ME31B: CHAPTER TWO

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Transcript ME31B: CHAPTER TWO

ME31B: CHAPTER TWO
BUILDING MATERIALS
2.1 INTRODUCTION
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A wide range of building materials is
available for rural building construction.
The proper selection of materials to
be used in a particular building can
influence
the
original
cost,
maintenance;
ease
of
cleaning,
durability and of course, appearance.
Proper selection of material is very
important and the following factors are
to be considered.
Properties of Building Materials
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a) Strength and Durability: This includes
tensile and compressive strength, hardness,
toughness, shear strength etc.
The materials should also be durable ie.
able to retain strength and other properties
over a considerable period of time.
b) Workability: Ease of working with the
material eg. Cutting and shaping.
This should be considered hand in hand with
skilled labour or otherwise available.
Properties of Building Materials Contd.
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c) Cost: Cheaper materials should be
preferred without sacrificing other
characteristics.
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d) Availability: Local materials should
be preferred, all other things being
equal over imported or transported
materials.
Other Properties of Building Materials
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Other properties to consider include
Esthetics,
Resistance to corrosion and to fire,
Easy transportability and
ease of
cleaning,
Resistance to environmental changes,
and to insect attack as well as
imperviousness.
2.2 Soil(Earth)
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This is one of the oldest materials for
building especially in rural areas.
Advantages of earth as a building
material are:
i) It is resistant to fire
(ii) It is cheap and readily available
(iii)It has a high thermal capacity which
enables it keep the inside of the
building cool when the outside is hot
and vice versa.
(iv) It is a good noise absorbent
Soil (Earth) Contd.
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(v) It is easy to work using simple tools
and skills.
The following properties of earth need
improvement for it to be a good building
material:
Look,
Crackness,
Weakness,
Lack of water proofness.
2.2.1 Methods of Improving Soil Properties
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Soil can be improved by compaction.
Adobe bricks(made from soil) can be
used to build. This improves the looks
and reduces cracking.
a) Cracks: Soil particles range from
clay to sand.
Clay is strong but cracks, while sand
does not crack but is weak.
A mixture of clay and sand can be used
to avoid cracking.
Improving the Soil Contd.
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This is the principle of brick making.
Cracks can also be controlled by using
fibres of certain plants e.g. straw. The
fibre reinforces the soil particles by
bridging the gap between them.
b) Weakness: Soil strength can be
improved by adding clay or chemicals
eg. cement. Other stabilizers include
lime, bitumen, rice husk ash, natural
fibres e.g. grass, straw, sisal, saw dust,
resins, cow dung and gypsum.
Improving Earth Contd.
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c) Water Proof-ness: The soil's
waterproof qualities can be improved
by adding some chemicals e.g.
benzene.
Emulsified asphalt can also be added
but it is very expensive.
The earth wall should be protected
from rain by extending the roof.
2.3 MASONRY
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Masonry is an art of building with stones,
bricks or concrete blocks.
2.3.1 Stones: Soft stones are preferred for
building to ease reshaping.
Stones generally show good strength, high
durability, low cost(especially if available in
site), resistance to fire, insects and
environmental changes.
Stones have high compressive strength and
hence good for walls, columns, foundations
etc.
2.3.2 Clay Bricks
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Bricks are made from clay; the clay
paste is placed in proper moulds,
hardened and burnt in kilns (oven) in
high heat.
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Bricks are generally strong, durable,
resistant to environmental changes, fire
and insects.
2.3.3 Mortar
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Used to put stone materials together. It is a
mixture of sand, water and a binding material
like clay, cement or lime.
Cement is more expensive that lime which is
more costly than clay.
Lime is produced by burning calcareous
stone (Caco3) in water.
CaCo3 + H 2O = Ca(OH)2
+ CO2
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Cement is a mixture of lime and clay.
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2.3.4 Cement or Concrete Blocks
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Made using mortar of cement and sand with
necessary quantity of water.
The blocks can also contain low sized gravel
also.
The blocks are generally made hollow with
the core making about 25% of the gross
area. These blocks are quite strong, durable
and have most other characteristics of a
good building material.
The core of the block increases the
acoustical and thermal properties and
reduces the self-weight.
2.4 CONCRETE
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Concrete is probably the most
important structural material today.
It has the following ingredients:
 i) Cement : Portland cement
 ii) Fine aggregate: Sand, granite
granules etc.
 iii)
Coarse aggregate:
Gravel,
crushed stone, broken bricks etc.
 iv) Water.
2.4.1 Ratio of Concrete
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Normal ratio of concrete is 1: 2: 4 (by
volume).
This means 1 part cement, 2 parts fine
aggregates and 4 parts coarse aggregates.
Fine aggregates fill the pores of coarse
aggregates, cement fills the pores of fine
aggregates.
Cement with water forms a very good
binding material which binds the concrete
into a homogeneous mass.
2.4.2 Water-Cement Ratio (w/c
ratio)
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The best w/c ratio is 0.45 to 0.65 by
weight.
Too high w/c ratio increases workability
of concrete but reduces strength of the
paste while
Very low values reduce workability and
binding ability leading to cracking of
concrete.
2.4.3 Processes in Concreting
a) Batching: This involves bringing all
the ingredients together in a proper
ratio. It is normally done by volume but
better results are obtained if the ration
is taken by weight.
