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Grading of Aggregates

Different Types of Aggregates
Aggregates is derived from igneous, sedimentary and metamorphic rocks or is manufacture from clays, slag etc. The properties of concrete are directly related to  those of  its constituents and should  be  hard,  strong, durable, and free from clay, loam, vegetables and other such foreign matters. The presence of clay or dirt coating prevents the adhesion of cement on the surface of aggregates and ultimately  retards the setting  and hardening  of cement  and reduces the strength, durability and soundness of concrete.

Depending upon their size, the aggregates are classified as
  1. Fine Aggregative 
  2. coarse aggregates.

Fine Aggregates: 

The material, most of when passes through 4.75mm I.S. sieve size, is termed as fine aggregates. It should not contain more than 1 to 8% of fine particles, which may be obtained from sea, river, lake or pit may be used as fine aggregates but care should be taken all its impurities must be removed.

Coarse Aggregates: 

The material whose particles are of such size as are retained on 4.75mm, I.S sieve are called coarse aggregates. The size of the coarse aggregates used depends upon the nature of work. The maximum size may be 23mm for mass concrete such as dams etc. and 63mm for plain concrete. Crushed hard stone and gravel is the common materials used as coarse aggregates for structural concretes. Coarse aggregates   usually   obtained  by  crashing  granite, gneiss, crystalline lime stone and good variety of sandstone etc.

Grading of Aggregates:

Grading of aggregates consists of proportionating the fine  and  coarse  aggregates  in  such  a  ratio,  so  as  to  get strongest and densest mix with the least amount of cement. voids  when  mixed  with  all  ingredients,  and  water  should render a concrete mass of easy workability.

The grading of aggregates are done by the following methods
  1. By trail: In this method, proportionating   of aggregates as to give heaviest weight for same volume, yield the densest concrete.
  2. By finesse modules method (sieve analysis method): in this method,  the  samples of  both coarse   and   fine aggregates are passed through a set of nine standard sieve and the percentage of sample retained on each of the said sieves is determined.  The total of these percentages divided by 100 gives the finesses modulus of sample.                                                                  
  3. By minimum voids method: This method is based on the fact, that so obtain dense concrete the quantity of cement should also be slightly in excess of voids more that the fine aggregates. In this method the voids in the fine and coarse aggregates are separately found out with the help of graduated cylinder and water. The percentage of voids I aggregate, “X” given by the equation.    
    Grading of Aggregates

  4. By  arbitrary  standards:  It  is  a  commonly  adopted method of propitiating the aggregates in a concrete mix for small works of moderate importance. This method is not recommended for large works or important works in this method, the volume of cement, sand and coarse aggregates are taken in the proportion of 1:n:2n respectively. The quantity of water to be used a varied suit the workability descried.

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