CONSTRUCTION MATERIALS
LECTURE 01
CONSTRUCTION MATERIALS
Construction material is any material which is used for construction purposes. Many naturally
occurring substances, such as clay, rocks, sand, wood, leaves etc, have been used to construct
buildings and other structures. Apart from naturally occurring material, many man-made
products are in use today.
The manufacturing of construction materials is an established industry in many countries and the
use of these materials is typically segmented into specific specialty trades, such as carpentry,
plumbing, insulation, masonry etc. They provide the make-up of habitats and structures
including homes and various infrastructures.
A wide range of materials is available for the construction of buildings and other structures.
The proper selection of materials to be used in a particular building or structure can influence the
original cost, maintenance, easy of cleaning, durability and appearance.
In ideal environments, most common construction materials are very durable and can last
indefinitely. However, design or construction deficiencies or lack of proper maintenance can
result in less-than-ideal conditions under which construction materials will degrade. Degradation
can take many forms, including chemical reactions, consumption by living organisms, and
erosion or mechanical wear. Traditional building materials – steel, concrete, and wood – usually
deteriorate and fail via well-known mechanisms. Even innovative materials that appear on
construction sites can degrade, either by these well-understood mechanisms or through exotic,
sometimes surprising, reactions and processes.
Several factors need to be considered when choosing the materials for a construction job,
including;
i)
ii)
iii)
Type and function of building or structure,
Specific characteristics required of the material used e.g great strength, water resistance;
wear resistance, attractive, appearance.
Economic aspects of the building/structure interms of original investments and annual
cost of maintenance.
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iv)
v)
vi)
vii)
viii)
Availability of materials in the area.
Availability of skilled labor required to install some types of materials.
Transportation costs.
Selection of materials with compatible properties, dimensions and means of installation.
Cultural acceptability or personal preference.
The following are basic construction materials
i)
ii)
iii)
iv)
v)
vi)
vii)
viii)
ix)
x)
xi)
xii)
xiii)
xiv)
xv)
Timber/wood
Stones
Cement
Aggregates
Water
Admixtures
Bricks/blocks
Concrete
Metals
Plastics
Glass
Adhesives
Bitumen
Lime
Paints
Timber/wood
Wood is a commonly used construction material in many parts of the world because of its
reasonable cost, easy of working, attractive and adequate life if protected from moisture and
insects.
However, forests are valuable natural resource that must be conserved, particularly in areas with
marginal rainfall.
As good a material as wood may be, there are regions where other materials should be
considered first, simply on a conservation basis.
Wood for construction is available from many different species with widely varying
characteristics.
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Some species are used in the form of small poles for light construction, while other species are
allowed to mature so that timber (lumber in some countries) may be sawn from large logs.
Timber Classification
Timber can be classified broadly into two categories a) hard woods b) soft woods
S/N
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
HARDWOODS
Lower growth rate (30-100 yrs)
Mostly high densityhjg
Very good durability
Comparatively expensive
Not readily available
Broad leaved tree
Deciduous
Generally hard
Covered seeds (protected) with some
sort of covering
Botanical name: angiosperms (flowering
plants)
SOFTWOODS
Higher growth rate (10-30 years)
Mostly low density
Poor durability unless well preserved
Comparatively cheap
Readily available
Needle leaved tree
Evergreen
Generally soft
Naked seeds (unprotected)
Botanical name: Gymnosperms
Types of hardwoods and soft woods
Some of basic types are as given below
Hardwoods
Paorosa (msekeseke)
Thimble (Mtondoo)
Itch (mkuti)
Congolse (Mkongo)
Meliaceae /Africanmahogany (Mkangazi )
(Pangapanga)
Papilionoideae/bloodwood
(Mninga)
(Mkulunga)
(Mpingo)
(Mturunga)
Moraceae
(Mvule/mvuli)
Camphor
(Mkulo/Mkarambaka/Mkenene)
Brown/wild olive
(Loliondo)
Balsa
Teak
Softwoods
Pine
Cypress
Eucalyptus
Gravellier
Podo
Cedrella
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LECTURE 2
SOFTWOODS AND HARDWOODS
Hardwood is not necessarily a harder material or denser and softwood is not necessarily a softer
material (less dense). For example, balsa wood is one of the lightest, least woods there is, and it
is considered a hardwood.
The distinction between hardwood and softwood actually has to do with plant production.
ANATOMY
A tree can be considered in the form of three subsystems which are roots, trunk and crown.
Each subsystem has an important role to play.
Root System
1) Absorbs moisture containing minerals from the soil for transfer via trunk to crown.
2) Provides sufficient anchorage for the trunk and crown to withstand wind forces.
Trunk System
1) Provides rigidity, mechanical strength and height to maintain the crown at a level
above the ground.
2) Transfers minerals/moisture from the ground to the crown.
3) Important part for obtaining timbers.
Crown System
Catchment area made up of leaves. This is where chemical reaction to form sugar and
cellulose for the growth of tree is taking place.
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Fig1: Tree subsystem
CROSS-SECTION OF THE TRUNK OF THE TREE
The following are the important parts of cross-section of the trunk of the tree
Pith: Central core of the wood tissue.
Heartwood: The inner concentric rings of the wood tissue
Sapwood: Sapwood contains more moisture and not as strong as heartwood but when seasoned
the difference is minor. It is inferior to heartwood in respect to durability, attracts insects and
provides food for fungal growth. On the other hand sapwood is very permeable and easily
impregnated with preservatives. It is brighter than the heartwood.
Rays: Cells which runf radially from the centre. They serve to store food and distribute it
horizontally.
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Fig. 2: The tree showing the main parts
Cambium Layer: A layer of cells where splitting of single cells into two cells is formed. The
process continues through the growing season and forms a ring known as annual rings.
Inner Bark: This is soft, moist and spongy and it transports the converted sap from leaves to the
growing parts of the tree.
Outer Bark: Rough texture, dense enough to provide protective coat.
USES OF TIMBER IN CONSTRUCTION
1) Roofs: Timber is used in the construction of roof members, e.g trusses. The sizes depend
upon the strength, span and other characteristics of the building. Beams (joists) are
usually made of 50x100mm or 50x150mm timber. For struts, timber of 50x75mm or
50x100mm is often used. Rafters are usually 50x100mm etc.
