Transcript Properties of Concrete

Session 1
Range of building materials
Properties of Concrete
Clay and none clay building products
Introduction
Introduction
In this module you will learn about the characteristics and quality standards of building materials
commonly used in residential scale buildings
There is a wide range of possible building materials available for our use and the performance of these
materials has an impact on the cost, aesthetics and function of the building.
A well designed, economical building takes the following factors into account the properties and behaviour
of building materials the initial and long-term costs the effects on the environment how the materials
interact with each other.
Introduction
Introduction
Well designed buildings take into account:
Properties and behavior of materials
Initial and long term costs
Effects of the environment
How building materials interact with each other
Introduction
Introduction
Learning outcomes
On completion of this unit, you should be able to:
Nominate the various factors that limit the life and durability
of building materials.
Understand the various physical, chemical and biological
factors that affect the performance of building materials in
use
Understand the basic characteristics of different building
materials.
The
Range of building materials
Introduction
Factors affecting the selection of building materials
The selection of materials is affected by a range of factors
including:
Economic
physical.
Let’s examine these factors in detail.
Factors affecting the selection of building
materials
Economic factors
Energy content
Building materials are sometimes described as having a
certain ’energy content’. This refers to the cost of their
production. timber or sand are materials having a ’low energy
content’, they do not require a primary manufacturing process.
By-products of other industrial processes (eg wood particles,
blast furnace slag and pulverised ash) also have a low energy
content.
Introduction
Factors
effecting the selection of building
material
Energy content
Other materials require energy in their production, and
therefore have a ’high energy content’. These include, glass,
bricks, plastics, metals and cement. This adds to their cost,
and if local supplies of the raw materials are exhausted or
unavailable, then purchase and transport costs are also added
to the overall cost.
Factors effecting the selection of building
material
Labour and Materials costs
The cost ratio for housing is approximately:
55 per cent materials
45 per cent labour
Factors effecting the selection of building
material
Labour and Materials
The choice of materials should not depend only on the
purchase and installation cost, but also on the cost of repair,
maintenance and replacement of short life-span products.
Less durable materials may be cheap to buy but repair or
replacement costs are usually high.
Cheap materials usually lower the value of a building,
whereas more durable materials, such as stone and brick,
mellow with age and give the structure a more aesthetic
appearance.
Factors effecting the selection of building
material
Conservation of resources
Most world resources of metals, rainforest timber, fossil fuel
and limestone are non-renewable and limited.
It is important for us as consumers not only to be aware of
those resources which are threatened or have bad effects on
the environment, but also to use those which are, with
management, safe both to our health and to the environment
as a whole. Where possible we should use renewable
resources, such as timber from re-planting programs.
It is also important that world fuel energy is not wasted by
unnecessary processing and transportation. As well as being
environmentally desirable, these savings mean cheaper
materials.
Factors effecting the selection of building
material
Physical properties
Materials have different characteristics, or properties.
These properties are affected by physical, chemical and
biological factors.
Here we will be looking at the following properties:
Density and specific gravity
Strength
Electrical conductivity
Thermal conductivity and capacity
Moisture absorption
Acoustics.
Factors effecting the selection of building
material
Density and specific gravity
Different substances have different densities. Iron is much
denser than aluminium which is why a piece of aluminium
is much lighter than a piece of iron of the same size.
Ice floats in water because the ice is less dense than the
water.
Density is measured by specific gravity.
Specific gravity is the ratio of the mass of a given volume
of a liquid or solid to that of the same volume of water. The
density of pure water is taken as 1 at 4°C.
Factors effecting the selection of building
material
Strength
A structure (eg a beam or a bridge) must be able to safely
support its own weight plus the load it carries without
distortion.
Distortion will reduce the efficiency of the structure or make it
unstable or look unattractive.
A structure can be made much stronger without increasing its
weight, by being made in a different shape.
Structures have different strength when used in different ways.
See, for example, in Figure 1.1, where the steel beam A is
much stronger than the steel beams B or C, even though
they all contain the same amount of steel.
Factors effecting the selection of building
material
Figure 1.1: Different types of steel beams
Factors effecting the selection of building
material
Some materials strongly resist being squashed. They are said
to have compressive strength. Concrete, stone and brick are
such materials. Other materials, such as steel, are strong
under tension and will resist being stretched.
The behaviour of concrete under pressure is illustrated in
Figure 1.2. Concrete cracks easily when stretched. It has low
tensile strength.
Factors effecting the selection of building
material
By using steel reinforcing in concrete, we combine the tensile
strength of steel with the compressive strength of
concrete, resulting in a product that is strong in tension
as well as being strong in compression (see Figure 1.3)
Figure 1.3: The tensile (under tension) strength of steel is
combined with the compressive strength of concrete
when reinforcing mesh or bars are used in concrete
Factors effecting the selection of building
material
A piece of 25 mm wide galvanised steel strap, which is often
used in bracing timber frames, is very difficult to stretch, but
crumples easily when compressed lengthways. It has high
tensile strength and low compressive strength.
Materials that are undergoing force are said to be stressed,
and their change in shape is called strain. An elastic material
is one which will recover its original shape when the stress is
removed. A steel spring is elastic. A piece of chewing gum is
not very elastic.
The response of materials to stress will depend on:
how stress is applied to them
whether the stress is continuous (eg a load-bearing arch)
whether the material is compressed, stretched or twisted
whether it is affected by moisture or temperature.
Factors effecting the selection of building material
Complete the check progress 1 questions in your Guide
Electrical conductivity
Materials that easily carry electricity through them are said to
be conductors. Materials that do not are non-conductors. For
example, most metals are good conductors and most plastics
are not. This is why electrical wiring is copper and the
protective sheathing is plastic.
Factors effecting the selection of building material
Thermal conductivity and capacity
The thermal properties of a material are concerned with how
a material reacts to changes in temperature. The thermal
properties include heat expansion or contraction, insulation,
heat storing ability, cooling, and reaction to frost, snow and
ice.
Thermal conductivity is a measure of how fast heat travels
through materials. This rate may be affected by density,
temperature, porosity and moisture content.
Factors effecting the selection of building
material
Moisture absorption
Some very porous materials will absorb moisture more readily
than others. However, most materials may take up moisture
from the air, from the ground (eg through poor dampcourses),
from damaged roofs or gutters, or by condensation.
Condensation from moisture in the air will form on surfaces
colder than the air.
Condensation often becomes trapped on the inner surface of
water-tight materials (eg flat-roof coverings, metal and glass
wall-cladding, foil insulation). This can be prevented by the
correct use of vapour barriers (materials which are designed to
prevent surface condensation by being placed on the warm
side of walls or ceilings in such a way that there is no gap in
them).
Factors effecting the selection of building
material
Acoustics
Insulation from noise can be achieved by the use of dense
materials, by avoiding openings directly onto noise areas and
by avoiding direct paths (eg a hall with a bend leading from a
noisy machine shop to the workers’ tea room or a hall with
lobbies or double doors would both reduce noise).
Some porous materials are used for modifying the acoustics
in a room but sound can only be prevented from travelling
from one space to another by the use of dense materials
.
On the inside of a building, double-glazed windows, heavy
curtains, wall-hangings and carpet all help absorb noise. On
the outside, walls, fences, hedges, trees and bushes may be
used to reduce traffic or industrial noise.
Factors effecting the performance of building
material
Complete the check progress 2 questions in your Guide
Building materials undergo changes over time and the following
factors affect their performance:
Movement caused by applied loads
Movement caused by temperature
Movement caused by moisture
Durability of the materials
Fire resistance
Compatibility of different materials.
Movements may be substantial and result in considerable
stresses. If these stresses are greater than the strength of the
material then, obviously, cracks or buckling will result.
Factors effecting the performance of building
material
Movement caused by applied loads
These loads may occur by design or by accident. They may be
caused by error in structural design or from overloading.
Movement caused by temperature
Most substances are affected by temperature changes,
expanding when heated and contracting when cool, but some
are affected more than others. This is called thermal
movement. Figure 1.4 shows a comparison of the relative
changes due to temperature in a number of materials.
Factors effecting the performance of building
material
Dark coloured materials set into light coloured ones
Dark coloured materials, when exposed to the sun, can heat
up and expand greatly, causing cracks in the material in
which they are set. Or else the dark materials may
themselves crack or buckle. For this reason, roof surfaces
(such as sheet metal) are best finished with a solar heatreflecting surface or paint. Coloured glass in a sunny wall
must be able to move freely, as it will expand and contract
with temperature changes. If the glass is set between metal
screws or beading that prevents this movement, it will crack.
Putty or silicone caulk allows such movement.
Factors effecting the performance of building
material
Factors effecting the performance of building
material
Movement caused by moisture
A change in the moisture content of most materials will
result in deformation: they will swell when wet and shrink
when dry. These changes, called moisture movements, can
result in warped, twisted, shrunk or cracked items.
Factors effecting the performance of building
material
Durability
Since all materials deteriorate over time to some extent, we
should be able to anticipate these changes and take them into
account when designing a structure, whether it is a house, a
shed or a cupboard. We should foresee normal wear and tear,
as well as the occasional very heavy stress caused by storms,
fire, flood or burglary for example.
