Construction materials: Glass

General properties

  • non-crystalline solid
  • brittle
  • elastic up to sudden fracture
  • soda-lime-silica glass constituents
    • silica-70%
    • lime-9%
    • soda-15%
    • magenesium,aluminium,iron oxides in small amounts
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Key features

lack of fit leading to stress concentrations that cause problem in design

sudden fracture encouraged by flaws

failure is stochastic (prediction is risk based or statistical)

panes deflect considerably

  • in Aus =< span/60

very high durability

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Large deflection theory

- a simple deflection theory is a straight line graph (stress vs uniform pressure)

- for glass a large deflection theory is used

- the graph for this decreaes in gradient at a certain point

- this means that at the design pressure, the quoted design stress is alot higher than the real design stress

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Float glass- process- Pilkington glass

  • 1) raw materials go in at 1500 C
    • silica sand, soda ash, limestone
  • 2) the melting furnace is 1600 C and produces molten glass
  • 3) in the float bath heating is controlled and decreases from 1100 C to 600 C which allows glass to flow and form flat ribbons of uniform thickness
  • 4) at the annealing LEHR, glass is subjected to a process of heating and slow cooling to make it tougher and less brittle 
  • 5) there's a temperature controlled kiln to slowly cool glass ribbons on rollers before cutting. the glass goes down to 200 C
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Annealed glass

annealed glass is the most commonly made glass

key properties:

  • density: 2500 kg/m3
  • elastic modulus: 70 GPa
  • poisson's ratio: 0.22
  • compressive strength: >1000 MPa
  • tensile strength: variable but alot lower than compressive
  • in bending failure: 40 N/mm2
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Toughened (tempered) glass

  • toughened glass is produced by heating annealed glass to 620 C and rapidly cooling it - quenching by jets of cooled air
  • the surface solidifies first leading to
    • tension core
    • compression in surface: 90-150 MPa
    • surface cracks dont propagate so can sustain higher stresses than annealed glass
    • high bending strength
    • may shatter due to impurities or deep scratches that penetrate the tension core
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Laminated glass

  • two or more pieces are bonded together by an interlayer
  • polyvinyl butyral and acrylic resin
  • pvb thickness= 0.4-6mm
  • bent glass uses pvb lamination
  • provides radiation thermal protection
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Uses of glass

structural

cladding

structural elements

aesthetics/ colour

heat transfer

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Glass fibres

  • drawn in molten state in an electric furnace through platinum brushes at high speeds
  • the finer the fibre the higher the tensile strength
  • filaments that are a few micrometers in diameter can have a tensile strength > 14GPa
  • twice as high tensile strength as steel but 1/4 of the weight
  • glass fibres are chopped o woven to form a strand
  • non corrosive
  • good impact resistance
  • dimensionally stable under thermal loading
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Glass fibres in concrete reinforcement

  • most suitable are glass, carbon and polypropylene
  • manufactured using resins to produce fibre composite rebars
  • high tensile strength bars/ strands used to replace steel to avoid corrosion problems
  • manufacturing problems include alkali attacks, long term performance and fatigue concerns
  • high strength
  • low stiffness
  • high cost

the alkali attack on glass fibres causes a chemical alteration and reduces the strength

a deposition of lime into filaments causes embrittlement and fracture

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