28 day crushing strength of cement is tested on 70.6 mm size cubes of mortar having cement to sand proportion of:
💪 Why Test Compressive Strength?
Compressive strength is the most important property of cement and concrete, as it measures the material's ability to resist squeezing or crushing forces. Since concrete is used almost exclusively for its strength in compression, this test is the primary indicator of its quality and suitability for structural applications.
To ensure that test results from different labs and different batches of cement are comparable, a highly standardized procedure must be followed. This includes the size of the test specimen, the curing conditions, the testing age, and, crucially, the composition of the mortar used.
📝 Detailed Analysis of the Options
The standard test for the compressive strength of cement requires creating mortar cubes and testing them in a compression testing machine. The specifications for this mortar are very precise to ensure consistency.
(a) 1:3
This is the correct answer. According to IS 4031 (Part 6), the standard mortar for the compressive strength test is prepared by mixing one part of cement with three parts of standard sand by weight. The sand used must conform to IS 650 (Standard Sand for Testing of Cement). These ingredients are mixed with a specific amount of water to create mortar cubes of 70.6 mm size. This specific size gives a surface area of approximately 5000 mm² (or 50 cm²), which simplifies load calculations. The cubes are then cured for 3, 7, or 28 days before being crushed.
(b) 1:4, (c) 1:5, and (d) 1:6
These are incorrect ratios for the standard laboratory test. While these leaner mortar mixes might be used in certain plastering or masonry applications, they are not used for the official determination of cement's compressive strength. Using a leaner mix would result in a lower strength reading that would not be comparable to the specified grade of the cement.
📊 Key Parameters for Cement Compressive Strength Test
| Parameter | Standard Specification |
|---|---|
| Mortar Proportion (Cement:Sand) | 1:3 by weight |
| Cube Size | 70.6 mm x 70.6 mm x 70.6 mm |
| Standard Sand | Conforming to IS 650 |
| Testing Ages | 3, 7, and 28 days |
💡 Study Tips
- The Magic Ratio is 1:3: For compressive strength testing of cement, the standard mortar mix is always 1 part cement to 3 parts standard sand. Commit this to memory.
- 70.6 mm = 50 cm²: Remember the unique cube size of 70.6 mm. This size is chosen because its face area is very close to 50 cm², making it easy to calculate the stress (Force/Area).
- Standard Sand is Key: The test doesn't use just any sand; it uses a specific "Standard Sand" to eliminate variability.
- Distinguish from Concrete Cubes: Don't confuse this test with the compressive strength test of *concrete*, which uses larger cubes (typically 150 mm) and includes coarse aggregate. This test is for the cement *itself*, using a standardized mortar.