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2) Walls: Walls can also be constructed with timbers. With the increasing cost of timber,
however concrete blocks are now generally preferred. The walls of most of temporary
buildings are still made of timbers.
3) Openings: Door and window frames and shutters are mostly made of timbers.
4) Floors and Stairs: A good number of most colonial buildings built in Africa in the past
had wooden floors and stairs. Today, wood is still used for floors particularly in
gymnasiums.
5) Formworks: Timber is one of the chief materials used for formwork especially in the
construction of concrete products. In water supply and sanitation the timber formworks
are used in the construction of various structures like chambers, septic tanks, soak way
pits and so on.
6) Supports for Soils and Structures: Timber/wood is used for supporting the sides of
excavation and as shores to safeguard structures from falling/failing. The excavations are
very common in constructing water supply and sanitation systems structures.
7) Scaffolding: These are temporary structures used to support workers and materials above
the ground during wall erections especially at places where they are difficult to reach and
work conveniently. Timbers/ woods are chiefly used for this construction.
8) Bridges: Timber can be used in the construction of temporary and low cost bridges or
crossovers.
9) Furniture: Timbers/woods are mostly used for manufacture of furniture, which others
form part of building, e.g built in wardrobes, cupboards etc.
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LECTURE 3
TIMBER CONVERSION
This is a process whereby the felled tree is converted into marketable size of timber. Or, timber
conversion is the process of turning a log into a pile of usable planks or boards.
This is done during the dry season. The trunk is not of the same diameter over the entire length,
so there is no equal number of marketable timbers on the whole length.
A series of planks is produced from each log, usually in the thickness range 25-75mm. The
cheapest and quickest way of doing this is by through and through/Plain sawing method.
Through and through sawing
This is one of the most popular methods of sawing. The log is cut in parallel cuts in the direction
of the grains.
It has the disadvantage that the sawn timbers will be subject to distortion on drying, particularly
if relatively wide.
Fig.3: Through and through sawing
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An alternative is to use quarter sawing method. This is far more expensive because of the need
to double (or more) handle the more wastage.
It is however stronger, more decorative and less prone to cup and distort.
Fig.4: Quarter sawing
MOISTURE CONTENT
This is the amount of moisture content in the piece of timber and is expressed in percentage
The moisture content of timber is given by
Moisture content= Mass of water present in a sample/ Mass of that sample when oven dry
The presence of moisture in timber not only increases the weight of timber but also results in
swelling of the timber which affects the strength of timber. It will be noted that, if the mass of
water contained exceeds the mass of dry timber, a moisture content of over 100% is obtained and
this is usual in newly felled tree (green).
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The sapwood tends to contain more moisture at the time of felling, the average moisture content
being about 130%, though values over 200% are possible.
In heart wood the moisture content is more likely to be around 50%. The moisture content of
green timbers varies considerably with species. Dense timbers, which include most hardwoods,
have small cavities and large cell walls; hence they can store less water.
The moisture content of timber in service is greatly influenced by the fact that wood is
hygroscopic, that is, it tends to attract water from a damp atmosphere and give up water to a dry
environment. In consequence timber will adopt equilibrium moisture content in a given
environment, depending on the relative humidity of that environment.
Since increase of temperature tends to reduce relative humidity, it also results in reduced
equilibrium moisture content. At moisture content in excess of 20%, timber becomes susceptible
to fungal attack; hence the importance of providing a dry environment will be appreciated.
DETERMINATION OF MOISTURE CONTENT
The accurate way of measuring the moisture content of timber is to oven dry it at 103+20C until
the mass is sufficiently constant (oven-dry method).
The moisture content of wood can be much more easily estimated by means of portable meters
based upon the relationship between electrical resistance and moisture content. A two- pin
electrode is pushed into the wood. At lower moisture contents electrical resistance is greatly
increased (moisture meter method).
SEASONING
This is a controlled reduction of moisture content of timber to a level appropriate to its end use.
Types of seasoning
There are two types of seasoning
Air Seasoning
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Fig 5: Air Seasoning
Timber is stacked in an open-sided shed which promote drying without assistance. This stack is
at 45cm from the ground and the timbers are spaced by spacers of 25mm square.
Advantages are
-It is a cheap method
-Little loss of quality of timber
Kiln seasoning
This employs a heated, ventilated and humidified oven. The essentials of the kiln are:
- Heat under proper control and sufficient to raise temperature to maximum required.
-Humidification also under proper control to meet all requirements.
-Air circulation uniform and of sufficient velocity.
TIMBER GRADING
This is the process of selection based on established system in order to achieve uniformity and
reliability of timber as a structural material.
It determines the strength class of timber. Structural timber must be stress graded in order to be
used as a construction material.
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Visual Grading
Manual process carried out by an approved grader
It classifies the timber in two classes GS and SS.
Stress Grading
GS-General Structure; SS-Special Structure.
Machine
Grading
The timber is graded directly into its strength class
Fig.6: Stress Grading Types
DEFECTS IN TIMBER/WOOD
Defects can be classified into two main groups.
Natural Defects: These occur during growing period.
Artificial Defects: These occur during conversion, seasoning and when timber is being used.
They can occur in timber at various stages either during growth, conversion, seasoning and
timber or wood in use.
These can reduce strength or mar the timber appearance.
Note
When designing in timber structures, it is vital to know maximum stress each member can take.
Timber without defects is obviously stronger than timber with defects. In practice, it would be
highly uneconomical to specify absolutely „clear‟ (defect- free) timber. Certain limited defects
have to be accepted.
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LECTURE 04
TYPES OF DEFECTS
Natural Defects: Fungal attack, cracks, burr and curl, knots, insect attack, growth rings, and
grain defects.
Artificial Defects: conversion defects, chemical defects, seasoning defects.
Fungal attack: Growth of all fungi in timbers requires a moisture content of at least 20%. The
enzyme action necessary for fungi growth cannot take place in completely dry timber. There are
numerous varieties, some attacking the growing tree or unseasoned timber, while others are able
to attack the wood in service.
It is decomposition of timber caused by fungi and other micro-organisms, resulting in softening,
progressive loss of strength and weight and often a change of texture or/ and color.
Fungi are living plants and require food supply, moisture, oxygen and suitable temperature.
