Concrete is made up of cement, coarse aggregate, fine aggregates, admixtures and water.
The excessive amount of silt content in the sand (fine aggregate) is harmful to the strength and durability of concrete.
Sand particles have a size between 4.75 mm and 150 microns. In sand, the fine materials of size less than 150-micron are known as silt.
Effect of Silt on Concrete:
The excessive presence of silt in the sand reduces the bonding between cement and aggregates. It increases water demand for concrete. This reduces the strength of the concrete.
It also modifies the setting times of concrete. It can cause cracks in hardened concrete. Hence, silt content is undesirable in the sand. Here is a simple method to find out the silt content in sand.
Test for Silt content in Sand as per CPWD specifications
This test gives the silt content in sand by volume.
Apparatus required: A graduated cylindrical jar (measuring cylinder) of 200 ml capacity.
Materials Required: Sand, Water and Salt
- 100 ml of the measuring cylinder is filled with sand.
- Water is then poured up to 150 ml of the jar.
- Dissolve a pinch of salt is in the water
- The jar is shaken vigorously by closing the mouth of the jar by palm and turning it up and down repeatedly.
- The mixture is then allowed to settle down by keeping the jar on a stable and flat surface for three hours.
- The silt in the sand will settle on the top of the sand in the jar.
- The percentage of silt is calculated by dividing the height of silt above sand by the height of sand below silt.
Observation Table Example:
|Height of Silt above sand Layer, (ml) A||4|
|Height of Sand below silt layer, (ml) B||98|
|Silt Content, A/B * 100||4/98 * 100 = 4.08 %|
The permissible limit of silt content
The silt content should be less than 8 % by volume.
What to do if silt content is more than the allowable limit?
The sand having silt content more than 8 %, should not be used in concrete. However, the sand can be used after washing. It brings the sand content below the permissible limit.
Frequency of Testing of silt content
The silt content should be tested daily and once for each truck at the time of unloading at sites.
- CPWD Specifications Vol. 1, Section 3, Clause 188.8.131.52