Building Materials
General Requirement of Materials
in Construction
HNTec in Construction and Draughting (Dual TVET)
Enabling Objectives
1.
Identify general performance requirements of materials for
buildings.
2.
Define the terms used to measure materials performance.
i. Structural safety (Strength, Stiffness, Toughness, Hardness, Creep)
ii. Health and safety
iii. Fire
iv. Durability
Introduction
What are the materials used in buildings?
Timber, gravels, sand, glasses, steel and so on.
Stone / Gravel
What are the general performance requirements of
materials for buildings?
Structural Safety The ability to withstand stress resulting from gravity, wind
thermal, or moisture movement, or other sources.
Health / Safety
There should be no risk to health due to chemical or physical
effects of the material both during and after construction.
Fire
The material must be have acceptably in resisting fire spread,
release of dangerous substances in fire and retaining
satisfactory structural stability.
Durability
The material should fulfil the above performance criteria as
required for the planned lifetime of the building.
There are a number of other important performance requirements.
For example, thermal comfort, weather exclusion, sound control,
serviceability, appearance, security, etc.
Performance requirements cannot be placed in order of
importance because any one of them may be more critical than
other for a particular element of a building, priority is normally
dictated by the precise function and location of a specific building.
Materials performance and its measurement
Structural safety
• Strength
This may be defined as the ability to resist failure or excessive
deformation under stress. There are several types of stress.
Bending
Compression
Tension
Shear
Stress is the force carried by unit area, is measured in Newton’s (N) per
square millimeter (mm2), expressed as N/mm2. Note that fracture is not
necessary for a strength ‘failure’, for example, steel for practical purpose, has
failed when it yields, through yielding does not initially damage the metal.
Yield is an increase of strain without any increase of stress.
Strain is the deformation caused by a force. It is a ratio (no units) expressed
as:𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑜𝑟 𝑑𝑒𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑙𝑒𝑛𝑔𝑡ℎ
𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ
• Stiffness
This term normally relates to elastic deformation, that is, deformation
which is recovered when the load is removed. High deformations, even if
elastic, may cause problems, for example, unsightly appearance or failure
of plaster coatings. The ability of a material to resist plastic deformation
is referred to as ‘stiffness’, normally measured by:
𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 𝐸
=
𝐴𝑝𝑝𝑙𝑖𝑒𝑑 𝑠𝑡𝑟𝑒𝑠𝑠
𝑆𝑡𝑟𝑎𝑖𝑛 (𝐾𝑁 / 𝑚𝑚2
• Toughness
This is the ability to absorb energy by impact or sudden blow. Strong
materials are not always tough e.g. cast iron. Weak materials can have
high toughness e.g. leather.
• Hardness
This is resistance to indention and is relevant to floor and wall surfaces.
Hardness depends on a combination of strength and stiffness properties.
• Creep
This is the effect of long-term stress under which some materials
gradually deform and eventually break. Materials subject to creep are
timber, clay, lead, concrete, thermoplastics and to a small extent, glass.
• Fatigue
This is the effect of load reversals such as vibration which lead to failure
at relatively low stresses. All materials subject to fatigue effects and in
some situations e.g. roads or floors subject to heavy moving loads, or
machine frames – fatigue may be the critical factor in design.
Fatigue is a term used to describe the loss of strength resulting from repeated applications of a force
which is less than would cause failure with a single applications.
Toughness and Strength are related.
A material may be strong and tough
if it ruptures at high strains
exhibiting high forces. The quality
known as toughness describes the
way a material reacts under sudden
impacts. Brittle materials may be
strong but not tough.
Strength indicates how much force
can the material support, while
toughness indicates how much
energy a material can absorb
before rupture.
In short, the opposite of tough is
"brittle.“
A good example of a tough material
that has low strength is rubber.
A good example for a strong
material that is brittle is porcelain.
Materials performance and its measurement
Health and Safety
This table indicates some of the possible safety hazards posed by materials,
together with recommendations for overcoming them.
Substance
Situation
Risk
Remedy
Lead
Formerly in paints, pipes
solders
Health risk if ingested,
especially to children
Remove existing lead
pipework. Specify lead –
free paints and solders
Radon gas
Diffuses from some
granite rocks through
ground floor into houses
Radioactive gas may lead Isolated ground floor by
to risk of lung cancer
membrane or prevent
ingress by positive
internal pressure
Substance
Situation
Risk
Formaldehyde gas
Present in some foams,
Can cause nausea
e.g. cavity fills; glues as in
chipboard
Ventilate new dwellings
Chlorofluorocarbons
(CFCs)
Used in refrigerants, air
conditioning systems,
propellants for aerosols
and foaming agents for
some plastics
Check specifications for
products which might
contain CFCs. Less
harmful substitutes now
available
Wood preservatives
During storage, transport Risk to human, animal or Use only safe situations
and at or soon after
plant life
under strict control
application
Asbestos
Not currently used but
Risk of lung cancer
may be present as
insulation or other forms
in buildings
Depletes ozone layer,
hence contributes to
global warming and
increases UV radiation at
earth’s surface
Remedy
Arrange for safe removal
if found
Fire
Combustion is in essence a simple process involving chemical reaction of a
fuel (combustible material which is usually organic, contains carbon) with
oxygen. To initiate the process, heat or a source of ignition is essential,
though once started, many combustion processes are self-sustaining because
heat is a by-product.
OXYGEN
The fire triangle.
If any one ingredient is missing the fire cannot start.
HEAT
FUEL
Durability
A material may be said to be durable in any one situation if it fulfils all its
performance requirements, either for the planned lifetime of the building, or for
a shorter defined period where this is acceptable – for example, where
replacement is straightforward, where there is a serve cost penalty of longer life
or where replacement arising from changing user requirements may be desirable.
It is often very difficult to predict the durability of individual components. Also,
since failures in a very small proportion of the items in use may be unacceptable,
the only safe course of action may be to ‘over-design’ them so that materials in
the worst likely situation should be satisfactory. In consequence, many buildings
last much longer than their design lifetime.
Modes of deterioration of the major materials groups
Type
Form of deterioration
Recommendation
Metals (non porous)
Surface deterioration by electrolytic Use resistant metal such as stainless
corrosion in damp conditions,
steel or apply coating to exclude
Severity depends on metal type and moisture
situation
Bricks, stone concrete (porous)
Moisture penetration followed by
frost or chemical action.
Deterioration at or below surface
Use non-permeable forms;
weathering to shed water,
impermeable coatings to prevent
water / chemical penetration
Timber and timber products
Fungal attack (damp conditions)
insect attack (normal conditions)
Exclude water. Apply preservatives
Polymeric materials
Ultraviolet degradation
Protect from sun, e.g. carbon black,
or use indoor / underground. Use
stabilisers or surface coatings