Fungi Types: There are two types of fungi that affect timbers, namely dry rot and wet rot.
Dry Rot (Brown Rot): Once the fungus has established itself it is much less dependent upon
external moisture supply. It will even moisten dry wood before it attacks it. The mycelium* will
develop into thick grayish cord which can penetrate brickwork to find fresh wood to attack. The
spores* are rust colored.
Wet Rot: This fungus requires wet conditions in which to exist. The sheet like fruit bodies may
vary from green to brown, the spores being produced in little pimples. Whereas dry rots produce
charred effect, wet rot produces darkened tracts across the grains. Wet rots require higher
moisture content in order to thrive, optimum values being in the region of 50%.
Prevention: Keep the timber dry, at least drier than 20% and this may be done by providing
thorough ventilation and of course, proper Damp Proof Courses (DPC). Frequent causes of
trouble are broken rain water pipes and drains.
Cure: Affected wood must be removed and burned. All surrounding construction should be
treated (if uninflammable) with a blow- lamp, but otherwise with proprietary antiseptic. The
remaining wood work should be treated with preservative as should any new work.
* The vegetative part of fungus, consisting of a mass of branching, threadlike hyphae. Hyphae- (In a fungus) one of
the threadlike elements of mycelium.
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2) Insect Attack: The insect‟s eggs are laid on the surface of timber in a crack or crevice and the
larva when hatch bores into the wood which produces its food. The larva makes a special
chamber near the surface and changes to a pupa. Finally it develops into the adult insect and
bores its way out through the surface. The bored hole reduces the cross sectional area, and thus,
the strength of the timber.
3) Knots: These are portions of branches enclosed in the wood by the natural growth of the tree;
they affect the strength of timber as the cause a deviation of grains and may leave a hole.
Fig.7: Knot Types
a) Live (Sound) Knot: is one free from decay, solid across its face and at least as hard as the
surrounding wood.
b) Dead Knot: it has its fibres intergrown with those of the surrounding wood to an extent of up
to one-quarter of the cross-sectional perimeter; a loose knot is a dead knot not held firmly in
place.
4) Shakes: these consist of a separation of fibres along their grains as a result of stresses
developing in the standing tree, or during the felling of the tree or seasoning. A cross shake
occurs in the cross grained timber following the grains. A heart shake is a radial shake
originating at the heart; a ring shake follows the line of growth rings and a star shake consists
of a number of heart shake resembling a star.
5) Checks: This is a seasoning defect, normally a separation of fibres along the grains forming a
crack or fissure in the timber, not extending through the piece from one surface to another.
6) Split: This is a seasoning defect, normally a separation of fibres along the grains forming a
crack or fissure that extends through the piece from one surface to another.
7) Resin Pocket: Lens-shaped cavities in a timber containing or that have contained resinous
substances.
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Fig.8: Timber warping, Shakes, check and split
8) Warp: A distortion in converted timber causing departure from its original plane. The
following are different forms of warping.
- Cupping: This is a curvature occurring in the cross-section of a piece of timber.
-Bow: This is a curvature of a piece of timber in the direction of its length. Other warping defects
which are resulted from poor seasoning are twisting and springing, as shown below.
9) Chemical Defects: Occurs in situation where timber is used in unsuitable position. It contains
some acids which react with metals and form stains.
10) Grain Defect: these occur in the form of twisted grains, due to either wind, or other forces.
11) Imperfect Manufacture: any defects blemish or imperfection incidental to the conversion or
machining of timber, such as variation in sawing, torn grain, chipped grain or chip marks.
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12) Waney Edge (Wane): This occurs due to unsound practice of attempting to use every
possible piece of timber converted from trunk, e.g wane.
Fig.9: The wane defect
TIMBER PRESERVATION
Few timbers are resistant to decay or insect attack for long periods of time and in many cases the
length of life can be much increased by preservative treatments.
The need for preservative treatment is largely dependent upon the severity of the service
environment.
The principal protective liquids are toxic oils, such as coal tar creosote, water-borne inorganic
salts such as copper/ chrome/arsenic which are suitable for exterior use; and Organic solvent
solutions such as copper and zinc napthanate, which are also suitable for exterior and interior
use.
Application
Preservatives can be applied by non-pressure methods such as brush application, spraying,
immersion and steeping.
An alternative is to use pressure method. The timber is subjected to air pressure, the
preservative admitted and a higher pressure applied causing the liquid to penetrate the timber and
compress air in the cells. When timber is extracted, the air trapped in the cells forces out excess
liquid leaving the cell empty but impregnating the cell wall.
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LECTURE 5
CEMENT
Cement is a binder, a substance used in construction that sets and hardens and can bind other
materials together. The most important types of cement are used as components in the production
of mortar in masonry, and of concrete which is a combination of cement, aggregates and water to
form a strong building material.
Source of cement
a) Natural Cement
b) Artificial cement
Natural cement: Natural cement rocks.
-
Calcareous (lime)- 75%- CaO
-
Argillaceous- 25% which includes mainly of Silica- SiO2, Alumina- Al2O3, and IronFe2O3.
Cements used in construction can be characterized as being hydraulic or non-hydraulic,
depending upon the ability of the cement to set in the presence of water.
The hydraulic properties of natural cement will be governed by the amount of clay contained.
Hydraulic cement
This is a material which sets and hardens when water is added. Hydraulic cements set and
become adhesive due to a chemical reaction between the dry ingredients and water. The
chemical reaction results in products (mineral hydrates) that are not very water-soluble and so
are quite durable in water and safe from chemical attack. This allows setting in wet conditions or
under water and further protects the hardened material from chemical attack.
Hydraulic cement does not rely on atmospheric action on drying for its strength. This property
and the related property of not undergoing chemical change by water in later life are most
important and have contributed to the widespread use of concrete as a construction material.
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Artificial Cement: Manufactured in cement industries.
Non hydraulic cement
This will not set in wet conditions or under water; rather, it sets as it dries and reacts with carbon
dioxide in the air. Non-hydraulic cement such as slaked limes (calcium hydroxide mixed with
water), harden due to the reaction of carbonation in presence of carbon dioxide naturally present
in the air. Calcium oxide is produced by lime calcinations at temperatures above 825oC for about
10 hours at atmospheric pressure.