Durability will be different for different exposures. A coat of
paint will last for many years inside a cupboard or less than a
year in a sunny exposed position in a heavily polluted
industrial area. We are all aware of the effect of salt spray on a
car. Buildings are similarly affected, though it is not always so
obvious.
Factors effecting the performance of building
material
Corrosion of metals
The effects of metal deterioration on surrounding materials can
be significant, and will be looked at in the context of these
materials when they are dealt with in later units.
Sunlight
Sunlight causes drying and cracking of timbers. It also fades
colours and pigments and its heating of dark coloured
materials can greatly speed up their breakdown.
Ultraviolet radiation causes breakdown of clear finishes, stains,
paints, rubber, some plastics and polythenes, tars and bitumen,
fabrics and canvas.
Metals, bricks and stones are largely unaffected by sunlight.
Factors effecting the performance of building
material
Biological agencies
Certain bacteria in the soil break down sulphur chemicals
which cause corrosion of metals such as iron, steel and lead.
Burrowing animals or birds making nests can tunnel
foundations, undermining footings; they can also excavate
loose unsound material allowing rain in or weakening
supports.
Tree roots and vines growing in cracks exert a very strong
and destructive force, expanding and extending cracks in
masonry, pipes, concrete or timber. They also hold moisture,
encouraging the growth of moulds and fungi, and the uneven
drying of brickwork (which causes uneven movements within
the wall).
Factors effecting the performance of building
material
Water and frost
Care should be taken in the selection of materials for use in
damp areas since some building materials react less well in
such situations than others. For instance, limestone and
marble slowly dissolve in water. Timber, chipboards,
hardboards and other similar wood products lose some of
their strength, and many flooring materials are less hardwearing when wet.
Water can encourage fungal attack and certain destructive
chemical reactions. Repeated wetting and drying causes
surface crazing and cracking of timbers. Water also often
carries destructive acids, salts and other soluble chemicals.
Factors effecting the performance of building
material
Salt crystallisation
Salts that are dissolved in water can come from the sea, the
ground and from some building materials. As moisture evaporates
from a surface, the salts are left behind in the form of powder or
crystals, called efflorescence. Sometimes this is just an
unattractive coating, usually white, but sometimes yellow, green or
brown. However, it can be destructive if allowed to persist for a
long time.
Salts crystallising on the surface of a porous material can cause
gradual erosion or flaking. This surface deterioration, called
fretting or spalling, often occurs in soft sandstones, bricks (such as
sandstocks) or in mortar layers in masonry. When moisture rises in
the walls of a building these salts cause paint to bubble and peel.
Fixing this problem can involve costly installation of dampcourses
and removal of all affected plaster or render from the walls.
Factors effecting the performance of building
material
Chemical action
Cause swelling, shrinking, weakness or damaged appearance. Due
to chemical changes within the material itself, or changes brought on
by attack from outside chemicals. Heat and moisture aid most
reactions.
The presence of aggressive gases, in the air or in factories or
dissolved in rainwater, can mean that some materials may need
special protection, or that other more suitable materials should be
used instead.
Factors effecting the performance of
building material
Groundwater, industrial wastes, soil, ash and wet clays are
some of the substances that can produce soluble sulphates
which attack cement products and metals.
Loss of volatiles
Volatiles are liquids and gases. Plastics, paints, varnishes,
finishes, mastic, rubber, tar and bitumen shrink and become
brittle when their volatiles are lost.
Factors effecting the performance of building
material
Abrasion and impact
In situations of abnormal impact or abrasion, suitable
materials and finishes need to be chosen. For example, a
concrete path or floor that will take heavy traffic requires
correct concreting techniques to be followed so as to
produce a hard, durable surface.
Vibration
Vibration caused by proximity to machinery or heavy
vehicular traffic can cause problems in light constructions
and with brittle materials.
Complete the check progress 3 questions in your guide
Factors effecting the performance of building
material
Fire resistance
Fire is usually the fastest, most destructive and
dangerous way in which a building can be damaged or
destroyed. It is a very important consideration for both city
and country dwellers.
Government bodies test materials and publish regulations
and codes which are implemented by local councils
concerned about fire hazards in public or private
buildings.
Fire hazard indices (published by the Experimental
Building Station as ’Notes on the Science of Building’,
Nos 66, 98, 136, 137, 142) are lists based on extensive
experiments on structures and materials.
Factors effecting the performance of building
material
Combustibility
Materials that ignite, that give off flammable gases or that
show considerable self-heating when exposed to a set heat
in a furnace, are called combustible.
Non-combustible materials, on the other hand, do not feed
the fire, and flame does not spread over them. Noncombustibility does not mean fire resistance. Table 1.1 lists
some combustible and non-combustible materials.
Non-combustible materials (such as steel) may expand and
disturb attached structures, or lose strength and collapse.
Other non-combustible materials may spall (flake) and shrink
or crack. On the other hand, some combustible materials
(such as timber) can often provide a useful degree of fire
resistance.
Factors effecting the performance of building
material
Fire resistance
Fire is usually the fastest, most destructive and dangerous
way in which a building can be damaged or destroyed. It is a
very important consideration for both city and country
dwellers.
Government bodies test materials and publish regulations
and codes which are implemented by local councils
concerned about fire hazards in public or private buildings.
Fire hazard indices (published by the Experimental Building
Station as ’Notes on the Science of Building’, Nos 66, 98,
136, 137, 142) are lists based on extensive experiments on
structures and materials.
Factors effecting the performance of building
material
Fire Resistance
Fire resistance is expressed as the amount of time in hours
and minutes a component survives a fire test of set
temperature before it can no longer perform its function. It is
considered to fail the test when any of the following occur:
It collapses.
It forms holes or cracks through which flame can pass.
It gets hot enough to ignite other combustible materials it is
in contact with and which the fire hasn’t yet reached.
Factors effecting the performance of building
material
How certain materials behave in fire
Timber
Timber easily ignites at about 221–298°C. However, some timber
(particularly large pieces, at least 100 by 75 mm in section or larger)
are resistant to the fire once the surface has been charred. Many
Australian hardwoods have this characteristic and, in fact, have proved
to be more fire resistant in buildings than steel. However, all timbers do
burn readily if temperatures stay high enough. Therefore, timber
buildings are not classified as fire resistant.
Factors effecting the performance of
building material
How certain materials behave in fire
Timber
Timber has good thermal insulation, preventing materials not in contact
with the fire from heating up to extreme temperatures. When hot,
timber does not expand in length (unlike steel) and neither does it
markedly lose strength.
Laminated timber structures glued with synthetic resins have similar
fire resistance to solid timber, although resistance will vary according
to the type of timbers and glues.
Factors effecting the performance of building
material
Stone
Stone blocks and slabs are usually satisfactory in fires, but
overhanging features and lintels are liable to fail. Free quartz (eg in
granites) explodes suddenly at 575°C and should not be present in
any stone that is required to be fire resistant. Sandstones behave
better than granite, but in drying they may shrink and crack, with 30–
50% loss of strength.
Plastics
Although many plastics are made in fire-retardant grades, all are
combustible and some give off large quantities of toxic smoke. PVC
(polyvinyl chloride) melts at fairly low temperatures, and most
thermoplastics (plastics that can be heated and shaped) char above
400°C and burn at 700–900°C.
Factors effecting the performance of building
material
Clay products
Most clay products perform well in fires, having been made
at kiln temperatures higher than most fires reach.
Brickwork failure is often caused by expansion of enclosed
or adjoining steel work.
Concrete
Ordinary Portland cement concrete disintegrates at 400–
500°C. However, how the concrete performs depends very
much on the presence of reinforcement and the type of
aggregate it contains.
Factors effecting the performance of building
material
Metals
Metals used in building are non-combustible, but they lose strength
when heated. Aluminium, lead and zinc melt in building fire
temperatures. As previously mentioned, the expansion of the hot metal
can cause problems. Also, the high thermal conductivity of metals
means that the temperature of surfaces remote from a source of heat
will approach the temperatures near the fire, causing fires to spread.
Steel
Mild steel behaves in an interesting way when heated. Up to 250°C, it
gains strength, then gradually returns to normal strength by 400°C.
After that, it rapidly weakens so that, at 550°C (referred to as the
critical temperature), it begins to fail.
Generally, structural steelwork must be protected with fire-resistant
encasements, such as concrete or brickwork.
Factors effecting the performance of building
material
Glass
Although glass is non-combustible, it readily transmits heat
and often shatters unpredictably at an early stage in a fire.
Toughened glass is not fire-resistant.
Glass fibre and rockwool
Resin-bonded glass fibres are combustible. Glass fibres
themselves melt at about 600°C.
Fibrous cement
This material tends to shatter when heated, sometimes
explosively. It does not contribute to making a fire-resistant
structure.
Factors effecting the performance of building
material
Paints
Generally, paint films are combustible and may help spread
flame over surfaces. However, as they are thin, they only
contribute a small amount to the fire load. When applied
to combustible materials, certain paints can reduce the
spread of flames. They delay but never prevent the
spread of flame.
