MATERIALS II

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MATERIALS II

• By
• Richard Cooper

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MATERIALS

• We will look at some aspects of -
• Concrete
• Plastics
• Timber
• Also at-
• Corrosion
• Cracks Stress concentration

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Metals

• Metals are not looked at specifically but covered
  by our discussions of environmental issues,
  corrosion and stress concentrations

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Environmental Issues
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• About half the energy used in Britain goes on
  heating and lighting buildings and of this over
  half - 30 of the whole - is used in peoples
  homes.
• A significant amount of energy is used in
  producing and transporting building materials and
  in the construction process itself.

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• This embodied energy accounts for a further 8
  of energy consumption.
• A buildings embodied energy content can be cut
  by up to a half by adopting some simple measures

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• Use second-hand materials when possible, or
  materials with a high recycled content, in
  preference to new materials
• Use minimally processed materials not highly
  manufactured products
• Use locally produced materials not materials from
  far away or imported products

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• These simple measures are a start but do not take
  into account the other environmental effects of
  using building materials
• The most commonly recognised approach to
  assessing materials is Life Cycle Analysis or LCA
  

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• LCA tracks the effect of materials through
  extraction, production, transport, construction,
  maintenance, repair, replacement and ultimate
  disposal
• LCA underlies the Green Guide to Housing
  Specification
• The Green Guide is published by the Building
  Research Establishment (BRE)

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• The Green Guide to Housing Specification analyses
  10 principal elements of a house - roof, external
  walls etc
• Together these contribute around 90 of the
  overall environmental impacts of a house over a
  60-year life

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Environmental Effects of Some Materials
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Concrete, Brick

• Pollution generated per kilogram by these
  materials is generally very low but they are used
  in large quantities,
• This leads to the environmental problems
  associated with them
• Energy used in transport
• Damage to the landscape and environment arising
  from extraction and disposal

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• Recycling brick, stone and concrete for
  aggregates reduces environmental damage from
  extraction and waste disposal
• Currently the maximum proportion of recycled
  aggregates to achieve the technical specification
  for concrete is around 20.

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Cement

• The production of cement is very
  energy-intensive.
• Many cement factories use refuse-derived fuels
• Poses a potential environmental threat because
  they produce dioxins and other pollutants

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• Portland cement is an almost universal product
  that enables concrete and high-strength
  load-bearing brickwork to be constructed
• Cement mortar for brickwork prevents the bricks
  being re-used on demolition.
• Lime mortars are far preferable as render and
  mortar for brickwork

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Metals

• Extraction of metals from ores is damaging to the
  environment and can release harmful substances.
• Refining metals requires large amounts of energy
  but metals are relatively easy to recycle and
  this is generally economically attractive

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• Aluminium requires vast amounts of electrical
  energy to extract but can be recycled to produce
  high-grade materials
• Steel can be recycled but to a lesser extent than
  aluminium

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• Steel needs coating to protect it but zinc
  coating in particular may cause environmental
  problems
• Corrosion can be avoided by adding chromium and
  nickel to produce stainless steel
• This process can, however, lead to emissions of
  these heavy metals which can be very
  environmentally damaging

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• Zinc stocks are expected to be severely depleted
  within decades at current rates of use
• Extraction releases cadmium and other heavy
  metals
• Zinc leaches into the environment from sheet zinc
  and galvanized surfaces
• Zinc can be recycled but this is not economically
  viable at present.

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• Lead is in very limited supply
• It is a hazardous material and its production and
  uses in paint and roofing produce pollution
• Almost all sheet lead is recycled

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• Copper in pipes and on roofs causes pollution and
  can kill water organisms
• Copper is long lasting and economically
  attractive to recycle, and this takes place on a
  large scale

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Synthetics

• The basic raw material is petroleum, the product
  of organisms that lived on earth many hundreds of
  thousands of years ago
• For all practical purposes it is an irreplaceable
  resource
• Extraction and transport have caused
  environmental disasters.