The calcium oxide is then spent mixing it with water to make slaked lime. The setting process
(reaction) takes a significant amount of time because the carbon dioxide in the air is small. The
reaction of carbonation requires the air be in contact with the dry cement, hence, for this reason
the slaked lime is non hydraulic cement and cannot be used under water.
Types of cement
Although Portland cement is the main cement used in concrete works, there are other types of
cements. The two types that are often mentioned are:
-Portland cements
-Non Portland Cements- eg blended cements
PORTLAND CEMENTS
The most important and used cements in concrete works are Portland Cements; so named
because of a similarity in appearance of concrete made with these cements to Portland stone.
MANUFACTURE
Portland cement is manufactured from heating a mixture of clay or shale and Chalk or
Limestone (slurry) in a kiln to a high temperature- around 1500 0C, such that chemical
combination occurs between them. About 5%gypsum (calcium sulphate) is added to the resulting
clinker in order to regulate the setting time of finished (manufactured) cement.
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Portland cement is made primarily from the combination of calcareous material such as lime
stone or chalk; and Silica, alumina and Ferrite found as clay or shale. The process of
manufacture consists essentially of grinding raw materials into a fine powder, mixing them (to
form slurry) intimately in pre-determined proportions in large rotary kiln at a temperature of
about 1500 0C when the material partially fuses into clinker.
The clinker is cooled and ground to a fine powder, with some gypsum (about 5 %) added, and
the resulting product is the commercial Portland cement used throughout the world.
Gypsum- Calcium Sulphate (CaSO4.H20) or Magnesium Sulphate (MgSO4.H2O)
TYPES OF PORTLAND CEMENT
The more commonly used Portland cements are
1. Ordinary Portland cement
2. Rapid hardening Portland cement
3. Sulphate resisting Portland cement
4.
Low heat Portland cement
5. White Portland cement
6. Water repellent Portland cement
7. Portland- pozzolan cement
8. Portland blast furnace cement
Ordinary Portland cement
Is the cheapest and most commonly used cement accounting for about 90% of all commercial
production. It is made by heating limestone and clay to a temperature of about 1500 0C to form a
clinker, rich of calcium silicates.
The clinker is ground to a fine powder with a small proportion of gypsum, which regulates the rate
of setting when the cement is mixed with water. This type of cement is affected by sulphates such as
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those present in ground water in some clay soils. The sulphates have a disintegrating effect on
ordinary Portland cement.
Rapid hardening Portland cement
It is similar to ordinary Portland cement except that the cement powder is more finely ground. The
effect of the finer ground is that the constituents of the cement powder react more quickly with water
and the cement develops strength more rapidly.
Rapid hardening cement develops in three days, a strength which is similar to that developed by
ordinary Portland in seven days.
The advantage of the early strength developed by this cement is the
possibility of speeding up
construction by, for example, early removal of formwork. Although rapid hardening is more
expensive than ordinary Portland cement, it is often used because of its early strength advantages.
Sulphate resisting Portland cement
The proportions of the constituents of the cement that are affected by sulphates are reduced to
provide increased resistance to the effect of the sulphates. This cement has a low C 3A content so as
to avoid sulphate attack from outside the concrete; otherwise the formation of calcium
sulphoaluminate and gypsum would cause disruption of the concrete due to an increased volume of
the resultant compounds.
The disintegration is severe where the concrete is alternately wet and dry, as in marine works.
Because it is necessary to control with some care, the composition of the raw materials of this
cement, it is more expensive than ordinary Portland cement.
Low heat Portland cement
It is used mainly for mass concrete works in dams and other constructions where the heat developed
by the hydration of other cements would cause serious shrinkage cracking. Because of the lower
content of C3S and C3A, there is a slower development of strength than with ordinary Portland
cement, but the ultimate strength is unaffected.
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White Portland cement
This contains not more than 1% of the iron oxide, which is responsible for the grey colour of OPC.
China clay, which is almost iron-free, is used instead of ordinary clay.
Firring and grinding are also modified to prevent colored matter being introduced. In consequence,
the cement costs are approximately twice as much as OPC. Setting is similar to OPC but, being
composed mainly of C3S, strength development may be faster than that of OPC.
The cement is used to produce white or coloured concretes-for the latter, pigments would be
incorporated. These concretes may be employed for their aesthetic or light- reflecting qualities.
Water repellent cement
This is made by mixing a metallic soap with ordinary or white Portland cement. Concrete made with
this cement is more water repellent and therefore absorbs less rain water than concrete made with
other cements and is thus less liable to dirt staining. This cement is used for cast and cast stone for
its water repellent property.
Portland- Pozzolan cement
These cements are made by intergrinding or blending pozzolans with Portland cement. ASTM
describes a pozzolan as a siliceous or siliceous and aluminous material which in itself possesses little
or no cementitious value but will, in finely divided form and in the presence of moisture, chemically
react with lime (liberated by hydrating Portland cement) at ordinary temperature to form compounds
possessing cementitious properties.
As a rule, Portland- pozzolan cements gain strength slowly and therefore require curing over a
comperatively long period, but the long strength is high.
Pozzolan may often be cheaper than the Portland cement that they replace but their chief advantage
lies in slow hydration and therefore low rate of heat development. Hence Portland-Pozzolan cement
or a partial replacement of Portland cement by the pozzolan is used in mass concrete construction.
Portland blast furnace cement
It is manufactured by grinding Portland cement clinker with blast furnace slags. The proportion of
slag being up to 65% by weight and the percentage of cement clinker not less than 35% by weight.
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This cement develops heat more slowly than ordinary Portland cement and is used in mass concrete
works as low heat cement. It has good resistance to the destructive effects of sulphates and is
commonly used in marine works.
Essentially, granulated blast furnace slag consisting of calcium, aluminium, silica and iron, the
same components found in cement but in different proportions, is ground to fineness greater than
Ordinary Portland Cement (OPC). The ground granulated blast furnace slag is dry-blended
together with OPC as per requirements. The slag's latent hydraulic properties are activated in the
presence of calcium hydroxide which is released when cement is mixed with water. This
produces a denser calcium silicate hydrate which possesses substantial properties that are
superior to OPC.
