If the engineer-in charge approves, the 10 cm cubes may be used for the work test of concrete provided maximum nominal size of aggregate does not exceed :
Correct Answer: C. 20 cm
📚 Detailed Explanation: Cube Size vs. Maximum Aggregate Size in Testing
The standard cube for compressive strength testing in India is 150 mm × 150 mm × 150 mm. IS 456:2000 permits the use of smaller 100 mm (10 cm) cubes for site work tests, subject to the engineer's approval, provided the aggregate size is not too large relative to the cube dimensions.
Why C (20 cm, interpreted as 20 mm) is correct: The standard rule is that the maximum aggregate size should not exceed 1/4 of the smallest dimension of the test specimen. For a 100 mm cube: 100 / 4 = 25 mm. However, IS 456 Table 16 specifies that 100 mm cubes are permitted when the maximum nominal aggregate size does not exceed 20 mm. The question options are in cm but should be interpreted as mm (a common source of confusion in older textbooks).
Cube Size and Permitted Aggregate Size
| Cube Size | Standard Use | Max Aggregate Size |
|---|---|---|
| 150 mm (15 cm) | Standard test cube (IS 516) | Up to 40 mm |
| 100 mm (10 cm) | Work test (with approval) | ≤20 mm |
Key Concepts for Students
- Results from 100 mm cubes are approximately 5–10% higher than 150 mm cubes due to the size effect in concrete strength testing.
- IS 516:1959 specifies the full procedure for making, curing, and testing standard cubes.
- Cylinder tests (diameter:height = 1:2) are used internationally; Indian practice uses cubes as the primary test.