Complete the check progress 4 questions in your guide
Factors effecting the performance of building
material
Compatibility of materials
The large range of new materials on the market today, many
of which are chemically based, plus widespread pollution, has
led to new chemical and physical problems with materials. A
material may break down many times faster than normal in
the presence of another particular substance. Problems do
not always show up until a product has been on the market
for a number of years. Incompatibility of building
Materials can be grouped roughly under the following
headings:
Corrosion of metals
Stains and discolouring effects
Problems with surface finishes
Chemical reaction between materials.
Factors effecting the performance of building
material
Corrosion of metals
Galvanic reactions: These occur between metals that have
different levels of electronegativity. This is often seen as
corrosion of one metal or a deposition of metal scale on the
other metal. Offcuts or filings of metals left around in moisture
can cause rapid destruction of nearby metal building
components. Some common galvanic reactions are listed
below.
Lead used with zinc or aluminium promotes corrosion.
Therefore, metal roof-flashings need to be carefully chosen.
Steel screws or nails should not be used with aluminium or
zinc roofing, unless they are zinc or cadmium coated.
Copper should not touch or drain onto zinc, aluminium,
zincalume or galvanised materials.
Factors effecting the performance of building
material
Corrosion of metals
As heat speeds up corrosion, different metals should not be mixed in
hot water systems.
Copper and brass are permanently resistant to water.
Aluminium: This becomes encrusted in coastal atmospheres. Mortar,
cement or concrete pit the surface of aluminium if splashed on it.
Industrial atmospheres: These are usually acidic and corrode all
metals.
Stains and discolouring effects
Copper: Water dripping off copper causes green stains.
Rust: Water running off exposed iron or steel will stain surrounding
surfaces.
Eucalypt timbers: When wet, many eucalypt timbers produce brown
stains on masonry.
Factors effecting the performance of building
material
Problems with surface finishes
When finishes won’t stick to the surface they are applied to, it
is usually due to the two being unsuitable for each other. The
surface may either be too smooth or it may be powdery or
flaky; or there might be a chemical incompatibility between the
surface and the finish.
Many silicone sealants will not accept paint.
Acid-resisting grouts (for floor-tiles) cannot be satisfactorily
cleaned from the tile surface.
Primers, undercoats, finish paints, lacquers, varnishes and
stains should all be used according to manufacturers’
instructions as many are incompatible with certain materials.
Factors effecting the performance of building
material
Testing of materials
The testing of materials is carried out by the manufacturer or supplier
before delivery (eg stress testing of timber). Upon delivery, an
inspection should be carried out with respect to the quality and
suitability for the construction situation it is intended.
Concrete is one material which is tested on site (the slump test), and
later laboratory tested for compressive strength at 28 days. Materials
such as paints, adhesives, glass and the like have been developed
and trialled under strict laboratory controls and conform to Australian
Standards.
Building The builder or supervisor of a project, needs to be
informed of all the information relating to products being used. Details
such as handling, storage, application, installation and warranties
should be kept in a product file and updated to provide ready access
to this information to avoid warranty problems associated with
incorrect handling and installation.
Factors effecting the performance of building
material
Handling and storage
Planning for storage and handling of materials on site is an important
job for building staff. Many materials are easily damaged if due care
is not taken in handling, and some can deteriorate if exposed to
moisture and direct sunlight.
Materials should be stored in accordance with manufacturers’
instructions; for example, stacked flat, off the ground, in a dry area or
in a secure area for flammable or toxic materials.
Transportation to the site and unloading arrangements need to be
given careful consideration and appropriate equipment must be
organised.
When handling materials on site, safe working practices must be
followed and all OHS regulations implemented.
Factors effecting the performance of building
material
Tolerances
All building work in Australia is covered by the Building Code of
Australia and many Australian Standards. These standards
have been developed for most building materials and detail
tolerances, application, testing (if applicable) and method of
installation. These tolerances should be followed and best
industry practice adhered to.
Complete the progress check 5 in your learner guide
Properties of Concrete
Properties of Concrete
Learning outcomes
On completion of this unit you should be able to:
•
•
•
•
•
Identify the properties of concrete
Understand the nature and purpose of the materials which
make up concrete
Identify the procedures used in the transport and
placement of concrete
Understand the reason for and the methods of curing
concrete
list the uses of concrete in residential construction.
Properties of Concrete
Cement
In Australia, all Portland cements are made to meet the
requirements of AS3972–1991 ‘Portland and Blended
Cements’.
General purpose cements:
Type GP—general purpose Portland cement
Type GB— general purpose blended cement.
Special purpose cements:
Type HE— high early strength cement
Type LH— low heat cement
Type SR— sulphate resisting cement.
Properties of Concrete
Special purpose cements
Type HE cement is used where high strength is required at an
early stage; for example, where it is required to move forms as
soon as possible or to put concrete into service as quickly as
possible. It is also used in cold weather construction to reduce
the required period of protection against low temperatures.
Type LH cement is intended for use in massive concrete
structures such as dams. In such structures the temperature
rise resulting from the heat generated during hardening of the
concrete is likely to be a critical factor
Type SR—sulphate resisting cement has better resistance to
attack by sulphates in ground water than other types because
of its special chemical composition.
Properties of Concrete
White and off-white cements
Off-white cement is in general use in cottage construction but
white cement usually proves cost prohibitive.
High alumina cement
High alumina cement is not a Portland cement. If mixed with
Portland cement it can give a rapid or ‘flash’ set. It is
characterised by a very high rate of strength development
accompanied by a high heat of hydration and by a greater
resistance to sulphate and weak acid attack than Portland
cements. Curing conditions require very close control for 24
hours after placement.
Answer the questions on the your guide
Properties of Concrete
Storage of cement
Cement will retain its quality indefinitely if it does not come in
contact with moisture. If it is allowed to absorb appreciable
moisture it will set more slowly and its strength will be reduced.
Therefore, storage of bagged cement requires storage facilities
to be as airtight as possible, and the floor should be above
ground level to protect against dampness.
The bags should be tightly packed to reduce air circulation, but
they should not be stacked against outside walls. If they are to
be held for a considerable period the stacks should be covered
with tarpaulins or water-proof building paper.
Doors and windows should be kept closed. A ‘first-in-first-out’
rotation of bags should be maintained at all times.
Properties of Concrete
Setting and hardening
Setting is the initial stiffening of the cement paste during
the period in which the concrete loses its plasticity and before
it gains much strength.
This period is affected by the water content of the paste and
the temperature. The more water in the paste the slower the
set, the higher the temperature the faster the set.
Hardening is the gain in strength which takes place after
the paste has set. It is affected by the type of cement used
and the temperature. High temperatures cause more rapid
hardening.
Properties of Concrete
Water
Water suitable for drinking will generally be suitable for
concrete making.
Aggregates
Aggregates used in concrete should consist of clean, hard,
durable particles strong enough to withstand the loads to be
imposed upon the concrete.
In general they should consist of either natural sands or gravels
or crushed rocks, although some manufactured aggregates
such as blast furnace slag and expanded shale and clays can
be equally satisfactory.
Commonly used crushed rocks include basalt, granite, diorite,
quartzite and the harder types
Properties of Concrete
Grading
Both coarse and fine aggregates should contain a range of particle
sizes. Graded aggregates produce more workable oncretes which are
less prone to segregation and bleeding.
Particle shape and surface texture
The particle shape and surface texture of aggregates affect the
workability. For workability, particles should be smooth and rounded.
On the other hand, angular materials result in greater strength, so
that, in the final analysis, there is little or no difference in
effectiveness. The ultimate decision is one of economics and
availability.
Maximum size of aggregates
The greatest economy is achieved when the largest maximum size
aggregate is used. The factors limiting size are the availability,
transporting and placing equipment to handle the larger sizes, and the
clear spacing between reinforcing bars and the clear spacing between
the reinforcement and the formwork.
Properties of Concrete
Manufactured aggregates
Blast furnace slag
If sound and free from excessive quantities of ferrous iron,
blast furnace slags are satisfactory concrete aggregates.
Generally they are angular in shape and require a higher
percentage of fines to produce workable concrete.
Lightweight aggregates
Expanded shale aggregates produce concrete having
approximately two-thirds the density of those made with dense
aggregates, but with comparable strengths. Lightweight
aggregates may be smooth and rounded or harsh and
angular, depending on the method of manufacture.
Properties of Concrete
Testing of aggregates
Since aggregates comprise up to 75 per cent of the volume of
concrete, their properties are obviously important. These
properties include size and grading as well as cleanliness.
The testing of concrete aggregates is generally carried out to
determine:
Presence of organic or other deleterious material which may
severely limit the strength of the concrete
Resistance to abrasion, which may limit the durability of the
concrete
the presence of any alkalis which may react with the cement
and cause expansion of the aggregate.
Properties of Concrete
Conclusion
Good concrete can be made from a wide variety of aggregates
provided these are clean and free from harmful impurities. As
the quality of concrete becomes higher, the quality of the
aggregate becomes more important and factors such as
grading more critical. Good aggregates, although sometimes
higher in initial cost, are generally more economical because of
the higher quality and lower overall cost of the concrete they
produce.
