Tuesday, April 3, 2018

STRENGTH OF CONCRETE


Definition
      Strength of concrete is its resistance to rupture
      Strength of concrete are-
      Strength in compression
      Strength in tension
      Strength in shear
      Flexural strength
Concrete is stronger in compression
      Concrete is stronger in compression but weak in tension
      Concrete consists of two phase : aggregate and cement paste
      At microscopic level, there is an interfacial zone between aggregate and hardened cement past called transition zone.
      The transition zone contains more water less cement so it is of poor quality and is weak.
      This zone reduces bond between aggregate and paste
      Now when compressive force is applied, the function of transition zone (the weak cement paste) is only to transfer force between aggregates
      But when tension force (Pulling force) is applied,  the weaker zone (transition zone formed around the aggregate) has to resist the force
      It means the transition zone of weak cement paste has to bind the aggregate, which it fails to do as it is weak
      And hence crack appears.
      That’s why concrete is strong in compression and weak in tension

Setting vs hardening of concrete
      Setting
      Setting means the concrete becomes stiff.
      It means the concrete is no longer fluid but still concrete may be weak
      Setting takes place within short period of time
      Concrete should be placed within the initial setting time
      Hardening
      Hardening is the process of gaining strength
      It is due to hydration i.e. formation of calcium silicate hydrate
      It may continue for long time


Factor affecting strength of concrete
      Porosity in concrete: Porosity is due to void (air) in concrete. The more porous the concrete is, the weaker it is.
      Water-cement ratio: As the water-cement ratio increases, the strength of concrete decreases but workability increases. So optimum water-cement ratio is chosen considering both workability and strength of concrete
      Quality of aggregate: The stronger the aggregate, the higher the strength of concrete
      Relative humidity: As the relative humidity decreases, the strength also decreases because without moisture, hydration reaction cannot proceed.
      Compaction of concrete: If concrete is well compacted, the air will be removed and the void decreases. Hence the strength of concrete increases.
      Age of concrete: As the age of concrete increases, the strength of concrete increases in a decreasing rate.
      Majority of strength is gained during first 28 days.


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