Sand or fine aggregate is one of the most important construction materials at any construction site. Its selection should be done very carefully as it contributes to the strength of various important mixtures like concrete, plasters and mortar. Sand should be tested for the presence of clay, silt, moisture and other deleterious materials (salts, coal, mica) that may affect the strength of a structure and cause avoidable rework.
A good quality construction sand has a particle size measuring about 150 microns to 4.75mm. Particles smaller than this are classified as silt. The presence of excess quantity (> 8%) of silt in sand reduces the bonding capacity of raw materials and affects the strength and durability of work. It is recommended to conduct silt content test for every 20m3 of sand.
To find out the Silt content in sand
A measuring cylinder (250ml)
Procedure for the Test
Firstly, a 50ml solution of 1% salt and water is prepared in the measuring cylinder. The addition of salt increases the settlement time of silt.
The sample of sand to be tested is then added to the cylinder until the level reaches 100ml.
50ml of the solution of salt and water is again added to the measuring cylinder.
Close the open end of the measuring cylinder and shake it well.
After a period of 3-4 hours, you will notice a layer of silt settled over the sand.
Now note down the volume V1 of the silt layer settled over the sand.
Note down the volume V2 of the settled sand.
Repeat the procedure a couple more times to get the average.
Percentage of Silt Content = (V1/V2) x 100
V1 – Volume of silt layer V2 – Volume of sand layer
For example, for a given sand sample, if V1 comes out to be 9 and V2 is 130, the % silt content would be 6.9 %.
The permissible value of silt content in Sand is 8%, hence the sand sample is ok and can be used for construction purposes.
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