A construction firm has decided to use burnt clay bricks of class 20 and above (as per IS 1077: 1977) for the construction of a residential complex. The table below gives the details of Samples with Id (S1, S2, S3, S4 and S5) taken from five different suppliers. The criteria to be adopted are the percentage of water absorption and average compressive strength of bricks corresponding to bricks of class 20 and above. Select the Id of the samples which can be recommended for use in brick wall construction
| Id | Water absorption (%) | Average compressive strength (kN/m²) |
|---|---|---|
| S1 | 18 | 17500 |
| S2 | 20 | 21000 |
| S3 | 12 | 20000 |
| S4 | 10 | 14000 |
| S5 | 15 | 27000 |
📋 Understanding the Criteria (IS 1077)
This question requires us to evaluate brick samples against the standards set by IS 1077 for a specific quality level, "Class 20". To do this, we need to know the requirements for this class.
- Compressive Strength: The class number itself indicates the minimum average compressive strength in N/mm². So, for Class 20, the strength must be at least 20 N/mm².
- Water Absorption: For high-quality bricks (Class 12.5 and above), IS 1077 specifies that the average water absorption after 24-hour immersion in cold water should not exceed 15% by weight.
- Unit Conversion: The strength is given in kN/m². We must convert it to N/mm².
1 N/mm² = 1000 kN/m². So, we need to divide the given values by 1000.
🔬 Detailed Analysis of the Samples
Let's check each sample against the two criteria: Strength ≥ 20 N/mm² and Absorption ≤ 15%.
B. S3 and S5 only
This is the correct answer. Let's verify:
- Sample S3: Absorption is 12% (≤ 15%, OK). Strength is 20,000 kN/m² = 20 N/mm² (≥ 20, OK). This sample is acceptable.
- Sample S5: Absorption is 15% (≤ 15%, OK). Strength is 27,000 kN/m² = 27 N/mm² (≥ 20, OK). This sample is acceptable.
Since only S3 and S5 meet both criteria, this option is correct.
Why other samples are rejected:
- Sample S1: Absorption is 18% (too high) and Strength is 17.5 N/mm² (too low). Fails on both counts.
- Sample S2: Strength is 21 N/mm² (OK), but Absorption is 20% (too high). Fails.
- Sample S4: Absorption is 10% (OK), but Strength is 14 N/mm² (too low). Fails.
Therefore, options A, C, and D are incorrect because they include samples that do not meet the required standards.
📊 Summary: Sample Evaluation for Class 20 Brick
| ID | Water Abs. (%) | Strength (N/mm²) | Meets Abs. (≤15%)? | Meets Strength (≥20)? | Recommended? |
|---|---|---|---|---|---|
| S1 | 18 | 17.5 | No ❌ | No ❌ | No |
| S2 | 20 | 21.0 | No ❌ | Yes ✅ | No |
| S3 | 12 | 20.0 | Yes ✅ | Yes ✅ | Yes |
| S4 | 10 | 14.0 | Yes ✅ | No ❌ | No |
| S5 | 15 | 27.0 | Yes ✅ | Yes ✅ | Yes |
💡 Study Tips
- Class = Strength: For IS 1077, the brick class (e.g., 10, 15, 20) directly tells you the minimum compressive strength in N/mm².
- Check Both Conditions: A brick must satisfy *both* the strength and water absorption criteria to be acceptable. Don't stop after checking just one.
- Watch the Units: Pay close attention to units. Converting from kN/m² to N/mm² (divide by 1000) is a common step in these problems.
- Higher Class, Stricter Rules: Remember that as the brick class (strength) increases, the requirement for water absorption becomes stricter (the maximum allowed percentage decreases).