CEMENT STORAGE
Cement will maintain its quality for a long time if kept away from moisture. On the other hand,
cement exposed to air will absorb moisture slowly which will cause distortion. At the factory,
cement is stored in silos, usually reinforced concrete containers several meters in depth.
The best method for site storage of cement is in bulk, and in an elevated bin 2 meters or more in
depth.
Bagged cement may also be kept safely for months if stored in a water proof shed with a dry
floor well off the ground. The bags should be placed close together to reduce circulation of air. If
the piles are to be more than seven, the bags should be placed alternately lengthwise and
crosswise to lessen the danger of overturning.
-For long storage, the tops and sides should be covered with a waterproof paper as an extra
precaution against moisture. When removing cement it is desirable to apply the “First In First
Out (FIFO)” rule, that is, remove the oldest cement first.
Note:
Cement is usually supplied in 50Kg paper bags.
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Test on Cement at construction site
The following are the quality test on cement at construction site:
Color test
The color of the cement should be uniform. It should be grey color with a light greenish
shade.
Presence of lumps
The cement should be free from any hard lumps. Such lumps are formed by the
absorption of moisture from the atmosphere. Any bag of cement containing such lumps
should be rejected.
Adulteration test
The cement should feel smooth when touched or rubbed in between fingers. If it is felt
rough it indicates adulteration with sand.
Temperature test
If hand is inserted in a bag of cement or heap of cement, it should feel cool and not warm.
Float test
If a small quantity of cement is thrown in a bucket of water, the particles should float for
some time before it sinks.
Setting test
A thick paste of cement with water is made on a piece of glass plate and it is kept under
water for 24 hours. It should set and not crack.
Strength test
A block of cement 25mmx25mm and 200mm long is prepared and it is immersed for 7
days in water. It is then placed on supports 15cm apart and it is loaded with weight of
about 34 kg. The block should not show signs of failure.
Date of Packing
Strength of cement reduces with time, so it is important to check the manufacturing date
of cement. Generally, the cement should be used before 90 days from the date of
manufacturing.
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LECTURE 6
STONES
Stones may be classified into any of three categories depending on its origin and composition.
These three categories are:
i) Igneous stones
ii) Metamorphic stones
iii) Sedimentary stones
Igneous stones
Igneous rock (stone) is formed through the cooling and solidification of magma or lava. Igneous
rock may form with or without crystallization, either below the surface as intrusive (plutonic)
rocks or on the surface as extrusive (volcanic) rocks.
Granites: these are examples of igneous stones used in buildings construction and they are
differentiated from other igneous stones by the nature of their coarse grains and their generally
lighter colors.
-
Granites are the strongest and most durable of all stones used in building works. They
have very low porosity and water absorption rate but are very difficult to work on.
Granites are used in walling to prevent erosion and for fences in water-logged areas.
They are also used for retaining walls.
Sedimentary stones
They are from the sedimentary rocks, which are types of rocks that are formed by the deposition
of material at the earth‟s surface and within bodies of water.
Various forms of sedimentary stones are used for construction purposes. Most sedimentary
stones are formed out of fragments of igneous rocks which are often deposited by water in layers
or strata.
As one layer is formed over another, the sediments harden and consolidate under pressure and
are bonded together by sandy or clayey pastes, or sometimes by line present in percolating water.
Two major types of sedimentary stones are sandstone and limestone.
All sedimentary rocks (stones) have bedding planes which facilitate extraction but limit the
thickness of stone obtained and may cause weakness through water admission (lay bedding
planes horizontally where possible).
They have lower extraction/cutting costs and easy to work to a fine finish.
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Metamorphic stones
Metamorphic rocks arise from the transformation of existing rock types, in a process called
“metamorphism” which means “change in form” under heat and pressure.
Properties variable according to origin but the best varieties are very durable. They have many
closely spaced bedding planes which permit cutting into fine sheets. Examples are:
Marble
This is a crystalline, metamorphic form of limestone. Marble is used in terrazzo flooring when it
is mixed with cement and water.
Slates
These are metamorphic forms of clay and shales formed a result of disintegration of weathered
igneous stones. Slates have a laminated structure and are easily split into thin slabs which can be
used for roof covering and damp proofing. They are also used as cladding and take polish
readily.
STONE MASONRY
Use of natural stone masonry has diminished greatly during the 20th century largely on account
of increased labour cost which have made the processes of extraction and cutting/dressing of
stones uneconomical.
Stone is now mainly used as a cladding material since some types have excellent appearance and
weathering properties and their use symbolizes quality of prestige.
In this context stone is fixed as generally relatively thin slabs to a structural frame and therefore
must be regarded as a cladding rather than the more traditional masonry.
PROPERTIES
The properties of stones vary greatly according to the classification and also according to the
location.
The performance of sandstone in particular can vary within wide limits according to the nature of
cementing material which may be based upon calcium carbonate, iron oxide or silica. The
general properties are strength, durability, impervious to water (generally), hardness etc.
AGGREGATES
Aggregates constitute the largest single material by weight and volume used in construction
works.
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Aggregates are gravels, crushed stones, blast furnace slags and sand which are mixed with
cement and water to make concrete.
ESSENTIAL CHARACTERISTICS
The two most essential characteristics for aggregates are durability and cleanliness; cleanliness
means free from organic impurities.
CLASSIFICATION
Aggregates are classified according to the size of particles in them, i.e fine aggregates and coarse
aggregates.
i) Fine aggregates
This consists of natural sand, or crushed stones, or crushed gravel, mainly passing
through a 5mm British standard (BS) sieve with a good proportion of the large
particles. This excludes the fine dust.
ii) Coarse aggregates
This is primarily natural gravel, or crushed gravel, or crushed stones that are mainly
retained on a 5mm BS sieve.
SIZE
The maximum size of coarse aggregate is determined by the class of work.
For foundations and mass concrete work the size can be increased possibly up to 40mm whereas
with the reinforced concrete the aggregates must be able to pass readily between the
reinforcements and it rarely exceeds 20mm.
SOURCES
FINE AGGREGATES
River banks
Rain water deposits
Pits
Crushed gravel
COARSE AGGREGATES
Gravel
Pumice
Breeze and clinker
Broken stones
Blast furnace slags
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27
LECTURE 7
WATER
Water is a chemical means by which cement is converted from a powder into a hardened material
with strength and durability. It is therefore needs to be of appropriate quality and two main
aspects may be considered.
