The compressive strength of 100 mm cube as compared to 150 mm cube is always...............
📏 The "Size Effect" in Concrete Testing
In materials science, the "size effect" is a phenomenon where the measured strength of a material changes with the size of the test specimen. For brittle materials like concrete, smaller specimens tend to show higher strength than larger specimens.
This is primarily explained by Weibull's "weakest link" theory. Concrete is not perfectly homogenous; it contains micro-cracks and voids. A larger volume of material has a higher statistical probability of containing a larger, more critical flaw that will initiate failure under a lower stress. A smaller specimen has a lower chance of containing such a critical flaw, and therefore appears stronger.
📝 Detailed Analysis of the Options
(a) Less
This is incorrect. Due to the size effect, smaller cubes exhibit higher compressive strength.
(b) More
This is the correct answer. A 100 mm cube has a smaller volume than a 150 mm cube. Therefore, it has a lower probability of containing a strength-limiting flaw. When tested, the 100 mm cube will consistently show a higher compressive strength value. The difference is generally considered to be around 10%.
(c) Equal
This is incorrect. While the concrete mix is the same, the specimen size has a measurable impact on the test result.
(d) None of these
This is incorrect as option (b) is the correct statement.
📊 Cube Size and Strength Comparison
| Cube Size | Standard For | Relative Strength |
|---|---|---|
| 150 mm | Standard cube size for concrete strength testing in India (IS 456). | 100% (Reference Strength) |
| 100 mm | Used when aggregate size is smaller (< 20mm) or for specific research. | ~110% (Approx. 10% More) |
💡 Study Tips
- Smaller is Stronger: This is the key takeaway for the size effect in concrete cube testing.
- Think Probability: Remember the reason: a bigger cube has a better chance of having a "weak spot" inside it.
- 150mm is the Standard: Know that the 150mm cube is the standard reference for characteristic strength (fck) in the Indian Standard code. The strength of other sizes is compared to this standard.
- Remember the 10%: The approximate 10% difference between 100mm and 150mm cubes is a commonly cited figure in exams.