S. No. MYTH REALITY
I Higher the compressive strength of concrete better is the quality, hence high grade OPC is essential to achieve them.
  • Even low grade cements can be used to develop high strength concretes.
  • Given the same grade of concrete, the concrete made of low grade cement is more durable than that with high grade cement.
II High grade cements offer higher strength concretes and thus the structures are durable.
  • The relation between strength and durability is not linear.
  • The durability of concrete with high grade cements is hampered due to following technical realities:
    1. High grade cements are associated with rapid hydration, releasing heat and lime rampantly.
    2. Hydration of high grade cements results in considerable quantities of surplus calcium hydroxide, which either leaches and leave porosity in concrete or creates conducive environment for deleterious chemical reactions with pollutants if left in the concrete matrix.
    3. High heat of hydration results in micro cracking, causing harm to the durability.
III Higher the quantity of cement (OPC), higher the strength and, in turn, higher the durability.
  • Higher the input of OPC, higher the heat of hydration and higher the lime leaching, hampering the durability.
  • It is not the quantity of cement (OPC) but the quantity of cementitious input that decides the durability.
  • Strength achieved through controlling the water cement ratio is more desirable than increasing the cement content.
  • By properly selecting materials, proportioning the mix and using compatible chemical admixtures (plasticisers and superplasticisers), high grade concrete can comfortably be designed, simultaneously protecting the durability of concrete.
IV Residential construction requires concrete with high compressive strength like M40 grade, etc.
  • Residential construction does not require high grade concrete unless and otherwise specified.
  • General requirement is concrete of M20 grade and certain structural members like beams and columns could be of M25, M30 grade concrete.
  • In the construction of high rise buildings, concrete high grade may be required and it needs to be specially designed keeping in view, various requirements of construction practices.
V Use of blending materials like fly ash is not needed in concrete
  • Blending of fly ash in concrete is very much essential because it converts the surplus Ca(OH)2 into strength rendering mineralogy through pozzolanic reactions.
  • The most economical route to enhance durability in concrete is through the blending materials like fly ash and GGBS in concrete 
VI Fly ash is inert and does not enhance any of the properties in concrete.
  • Fly ash is not inert but has dormant pozzolanic characteristic that gets activated to chemically react with calcium hydroxide, in the presence of moisture at ordinary temperatures, to form compounds possessing cementitious properties.
  • The reactive silica and alumina of fly ash react with surplus calcium hydroxide in cement matrix, thereby densifying the cement gel.
  • The densified cement gel makes the cement paste impermeable, thereby enhancing durability of concrete.
  • Fly ash plays two roles in concrete: reactive portion participates in the pozzolanic chemistry contributing for secondary mineralogy and coarse and crystalline portion acts like micro aggregate, filling the pores. Both these phenomena contribute for impermeability of concrete, eventually helping to enhance the durability of concrete.

 

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