Determination of Silt Content in Fine Aggregate

Objective : To determine silt content in a given sample of fine aggregate by sedimentation
method.

Reference : IS : 2386 ( Part II ) - 1963

Theory :
This is a gravimetric method for determining the clay, fine silt and fine dust, which includes particles up to 20 micron. Differences in the nature and density of materials or in the temperature at the time of testing may vary the separation point.

Apparatus :
A watertight screw-topped glass jar of dimensions similar to a 1-kg fruit preserving jar, A device for rotating the jar about its long axis, with this axis horizontal, at a speed of 80 ± 20 rev/min, A sedimentation pipette, A 1000-ml measuring cylinder, scale, well-ventilated oven, Taping rod etc.

Chemical :
A solution containing 8 g of sodium oxalate per liter of distilled water shall be taken. For use, this stock solution is diluted with distilled water to one tenth (that is 100 ml diluted with distilled water to one liter).

Figure: 

Procedure :

1. Approximately 300 g of the sample in the air-dry condition, passing the 4.75-mm IS Sieve, shall be weighed and placed in the screw-topped glass jar, together with 300 ml of the diluted sodium oxalate solution. The rubber washer and cap shall be fixed, care being taken to ensure water tightness.

2. The jar shall then be rotated about its long axis, with this axis horizontal, at a speed of 80 ± 20

rev/min for a period of 15 minutes.

3. At the end of 15 minutes, the suspension shall be poured into the 1 000-ml measuring cylinder and

the residue washed by gentle swirling and decantation of successive 150-ml portions of sodium

oxalate solution, the washings being added to the cylinder until the volume is made up to 1000 ml.

4. The suspension in the measuring cylinder shall be thoroughly mixed by inversion and the tube and

contents immediately placed in position under the pipette.

5. The pipette A shall then be gently lowered until the tip touches the surface of the liquid, and then

lowered a further 10 cm into the liquid.

6. Three minutes after placing the tube in position, the pipette A and the bore of tap B shall be filled by opening B and applying gentle suction at C.

7. A small surplus may be drawn up into the bulb between tap B and tube C, but this shall be allowed to run away and any solid matter shall be washed out with distilled water from E.

8. The pipette shall then be removed from the measuring cylinder and its contents run into a weighed

container, any adherent solids being washed into the container by distilled water from E through the

tap B.

9. The contents of the container shall be dried at 100 to 110°C to constant weight, cooled and weighed.

10. Calculations— The proportion of fine silt and clay or fine dust shall then be calculated from the

following formula:

W = weight in g of the original sample,

W = weight in g of the dried residue,

V = volume in ml of the pipette, and

0.8 = weight in g of sodium oxalate in one litre of the diluted solution

Conclusion / Result :

The clay, fine silt and fine dust content of given sample of fine aggregate is found to be ……. %

Limits :

As per IS 383 and CPWD permissible amount of silt in sand is 3% by mass and 8% by volume respectively.