Plaster can be defined
as lean mortar, mostly used to cover masonry surfaces (brickwork or stone
works).
Mostly,we use plaster for interiorwalls of construction(houses,lintels,flooring,ceiling,etc)
They are lean mixtures of binding materials (lime or cement) with filler materials such as sand or
crushed stone.
And the process of covering surfaces with plaster is called plastering.
They are specially prepared for two reasons:
1. For decoration.
2. For protection.
As a decorative finish, plasters are used to give many attractive colors and finish designs for construction and adds to asthetics in terms of design.
In protective cover, plaster protects bricks or stones from direct destructive attacks of air, rain,moisture and harmful gases from factories or industries.
Plastering refers to the protection provided for a wall or ceiling in terms of coating the surface by laying plaster (cement plaster). Plastering is done to remove surface imperfections due to brickwork and to provide smooth surface for painting.
Different types of plastering materials are used,but cement plaster is used extensively.Some other types of plastering materials include lime plaster,clay plaster, etc.
HOW TO APPLY CEMENT PLASTER ON WALL SURFACE:
Cement plaster is usually applied in
single coat or double coat.Cement plastering is commonly used as an ideal
coating for the exterior and interior surface of a wall.
Double coat plaster is
applied where the thickness of the plaster is required to exceed 15 mm or when
it is necessary to obtain a very fine finish.
The process of applying a double
coat cement plaster to the wall surface consists of the following steps:
Step 1 - Surface Preparation for
Plastering.
Keep all the mortar joints of the
wall thick, to give a good bonding to hold the plaster.
Clean all wall joints and surfaces with a wire brush, there should be no grease or grease etc. on the wall surface.
Roughen the entire
wall to be plastered.
Wash the mortar joints and the
entire wall to be plastered, and keep it wet for at least 6 hours before
applying cement plaster.
Step 2 - Ground work for
plaster.
The dots are fixed horizontally on
the wall and then vertically at a distance of about 2 metres, covering the
entire wall surface.
Through the plumb-bob, check the
verticality of the dots.
After fixing the dots, vertical
strips of plaster, known as screeds, are formed between the dots. These serve
as screw gauges to maintain plaster thickness.They hold the plasters and give
uniformity of surface.
Step 3 - Applying first coat (or
under coat or coat).
The ratio of cement and sand for the
first coat plaster varies from 1: 3 to 1: 6.
In the case of brick masonry the thickness of the first coat plaster is in the normal 12 mm and in the case of concrete masonry the thickness varies from 9 to 15 mm.
Apply the first coat of plaster between the spaces formed by screws on the surface of the wall. This is done through trowel.
Level the surface through flat wooden floats and straight edges of wood.
After leveling, leave the first coat to set but not to dry and then thicken it with a scratch tool to key the second coat of plaster.
Step 4 - Second coat (or
finishing coat or fine coat).
The thickness of the second coat or
finishing coat may vary between 2 and 3 mm.
The ratio of cement and sand for the second coat of plaster varies from 1: 4 to 1: 6.
Before applying the second coat, moisten the first coat evenly.
To eliminate joining marks,the finishing coat should be applied from starting at top towards bottom.
Care
to be Taken After Plastering:
Curing the works
for a minimum of 10 days to develop strength and hardness.Write the date
of plastering on the wall/ceiling with a permanent marker. (So you can track
the curing days).
All mortar
waste stuck to walls, floor, and MS sheet shall be cleared after the day’s
work.
How to choose the right plastering
material for House works?
Two major factors to be considered
in any type of plaster are surface protection and material cost. If the
plastering quality increases and exceeds, the cost is affected. If price is
considered and the quality of the plaster is given less importance then surface
safety is compromised.
It should be durable enough to
sustain any climate change throughout the life span of the structure.
Plastering materials should be inexpensive
and economical.
The plastering material must have
excellent workability that can be applied during any weather conditions (Hot or
Cold).
Points in plastering task calculations should be remembered:
The overall thickness of the plastering should be a minimum
of 20 mm with two coats.
The ratios mentioned in the plastering are the volumetric
ratios of cement and sand (former cement: sand = 1: 5, 1 part of cement and 5
parts of sand in a mortar).
