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       Natural sand are weathered and worn out  particles of rocks and are of various grades or size depending on the accounting of wearing.  The main natural resource of sand is river.  Now a days dams are  constructed on every river, so these resources are erasing very fast.  So it has become must to produce sand by machines.  Now a days good sand is  not readily available,  distance, transportation is required  to get good sand.  Those resources are also existing very rapidly. 
   
 
     The sand should be sharp, clean and course.  The grains should be of durable material.  The grain sizes must be such that it should give minimum voids.  The presence of clay and silt retards the setting of the cement and makes the mortar weaker and the walls  or the slab leaks and holds dampness.


 
     The sand is the mortar  does not   add to its strength but it used as an adulterant for economic.  But it prevents the shrinkage and cracking of mortar in setting.  The amount of cement required will be less when the void 
in the sand is less and then the sand will be more economical.  The machine crushed sand is cubical and angular in shape.  Some times it is flaky , which is troublesome in working.  There is no plasticity in the  mortar.  Hence the mason are not ready to work with machine made crushed stone sand.  For the same reason inferior river sand is used. Screening from crushed stone often contains a high percentage of dust may tend to be flaky.  The particles  from  crushed stone are of cubical shape and angular and flaky.  The angular shape of sand gives difficulty in working.  Flaky and angular particles may produce harsh concrete, and if not done carefully may result in spongy concrete. 
   
 
There is standard specification for Fine aggregates (Sand). It is divided in four gradations. Generally known as Zone I, Zone II, Zone III and Zone IV. There is sieve Designation for each grade. Gradation is made as per the use of the sand. There are testing sieves for testing the sand. A set of Sieves with square hole is available. Followings are the sieves

1) 4.75 mm, 2) 2.36, 3) 1.18, 4) 600 microns, 5) 300 microns, 6) 150 microns 7) pan.

Specific percentage are designated for each size for each Zone sand in terms of material retained or passed `from the sieves.

      •  Zone One Sand :
 
Sieve size Percentage retained Percentage passing
4.75 0-10 % 90-100 %
2.36 5-40 % 60-95 %
1.18 30-70 % 30-70 %
600 microns 66-85 % 15-34 %
300 microns 80-90 % 5-20 %
150 microns 90-100 % 0-10 %
   
      •  Zone Two Sand :
 
Sieve size Percentage retained Percentage passing
4.75 0-10 % 90-100 %
2.36 0-25 % 75-100 %
1.18 10-45 % 55-90 %
600 microns 41-65 % 35-59 %
300 microns 70-92 % 8-30 %
150 microns 90-100 % 0-10 %
   
 

Where concrete of high strength and good durability is required Sand used should be of proper gradation. The concrete mix design should be done properly. When the sand of finer grade is used, the ratio of finer to coarse aggregates should be reduced. The fine to coarse ratio depends upon the particle shape, surface texture of both fine and coarse particles. The gradation of coarse and fine aggregate is very important. Very fine sand is not recommended for concrete purpose.

The fines content in the sand below 600 microns should be about 30 to 50 %. This will be a ideal sand for concrete work as well as masonry and plaster work.

At present it is generally observed that the fine content in river sand is less then the required percentage. Fines below 600 microns are very less. (Some Engineers and Concrete designers recommend to use stone dust to compensate this shortage in River sand. Which is known as toning of sand. Addition of fine dust should be as per mix design.)

 
 
     
   
 
 
 
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