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For 1 m of concrete, take approximately 1
m3 of coarse aggregate.
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With 1: 2: 4 ratio, 0.5 m3 of sand and
0.25 m 3 of cement are used.
Water is normally added by experience.
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Processes in Concreting: Batching
Contd.
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With these mixtures, a little more than 1
m3 of concrete is obtained.
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For more accurate results, for 1 m3 of
concrete, take 0.96 m3 of coarse
aggregate and others accordingly using
the mixing ratio.
Processes in Concreting: Mixing
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Mixing can be done manually or
mechanically.
In manual mixing, cement and sand are
mixed thoroughly and then mixed with
the coarse aggregate, and then water
is added and mixed thoroughly.
In mechanical mixers, all aggregates
and water are added in the rotating
mixer and rotated to mix them
thoroughly.
Other Processes in Concreting
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c) Conveyance or Transport: Done using
barrows or mechanical dumpers. Should be
done quickly.
d) Placing: Before placing concrete in
sites, the formwork is to be checked, cleaned
and oiled.
e) Compaction: Concrete must be
compacted adequately to secure maximum
density.
This is done either manually or using rods or
mechanical vibrators.
Processes in Concreting: Curing
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f) Curing: The setting of cement and
hardening of concrete depends on the water
content.
The placed concrete has to be protected
against evaporation.
Curing is done by keeping the exposed
surface of concrete wet continuosly.
It is done by flooding in case of slabs or by
continuous spraying in other cases.
Curing is done for 7 to 10 days, but 21 days
of curing gives maximum strength.
2.4.4 Reinforcement of Concrete
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Concrete is strong in compression but
weak in tension.
Reinforcement involves inserting steel
bars into concrete to make it resist
tension.
Beams, slabs etc. need reinforcement
in order to make them resist bending
which produces tension.
Example
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Find the required quantities of cement,
sand and coarse aggregate required to
concrete a slab 10 m long, 5 m wide
and 12 cm thick. Calculate the quantity
of water required also. Specific gravity
of cement is 1.5; ratio of ingredients is
1 : 2 : 4 and water cement ratio is 0.5.
Solution
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Total volume of concrete needed = 10
m x 5 m x 12/100 m = 6 m3
Assume 0.96 m of coarse aggregates
per m3 of concrete
Coarse aggregate required is 6 x 0.96
= 5.76 m3
Fine aggregate required is 5.76 x 2/4
= 2.88 m3
Cement required = 5.76 x 1/4 =
1.44 m3
Solution Concluded
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Note: Bulk density of water is 1000kg/m3 ;
with specific gravity of 1.5; bulk density of
cement is 1.5 x 1000 = 1500 kg/m3
Wt. of cement = 1.44 m3 x 1500 kg/m 3 =
2160 kg.
Assuming 1 bag of cement weighs 40 kg,
Cement required = 2160/40 = 54 bags.
w/c ratio is 0.5, water required = 0.5 x wt.
of cement = 0.5 x 2160 = 1080 kg = 1080
litres
Note: 1 litre of water weighs 1 kg.
2.5 WOOD
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Wood is a traditional building material.
The quality of wood varies widely.
Wood is generally durable, strong,
dependable, workable and possesses
many other characteristics.
It is, however, highly inflammable and
liable to insect attacks if not properly
protected. Wood is mainly used for
roof skeleton, doors and windows,
walls and partitions in small buildings.
Properties of Wood Contd.
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Wood may be hard or soft.
Hard ones include ebony, mahogany,
etc. while soft ones include Douglas
fur, palm tree etc.
Strength of wood increases with
decreasing moisture content.
By seasoning (drying), the strength can
be improved. Wood is highly workable.
Plywood
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Produced by gluing many layers of split
wood together.
The grains are arranged alternatively
to eliminate distortion.
The plywood is a very good modern
construction material whose quality
depends on type of wood used, glue
used and pressure of steam used in
gluing.
Wood can be cut into many shapes.
a) Plank: The thickness is small when compared to its width.
t
t <<B
B
Plank
b) Beams: Width and thickness dimensions are almost same.
Beams
Shapes of Wood Contd: Block
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c) Blocks: Very thick eg. 25 x 25 mm(widthdepth) or 20 x 25 mm or 30 x 30 mm
BLOCK
BLOCK
Many grades of wood exist depending on the
strength.
2.6 METALS
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These are man-made materials whose
properties are known. The most widely used
metal is steel.
2.6.1 Steel: Steel is very strong, highly
durable, resistant against weather, fire and
insects and possesses almost all good
characteristics.
Steel is used in reinforcement in concrete,
gates, windows, roof trusses, steel sheets for
partitions and tanks etc.
2.7 Other Building Materials
(Synthetic Materials)
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a) Asbestos Cement: Used for making
roof sheets, drain pipes and accessories. It
is also used for partitions and ceilings.
b) Plastics: used in water supply and
sanitary systems. Also electric fittings.
c) Ceramics, fibre glass etc. are new
materials used in some restricted areas.
2.8 PROTECTION OF
BUILDING MATERIALS
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Plastering with cement mortar, lime mortar
etc. is common to protect the super
structure.
Painting is used to protect steel from
rusting, timber from insect attack and
shrinkage etc.
Painting of walls, ceilings etc. are done for
light control and beauty.
Galvanizing and other protective coverings of
certain materials especially metals are
common.