In concrete construction, the quality of sand plays a crucial role in determining the strength, durability, and appearance of the final structure. Often overlooked, sand—technically referred to as fine aggregate when it passes through a 4.75 mm IS sieve—is just as important as cement and coarse aggregates.
Fine aggregate can be sourced from natural sand, crushed stone sand, crushed gravel sand, fly ash, marble dust, or even broken bricks. To ensure high-quality concrete, the sand must be hard, durable, chemically inert, clean, and free from harmful impurities such as clay balls, organic matter, iron pyrites, alkalis, salts, coal, mica, and laminated materials. The total amount of deleterious substances should not exceed 5% by mass.
To maintain these standards on-site, several tests are conducted regularly to assess the quality of sand before its use in concrete or mortar.
Field Tests for Sand Quality
There are four primary field tests performed to evaluate the quality of sand at construction sites:
1. Organic Impurities Test
This test checks for the presence of decayed vegetation, coal dust, or other organic contaminants that may interfere with the setting and hardening of concrete. It is typically carried out for every 20 cubic meters (cum) of sand delivered to the site.
2. Silt Content Test
Silt content should not exceed 8%. If it does, the sand must be washed to reduce silt levels within acceptable limits. This test is also conducted for every 20 cum of sand.
3. Particle Size Distribution Test
Also known as grading analysis, this test determines whether the sand falls into one of the prescribed grading zones. It can be performed either on-site or in a laboratory and is required for every 40 cum of sand.
4. Bulking of Sand Test
Moisture in sand causes an increase in volume, known as bulking. This test helps adjust the quantity of sand used during concrete mixing and is conducted for every 20 cum of sand.
Detailed Testing Methods
1. Silt Content Test
Excess silt reduces workability and weakens the bond between cement paste and aggregate. In this test, sand is mixed with a saltwater solution, and after settling, the height of the silt layer is measured. If the silt content exceeds 8%, the sand must be washed before use.
2. Grading of Sand
Fine aggregates are classified into four grading zones based on sieve analysis results:
| IS Sieve | Zone I (%) | Zone II (%) | Zone III (%) | Zone IV (%) |
|---|---|---|---|---|
| 10 mm | 100 | 100 | 100 | 100 |
| 4.75 mm | 90 – 100 | 90 – 100 | 90 – 100 | 90 – 100 |
| 2.36 mm | 60 – 95 | 75 – 100 | 85 – 100 | 95 – 100 |
| 1.18 mm | 30 – 70 | 55 – 90 | 75 – 100 | 90 – 100 |
| 600 µm | 15 – 34 | 35 – 59 | 60 – 79 | 80 – 100 |
| 300 µm | 5 – 20 | 8 – 30 | 12 – 40 | 15 – 50 |
| 150 µm | 0 – 10 | 0 – 10 | 0 – 10 | 0 – 15 |
A deviation up to 5% in any sieve (except the 600 µm sieve) is acceptable and still considered to fall within the respective grading zone.
3. Deleterious Materials Test
The presence of certain substances like iron pyrites, alkalis, salts, coal, mica, and soft fragments can severely affect the performance of concrete. These materials should not be present in quantities that cause corrosion or weaken the structure.
- Clay, fine silt, and fine dust : Should not exceed 5% by mass (as per IS 2386 Part-II).
- Organic impurities : Determined by comparing the color of the liquid extract with a standard reference; if darker, the sand is unsuitable.
4. Bulking of Sand Test
When sand is damp, its volume increases due to the formation of a thin film of water around each particle. This phenomenon is called bulking. Since volumetric batching is common on-site, it’s essential to account for this increase to maintain the correct proportions.
| Moisture Content (%) | Bulking (%) |
|---|---|
| 2 | 15 |
| 3 | 20 |
| 4 | 25 |
| 5 | 30 |
For example, if the moisture content is 4%, the volume of sand must be increased by 25% to compensate for bulking.
Conclusion
Ensuring the quality of sand is a critical step in achieving durable and strong concrete structures. By conducting regular field tests—such as checking for silt content, grading, organic impurities, and bulking—construction teams can prevent costly rework and ensure compliance with national standards like IS 2386.