Object: To determine the quantity of silt and clay in a given sample of fine aggregate.
Apparatus:
- Measuring cylinder – 250ml
- Sand – Specimen
- Water
- Salt Solution.
Procedure:
- The measuring cylinder should be free from moisture and dry.
- Fill the water up to 50 ml with 1% salt solution.
- Then add sand to it until the level reaches 100 ml.
- Again add salt solution up to the level of 150 ml.
- Close the mouth of the cylinder and shake well then keep the cylinder on a flat surface.
- The silt content shall be settled over the sand layer after 2 hours.
- Now note down the silt layer alone volume as V1 ml (settled over the sand)
- Then note down the sand volume (below the silt) as V2 ml
- Repeat the procedure two more times to get the average.
How to calculate the percentage of silt?
- V1 = Volume of Silt layer (Silt settled over the sand layer).
- V2 = Volume of the Sand layer (which is settled under the silt).
Percentage of Silt = (V1/V2) x 100
The permissible Silt content in sand percentage is only 6%.
Observation Table:
S.NO | DESCRIPTION | SAMPLE NO | ||
SAMPLE 1 | SAMPLE 2 | SAMPLE 3 | ||
1 | Volume of Sample (V1) ml | |||
2 | Volume of Silt (V2) ml | |||
3 | Percentage of Silt (V2/V1) x 100 | |||
Average | ||||
Technical discussion:
EFFECT OF SILT FINES ON CONCRETE PROPERTIES:
- Increase in the water/cement ratio results to a decrease in the compressive strength of concrete
- The increase in sand fine content has also been found to decrease the compressive and tensile strengths of concrete
- Inadequate sand fine content also has an adverse effect on the strength of concrete due to poor adhesion
- The decrease in slump value as a result of an increase in fines percentage in the sand can be attributed to the fact that finer particles have a larger surface area which absorbs more water in the concrete mix.