Properties of Concrete
There are several properties of concrete which affect its
quality.
These are:
• Compressive strength
• Tensile strength
• Durability
• Workability
• Cohesiveness.
• Let’s examine these properties in detail.
Properties of Concrete
Compressive strength
Compressive strength remains the common criterion of concrete
quality and will frequently form the basis of mix design. For fully
compacted concrete made from sound clean aggregates the strength
and other desirable properties under given job conditions are
governed by the net quantity of mixing water used per bag of cement.
This relationship is known as the water/cement ratio, that is, the
quantity of water in the mix to the amount of cement present.
Example: A concrete mix having a water/cement ratio of 0.5:1 would
require 20 litres (20 kg) of water for each 40 kg bag of cement.
The ultimate strength of concrete depends almost entirely on the
water/cement ratio, for as the ratio increases the strength of the
concrete decreases.
Properties of Concrete
Tensile or flexural strength
This is the measure of the concrete’s ability to resist flexural or
bending stresses.
The tensile or flexural strength of concrete is dependent on the nature,
shape and surface texture of the aggregate particles to a much
greater degree than does the compressive strength.
Durability
Concrete may be subject to attack by weathering or chemical action.
In either case the damage is caused largely by the penetration of
water or chemical solutions into the concrete and is not confined to
action on the surface. The resistance to attack may therefore be
increased by improving the watertightness of the concrete. This is
achieved by lowering the water/cement ratio, assuming the concrete is
fully compacted.
Properties of Concrete
Workability
The workability of concrete, or the effort required to handle and
compact it, depends on several factors, as follows:
Water/cement ratio: The higher the water/cement ratio, the more
workable concrete becomes. However, the water/cement ratio
should be fixed by considerations other than workability (eg
strength and durability), and should not be increased beyond the
maximum dictated by these considerations.
Cement content: The cement paste in concrete acts as a lubricant,
and at a fixed water/cement ratio, the higher the cement content,
the more workable the concrete becomes. It follows then that any
adjustments to increase workability should be made by increasing
the cement and the water content at a constant water/cement
ratio.
Grading of aggregates: Grading tends to produce more workable
concrete.
Properties of Concrete
Particle shape
Particle shape and size of aggregates:
Smooth, rounded aggregates will produce more workable
concrete than rough, angular aggregates. Also, for a given
water/cement ratio and cement content, workability increases
as the maximum size of the aggregate increases.
Properties of Concrete
Cohesiveness
The cohesiveness of concrete means the ability of plastic concrete to
remain uniform, resisting segregation (separation into coarse and
fine particles) and bleeding during placing and compaction.
Concrete in the plastic state should be cohesive to prevent
‘harshness’ of the mix during compaction, and to avoid segregation
of the coarse and fine components during handling.
Segregation may occur during transporting over long distances,
discharging down inclined chutes into a heap, dropping over the
reinforcement or falling freely through a considerable height and
placing in formwork which permits leakage of mortar.
Maximum cohesiveness usually occurs in a fairly dry mix, so as a rule
the wetter the mix the more likely it is to segregate. Segregation can,
however, occur in very dry mixes.
Properties of Concrete
Testing of concrete
Concrete is tested on the site or in the laboratory to
determine its strength and durability or to control its
quality during construction.
These tests must be carried out carefully and in the correct
manner or the results may be misleading and cause
unnecessary delays while they are being checked.
Worse still, faulty tests may result in either substandard
concrete being accepted or even good concrete being
rejected.
There are several ways in which testing can be carried out:
by sampling
by slump testing
by compression testing.
Properties of Concrete
Sampling
To make a composite sample from the discharge of a mixer or truck,
three or more approximately equal portions should be taken from
the discharge and then remixed on a non-absorbent board.
The sample portions should be taken at equal intervals during the
discharge and none should be taken at the beginning or the end. The
concrete at these points may not be truly representative of the whole
mix.
When sampling freshly deposited concrete, a number or samples
should be taken from different points and recombined to make a
composite sample. Care should be exercised to make certain the
sample is representative by avoiding places where obvious
segregation has occurred or where excessive bleeding is occurring.
Properties of Concrete
Slump testing
The slump test is a measure of the consistency or mobility of
concrete and is the simplest way of ensuring that the concrete
on the site is not varying.
It should be done often as an overall control on the various
factors that can affect the result. Most important among these
factors is the water content of the mix, variation of which can
result in varying strengths of concrete.
A consistent slump means that the concrete is under control. If
the results vary it means that something else has varied,
usually the water, which can then be corrected.
Details are contained in your guide
Properties of Concrete
Compression testing
The strength of concrete is determined by making
specimens, curing them, and then crushing them to
ascertain their strength. The preparation of specimens is
most important as a badly prepared specimen will nearly
always give a low result.
Compressive test specimens are normally cylinders 150
mm in diameter and 300 mm high.
Details are contained in your guide
Complete the questions in your guide
Properties of Concrete
Proportioning and mixing
Design strength
The designer of a concrete structure determines during the
design stage, the concrete properties that are necessary to
ensure that the structure performs in the desired manner.
Since compressive strength is usually the most important
property required and since most other desirable properties are
directly related to it, it is usual for the designer to specify the
minimum compressive strength required, usually at 28
days.
The ‘design strength’ is the minimum strength required by the
designer.
Properties of Concrete
Target strength
The mix designer must design a mix which will produce concrete with
a strength in excess of the design strength:
It is known that when a series of compressive tests are made from
samples of concrete taken from time to time through the course of a
job, the results will be scattered to either side of an average value.
This means that the concrete produced is never completely uniform in
quality some weaker than the average strength and some stronger.
Since the designer has specified the minimum strength required, the
mix designer must aim at an average strength, between the target
strength and the design strength.
Generally, a target strength 33 per cent higher than the design
strength meets the requirements of the building codes.
Properties of Concrete
Specification of concrete
In writing the specification to ensure that the concrete has the
properties required, the designer has two alternatives:
specify the concrete by strength (the usual method)
specify concrete by proportions.
Concrete specified by strength
The designer specifies the minimum compressive strength
required in the concrete and the age at which the concrete
should have this strength, usually 28 days.
Properties of Concrete
Batching
All materials, including water, should be accurately measured
to ensure that concrete of uniform quality is produced.
Batch proportions are often specified in relation to the bag of
cement; for example, one 40 kg bag of cement to so many
kilograms of coarse aggregate and so many kilograms of fine
aggregate with perhaps 20 L or 20 kg of water.
One Litre of water has a mass of one kg and is not subject to
variation.
With mass batching, there is no need to make allowance for
the bulking of damp sand but allowance must be made for the
non-absorbed water held by the aggregates as this moisture
forms part of the mixing water.
Properties of Concrete
Notes on mixing contained in your guide
Premixed concrete
Premixed concrete is used almost universally on residential
building sites. The use of premixed concrete has advantages
which include:
Better quality control is possible at a large plant than under
most site conditions.
Premixed concrete is controlled by AS1379–1991 Specification
and Manufacture of Concrete, which should be referred to for
information on methods of ordering, mixing and delivery.
Properties of Concrete
Transporting concrete
Irrespective of the methods used to transport, place and
compact the freshly mixed concrete, the following
requirements are basic to good practice:
The concrete must be transported, placed and compacted
with as little delay as possible.
The concrete must not be allowed to dry out before
compaction.
There must be no segregation of the materials.
The concrete in the forms should be fully compacted.
Properties of Concrete
Pumps and pipelines
Pumps and pipelines enable concrete to be transported across
congested sites and where space is limited.
Concrete for pumping must be of medium workability with a
slump of 70 mm to 120 mm and must be free from any
tendency to segregate. The introduction of fly ash to the
concrete improves pumpability and workability of the mix,
and therefore adds appreciably to the distance concrete
can be pumped.
More detailed information is contained in your guide
Properties of Concrete
Placing concrete
Certain precautions must be taken when placing concrete, to
ensure that:
Formwork and reinforcement is not damaged or dislodged
The concrete is free from segregation
Other qualities of the concrete are not impaired.
Study the notes in your guide
Properties of Concrete
Compacting
It is essential that concrete be properly compacted to
ensure maximum density. Air holes must be eradicated, voids
between aggregate particles must be filled and all aggregate
particles must be coated with cement paste.
Thorough compaction results in:
Maximum strength
Watertight concrete
Sharp corners
Good bond to reinforcement
Protective cover to reinforcement
Good surface appearance.
Properties of Concrete
Vibration
Concrete is usually vibrated to achieve good compaction.
There are three types of vibrators:
Immersion vibrators
Form vibrators
Surface or screed vibrators
The immersion vibrator is driven either electrically,
mechanically or pneumatically and is probably the most
efficient type of vibrator as it vibrates the concrete directly by
immersion in the concrete. They are particularly suited to the
compaction of large volumes of concrete.
Properties of Concrete
Curing
Concrete increases in strength and other desirable properties
with age, this is so only so long as drying is prevented.
