure the slump.
·
Hint:You need to remember this word(Workability)a word
problem might be asked and you need to understand the concept.
·
Water Cement ration-
·
Water/Cement
ratio (w/c) is the ratio
of water to cement in any concrete
mix. High water cement
ratio would decrease the strength of concrete and increase workability.
·
On the
other hand, low water cement
ratio would decrease work ability and
· increase strength.
·
Creep
·
Creep is the strain
that occurs under
a sustained load.
Concrete will continue to deform, but at a rate
that diminishes with
time. It increases with increasing the w/c ratio.
Creep increases deflection of concrete beams.
·
Admixtures -
· Accelerating admixtures are used to reduce the time of setting and accelerating early strength and often used in cold weather, when it takes too long for concrete to set naturally.
· Water reducers will lubricate the mix. Reduction in water content of a mix may result in either a reduction in w/c ratio for a given slump and cement content or increase slump for same w/c ratio and cement content. Having less water and same cement content, concrete will attain greater strength. Water reducers are preferable in Hot weather conditions.
·
Structure Advantage and Disadvantage of Concrete -
·
1-Concrete is extremely strong
in compression and therefore has a high compressive strength
·
Structure Disadvantages to using concrete -
·
1-Concrete is a brittle
material and can crack or break with
little warning 2-Concrete is very weak
when a tension force is applied to it since
it has a
·
very low tensile strength. (To address this
weakness, steel is often embedded within the concrete at locations where
tension forces are known to exist,
making reinforced concrete. In a simple
concrete beam, the steel would
be placed along the bottom
of the beam.)
·
3-Because a certain
amount of time is needed
for hydration to completely
occur, concrete members
do not gain their full strength until
much time has passed.
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