To determine quantity of silt in fine aggregates in the field
There are two types of harmful substances preset in fine aggregates i.e. organic matter produced by decay of vegetable matter and/or clay and silt, which form coating thus preventing a good bond between cement and the aggregates. If present in large quantities, result in the increase water-cement ratio and finally affecting the strength of concrete.
Field test is generally conducted in order to determine the volumetric percentage of silt in natural sand for percentage up to 6%, otherwise more detailed test as prescribed by standard code are required to be conducted.
Measuring cylinder of 250 ml capacity.
Fill 1% solution of common salt and water in the measuring cylinder up to 50 ml mark.
Now add sand to be tested to this solution till the level of the salt solution shows 100 ml mark.
Top up the level of salt solution up to 150 ml mark.
Shake the mixture of sand and salt solution well and keep it undisturbed for about 3 hours.
The silt being of finer particles than sand, will settle above the sand in a form of layer.
Measure the thickness of this silt layer.
Recording of Observation and Calculation
Volume of sample, V1 (ml)
Volume of silt after 3 hours, V2 (ml)
% silt by volume =(V2/V1)*100
If the average value of silt content exceeds 6% by volume, the aggregate requires washing before use.
Utmost care should be taken while taking sample for test