The hydration of cement is a chemical reaction and this
reaction will cease if the concrete is permitted to dry.
Evaporation of water from newly placed concrete not only
stops the process of hydration, but also causes the concrete to
shrink, thus creating tensile stresses at the drying surface; and
if the concrete has not developed sufficient strength to resist
these stresses, surface cracking may result.
Properties of Concrete
Curing
As in many other chemical reactions, temperature affects the
rate at which the reaction between the cement and water
progresses; the rate is faster at high temperatures than at lower
temperatures.
It follows then that concrete should be protected so that
moisture is not lost during the early hardening period and should
also be kept at a temperature that is favourable to hydration.
Properties of Concrete
Curing methods
Curing methods can be classified as follows:
The supply of additional moisture to the concrete during the early
hardening period.
Sealing the surface to prevent loss of moisture from the
concrete.
Ponding
Sprinkling
Wet coverings
Waterproof paper, plastics
Curing compounds
Properties of Concrete
Length of curing period
For most structural purposes, the curing time for concrete
varies from a few days to two weeks according to
conditions; for example, lean mixes require longer curing
time than rich mixes and temperature affects the curing time
as does the type of cement used.
Since all the desirable properties of concrete are
improved by curing, the curing period should be as
long and as practicable in all cases.
Answer the questions in your guide
Properties of Concrete
Reinforced concrete
Basic principles
Concrete, Is strong in compressive strength, and comparatively
weak in tensile strength. To overcome this weakness in
tension, concrete which is to be subjected to tensile stresses is
reinforced with steel bars or mesh which is so placed that it will
resist such stresses.
The designing and detailing of reinforcement is the job of the
designing engineer and will not be dealt with in any great detail
here, but it is important that those who supervise the fixing of
reinforcement on the job have an appreciation of the basic
principles of reinforced concrete.
Properties of Concrete
Figure 3.5: Types of stress found in a structure
Properties of Concrete
Reinforced concrete design combines the steel reinforcement
with the concrete in such a manner that enough steel is included
to resist the tensile stresses and excess shear stresses while the
concrete is used to resist the compression stresses.
The bond between concrete and steel directly counteracts any
tendency for the concrete to stretch and crack in a region
subjected to tension
Concrete and steel expand and contract the same amount. If this
were not so, the different expansion rates would break the bond
between the two materials and so prevent the transfer of tensile
stresses to the steel
Concrete has a high fire-resistance and protects the steel from
the effects of fire.
Properties of Concrete
Design of reinforced concrete
In order to be effective, the tensile reinforcement must be
prevented from sliding in the concrete. The adhesion or bond
between the concrete and the steel is related to the surface
area of the steel embedded in the concrete.
Adequate anchorage is effected by extending the rods past
the critical points (where no longer required to resist tensile
and shear stresses) and by the use of:
Standard hooks
Plain rods extended into the supports (rarely used)
Deformed bars (rolled with lugs or projections).
Study Figure 3.6 in your guide
Properties of Concrete
Good formwork
The guiding principles for the production of good formwork
are:
Quality
Safety
Economy.
Properties of Concrete
Quality
First quality formwork should be:
Accurate: True to the shapes, lines and dimensions required by the
contract drawings.
Rigid: Forms must be sufficiently substantial so as to prevent any
movement, bulging or sagging during the placing of the concrete.
Tight-jointed: If joints are not tight, they will leak mortar. This will
leave blemishes in the shape of fins on the surface of the
concrete and may result in honeycombing of the concrete close
to the leaking joint.
Well-finished: The quality of the finish of the concrete is dependent
on the finish of the forms. Nails, wires, screws and so on should
not be allowed to mar the surface of the finished concrete.
Properties of Concrete
Safety
Strength: For the safety of the workers and of the structure,
the formwork must be strong enough to withstand not only
the mass of the wet concrete but also the live loads of
workers, materials and equipment. It is impossible to over
emphasise how important this aspect of safety really is.
Soundness: Materials must be of good quality and durable
enough for the job. The time will come, no doubt, when it
will be essential to use for structural load-bearing
members, only timber that has been tested with the
mechanical stress grading process.
Properties of Concrete
Economy
For economy, formwork should be:
Simple: Formwork should be designed for simplicity of erection and
removal.
Easily handled: Shutters and units should be light enough to permit
easy handling.
Standardised: Where standardisation of formwork is possible, the
ease of assembly and the possibility of reuse serve to lower the
formwork cost.
Reusable: Formwork should be designed for easy removal and in
sections that are reusable. This will minimise the amount of
waste material and thus decrease the cost of the formwork.
Properties of Concrete
Supervision
Note:Study the notes in the guide carefully regarding
supervision
Surface Treatments
There are many proprietary surface treatments available, some
prevent adhesion to the formwork, others provide architectural
finishes.
Properties of Concrete
Stripping times
The time of the removal of forms is generally specified by the
architect or engineer
Forms can usually be safely stripped when the concrete has
developed about two-thirds of its 28-day strength.
However, the earliest possible removal of forms is desirable
for the following reasons:
To allow the reuse of forms as planned.
In hot weather, to permit curing to begin.
To permit any surface repair work to be done while the
concrete is still ‘green’ and favourable to good bonding.
Vertical forms can generally be removed before the forms to
the soffits of beams and slabs.
Properties of Concrete
Table 3.1: Times for stripping formwork and supports
Study the chart in your guide
Properties of Concrete
Concrete finishes
Many types of off form finishes:
Smooth
Wood grain
Architectural patterns
Textured and patterned surfaces
Properties of Concrete
Joints in concrete construction
If the concrete is allowed to stiffen to the extent that it cannot
be worked, then a joint must be made. Other cases will occur
when it is necessary, for structural reasons, to break the
continuity of placing and to form a joint.
Joints can be of two general types:
Construction joints: Bond the new concrete to the hardened
concrete in such a manner that the concrete appears to be
monolithic and homogenous across the joint and allows for no
relative movement of the concrete on either side of the joint.
Control joints: These allow for relative movement on either
side of the joint, thus they can be either construction joints or
expansion joints.
Properties of Concrete
Construction joints
In practice, it is very difficult to obtain a perfect bond at a joint and
a plane of weakness will always occur at a construction joint.
For this reason, they should be avoided wherever possible.
While unscheduled interruptions are often unavoidable during
placing, making an unplanned construction joint necessary,
some breaks in the continuity of placing may be foreseen
either in the design stage or just before commencement of
construction, thus allowing the position of many joints to be
planned.
Good planning will aim to interrupt placing in a position suitable for
a control joint and so eliminate the need for a construction
joint.
Properties of Concrete
Location of construction joints
Where construction joints are necessary in structural members
they should be made where the shear forces are at a
minimum. The joint should be at right angles to the axis of the
member so that axial forces act normally to the joint and do
not tend to cause sliding along a weakened plane.
Concrete for columns should be poured continuously to just below
the soffit of the beam, drop panel or capital, and the concrete
left for at least two hours to settle before fresh concrete is
placed.
The whole floor system around the head of the column should
then be cast in one operation after suitable preparation of the
joint.
Properties of Concrete
Construction joints in beams should be made in the
middle third of the span and on no account should they
be made at or near the supports or over any other
beam, column or wall since shearing stresses are
usually very high at these positions.
When a construction joint is required in a floor slab it
should be made near the middle of the span.
Properties of Concrete
Making vertical construction joints
When making a construction joint in a beam or slab, the
concrete must not be allowed to assume its natural angle
of repose, but should be taken up to a suitable stop board
so as to form a vertical joint. To assist the transfer of load
across the joint, either dowels or a keyway to aid
mechanical bonding may be used at about mid-depth of
the beam or slab. This is recommended in sections over
150 mm deep. Reinforcement must not be cut at a
construction joint but must be left continuous in the
member.
Properties of Concrete
Making vertical construction joints
Properties of Concrete
Watertight construction joints
A correctly made horizontal construction joint in a wall should
not require sealing, but if the joint is to be in contact
with
water and particularly if subjected to hydraulic pressure,
effective sealing will be necessary because of the tendency of
the joint to open up as the concrete shrinks.
This can best be carried out by using a water stop. PVC water
stop membranes extending into the concrete equally each
side of the joint and welded or glued together at the ends to
form a continuous diaphragm are commonly used.
Properties of Concrete
Contraction joints
A contraction joint is a concrete joint made so that the concrete is free
to shrink away from the joint while all other relative movement across
the joint face is prevented.
As concrete sets, hardens and dries out, it shrinks. If no provision is
made to relieve the drying-shrinkage tensile stresses within the
concrete, cracking will occur when these stresses exceed the tensile
strength of the concrete. If the concrete is completely unrestrained,
cracking will not occur, but very few structures are completely
unrestrained.
Contraction joints are most needed in unreinforced concrete structures
because reinforcement considerably increases the tensile strength of
concrete, restrains overall shrinkage movement and prevents the
formation of large shrinkage cracks.
Properties of Concrete
Location of joints
Contraction joints should be located where it can be
expected that the severest concentration of tensile stresses
will occur, such as:
Where abrupt changes in cross section occur.