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• Refining and manufacture use energy, release
  organic hydrocarbons and create waste.
• Synthetics do not generally cause problems during
  use but rather, after demolition.
• Many synthetics can be recycled but the recycled
  material must be uncontaminated

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• Polyethylene and polypropylene are simple
  polymers whose production creates little
  pollution and uses relatively little energy
• The additives used are relatively harmless.
• The materials can be both recycled and
  incinerated.

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• PVC requires less energy to produce than many
  other synthetics but during its production,
  dioxins are created and released
• PVC products must generally be either burned or
  buried at end of life
• PVC can be recycled but in practice it contains
  such a variety of additives that uncontaminated
  high quality material is very hard to produce

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• Most other plastics have similar issues to those
  noted above
• Many of the additives employed to enhance
  properties or reduce costs are also hazardous

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Timber

• This is the most important renewable raw material
  used in construction.
• Production requires little energy and produces
  little pollution
• The environmental effects arise from poor
  forestry management, preservative treatment and
  transport distance

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• Sustainable wood means wood from forests that are
  managed sustainably
• Durable wood does not require treatment
• Tropical wood from sustainable sources may have
  beneficial economic and social effects
• Few sources of sustainable tropical wood exist.
  The only reliable guarantee is wood certified by
  the Forestry Stewardship Council (FSC)

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• Many forests are poorly managed
• European forests tend to be better managed and
  are close to hand, but they produce relatively
  little durable wood
• Timber can be very durable if the timber elements
  are carefully detailed to protect them from
  constantly moist conditions, and if maintenance
  is good.

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• Manufactured timber products (MDF, Chipboard etc)
  use bonding agents that generate pollution
• Formaldehyde can be released into home
• Hardboard and similar boards have low
  environmental impact as they are bonded at high
  pressure and temperature by the resins occurring
  naturally in the timber

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• Synthetic resin boards used for cladding take
  more energy to produce than other board materials
  and contain a high level of resin for bonding
• They are, however, maintenance free and more
  resistant to impact damage than, for example,
  cement-bonded particleboard

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Summary

• The energy embodied in the construction of the
  home itself is a significant proportion of the
  total energy used and resulting emissions during
  the lifetime of a low-energy building.
• It can be reduced by using materials that are-
  second-hand - minimally processed - locally
  produced.

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• Materials should be specified which have minimum
  environmental impacts.
• This can be established by considering the Life
  Cycle Analysis of the material.

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Cost Implications

• Difficult to generalise.
• Some materials with low environmental impact may
  cost more
• Many cost less because they use fewer resources
  and are used in or near their natural state.

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Benefits

• Reduced environmental impacts from pollution, and
  less resource depletion.
• The Green Guide to Housing Specification,
  Building Research Establishment Watford
• Green Building Handbook, Volumes 1 2 by Woolley
  et al. (E FN Spon, London, 1997 and 2000)

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Back To Materials Properties
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CONCRETE

• Made from Cement, Water Aggregate
• Aggregates are most often stone such as river
  bed gravel, but may be crushed brick or reclaimed
  concrete. Can even be steel or lead shot or even
  polystyrene beads.

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• Cement may be Ordinary Portland Cement (OPC) or
  other special type suitable for the application
• Water should be free from impurities

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• Concrete has low tensile strength and must be
  reinforced for use in any area that is in
  tension. This includes beams and lintels
• Reinforcement is usually from steel bars but may
  be from glass or steel fibres
• Corrosion of reinforcement can give major problems

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• Concrete has great strength in compression
  generally needing no reinforcement in that case
• Concrete must be well compacted when wet to gain
  its full strength

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• Dense well compacted concrete can only be made
  with sufficient water in the mix
• Excess water will reduce concrete strength
• Determination of the correct quantity of water is
  very important
• Never add more water to a truck mixer without
  proper authorization

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• Remember that concrete moves due to environmental
  changes
• Increase in moisture or temperature will cause
  expansion which must be considered during design
  and properly dealt with during construction

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PLASTICS

• Three main groups
• Thermoplastics
• Lose rigidity when heated
• Thermosetting plastics
• Remain rigid when heated but will break down and
  char eventually

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• Elastomers
• Thermosetting type with high elasticity but low
  modulus of elasticity

