A house built using Eco friendly materials. Image credit asiagreenbuildings.com

A house built using Eco friendly materials. Image credit asiagreenbuildings.com

ZIMSEC O Level Combined Science Notes: Factors to consider when choosing construction materials

  • The choice of building material depends mainly on the properties of the materials and
  • The purpose of the structure which is being constructed
  • For example concrete is used to make large buildings
  • Fiberglass is used to make cars and water tanks
  • Wood is used to make roofing trusses
  • Stone is used to make walls
  • Some of the properties of materials that have to be considered are:

The strength of the material

MaterialsTensile strength/Mega newtons (Million Newtons)
Cement/concrete4
Brick5
hemp rope82
wood:
  • along grain
  • across grain

  • 103

  • 3.5
plastics350-1050
flax700
aluminum70
copper140
cast iron140-300
aluminium alloy140-600
mild steel400
high tensile steel1550
  • The table above was adapted from Focus on Science
  • It shows the strength of various construction materials
  • The strength of a material can be found by finding the amount of force required to break it
  • As shown different materials have different breaking strengths as shown by the table
  • Engineers must know the strength of materials when they design structures such as bridge
  • Steel is strong in both tension and compression
  • Wood is weak in compression and strong in tension
  • Concrete is weak in tension but strong in compression
  • it can be combined with steel to make reinforced concrete which is strong in tension and compression
  • Civil engineers should use the strongest material after considering the forces involved in that part of the structure

Flexibility

  • Flexibility is another factor that engineers need to consider
  • Flexibility is the ability of a material to bend without breaking
  • Rubber is an example of a flexible material
  • It is used where flexibility is required for example rubber soles of shoes
  • Materials that are not flexible are known as rigid
  • Examples of rigid structures include steel and concrete
  • Some structures e.g. walls of building require rigid materials
  • Wood is slightly flexible but it has low shear strength
  • Twisting forces will easily break it making it unsuitable for structures that experience shear forces

Corrosion

  • Materials are constantly being weakened, corroded and decay due to
  • sunlight, water, pollution, living organisms such as termites and bacteria as well as fire
  • When materials are erected they are immediately attacked by water and oxygen
  • Iron which is commonly used as a construction material rusts when subjected to water/moisture and oxygen
  • It can be protected by electroplating, painting and alloying
  • Copper is low on the reactivity series which makes it less susceptible to corrosion
  • Aluminum is protected from corrosion by a thin layer of aluminum oxide that forms a protective covering
  • So even though it is higher in the series it does not react
  • Steel is more resistant to corrosion than iron
  • Where corrosion is undesirable such as on bridges steel or materials that do  not corrode easily are to be preferred

Corrosion by pollutants

  • Where corrosive pollutants are found materials that are resistant to these pollutants must be considered
  • For example sulphur dioxide is produced when coal is burnt
  • The gas combines with water and oxygen in the atmosphere to form sulphuric acid
  • This falls as acid rain and attacks most materials and corrodes them
  • Nitrate factories  e.g Sable Chemicals also produce nitrates
  • which are expelled via chimneys and combine with rain water to form
  • acids that are even corrosive than sulphuric acid

Destruction by living organisms

  • For example wood is attacked by living organisms
  • Such as termites, fungi etc
  • This destroys and weakens many wooden structures especially given Zimbabwe’s warm moist climate
  • Wood can be protected by
  • Painting it or dipping it in creosote
  • which protects the wood and makes it last longer

Other factors

  • Other factors to consider include:
  • Cost
  • Weight of material
  • Or even the physical  appearance of the material
  • Environmental impact of using that material i.e. is it Eco friendly
  • Durability
  • How easy is it to maintain

To access more topics go to the Combined Science Notes page


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