On irregularly shaped floors and slabs (eg T, H, L and U
shapes), to divide them into rectangular shapes.
Where structures are weakened by openings.
In long structures such as walls and road pavements, which
are not sufficiently reinforced to prevent the formation of
shrinkage cracks.
In large areas of pavement or slab on the ground.
Properties of Concrete
Construction of joints
A vertical plane of weakness is purposely formed in the slab
or wall. Vertical movement is controlled by forming a keyed
joint or by using non-ferrous dowels with one end capped
and coated so that they are free to slide. The bond between
new and existing concrete at a contraction joint must be
broken.
Properties of Concrete
Dummy contraction joints
A dummy contraction joint is a plane of weakness
built into a structure by means of a groove, either
sawn or formed with a grooving tool.
This joint functions as a contraction joint by localising
shrinkage cracks to beneath the groove. The
irregularity of the crack serves to transfer loads
across the joint and prevents relative movement in the
plane of the joint.
Since this type of joint is an alternative to a full depth
contraction joint, the location should be the same as
for contraction joints.
Properties of Concrete
Expansion joints
An expansion joint is formed by creating a gap between
the two surfaces of the concrete to allow for expansion.
The gap is usually filled with a compressible filler and all
relative movement in the plane of the joint is prevented.
Expansion joints are generally provided in structures
exceeding 30 m length, in unreinforced or lightly
reinforced road pavements and as sliding joints
between a roof slab and a supporting wall.
Answer the questions in your guide
Clay, Non-clay bricks,
blocks and Stone
Clay, Concrete & Stone
Clay has endured as a building material and even in early times its
use was widespread (eg bricks, tiles, pipes and accessories). The
shaping of plastic clay and then hardening it by drying and firing,
was perhaps humanity’s earliest form of manufacturing but it was
not until the late nineteenth century that machines became involved
in the manufacturing process.
Learning outcomes
On completion of this unit, you should be able to:
•
identify the uses of clay products in the building industry
•
understand the role of brickwork in the building industry, and be
familiar with the range of brick types and the different styles of
bonding and tinting
•
describe the desirable qualities of stone for specific
applications
•
discuss the uses and limitations of stone as a building material.
Clay
Clays are natural materials made up of very small crystalline
mineral fragments. The shape, size and type of these
fragments gives clays their plastic quality which allows them
to be moulded and shaped when wet. These mineral
fragments are also responsible for the hard, stony nature of
clays after they are fired at high temperatures.
Clay products
When clay has been changed by heat (firing), the products are
called ceramics. During firing, water is driven off, some
recrystallisation of minerals takes place, and glass is formed from
quartz sand present in the clay. The result is a hard, insoluble
material. The higher the firing temperature, the more
recrystallisation occurs and the more glass is formed, resulting in
greater hardness and density.
The minerals present in the clay will determine its colour when fired.
Ceramics are also coloured by having a specially prepared coating,
or slip, applied before firing, which results in a glaze of the required
colour or texture.
Different products require different firing temperatures, as shown in
Table 4.1.
Table 4.1: Table of firing temperatures and uses of various
ceramics
Uses of ceramics in building
There are five types of ceramics, apart from bricks, that are mainly
used in building:
•
•
•
•
•
terracotta
fireclay
stoneware
vitreous china
porcelain.
Table 4.2 shows how these different ceramics are made and used
Carry out activity 1 in your guide
Table 4.2: Features, firing temperatures and uses of ceramics used in building
Type of ceramic
Features
Firing temperature
Uses
Terracotta
Yellow to brownish red
clays, which may be glazed
or unglazed. Terracotta
roofing tiles, although
brittle, are stable in high
climatic temperatures and
do not contaminate run-off
water
Fairly low temperature
Main use is for floor and
roofing tiles and air bricks
(ventilators). Over the
years, the most common
pattern seen in Australia
has been the French or
Marseilles pattern (see
Figure 4.1).
Fireclay
Usually a creamish colour,
it can withstand high
temperatures over a period
of time without cracking
Stoneware
Harder, and less absorbent
than fireclay. Contains
more glass
Flue liners and firebricks in
stoves, fireplaces, kilns and
furnaces
Fired at a higher
temperature than fireclay
Drainpipes and fittings.
Bricks
Bricks used in construction are made from:
• clay or shale
• cement/concrete
• sand and lime (calcium silicate).
Methods of brick manufacture
Bricks are no longer made by hand but these are sometimes
available second-hand from demolition sites. They are soft, porous,
rather irregular in shape and, if protected from the weather, retain a
pleasing warm appearance.
There are, now, two main methods of brick manufacture:
 the dry pressed method
 the plastic or extruded process.
Dry pressed method
In this method, almost-dry clay powder is pressed into moulds and
then fired. Most dry-pressed bricks have an indentation (called a
frog) resulting from the shape of the mould (see Figure 4.2).
Plastic or extruded process
With the plastic or extruded process, a soft, moist mix is extruded
through a die in the form of a long clay column which is then cut into
brick-sized pieces by wires in a frame (see Figure 4.3). Extruded
bricks have a much higher average compressive strength because
the proportions between the raw materials are more accurate.
Brick classification
Bricks are graded A, B or C, according to their compressive strength (with grade A being
the strongest) and are classified according to type as shown below:
Clinkers
overburnt and very hard but often distorted in shape; usually unsuitable for
regular brickwork; often used for feature walling.
Callows
underburnt, light in colour, soft, very absorbent; inferior for most structural
purposes.
Commons
general purpose bricks; hard in texture but often with flaws developed
during manufacture.
Select commons
best quality commons, with sharp arises and fairly uniform colour; suitable
as a substitute for face bricks.
Face bricks
good quality bricks, with smooth or texture faces in a variety of styles and
colours.
Sandstock
imitation (mechanically-made) or hand-made bricks.
Brickettes
small-face bricks, with plain and textured faces; often used for fireplace
facings and ornamental feature work.
Brick classification
The different types of brick can best be illustrated by looking at
appropriate product literature.
With modern methods of applying a surface coating to a compatible
colour base, bricks are now available in many colour shades, from
black, through reds and yellow to white.
There are also purpose-made bricks which are made in special
shapes (eg bullnose or squint).
Brick quality and standards
The quality of good bricks is determined by their texture and
hardness and their size and shape.
They should have an even, granular texture, be well-fired and free
from flaws (eg face blisters or shrinkage cracks). Two bricks, when
struck together, should give a clear ringing sound.
They should also have regular shaped faces and sharp arises (see
Figures 4.2 and 4.3) and fall within a standard size range.
Brick sizes:
Metric modular brick 290 (90 ( 90 mm
Metric standard brick230 (110 (76 mm
The long face (called the stretcher) of a standard metric brick measures 230 (76 mm, and
the short face (called the header) measures 100 (76 mm.
A closer (quarter brick) measures 50 (76 (110 mm (standard metric) and a queen closer (a
standard metric brick split lengthways showing a closer face at each end) measures 50
(76 (230 mm (see Figure 4.4).
Laying bricks
Bonding is the way the bricks forming a structure are held together. Good
bonding depends on the chemical bond between the bricks and mortar and
on the mechanical bond resulting from how the bricks are laid.
The depth of mortar between bricks is usually 10 mm, providing a
horizontal joint (called a bed joint) and a vertical joint (called a perpend).
Jointing is the term usually given to the surface finish of the mortar set
between bricks. Such finishes vary according to trends. Tuck pointing used
to be common about the turn of the century but has since faded from
popularity. The most common forms of jointing in use at present are:
• ironed
• flush jointing
• raked jointing (see Figure 4.7).
Many different methods of laying bricks are used, some more
effective than others. Bonding is provided by the way the bricks
overlap each other and interlock, and it should:


distribute the load evenly throughout the mass of brickwork
tie the mass of brickwork together as an integrated unit
provide a pleasing arrangement of bricks and joints.
Two types of bonding are illustrated in Figures 4.8 and 4.9. The
stack bond (see Figure 4.8), for example, provides little mechanical
bond between the bricks (because it creates a vertical downward
thrust), whereas with stretcher bond (see Figure 4.9) the load is
more evenly distributed throughout the brickwork.
Figure 4.8: Stack bond
Figure 4.9:
Stretcher bond
Accessories for brickwork
There are a number of different accessories which are used with
brickwork:
• wall ties
• damp proofing
• anti-termite caps
• ventilators
• lintels
• piers.
Let’s look at how they are used.
Wall ties
Wall ties tie the two walls of a double brick wall together, so that they do
not move apart from each other.
The most common type is 4 mm or 3.15 gauge galvanised wire bent to
shape, with a kink (or drip) which should be positioned pointing down in
the cavity between the two walls to prevent moisture passing along the
inside wall (see Figure 4.11).
Wall ties should be spaced no more than 1 m apart and staggered every
fourth course in height, with a minimum number of four ties per square
metre. The ties should be at least 6 mm higher on the inner walls than on
the outer walls. If the cavity width is greater than 75 mm, special length
ties are used.
