Strengthening of concrete

Concrete

• Concrete is a mixture of water, cement, aggregate and cement.
• The cement holds everything together.
• It's a cheap material that is commonly weak in tension but strong in compression.
• To combat this weakness in tension, it can be reinforced and prestressed.

Reinforced concrete

• Reinforced concrete is when concrete is combined with a reinforcement to strengthen its tensile strength.
• The most common type of reinforcement is ferro-concrete
  • This is where some form of iron, mainly steel is inserted as the fibre in the concrete.
  • Other materials that can be used include glass, plastic and graphite.
• The material used to reinforce concrete must be high in tension (Or else why bother) and be compatible with concrete (i.e. have similar thermal expansion rates. etc.)
  • Steel has a similar expansion rate to concrete, and thus would remove the risk of internal stresses
• It would also have to stick well to concrete.
  • For steel, it is corrugated (like crinkle cut chips) to improve its adhesiveness.

Prestressed concrete

• Prestressed concrete is when the reinforcements in reinforced concrete has tension applied to it before it is in use.
  • The reinforcements are usually steel cables or rods
• This process creates a clamping stress as the steel rods attempt to return to an unstressed state, which balances potential tensile bending stresses.
  • This allows it to tolerate larger bending stresses and lead to longer spans.
• There are several ways of doing this:
  • Pre-tensioning: This is when the concrete is cast around tensioned members.
    • This allows for a good bond between concrete and fibres
    • It also protects the fibres from corrosion.
  • Bonded Post-tensioning: This is when the fibres are put in compression after the concrete has hardened.
    • These fibres are stressed using hydraulic rams
  • Unbonded Post-tensioning: Similar to the above, but the fibres are given free movement
    • This is done by coating the fibres in grease and placing them within plastic sheathes.