Compressive Strength Test of Cement as per Indian Standard (IS 4031: Part 6)

Compressive Strength Test of Cement as per Indian Standard (IS 4031: Part 6)

Aim
The aim of this test is to determine the compressive strength of cement using cement mortar cubes. The compressive strength is a crucial property of cement as it gives an idea of the cement’s performance under load and its suitability for different types of construction.

Apparatus

1. Compression Testing Machine (Capacity: 2000 kN)
2. Vicat Apparatus
3. Standard IS Moulds (70.6 mm cube size)
4. Weighing Balance
5. Trowel
6. Vibrating Table (Optional)
7. Measuring Cylinder
8. Gauging Trowel
9. Cement, Sand (standardized), and Water

Theory
The compressive strength of cement is a measure of its ability to withstand axial loads when applied. Cement's strength is an important factor that affects the durability and load-bearing capacity of concrete. The test is conducted by preparing cement mortar cubes made of cement, sand, and water in a specified ratio (1:3 by mass) and testing these cubes under compression at various time intervals (3 days, 7 days, and 28 days).

The compressive strength is calculated by dividing the maximum load applied to the cube by the surface area of the cube face (70.6 mm × 70.6 mm). Higher compressive strength indicates better performance of the cement under load-bearing applications.

Procedure

1. Preparation of Mortar: Mix cement and standardized sand in the ratio of 1:3 by mass. Add water, which is 0.4 times the total mass of cement and sand, and mix thoroughly to obtain a uniform paste.
2. Filling the Moulds: Fill the prepared mortar into the standard IS moulds (70.6 mm cube size) in two layers, compacting each layer using a tamping rod or vibrating table.
3. Curing the Specimens: After filling the moulds, keep the specimens at a temperature of 27°C ± 2°C for 24 hours. After this period, remove the cubes from the moulds and immerse them in clean water for curing until testing.
4. Testing: After curing, test the cubes for compressive strength at specified time intervals (3 days, 7 days, and 28 days). Place the cube in the compression testing machine and apply load uniformly at a rate of 35 N/mm² per minute until failure.
5. Recording the Failure Load: Record the maximum load (P) at which the cube fails.

Observation Table

Age of Cube (Days)	Mass of Cube (g)	Failure Load (kN)	Compressive Strength (N/mm²)
3	700	500	10.0
7	700	1000	20.0
28	700	1500	30.0

Sample Readings

• For a 3-day-old cube:

  • Failure Load = 500 kN
  • Cross-sectional area of the cube = 70.6 mm × 70.6 mm = 4996.36 mm²
  • Compressive Strength = Failure Load / Cross-sectional area
  • Compressive Strength = 500 kN / 4996.36 mm² = 10.0 N/mm²

• For a 7-day-old cube:

  • Failure Load = 1000 kN
  • Compressive Strength = 1000 kN / 4996.36 mm² = 20.0 N/mm²

• For a 28-day-old cube:

  • Failure Load = 1500 kN
  • Compressive Strength = 1500 kN / 4996.36 mm² = 30.0 N/mm²

Reporting the Results
The compressive strength of cement is reported for 3, 7, and 28 days. The values are calculated based on the formula:

$$\text{Compressive Strength (N/mm²)} = \frac{\text{Failure Load (kN)}}{\text{Area of Cube Face (mm²)}}$$

For example:

• 3-day compressive strength: 10.0 N/mm²
• 7-day compressive strength: 20.0 N/mm²
• 28-day compressive strength: 30.0 N/mm²

Conclusion
The compressive strength of cement is a critical property that reflects its ability to resist loads. For Ordinary Portland Cement (OPC), the minimum compressive strength required as per IS 4031: Part 6 is:

• 16 N/mm² for 3 days,
• 22 N/mm² for 7 days, and
• 33 N/mm² for 28 days.

In the example test, the compressive strength values meet the minimum requirements, indicating that the cement is suitable for structural purposes. The results of this test help in assessing the quality and usability of cement in construction.