Figure 4.11: A wall ties
Damp-proof courses
Damp-proof courses are provided:
•
horizontally in walls and on piers to prevent upward seepage of
water from the ground or through concrete in contact with the
ground
•
vertically as vapour barriers to prevent penetration of moisture
through a wall
•
through walls and across cavities as flashing to control moisture
from a roof or parapet or around windows, door heads and sills.
Anti-termite caps
Anti-termite caps made of galvanised iron are used on all piers
under floor timbers.
Ventilators
Ventilators made of terracotta or concrete with wire mesh are set
into brickwork to provide under-floor ventilation as close as
possible to the underside of the floor, or ventilation into the cavity
of double brick walls.
Reinforcement
Reinforcement should be placed in footings and walls where tension
stress is likely to occur, because brickwork is weak in tensile
strength. The types of reinforcement available are:





wire mesh
welded wire
fabric mesh
expanded metal
steel rods, generally used for vertical reinforcement
Figure 4.12: Reinforcement types
Lintels
Lintels are steel bars, steel angles and so on, used over doors,
windows, fireplaces or other openings to support the brickwork
above (see Figure 4.13).
Piers
Piers are brick columns which provide above-ground support for
other structural members, usually floors. They are of two types,
attached and isolated.
An attached (or engaged) pier is built attached or bonded to a wall.
It may be used to stiffen or supply lateral support to the wall and
carry a superimposed load by providing an additional bearing area.
An isolated (or sleeper) pier is free-standing and usually carries
some structural load but it may also be purely decorative (ie non–
load-bearing). In order to maintain stability, attention must be paid to
the relationship between the height of the pier and the size of the
base dimension. Tables can be obtained to provide guidance in this
respect.
Figure 4.14: An isolated (sleeper) pier
Unfired clay or soil construction
Carry out the check progress 2 in your guide
Clay in mud and soil has a very long history as a building material.
Wet or moist soil or clay is put into forms or moulds and allowed to
sun-dry (cure). Mud brick (adobe), rammed earth (pisé), pressed
blocks, wattle and daub, and cob are the five most common
methods used.
Mud brick (adobe)
Mud brick walls are probably one of the oldest and most popular
forms of earth housing. Wet mud is placed in boxes (forms) which
are removed shortly after, and the blocks are allowed to cure for
about a month before being used. The blocks are bonded with a
mortar of the same mud that was used for making the blocks.
Rammed earth (pisé)
Moist soil is rammed into position between heavy wooden forms.
The forms are moved along or up as work progresses. The
ramming may be done by hand or with pneumatic tampers.
Machine-made (pressed earth) blocks
The method involves the use of a hand-operated machine to press
the soil into bricks or blocks which are then allowed to sun-cure
before being laid in courses like any other brick or block.
Wattle and daub
With this method, a wall of reeds or branches is woven over a
timber frame and mud is plastered on the inside and outside of the
weave. Although very cheap and fast to build, the mud often cracks
and needs constant maintenance. White ants easily destroy the
timber frame, and the buildings are not usually very durable.
Materials added to stabilised earth
Cement is often used in adobe, pisé, pressed block construction
and in soil floors to improve inferior soils. The soil needs to be
pulverised first. The cement (5–12% by weight) and water are then
added and amounts made must be in smaller batches than for
straight mud, since the concrete ‘goes off’.
Bitumen added to soil acts both as a binding and waterproofing
agent.
Non-clay bricks and blocks
Concrete bricks and blocks
These are manufactured from graded sand, aggregate, Portland
cement and water; fly ash is often used as a cementing agent. They
are made in a variety of solid and hollow shapes but in standardised
metric sizes, so that a block or half block, with the addition of 10 mm
of mortar, measures whole units of 100 mm or 50 mm.
Table 4.6: Concrete block and brick sizes
Brick type
Length
Height
Width
Standard blocks
390
190
290, 190, 140,
90
Half-high blocks
390
90
190, 140, 90
Metric modular bricks
290
90
90
Standard bricks (same
size as standard clay
bricks)
230
76
110
Concrete bricks, blocks and paving are very versatile with the
advantage that they are not usually difficult for unskilled workers to
use. They come in a variety of textures and colours.
Blocks are usually used hollow and unreinforced. They can easily be
reinforced, if required, by using steel reinforcement and filling the
central core with concrete.
Concrete blocks shrink and swell with temperature and humidity
variations and this has to be allowed for, particularly in external work.
Paving blocks are available in new interlocking systems that make
very hard-wearing, attractive roads or footways and which give good
access to buried service piping.
Concrete roofing tiles are also available in a range of colours and
shapes and are widely used.
Calcium silicate bricks
Calcium silicate or sand-lime bricks are also used, though not yet in
the same quantities as clay bricks.
They are usually whitish or grey in colour, but their physical
characteristics are different.
Stone
The main rock groups
Rocks, referred to in building as ‘stones’, can be divided into three
groups, according to how they are formed in nature:
 igneous rocks
 sedimentary rocks
 metamorphic rocks.
Igneous rocks
Igneous rocks are all formed from molten rock which has cooled and
hardened. For example, rocks such as basalts, volcanic glass and pumice
(cooled glassy froth) are formed from volcanic lava. Rocks such as granite
are formed from molten rock that has cooled and hardened underground.
Types of igneous rocks
Granite
Granites and granite-like rocks are hard rocks and are made up of a mosaic
of fairly large crystals of various minerals easily visible to the naked eye.
Granites are usually light grey or pink in colour but can vary through to quite
deep reds. The trade term ‘granite’ is also used to cover a number of darker
rocks including gabbro, a black rock known as ‘black granite’.
Examples
•
Moruya granite—pale grey (used for the Sydney Harbour
Bridge pylons).
•
Mudgee granite—deep reds.
•
Bathurst granite—reds and pinks, various types from the
area around Bathurst.
•
Riverina grey—from the Tocumwal area; and pinks from
Berrigan.
Trachyte
Trachyte has smaller grains than granite.
Examples:
Bowral trachyte—dark olive green or dark grey, occasionally
streaked with beautiful veins of glassy crystals and quarried at ‘The
Gib’. Bowral trachyte has been used on a number of Sydney
buildings and for the piers of the old Hawkesbury bridge.
Canoblas trachyte—a very hard and durable stone, it polishes to a
soft grey or buff base colour with small pink and black spots and is
from Orange (held in great repute by local builders, it makes a good
flagging stone and was used as such on the front of most of the
older important buildings in Orange).
Basalt
Very dark to black, fine-grained igneous rock. Basalts are often
called ‘bluestone’ or ‘blue metal’. They have been quarried from
Orange, Kiama, Dundas, Stirling (near Inverell) and Uralla and
used extensively in Melbourne (eg St Patrick’s Cathedral) and
other parts of Victoria.
Dolerite
This is similar to basalt but coarser grained. It is used extensively
as road metal, gravel and aggregate in concrete.
Sedimentary rocks
Sedimentary rocks are most often made up of bits of other rocks,
usually as a result of erosion. For example, layers of mud and sand
(the result of other rocks being worn down) become buried deep in
the earth and are compressed and hardened to form shale and
sandstone.
Sandstone
Formed in nature by sand grains which are cemented together, sandstone is a popular
building stone when available, as it is fairly easily worked, very attractive in
appearance and not very heavy. Many sandstones, however, are too soft and crumbly
to be useful.
Sandstones are porous, allowing dampness to soak through: so, when used as
footings, they must have good damp-proof course. Inadequate or non-existent dampproofing has resulted in rising damp problems in many old buildings with sandstone
footings.
The predominant building stones used around Sydney have been the Sydney and
Gosford sandstones. As these two stones are basically identical, descriptions of
Sydney sandstone apply equally to Gosford sandstone.
Sydney sandstone is one of the finest building sandstones in Australia. Its colour is
usually a pale yellowish or buff colour to pinkish or brownish tones, with colour
variations within it. It is easily seen in many road cuttings around the Sydney area,
such as the expressways north of Sydney, in the Blue Mountains, and approaches to
the Harbour and Gladesville bridges.
Other areas in NSW where sandstone has been quarried include Marulan (used for St
Saviour’s Anglican cathedral, Goulburn); Bundanoon, one of the best sandstones in
NSW for large buildings, its colour varies from white to pink (used for the base of the
soldier’s memorial and town hall in Goulburn and St Michael’s cathedral, Wagga
Wagga); Yass; Canberra; Frogshole; Galong; Grong Grong; Milparinka; Mendooran;
Newcastle (identical to Sydney sandstone); and Ravensfield.
Limestone
Limestones are sedimentary rocks formed from coral, sea shells
and deposits of calcite (the mineral of which shells and coral are
made).
Limestone as a building stone is worked and sold under the general
name ‘marble’. However, limestone is also mined extensively for
manufacturing lime and cement.
Metamorphic rocks
Metamorphic rocks are formed as a result of changes which have
been usually brought about by heat and/or pressure in the earth’s
crust. For example, when shale (a sedimentary rock) is compressed
it becomes a metamorphic rock, slate; sandstone, when heated up,
perhaps by volcanic lava, turns into quartzite; and limestone
becomes marble under pressure.