Organic contamination
The quality of water is important because impurities in it may interfere with setting of the
cement, may adversely affect the strength of concrete or cause staining of its surface, and may
also lead to corrosion of reinforcement. For these reasons, the suitability of water for mixing and
curing purposes should be considered.
Dissolved salts
Sea water, for example, may contain dissolved salts such as sulphates which can react with the
hydrated cement or chlorides which tend to accelerate hydration and increase the risk of
corrosion of embedded steel.
Sea water (or any water containing large quantities of chlorides) tends to cause persistent
dampness and efflorescence. Such water should not be used where appearance of concrete is of
importance or where plaster finish is to be applied.
On the other hand, when reinforced concrete is permanently in water, either sea or fresh, the use
of sea water in mixing seems to have no ill-effects. However, it is generally inadvisable to use
sea water for mixing.
-
In general, flowing water or drinking (potable) water should be suitable for production of
concrete but, if in doubt, cubes can be made to the required specification and checked
against similar cubes made with distilled water.
MORTARS
Mortars may be defined as a mixture of sand, a binder such as lime or cement, or both which
forms a hardened mass after mixing with a suitable proportion of water.
USES: It is used in the beds and side joints of brick/block/stone work in order to bind them
together; to distribute the pressure throughout the brick/block/stone work; and to increase the
heat and sound insulation by filling the joints. It is also used as finishing for floors, walls and
ceilings.
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REQUIREMENTS
The essential requirements for a good mortar are as follows;
a) The particles should be evenly graded from coarse to fine.
b) The aggregate should be clean and free from all clay and earthly matters and presence of
soluble salts. These may give rise to efflorescence and dampness in the wall.
c) Strong mortar is not always necessary and may be detrimental. It is advisable that the
strength of the mortar should be slightly less than the strength of the bricks/blocks/stones.
d) It should be durable and resistant to frost and chemical attack.
e) It should be workable so that the brick layer can handle it easily.
f) It should retain plasticity long enough for the bricks to be laid on
TYPES OF MORTAR
1) Clay Soil Mortar: It is made by mixing clay soil and water. This is worked into a
plastic mass which is used to bond the bricks/blocks/stones.
2) Lime Sand Mortar: The water combines chemically with lime, changing it to
hydrated lime which has binding properties. This hydrated lime is mixed with sand to
form the so called lime sand mortar. This is suitable for bonding clay bricks in
internal walls. There are four suitable ratios for various works; these are 1:3, 1:4; 1:5;
or 1:6.
3) Cement Sand Mortar: This is a cement sand mixture used for bonding
bricks/blocks/stones. It is also suitable for finishing works e.g plastering. The
following ratios are in common use 1:3; 1:4; 1:5; 1:6; 1:7 or 1:8. Strong mortars of
the ratio 1:3 or 1:4 are used in works requiring high strength, but for general
brickwork up to 1:8 ratios may be used.
4) Cement, Lime, Sand Mortar (Compo Mortar)
This is the commonest mortar for general purposes. It is composed of cement, lime,
and sand, and common ratios are 1:1:6; 1:3:9 and 1:4:12.
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CONCRETE
Concrete is essentially a mixture of cement, aggregate and water in a given (suitable)
proportions. Sometimes, other materials are added at the mixer, referred to as admixtures.
Admixtures: These are materials other than cement, water or aggregates added at the mixer in
order to improve concrete properties.
Water/Cement ratio
Water is an essential ingredient of concrete. It causes the chemical action which results in the
setting and hardening of cement to take place and also enables the concrete to be sufficiently
plastic for easy placing and compacting. While too much water in concrete weakens and causes it
to deteriorate fast, too little makes it difficult to mix and place. The right proportion of water to
cement must therefore be established. This is known as the water/cent ratio. This is expressed by
weight and for most concrete the limits of the ratio lie between 0.4 and 0.65. Outside these
limits, the concrete will either be too stiff or too wet.
Aggregate /Cement ratio
Aggregate /Cement ratio= Mass of aggregate in concrete sample/ Mass of cement in that sample.
For most concrete mixes the aggregate cement ratio would be in the range of 4-10, small ratios
indicating rich, expensive mixes and high ratios lean, cheaper mixes.
CONCRETE OPERATIONS
The sequence of operations is as follows: The correct quantities of cement, aggregates and water,
possibly also of admixtures, are batched and mixed. This produces fresh concrete, which is
transported from the mixing place to its final position (location). The fresh concrete is then
placed in the forms, and compacted so as to achieve a dense mass which is allowed and helped to
harden.
BATCHING
This operation is concerned with measurements of dry materials and requires a careful attention.
Batching can be carried out by either volume or mass (weight).
a) Volume batching
The simplest way of measuring materials is to use the gauge-box, which can be made to
suit a 50kg bag of cement. Gauge boxes have four handles, sides, but no bottom or top.
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They are placed on the mixing surface which should be hard and clean. They are filled
and struck off level. The box is then lifted by means of handles, and the resulting
accurately-measured pile is then leveled off until the required overall quantity of
aggregate is obtained or the materials are shoveled into the mixer.
b) Mass (weight) batching
Some machine mixers are fitted with skips or hoppers that act as gauge boxes for
weighing the materials. A large dial records the weight of cement, sand and aggregate as
each is placed in the skip. When operated by a trained operative working under a capable
site supervisor, it is quick and effective.
Weight – batch mixers, however, are not popular in all areas.
Weight – batching is, however, a more accurate method.
Fig. 10: Gauge box and Spade
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MIXING
Concrete may be mixed by hand or machine.
a) Hand mixing
-
Place the measured of quantities of fine and coarse aggregate on a platform. (on site,
the platform is usually a layer of lean concrete worked to a flat surface). Each
measured quantity is leveled off until the required overall quantity of aggregate is
obtained.
-
Next, measure the cement, place on top of the heap of aggregates and spread evenly
with mixing shovels/spades.
-
The materials should be turned over completely with shovel at least twice dry. When
mixing by shovels only, it is necessary to turn over the materials together dry a couple
of times before the water is added.
-
When the heap shows an even colour throughout the mix is ready for addition of
water. Water is added to a chasen formed in the middle of the heap preferably in
sprays from a garden watering-can or a hosepipe. Water added from buckets on to the
heap tends to wash away the cement in the mix before the mixer has time to mix the
concrete.
