Fine Aggregate

  • Fine aggregate is an essential ingredient in the making of concrete, and this is the mainstay of the building industry. Hence it must be procured with all care and vigilance. Here are a few points to remember while you purchase sand for buildings, Firstly ensure the sand is free from clay lumps, silt and other organic impurities. Secondly assess that the moisture content in sand. It is desirable to avoid using very wet and fine sand, if the sand is wet, decrease the quantity of water to be added.
  • Natural sand is the best but, due to its higher demand, we go for other alternative materials i.e., manufactured sand, crushed stone sand etc.
  • The quality of the concrete and the mortar comes under the scanner when you find cracks in the construction, the chief reason for growing such cracks in concrete is the presence of silt and clay in more than the specified limits.

Limits of Impurities

Finer materials

silt and clay are a finer particle of size less than 0.075 mm, the main problem associated with these particles is, it forms a coating on the surface of fine and coarse aggregate, which is the leading cause of weakening the bond between the aggregate and cement. the other issues with these particles are, it increases water demand due to its high surface area. Hence IS 383 recommends that it shall not be more than 3 per cent.

Clay Lumps

if clay lumps present in the concrete increases the water demand of concrete. Secondly, it affects the durability and wears resistance of concrete; Hence IS 383 recommends that it shall not be more than 1 per cent.

Coal and lignite

due to its low density, it mostly comes on the surfaces of the hardened concrete which can be easily removed by the weathering agent. Furthermore, the excessive amount of such particles may seriously affect the durability of the concrete; Hence IS 383 recommends that it shall not be more than 1 per cent.

Mica

presence of mica in sand causes weak strength and bonding properties in concrete. Hence IS 383 recommends that it shall not be more than 1 per cent when there is no test conducted for strength and durability, if the test is conducted, the quantity of mica in the sand can be increased up to 3 per cent of mass.

Grading zone of fine aggregate

Why grading is essential, we can find out the grading, by doing sieve analysis, and sieve analysis of the fine aggregate provide information about the quantity of the particle, with its specific surface area, which is in that grading. As we know, when the specific surface area of the particle increases, it increases the water demand, or in other words, decreases workability and increases w/c ratio. Because the strength of concrete decreases with increases of water-cement ratio. Hence strength of concrete reduces. As a result of this, the use of fine aggregate (Grading Zone IV ) for RCC work is not preferable due to its high specific surface area. Furthermore, Use of fine aggregate (grading zone 1) for RCC work is preferred to design a mix of high strength concrete

Fineness modulus of fine aggregate

Fineness modulus is a number that gives us the understanding of the particle size of aggregate. the basic knowledge from the fineness modulus is, the lower the fineness modulus, the finer the particle and the higher the fineness modulus, the coarser the particle. But, is it possible that a single number can adequately describe the gradation curve. Is it a magical number. Of course, the answer is “NO“. As different gradation curve maybe have a single fineness modulus number. thats why IS 383:2016 does not describe fine aggregate through fineness modulus.