Cement has a dry
density of 1440kg/㎥
Weight of each bag of cement = 50 kilograms or 110 pounds
Quantity of each cement bag =50 kg/1440 = 0.0348㎥
Dry density of sand = 1600
kg/㎥
The plastering is done in two layers (two coats): the first
coat of plastering is kept with a minimum thickness of 12 mm (usually between
12–15 mm) and this coat is used as a rough coat or primary coat.
The second coat should be kept with a thickness of 8 mm and
is called the plaster finish coat or secondary coat.
Total plaster = first coat + second coat
Steps involved in plastering
calculation:
1. Find the total area of the
plastered wall in sqm.
2. Consider plastering ratio and
thickness.
3. Calculate the total amount of
plastering in terms of volume.
4. Find the volume of cement and
sand individually for both coats
5. Calculate the total volume of
cement and sand required for plastering.
Considering the following values for the purpose of calculation:
The width of the wall is 10 metres.
The height of the wall is 10 metres.
The ratio of the first coat of
plastering (cement: sand = 1: 5) with a thickness of 12 mm.
The ratio of plastering to secondary
coat (cement: sand = 1: 3) with a thickness of 8 mm.
Step 1: Calculate the Area of Plastering
Area = width x height = 10 x 10 = 100 ㎡
Step 2: Find the Volume of Plastering
Volume of First Coat = Area of Plastering x Thickness of Plastering
= 100㎡ x 12mm (as 1m =1000mm)
= 100×0.012 = 1.2㎥
Hence, Volume of First Coat of Plastering = 1.2㎥
The volume of Second Coat = Area of Plastering x Thickness of Plastering
= 100㎡ x 8mm (as 1m=1000 mm)
= 100×0.008 = 0.8㎥
Hence,Volume of Second coat of plastering = 0.8㎥
Step 3: To find the quantities of
cement and sand for first and second coat.
First coat:
The ratio of the first coat of
plastering (cement: sand = 1: 5) with a thickness of 12 mm.
Total sum =1+5=6
Quantity of Cement required for
First coat =
(cement/ Total sum) x Volume of First Coat = 1/6 x 1.2
Quantity of Sand required for First
coat =
(sand/ Total sum) x Volume of First
Coat = 5/6 x 1.2 = 1 ㎥
Second coat:
The ratio of plastering to secondary
coat (cement: sand = 1: 3) with a thickness of 8 mm.
Total sum =1+3=4
Quantity of Cement required for
second coat =
(cement/ Total sum) x Volume of second Coat = 1/4x 0.8=0.2 ㎥
Quantity of Sand required for second
coat =
(sand/ Total sum) x Volume of second
Coat = 3/4 x 0.8 =0.6 ㎥
Before step - 4, you need to know
something :
Total quantity of cement for
first + second coat as calculated above
=0.2 + 0.2 =0.4 ㎥
Total quantity
of sand for first + second coat as calculated above
=1 + 0.6 =1.6
Volume of 1 bag of cement (50kgs) =
0.0348㎥
For 0.4m3 = 0.4 x 50 / 0.0348
= 574kgs = 11.4 bags
Similarly, for Sand 1㎥ = 1600 kgs.
1.6 m3 = 1.6 x 1600
= 2560 kgs = 2.5 tonnes
Quantity
of Water required = 627 litres.
Step 4:To find the
quantity of water required for plastering:
Amount of water to
be added in mix depends upon the moisture content present in cement, sand &
atmosphere.
Quantity of water = 20% of total dry
material (Cement+Sand)
= 20% of (574+2560) = 0.2 x 3134 =
627 litres.
How many cement bags are required
for one metre of plastering?
From above, 100㎡= 574 kgs of cement
1㎡ = 574/100 = 5.7 kgs
Final result:
For 100㎡ of
Wall, first & second coat of cement mortar ratio is 1:5 & 1:3.The quantity of cement,
sand & water required for Plastering-then we have :-
The Quantity of cement required =
574 kgs
The Quantity of Sand (Fine
aggregate) required = 2560 kgs
Quantity of Water required = 627
litres
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