Slate
Slate is formed by immense pressure in the earth’s crust compressing and
altering clay rocks such as shale. Slate splits easily in layers in one
direction, like pages of a book. Some coloured shales are marketed for
paving as ‘slate’, but a true slate is usually grey, greenish grey, bluish or
purplish in colour.
Slate is fairly soft and easily scratched, but has a very pleasing
appearance when well laid and cared for. Its softness is obvious when we
see how the centre of the tread in the grey slate steps in old buildings are
often worn away with use.
The high labour cost of cutting and laying slate roofs led to a decline, but
its recent popularity for floors, wall facings and fireplace surrounds has
renewed interest in it as a marketable product.
In the early days of the colony of NSW, slate was brought out from Britain
as ship’s ballast. It was then used to roof many early Sydney buildings
(some fine old slate roofs are still to be seen around NSW). Gradually
Australian deposits were found and worked—at Chatsbury, Gundagai,
Towrang, Black Mountain, Bathurst and Mudgee.
Marble
Limestone, acted on by heat and pressure in the earth’s crust,
changes its structure and pattern of colour and becomes marble.
Marble and limestone are both quarried for building stone as
‘marble’, so we will look at them together. Their colours vary from
almost pure white through nearly every possible shade of greys,
greens, yellows, reds and blues to black. They are used for making
cement, for ornamental and monumental stones, statues and
building stones.
Carry out check progress 3 in your guide
Stone classified for building purposes
In the building industry, special terms are used to describe different
types of stone. These terms might indicate the quarry location, the
colour, texture, pattern or use of the stone.
Trade terns
Some terms, such as the following, have a different meaning in the
building industry to their geological meaning.
Granite any medium- or coarse-grained igneous rock used as dimension
stone.
Sandstone sedimentary rock made of sand-size grains; sandstones with
thin, even, regular bedding along which the rock is easily split are termed
‘natural flagstones’; in NSW sandstone which splits with equal ease in any
direction is called ‘freestone’.
Marbleany limestone or marble which is able to take a polish and is used
decoratively; also includes the metamorphic rock serpentine, termed
‘serpentine marble’.
Dimension stone
This term refers to natural rock used as ‘building stone’, ‘ornamental
stone’ and ‘monumental stone’. It is generally quarried in blocks or
slabs and marketed in a variety of sizes and finishes according to
customers’ needs. The main varieties of dimension stone quarried
and used in NSW are granite, marble, sandstone and slate.
Although most varieties of dimension stone are widespread in NSW,
economically viable deposits are not common. Suitable sandstone
deposits are available fairly close to Sydney, but the other stones
are located in isolated areas a long way from major markets. These
localities include: Wombeyan (marble); Yass (limestone); Mudgee
(granite); Eugowra (granite); Bowral (trachyte); Bundanoon
(sandstone); Tumut (marble); Mulyandry (granite); Middle Arm
(slate).
Requirements of dimension stone
These may vary from one project to another but, in general, are as
follows:
• It must be able to be extracted in large blocks free from joints
and imperfections.
• It must be sound and durable.
• It should be uniform in colour and texture.
• It must have aesthetic appeal (difficult to describe, but such
things as colour, pattern, texture and finish are important).
• Stones used for certain purposes must be capable of taking and
keeping a polish. Only ‘granites’ and ‘serpentine marble’ keep a
polish when exposed to weather.
• It must be available in quantity so that sufficient reserves exist of
fairly uniform stone to meet large orders and future demands for
maintenance or restoration work.
Economic outlook
Dimension stone is a moderate to high-cost material. It is often
passed over for cheaper load-bearing materials such as steel and
reinforced concrete.
Other dimension stones likely to be in demand include good quality
purple and green slate for decorative purposes and good quality
white marble, black marble, gabbro and granite.
Construction materials
Construction materials are low-cost minerals and rocks that are
extracted in bulk. They require little processing and are used for
construction purposes. Such materials include the following:
Coarse aggregate: Crushed and broken stone, prepared road base
and gravel. Usually igneous rocks are used in NSW, though
sedimentary sandstones have also been used successfully. The
most important deposits are those situated near the larger cities.
Much of the coarse aggregate is used in concrete.
Fine aggregate: Construction sand, which is usually dug from rivers,
beaches or dunes and must be clean, with no soil or salt. It is used
mostly for concrete, mortar, sand-lime bricks and fillers.
Unprocessed materials: These include weathered rock, gravel, soil
and loam. They are used mainly for road-making and site-filling.
General properties of stone
Most natural stones are very good load-bearers and make good
footings, walls and pylons.
The amount of thermal expansion is very low for marble and slightly
greater for sandstone, slate and granite. However, allowance should
be made for thermal movement.
Some stones, especially igneous rocks (such as granites, trachyte
and basalt), are not all porous and therefore do not allow moisture
penetration. Others, like sandstone, can be very porous.
Most natural stones are very durable—a property which can,
however, be adversely affected by certain environmental factors.
Factors causing deterioration in stone
Atmospheric pollution
Sulphur chemicals in the air or soil dissolve in rainwater and form
weak sulphuric acid which will slowly dissolve marble, limestone,
calcareous sandstones and mortars.
Salt
Salts dissolved in water seep into rocks and dry out, forming
crystals. These growing crystals cause pressure in porous rock or
in mortar and, as they expand, can cause progressive decay.
Frost
Porous rocks in which water freezes will crack and disintegrate,
often very quickly. However, frost action is not a problem in most
parts of NSW
Factors causing deterioration in stone
Solubility
Limestone, marble and calcareous sandstone will slowly dissolve in
water.
Wetting and drying
Repeated wetting and drying of porous rocks can cause slow
surface crumbling and should be guarded against. (This also
weakens mortars.)
Corrosion of metals
As iron and steel rust, they swell. Where iron or steel rods, bolts or
bars are fitted into or between pieces of masonry and allowed to
rust, serious damage is caused in stone structures.
Some metals also form salts as they corrode which can be
destructive to surrounding stonework.
Vegetation
Most plants, including lichens and mosses, do little damage to
stonework, but they do hold moisture, which may be a problem with
mortars and porous rocks. Ivy, however, because of the way its
roots penetrate cracks and cavities, can cause serious damage.
Finishes and maintenance of stonework
Surface finishes
Figure 4.15 gives some idea of the range of tooling that can be done on
stone with, usually, a mallet and various chisels. Today, with
mechanisation, sawn and polished faces are used fairly frequently,
especially with monumental work.
Rubble walling
Walls may be built either as:
•
•
•
•
random rubble (uncoursed)
random rubble (coursed)
square rubble (uncoursed)
square rubble (built in courses).
Ashlar walling
Walls may be built either as:
•
•
•
random ashlar
ashlar (regular coursed)
ashlar (irregular
coursed).
Maintenance
Outside stonework should be cleaned regularly and defective joints raked out and
refilled (reappointed) with a sand-lime mortar, not a cement mortar mix.
Methods of cleaning various stones are outlined in Table 4.7. Note that caustic soda
and soda ash are very damaging and must never be used on any stone.
Stone
Method
Comments
All types
Hydrofluoric acid (5%
concentration)
Sandblasting, dry
Mechanical abrasive
tools
Risks damage to adjacent
materials. Fast method, no
staining, very dusty. Sandblasting and abrasive tools
produce a lot of dust
Limestone
and marble
Clean water spray, mild
detergent, dry and
polish with soft cloth
Slow, not suitable for heavy
encrustations
Preservation
Most stone is fairly durable, so fast decay usually occurs from wrong
choice of stone, defects in design, or neglect. These errors should
be corrected before attempting to ‘preserve’ the stone. For example,
salts should not be sealed in, but should be removed by repeated
sponging with water. Get qualified advice before using surface
sealers as they can sometimes do more harm than good if not
appropriate to the problem.
Alternative materials
Dimension stone faces considerable competition from cheaper
materials, in particular exposed aggregate panels and other
concrete-based products. Steel and concrete have virtually
replaced dimension stone as a major load-bearing construction
material.
Artificial stone
Economic reasons, together with the greater range of architectural
finishes available, have brought about a greater use of synthetic
and artificial stones, such as the following.
Alternative materials
Cast synthetic stone
Pure polyester resin or a mixture of polyester resin and
acrylic is moulded into stone-like material which can be cast
in single pieces. In situations where the use of stone would
require a number of separate stone sections to be jointed
together (eg in panels or columns) this method offers
distinct advantages. It is also not as hard or as cold as stone
and can be worked with wood tools. It can be produced in a
variety of shapes and sizes and is usually used to imitate
marble.
Carry out the check progress 4 in your guide
Summary
In this unit you have learned about the uses of clay products, the
role of brickwork in building, the range of brick types, bonding and
tinting of bricks and about stone and its applications and limitations
as a building material.
You will have come across many terms used to describe the
processes and products linked with clay and stone. You may need
to read back over this unit to refresh your memory of some of these.
Carefully revise the meaning of those that you are unclear about.
You should be able to define all the materials we have looked at,
know their qualities and how they are used.
In the next unit you will learn about the role of mortar in the building
industry.