-
The mixture is further turned with shovels until it reaches a plastic state without being
too water, at least twice.
-
A well mixed concrete should be capable of standing in a heap. Concrete that flows
and drains with water is unsuitable for most purposes.
-
The concrete is now ready to be placed into position.
b) Machine mixing
When mixing by machine, the mixer should not be going too fast, mixing time being two
minutes.
Many small sized contractors and private house builders are denied of the advantages of
machine mixing of concrete because of the high cost of purchasing or hiring, where
hiring is possible. Machine mixing is faster, saves materials and a better mixture is
obtained.
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-
The revolving drum type of machine is used for concrete mixing, the capacity of the
machine being chosen to meet the required quantity of the concrete on the job and the
speed at which each batch can be laid.
-
There is no advantage to be gained by mixing the materials dry first so the required
amount of water is first added into the drum. This moistens the drum and removes
any fresh mortar or concrete adhering to the sides.
-
The remaining materials are then measured into the drum in their correct proportions.
-
The fully loaded drum is allowed to mix for about two minutes by which time the
concrete is thoroughly mixed and ready to be laid in position.
The optimum mixing time depends on the type and size of the mixer; speed of rotation and on
the quality of blending of ingredients.
Prolonged mixing
If mixing takes place over a long period, evaporation of water from the mix can occur, with a
consequent decrease in workability and an increase in strength. A secondary effect is that of
grinding of aggregate, particularly if soft: the grinding thus becomes fine and the workability
lower.
Water
The amount of water to be used should be as a general rule, just sufficient water to make the mix
reasonably plastic and workable.
Fig.11: A simple concrete mixer
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LECTURE 8
TRANSPORTING
The system whereby the concrete is transported from the mixer or mixing place to the point of
placing depends on the type of job being done, the nature of the site, the equipment available, the
distance to be covered below or above ground, and the cost of labor at the time. Concrete can be
transported in the following ways:
1) Carried on heads
It is frequent on building sites to see a file (line) of laborers transporting concrete in head
pans from the mixer to the point of placing. The number of people involved on each job
depends on the size of the job, as this method is used for both medium and small sized
jobs. This type of labor is most often used for foundations of housing and the casting of
floors up to first floor and roof.
-
Hot weather and strong warm winds dry out concrete very quickly and this must be
borne in mind when transporting concrete in this way. If concrete is not transported
quickly after mixing, it may stiffen and become difficult to place.
2) Wheel barrows
Hand pushed wheel barrows are frequently used to transport concrete to the placing point,
or to a hoist, from where the concrete and wheel barrow are hoisted to the point of
placing.
-
On swamp or water -logged sites, a barrow-way may have to be constructed so that
the wheel barrow is pushed along a dry surface. This obviously slows things down.
-
The barrow-way is usually made of planks placed end to end and firmly wedged in
position. Again, if the wheel barrow is to travel over rough ground, precautions must
be taken against segregation of the concrete.
3) Mechanical plants
These include powered barrows or dumpers, lorries and cranes using skips, belt
conveyors etc. These are fairly fast means of transporting concrete. Where powered
barrows or dumpers are used, the route taken to deliver the concrete must be fairly even
and be free of stones and broken blocks or other wastes lying about on the site. This is to
prevent the concrete being segregated through vibration along the way.
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During the dry season, the sunshine can be very hot making the moisture in the concrete
evaporates quickly. If the concrete is to remain out of doors for long it should be covered
up.
Cranes where easily available are a convenient way of transporting concrete. The cost
precludes their use in concrete works, except on large contracts.
PLACING/POURING CONCRETE
Mixed concrete should be placed in its final position within half to one hour of mixing. This is
because the initial setting time of ordinary Portland cement takes place within this period. After
this time, the concrete has lost some strength which can never be regained, and the strength of
the mature concrete is adversely affected.
Concreting in hot dry weather such as is found in Africa poses particular problems with regard to
placing. Moisture evaporates too rapidly and this can result in a weak finished product unless due
precautions are taken. Most designers specify that placing of concrete should be done soon
after mixing and not later than half an hour after mixing. This specification is ideal and
should be adhered to if best results are to be expected from the finished product.
When concrete is being laid in foundation trenches, apart from the fast evaporation of moisture
from the concrete, the dry trench absorbs moisture from the concrete. To limit this absorption,
the trench should be watered down before the concrete is placed into it. This is referred to locally
as “quenching the thirst”.
Concrete should not be dropped from a great height as this has the tendency to segregate the
aggregates- heavy particles falling first. A height of 1m is usually specified as the maximum
from which wet concrete should be allowed to drop freely.
A free fall from a height of more than 1meter (e.g casting of columns) should be discouraged and
the use of pipes is recommended instead to avoid segregation.
In slopping surfaces segregation can be avoided by placing a baffle against slope.
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Fig.12: Concrete chute
COMPACTING CONCRETE
The concrete after being placed in position is consolidated/ compacted by using tamping rods,
rammers, or vibrators. This must be done thoroughly in order to obtain a dense concrete, i.e one
free from honeycombs or voids.
Unless special care is taken, it is difficult to ensure that the concrete is fully compacted when it is
being done by hand (tamping rods or rammers, etc.). Therefore, it is much more satisfactory to
use machines. These are called vibrators, and their names suggest, they vibrate very rapidly and
settle the concrete with the high degree of compaction.
“Wherever possible use vibrators to ensure proper compaction and elimination of voids”. Where
this is not possible, tamp thoroughly by rodding, especially around reinforcements and in
awkward corners. Once the voids have been removed stop tamping or vibrating, otherwise
segregation may occur and laintence will appear on the surface. This indicates a weakening of
concrete.
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Vibrators
Vibrators may be of the internal type consisting of a poker at the end of a flexible drive and
operated from an electric, diesel or petrol motor. The poker is immersed in the concrete at
intervals and once the air bubbles cease to rise, vibration should stop at that place.
Another type of vibrator is clamped to the formwork and the vibration is operated from the
outside of the concrete. This is very efficient, but the formwork must be strong enough to resist
the vibration.
The electric hammer is a simple type of vibrator which is also used on the outside of the
formwork. This type is not, however, quite as efficient as the two previously mentioned.