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• These are all organic compounds.
• The molecules are long chains of atoms joined by
  strong covalent bonds. The way the chains join to
  each other is what gives the type of material

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• If the long chains are held by weak Van der
  Waals forces we have a thermoplastic
• If the long chains are held by covalent bonds we
  have thermosetting plastics
• If the long chains are coiled and twisted
  together held mainly with Van der Waals forces
  we have an elastomer

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• All of these plastics types are susceptible to
  degradation due to ultra violet radiation, heat
  radiation and solvent damage
• They also suffer from large thermal movement

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TIMBER

• A natural material used both structurally and non
  structurally
• Two main categories, hardwoods and softwoods.
  Though not all hardwoods are hard and not all
  softwoods are soft!
• To ensure that you get the material you expect it
  is best to specify using the true botanical name

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• Properties of timber are mainly controlled by
  four things-
• Density
• The more dense the stronger, can be judged by
  counting the number of rings per 25mm
• Moisture content
• M/C will effect strength, susceptibility to
  fungal and insect attack and movement. Timber
  always tries to reach equilibrium with
  surroundings

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• Slope of grain
• If the grain is parallel with the axis of the
  timber it has zero slope and strength is at max.
  As slope increases then strength reduces. A slope
  of 6 reduces strength by about 10
• Defects
• Mainly knots and splits, reduce strength in
  direct proportion to their size

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• For structural applications timber must be graded
• Standards specify what is required but
  fundamentally two methods are used-
• Mechanical stress grading or Visual stress
  grading

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Specific Properties of Timber

• Look at the material properties in the timber
  handout
• The specific strength (strength divided by
  specific gravity) of timber is very good
• Almost three times the specific tensile strength
  of common construction materials
• If you want light and strong this is a good
  material

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Specific Properties of Materials
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CORROSION

• This can give us major problems in construction
• Nearly always caused by flow of electricity
  (electrons) between areas on a metal surface, or
  between different metals, through a fluid called
  an electrolyte

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Anode Cathode Processes
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• These areas are known as anodes and cathodes
• Corrosion always occurs at the anode
• Anodes and cathodes can be formed by stress,
  surface imperfections, environmental conditions
  (flow of water for example) or by combining
  different metals

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• An electrolyte is almost always present in the
  form of moisture in the air which is (almost)
  always acidic

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• In neutral electrolytes corrosion slows and stops
  because hydrogen bubbles collect round the
  cathode and separate it from the electrolyte
  (Cathode polarization or The flat battery
  effect)
• If oxygen is present it combines with the
  hydrogen and corrosion continues

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• Dissimilar metals in electrical contact will
  corrode in an electrolyte
• A single metal in an electrolyte will corrode if
  anodic and cathodic areas are formed
• If oxygen is present in the electrolyte then
  polarization will not be able to prevent
  corrosion

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Preventing Corrosion

• Dont mix dissimilar metals (Those that are far
  apart on the electrochemical series)
• If you must mix metals then ensure that the
  cathode stays polarized by excluding oxygen
• Coat metals to prevent the electrolyte touching
  the surface ( Paint, Metal coating)
• Use a sacrificial anode

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• Any metal lower on the electro chemical series
  can be a sacrificial anode
• Zinc is often used as in zinc coated corrugated
  roofing

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The Electro Chemical Series
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Typical Bimetallic Corrosion
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CRACKS STRESS CONCENTRATIONS

• Any discontinuity in a material in tension will
  cause a stress concentration out of all
  proportion to the loss of area
• A round hole will increase stress by a factor of
  three at the edge of the hole
• A crack may increase the local stress by a factor
  of ten or more. Hundreds for a sharp crack

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Stress Concentration At Crack
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• Brittle materials (glass, concrete, iron) are
  very sensitive to cracks
• Ductile materials (mild steel, copper, pure
  aluminium) can be safe even with long sharp
  cracks
• Materials like timber, bone, plastics composites
  and other fibre reinforced materials can cope
  with cracks by using special crack stopping
  techniques

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Toughening Effect of Fibres
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• Remember that even a small notch or defect can
  change a tough strong material into a dangerous
  brittle one

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The Danger of Cracks
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The End