The coefficient of hardness of stones used in road work should be greater than
🛣️ Understanding the Coefficient of Hardness
The Coefficient of Hardness is a value that measures a stone's resistance to abrasion and wear. For road construction, this is a critical property because the stones (aggregates) in the road surface are constantly subjected to the grinding action of traffic.
This value is determined in a laboratory using Dorry's Abrasion Test. The test involves a specific procedure and formula to arrive at the coefficient.
The Dorry's Test Procedure & Formula
The test is performed as follows:
- A cylindrical sample of the stone (usually 25 mm in diameter and 25 mm high) is prepared.
- The initial weight of the sample is accurately measured.
- The sample is placed in Dorry's testing machine, where it is pressed against a rotating metal disc under a constant pressure (1250 gm).
- Standard sand is sprinkled on the disc to act as the abrasive agent.
- The machine is run for 1000 revolutions.
- After the test, the sample is removed and weighed again. The difference between the initial and final weight is the "Loss of Weight" in grams.
The formula to calculate the Coefficient of Hardness is:
Coefficient of Hardness = 20 - (Loss of Weight in grams / 3)From this formula, you can see that a smaller loss of weight results in a higher coefficient of hardness, indicating a more durable stone.
🔬 Detailed Analysis of the Required Values
D. Greater than 17
This is the correct requirement for high-quality road work. Stones with a coefficient of hardness above 17 are considered very hard and durable. They are essential for constructing road surfaces (pavements) that can withstand heavy traffic loads and resist wear and tear for a long time.
A, B, C (10, 12, 15)
These values represent stones that are too soft for use in the wearing course of a major road. Using such stones would lead to rapid deterioration of the road surface.
- Less than 14: Stones with a coefficient of hardness below 14 are considered soft and are not suitable for road work.
- 14 to 17: These stones are considered of medium hardness. They might be acceptable for roads with very light traffic or in lower layers of the pavement, but they do not meet the standard for a durable surface course.
- Therefore, a value greater than 17 is the standard requirement to ensure a long-lasting and safe road.
📊 Summary: Hardness for Road Aggregates
| Coefficient of Hardness | Stone Quality | Suitability for Road Work |
|---|---|---|
| < 14 | Soft | Not suitable |
| 14 - 17 | Medium | Suitable for light traffic / lower layers |
| > 17 | Hard | Required for road surfaces |
💡 Study Tips
- High Traffic = High Hardness: Remember that major roads with heavy traffic need the hardest stones. Look for the highest value in the options as the minimum requirement.
- "17 is the Highway to Heaven": Think of 17 as the magic number that makes a stone worthy of being on a major highway.
- Inverse Relationship: Remember the formula. A low loss of weight means a high coefficient of hardness.