Fig.13: Poker vibrator
CONCRETE CURING
Definition: curing is the term used for stopping freshly poured concrete from drying out too
quickly. This is done because concrete, if left to dry out of its own accord, it will not develop the
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full bond between all of its ingredients. It will be weaker and tend to crack more. The surface
will not be as hard as it could be.
Also, curing is the process of keeping concrete damp for some days in order to enable it gain
more strength. It has been established that the concrete strength increases with age provided that
it is kept damp.
Also, curing is the process by which cement is enabled to hydrate. Since water is involved in the
process it is important to maintain the concrete in a saturated condition for sufficient length of
time to develop a satisfactory hydrate structure. This is essential not only in relation to strength
development but also to enable the concrete to withstand the stresses caused by shrinkage.
-
During the process of curing, concrete absorbs the water necessary for its chemical action
to reach its full strength. The strength of concrete increases more rapidly in the first few
days after setting, thereafter, strength increases at a slower rate.
-
After the concrete has been placed, compacted and hardened, the surface should be kept
damp so that it can achieve its maximum strength and not shrink too much due to drying
out too quickly, which may result in cracks.
-
The concrete should also be protected against the wind and sun, which also cause surface
cracking and from cold weather which will delay the setting rate of the concrete and may
affect its ultimate strength.
-
During the curing period, you must take certain steps to keep the concrete moist and as
23oC (73oF) as practical.
Improved properties
The properties of concrete such as freeze and thaw resistance, strength, water tightness, wear
resistance, and volume stability, improve with age as long as you maintain the moisture and
temperature condition favorable to continued hydration.
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Period of curing
The length of time that you must protect concrete against moisture loss depends on the type of
cement used, mix proportions, required strength, size and shape of concrete mass, weather
and future exposure conditions.
The minimum period for this is usually considered to be about one week, though it may be
longer with cements which hydrate slowly, such as pozzolanic type. “The period can vary from a
few days to a month or longer”
Curing methods
1) Ponding: Is done by forming a dam wall of sand around the concrete formation and then
flooding with water. This method has the following disadvantages;
-
It takes a fair bit work to do, and then quite often a breach occurs and the water runs off
the slab.
-
Usually this can only be done for a few days as it inhibits other works and the pressure is
usually on to get the walls up.
A possible drawback of this method, especially if soil or clay is used, is the chance of staining
the concrete.
2) Spray water on the formation
A simple way is just keep water sprayed on to the slab with garden sprinkler or hand held
hose pipes. Following are some disadvantages that you need to consider if you intend to
use this method.
a) This method is very wasteful of water.
b) Again, it can only be done for a short time (period) usually. The cured area should be
wet all the time, that is you should not let it dry out at all, almost impossible to do.
3) Sand layer
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Sometimes a thin layer of sand is used to cover the surface of the concrete. The drawback
in this method is that one must necessarily wait until the concrete has achieved some
considerable setting before the sand is placed (spread) over it, to avoid causing bruises to
the surface, or embedding the surface with sand.
4) Hessian or matting
After the concrete has been placed into position, these materials are used to cover. The
hessian or mats are before and after placing, the aim being to keep water in close contact
with the concrete until normal setting takes place. They are suitable for columns and
beam curing.
5) Plastic Sheeting
This material presents the moisture already contained in the concrete from escaping into
the air. Cover the concrete shortly after placing, taking necessary care so that the sheeting
is not blown away by wind. This is one of the most efficient methods of curing concrete.
Usually a spray from a hose pipe is used to wet the surface and the plastic is laid on, with
generous laps at the joints. A major benefit of plastic sheeting is that it does not stop
other works like building the walls and it can be left in position for weeks if need be.
6) Curing Compounds
There are now existing membranes curing compounds marketed under various names
which offer specialized ways of keeping moisture inside the concrete until normal setting
is achieved. They are usually applied 15 to 30 minutes after finishing the concrete. They
are widely used in road works. They are waxy emulsions that can be sprayed on to the
fresh concrete with a hand pump type spray.
BRICKS AND BLOCKS
Bricks are still the most popular form of walling unit for domestic construction. Their limited
size and variety of colors and texture make them an attractive proposition. There is a wide range
of bricks available, varying in the materials used, methods of manufacture and forms of bricks.
40
Bricks may broadly be described as building units which are easily handled with one hand. By
far the most widely used size is the single standard metric brick of actual size
215mmx102.5mmx65mm.
Manufacture
Bricks are made from several materials, chief of which are clay and shale; concrete; and cement
sand.
Frog
Arrises
Header face
Stretcher face
Fig.14: A brick
BLOCKS
Concrete blocks, sometimes called sand-crete blocks are made from the mixture of hard durable
and clean sand cement and water or from concrete. On setting and hardening the blocks attain
sufficient strength to be used as walling units.
Blocks may also be manufactured from clay and are termed clay blocks, or a mixture of soil and
cement as pressed blocks.
Size of Blocks: A variety of size exists among local building blocks. Some being made to suit
the owner‟s handling convenience. These are non-standard as they are not factory-produced.
The actual dimensions are without joints while the nominal dimensions allow for the thickness of
joints.
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TYPES OF BLOCKS
ACTUAL SIZE (LHW)
NOMINAL SIZE (LHW)
Dense aggregate blocks
440x140x90mm
450x150x100mm
440x215x215mm
450x225x225mm
440x215x140mm
450x225x150mm
440x140x90mm
450x150x100mm
440x215x140mm
450x225x150mm
440x150x115mm
450x160x125mm
440x140x65mm
450x150x70mm
440x215x140mm
450x225x150mm
440x215x65mm
450x225x75mm
Light weight aggregate block.
Load -bearing
Non-Load bearing. Light
weight blocks.
Teach yourself questions
1) Differentiate blocks from bricks.
2) Why is the curing of concrete important?
3) What will happen if a newly constructed concrete member has been forgotten to be cured
in the first few days?
4) What is segregation in concrete? List all possible causes of segregation. List all possible
causes of segregation in concrete.
5) Sketch a brick and show all the important parts.
ASSIGNMENT 01
Write short notes on the following materials a) Bitumen, b) Paints c) Glass d) Plastic
e) Metals and f) Adhesives. In each case explain manufacturing process, properties
and uses.
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